Complete Class 12 Chemistry Notes of Benzene and Phenols according to the Sindh Board, Federal and Punjab Board Syllabus 2025. Includes structure, resonance, chemical properties, electrophilic substitution reactions, and preparation methods with examples perfect for revision and board exam preparation..
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🔥Summary of Benzene — Class 12 Chemistry Notes
| Topic | Key Points / Explanation |
|---|---|
| Molecular Formula | C₆H₆ |
| Structure | Planar hexagonal ring; each carbon is sp² hybridized |
| Bonding | 6 σ-bonds + 3 delocalized π-bonds |
| Resonance | Benzene shows delocalized π-electrons, making it unusually stable |
| Aromaticity | Follows Hückel’s rule (4n+2 π electrons) where n = 1 |
| Stability | Highly stable due to resonance; does not undergo addition easily |
| Physical Properties | Colorless liquid, sweet smell, flammable |
⚗️ Important Reactions of Benzene
| Reaction Type | Equation / Explanation |
|---|---|
| 1. Nitration | C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O (with H₂SO₄) |
| 2. Halogenation | C₆H₆ + Cl₂ → C₆H₅Cl + HCl (FeCl₃ catalyst) |
| 3. Sulphonation | C₆H₆ + H₂SO₄ → C₆H₅SO₃H + H₂O |
| 4.Friedel–Crafts Alkylation | C₆H₆ + CH₃Cl → C₆H₅CH₃ + HCl (AlCl₃ catalyst) |
| 5. Friedel–Crafts Acylation | C₆H₆ + CH₃COCl → C₆H₅COCH₃ + HCl (AlCl₃ catalyst) |
| 6. Combustion | 2C₆H₆ + 15O₂ → 12CO₂ + 6H₂O (sooty flame) |
🧾 Summary of Phenols — Class 12 Chemistry Notes
| Topic | Key Points / Explanation |
|---|---|
| Structure | Phenol = Benzene ring with –OH group directly attached |
| Formula | C₆H₅OH |
| Nature | Weakly acidic due to partial ionization of –OH |
| Hydrogen Bonding | Causes high boiling point and solubility in water |
| Resonance | Lone pair on oxygen delocalizes into the aromatic ring |
| Aromatic Character | Similar to benzene; stabilized by resonance |
⚗️ Important Reactions of Phenols
| Reaction Type | Equation / Explanation |
|---|---|
| 1. Reaction with Sodium | 2C₆H₅OH + 2Na → 2C₆H₅ONa + H₂ |
| 2. Reaction with NaOH | C₆H₅OH + NaOH → C₆H₅ONa + H₂O |
| 3. Bromination | C₆H₅OH + 3Br₂ → 2,4,6-tribromophenol + 3HBr |
| 4. Nitration | C₆H₅OH + HNO₃ → 2-nitrophenol + 4-nitrophenol |
| 5. Kolbe’s Reaction | C₆H₅ONa + CO₂ + H₂O → o-hydroxybenzoic acid (salicylic acid) |
| 6. Reimer–Tiemann Reaction | C₆H₅OH + CHCl₃ + NaOH → o-hydroxybenzaldehyde |
🧩 Quick Revision Points
📘 Suggested Reading
https://learnchemistrybyinamjazbi.blogspot.com/2024/11/iupac-nomenclature-of-organic-compounds.html
https://learnchemistrybyinamjazbi.blogspot.com/2025/11/class-12-chemistry-notes-benzene-and.html
https://learnchemistrybyinamjazbi.blogspot.com/2025/11/class-12-chemistry-notes-alkynes.html
https://learnchemistrybyinamjazbi.blogspot.com/2025/11/xii-chemistry-notes-ozonolysis-or.html
https://learnchemistrybyinamjazbi.blogspot.com/2025/02/xii-chemistry-model-test-questions.html
https://learnchemistrybyinamjazbi.blogspot.com/2024/10/alkyl-group.html
https://learnchemistrybyinamjazbi.blogspot.com/2024/09/diagonal-relationship-of-representative.html
https://learnchemistrybyinamjazbi.blogspot.com/2024/08/general-group-trends-of-representative.html
https://learnchemistrybyinamjazbi.blogspot.com/2024/08/flame-test-for-s-block-elements.html
https://learnchemistrybyinamjazbi.blogspot.com/2024/08/xii-chemistry-model-test-questions-xii.html
https://learnchemistrybyinamjazbi.blogspot.com/2024/08/ixxii-valency.html
https://learnchemistrybyinamjazbi.blogspot.com/2024/08/xii-completes-chemistry-notes-nationals.html
https://learnchemistrybyinamjazbi.blogspot.com/2024/03/xii-chemistry-model-test-questions-test.html
https://learnchemistrybyinamjazbi.blogspot.com/2024/04/xii-environmental-chemistry-chapter-12.html
https://learnchemistrybyinamjazbi.blogspot.com/2024/04/xii-chemistry-environmental-chemistry.html
https://learnchemistrybyinamjazbi.blogspot.com/2024/04/xii-chemistry-chemical-industries.html
https://learnchemistrybyinamjazbi.blogspot.com/2024/04/xii-model-test-questions-test-on.html
Introduction to Benzene and Its Derivatives and Information about Benzene
Molecular Orbital Structure of Benzene
🔥Total Valence Electrons in Benzene
There are six carbon and six hydrogen atoms in benzene. Each carbon atom contributes four and each hydrogen atom contributes one valence electrons in benzene, so there are thirty valence electrons in benzene (C = 24, H = 6).
🔥Nature of hybridization
According to molecular orbital treatment, the nature of hybridization in each of the six carbon atoms in a ring of benzene should be sp2 as each carbon atom is linked with three other atoms (Two carbon and one hydrogen) and has three sp hybrid orbitals.
🔥Evidences of sp2-hybridization
X-rays analysis and other evidences show that benzene molecule is a regular planer hexagon with all the six hydrogen atoms lying in the plane of ring with C–C–C bond angles of 120°. This clearly indicates that each carbon atom forms a Hybrid Orbital by mixing one 2s and two 2p orbitals, called sp2-hybrid orbital.
🔥Formation of sp2-hybrid orbital
The hybridization in benzene can be explained by considering the electronic configuration of carbon. sp2-orbital hybridization in each carbon atom forms three sp2-hybrid orbitals at the mutual angle of 120°. The unhybridized 2pz orbital of each carbon remain at right angle to the plane of sp2-orbitals. Thus each carbon has three hybrid and one unhybrid orbitals.
🔥Sigma Bond Formation in Benzene
Two of three sp2-hybrid orbitals of each carbon atom overlap linearly with sp2-hybrid orbitals of two adjacent carbon atoms through sp2-sp2 overlapping forming two C–C sigma bonds while remaining one sp2-orbital of each carbon atom overlap linearly with 1s orbital of a hydrogen atom through sp2-s overlapping forming one C–H sigma bond. Thus, there are six C–C s-bonds (as there are six carbon) and six C–H s-bonds in benzene i.e. there are total twelve sigma bonds. [Twelve electrons are thus located in six C–C s-bonds and another twelve to six C–H s-bonds. This account for twenty four electrons. The remaining six electrons are present in the form of p-electrons in unhybridized orbital]. All sigma bonds in benzene are coplanar and have bond angle of 120o. In benzene, the carbon-carbon bond length as well as carbon-carbon double bond (C=C) length is approximately 1.39Å.
The Kekule
structure of benzene as cyclohexa-1,3,5-triene suggests two bonds
lengths for the separate single and double bonds i.e. C–C bond length = 154 pm and
C=C bond length = 134 pm. Experiment shows only one C – C bond length of 139
pm, between the bond the bond lengths for single and double carbon-carbon
bonds.
This shows that
each carbon=carbon bond in the benzene rings is intermediate between a single
and a double bond.
🔥Pi Bond Formation and Delocalized p-Bond in Benzene
The six non-hybridized pz-orbitals of six carbon atoms that are orientated perpendicular to the sigma bonds undergo side-wise or lateral overlapping (as in alkene) with one another above and below the benzene ring forming six delocalized molecular orbitals in the form six p-bonds where are half of them are located above the plane while other half below the plane of sigma bonds; enveloping the six C–C s- bonds. the six p-electrons in pi molecular orbitals get associated with all the carbon atoms forming a continuous circular sheath of p-electronic cloud forming delocalized/diffused electronic cloud above and below the plane of the ring.
🔥Different Ways of Overlapping Unhybrid Pz-Orbitals
The 2pz1 electron of two adjacent carbon atoms are shared forming a p-molecular orbital. The electrons in these p-orbitals can be shared in two ways as shown in the figure:
The overlap of these p-orbitals result in
the formation of a fully-delocalized p-bond which extends
all over the six carbon atoms. Delocalized pi electrons are not confined to a
specific bond but instead spread out over a larger region of a molecule. [These
electrons of p-orbitals are
also known as Diffused or Delocalized electrons while the process of formation
of p-bond spreading
all over six carbon atoms is known as Delocalization. Due to this
delocalization of p-electrons in benzene, a more stronger p-bond than normal p bond is obtained and more stable molecule
is formed]. Delocalized pi electrons are commonly found in molecules with
alternate single and double bonds like benzene.
🔥Conclusion
The molecular orbital approach about
structure of benzene provides the following information:
1. Benzene is planar hexagonal molecule.
2. the continuous sheath of delocalized p-electrons is responsible for extra-stability and peculiar behaviour of benzene molecule.
3. Actual structure of benzene is a hybrid of two resonating Kekule’s structures.
Chemical Reactions of Benzene
Benzene undergoes following reactions:
(A)
Electrophilic Substitution Reactions
(B)
Addition Reactions
(C) Oxidation Reactions
Electrophilic
Substitution Reactions (E.S.R.)
💥Electrophiles
Electrophiles is an electron-deficient species that seeks to gain an electron pair. Some common examples of electrophiles are chloro cation (Cl+), alkyl cation (R+), acyl cation (RCO+), nitronium ion (NO2+), and sulphonium ion (SO3H+).
💥Definition of electrophilic
substitution reactions
The fundamental reaction type exhibited by aromatic compounds (e.g. benzene) is electrophilic aromatic substitution (EAS) also known as Electrophilic Substitution Reactions denoted by ESR or SE.
electrophilic substitution reactions or aromatic substitution reactions are a type of organic substitution reaction involving the replacement of one or more electrophilic hydrogen from the ring of an electron-rich compound like benzene and its derivatives by an electrophile (E+) of unsymmetrical reagent (of the type E–Nu) in the presence of Lewis acid catalyst to give substituted product (monosubstituted or poly-substituted benzene).
Benzene ring with
its delocalized p electrons is an
electron-rich system which is attacked by electrophiles (electron-loving
species) giving substituted products.
💥Reason of Undergoing E.S.R
The delocalized p-bonds in benzene, stabilizes the ring and retards the addition reactions. The six pi electrons of p-orbital of benzene are spread out over all six carbon atoms making benzene less reactive towards addition reaction. In addition, high resonance energy (150 kJ/mol) of a delocalized p-bond gives stability to benzene towards addition reactions. The highly delocalized pi bond of make benzene more reactive towards electrophilic substitution reactions by which Resonance Stabilized Ring System is preserved.
💥Details
Benzene (and its homologues) possess extensively delocalized p-bonds (which extends all over the six carbon atoms) in the form of continuous sheath of p-electronic cloud. The continuous sheath of highly diffused and fully delocalized p-bonds is responsible for extra-stability (hyper-stability) of benzene ring. The delocalized p-bonds in benzene, stabilizes the ring and retards the addition reactions. The stability of the benzene ring is due to very high value of resonance or delocalization energy (36 kcal/mol or 150 kJ/mol) of the delocalized p-electronic system.
Most of the
addition reactions cannot supply the required amount of energy, hence no
addition reactions take place. Thus high resonance energy of benzene retards
the addition reactions and acts as a barrier for the addition reactions and
stabilizes the benzene ring.
Therefore, benzene undergoes substitution reactions by which
Resonance Stabilized Ring System is preserved.
💥General Pattern of the
Mechanism of E.S.R
The electrophilic substitution reactions of benzene are due to delocalized p-electrons. Due to presence of p-electrons, benzene in its reaction may serve as a nucleophile. However, the highly stable, delocalized p-electrons of benzene ring are not readily available for the nucleophilic attacks like the p-electrons of alkenes. As the p-electrons are stable than alkene, a more powerful electrophile is required to penetrate and break the continuous sheath of p-electron cloud in benzene. Hence in benzene ESR occur by which resonance stabilized ring system is preserved.
Regardless of the electrophile used, all electrophilic aromatic substitution reactions occur by the same two-step mechanism — addition of the electrophile E+ to form a resonance-stabilized carbocation (phenonium ion), followed by deprotonation with base into MSB.
The E.S.R. of benzene involves following three steps:
1. Formation of
Strong Electrophile by Catalytic Heterolytic Cleavage of attacking reagent with
LAC
2. Formation of s-complex /Phenonium/Arenium
Ion by the attack of Strong electrophile
on benzene
3. Abstraction of a Proton or De-protonation and Regeneration of Catalyst with the restoration of Cyclic System or Aromaticity of Benzene i.e. Re-aromatization of ring giving MSB
Step-I (Formation of Strong
Electrophile by Catalytic Heterolytic Cleavage of attacking reagent with Lewis
acid catalyst)
The first step of ESR involves the formation of a strong and powerful electrophile (E+) by the reaction of Lewis acid catalyst with the attacking unsymmetrical reagent (E–Nu) involving heterolytic fission. This electrophile has ability to pull p-electrons of benzene.
Step-II (Formation of s-complex / Arenium Ion by the attack of Strong electrophile on benzene)
The strong electrophile (E+) attacks on p-system of benzene thereby producing an intermediate unstable cyclohexadienyl carbocation which is called Arenium ion (benzonium ion or phenium ion) or s-complex by taking up two electrons of p-system to form a sigma bond between it and one of the carbon of benzene ring. The formation of arenium ion breaks cyclic system of p-electrons because due to change of hybridization of one carbon (which is linked to the electrophile) and it becomes sp3-hybridized. This causes instability in the ion due to loss of aromaticity (or delocalization of p-bond).
The positive
charge in this adduct is actually delocalized over three of the
carbon atoms; the ion can be described as a resonance hybrid of three structures.
The arenium ion is a resonance hybrid of three allylic-type structures, each of which
has a positive charge on an ortho or para carbon of the ring.
Step-III (Abstraction of a Proton
and Regeneration of Catalyst with the restoration of Cyclic System or
Aromaticity of Benzene)
Arenium ion formed is not stable due to loss of cyclic structure. This arenium ion
loses a proton from the carbon atom to which electrophile (E+) has
attached resulting in the regeneration of double bond (i.e. cyclic ring
structure) which restores the aromaticity of the ring and gives monosubstituted
derivative of benzene.
[It may react with Nu- but this would result the resonance or stabilization energy of benzene is lost as the product would no longer be aromatic]. The carbocation intermediate is deprotonated by a weak base, restoring aromaticity. The aromaticity is prevented by the loss of proton from the sp3-hybiridized carbon of benzonium ion allowing both electrons from the C–H bond to go back into the ring and regenerates the cyclic aromatic ring structure (i.e. double bond) thereby giving monosubstituted derivative of benzene (MSB). This re-aromatization stores or preserves the stability of the ring].
💥Summary
💥Summary of Mechanism of Different Electrophilic Substitution Reactions of Benzene
1. Halogenation
2. Nitration
3. Sulphonation
4. Friedel-Craft Alkylation
5. Friedel-Craft Acylation
🔥📘1. Halogenation
of Benzene
💥Definition of Halogenation
Halogenation of benzene is a substitution reaction which involves the replacement of its one hydrogen atom by a halogen atom in the dark in the presence of Lewis acid catalyst such as FeX3 yielding halobenzene.
💥Order of Reactivity of Different Halogens
The order of reactivity of different halogens are F2 > Cl2 > Br2 > I2. Fluorination is too vigorous to control while Iodination does not give good yield. Thus only chlorination and bromination of benzene can be carried out.
💥Chlorination and Bromination of Benzene
Benzene gives substitution
reaction with halogens like Cl2, Br2 at room temperature
in the presence of Lewis Acids catalysts such
as FeX3 and in the absence of light (in dark) in which one
hydrogen atom of benzene is replaced by a halogen atom to yield halobenzene (chlorobenzene (90%) and
bromobenzene (65-75%)
respectively). [Fluorination is too vigorous to control while iodination
does not give good yield].
Various Lewis
Acids catalysts collectively known as ‘halogen carriers’ such as FeX3,
AlX3 or a mixture of AlX3 and Fe powder (Fe + AlX3
→ FeX3 + Al) or merely iron powder (2Fe + 3X2 → 2FeX3)
is used.
💥Function of Lewis Acid Catalyst
The function of Lewis acid catalysts such as FeX3 is to provide a powerful electrophile i.e. a positively charged halogen ion (X+).
Lewis acid
catalysts such as FeX3 function by polarizing the halogen molecule
(either partially or completely) to provide an electron-deficient powerful
electrophile i.e. a positively charged halogen ion (X+). Lewis acid
catalysts can stretch the X–X bond in such a way that attack on p-electrons of benzene is facilitated.
💥Iodination of benzene
Iodination of benzene is usually brought about by refluxing benzene and
iodine in presence of concentrated
nitric acid. The acid serves to provide the active iodinating species, possibly
Iodination requires an acidic oxidizing agent like nitric acid which oxidizes the iodine to an iodonium ion
🔥📘2. Nitration of Benzene
💥Definition of Nitration
Nitration is the name we give to the process of attaching the nitro group (NO2) to a molecule. The introducing a nitro group (–NO2 or +NO2) of nitric acid in an aromatic ring or benzene is called Nitration. It is the type of electrophilic substitution reaction of benzene with conc. Nitric acid involving the replacement of its one hydrogen by a Nitronium (–NO2 or +NO2) ion to produce nitrobenzene.
(Nitration is an important substitution reaction in the aromatic series. Subsequent reduction of the nitro group to the amino group (–NH2) can be readily carried out, and other function groups can be derived from this versatile amino group).
💥Reagent for Nitration (Nitrating Mixture) and Product of
Nitration
The nitration of benzene is carried out by warming benzene at 50–55°C (30–40°C) with a 1:1 mixture of conc. HNO3 and conc. H2SO4, called nitrating mixture producing nitrobenzene.
(In contrast to nitration of alcohols, the nitration of benzene produces relatively stable nitro compounds that are much more difficult to detonate. For example, the high-explosive TNT (2,4,6-trinitrotoluene) is formed by triple nitration of toluene. Another explosive, RDX comes from nitration of trihydro-1,3,5-triazine).
The key reagent for nitration is nitric acid, HNO3. By itself, nitric acid is a relatively slow-acting electrophile, especially in the presence of a poor nucleophile such as benzene. [Note – in the case of phenol and other aromatic rings with strongly activating groups, HNO3 by itself is sufficient for nitration].
💥Nitration of Toluene
Toluene on nitration with conc. HNO3 and conc. H2SO4, gives a mixture of ortho and para-nitrotoluene.
💥Function of Nitric Acid (Nitrating agent)
The nitric acid
serves as a source of nitronium ion
💥Function of Sulphuric acid
The function of
Sulphuric acid is two folds:
i) It dilutes or consumes the water formed in the overall reaction.
ii) It increases the reaction rate by increasing the concentration of
electrophilic nitronium ion +NO2
💥Mechanism of Nitration
🔥📘3. Sulphonation/sulphonylation
💥Definition of Sulphonation
The introducing a sulphonic acid group or
sulfonyl group (S+O3H
or –SO3H) of Sulphuric acid in aromatic ring or benzene is called
Sulphonation.
💥Sulphonating agent and Electrophile in Sulphonation
In sulphonation of
benzene, the electrophilic reagent is sulfur
trioxide, SO3 (a
gas) which can be introduced by bubbling through the solvent. On its own,
SO3 is not particularly reactive with aromatic rings, but addition of an acid can increase electrophilicity (and reaction rate) considerably (acid
catalysis). Like nitric acid, sulfur trioxide is “activated” by the addition of a
proton from sulfuric acid. [combination
of SO3 and H2SO4 is called “fuming sulfuric acid/oleum”]. What really give each reaction its unique flavor is
how the electrophile is activated, either by Lewis acid catalysis
(chlorination, bromination, alkylation and acylation) or Bronsted acid
catalysis (nitration, Sulphonation).
💥Sulphonation
with Concentrated Sulphuric acid
[Benzene undergoes sulphonation with conc.
Sulphuric acid at 50-60°C to give benzene sulphonic acid].
Sulphonation of
benzene by 98% concentrated H2SO4 alone is appreciably
reversible and takes place very slowly and refluxing for about 8-20 hours is
required giving benzenesulphonic acid. In this aromatic sulphur compound,
carbon is linked to sulphur (C-SO3H) whereas in ethyl
hydrogensulphate carbon is linked to oxygen (H3C–H2C–O–SO2–OH).
💥Sulphonation
with Fuming Sulphuric acid/Oleum
Sulphonation of benzene can be carried out
at room temperature by treating it
with fuming sulphuric acid/oleum (H2SO4.SO3) in cold quite rapidly to
produce benzene sulphonic acid. Fuming sulphuric acid contains dissolved sulphur trioxide (SO3) which
is the active sulphonating agent.
💥Sulphonation of toluene
Sulphonation of toluene gives a mixture of ortho and para-methylbenzene sulphonic acid.
💥Alternative
Mechanism of Sulphonation of Benzene
The sulphur
trioxide electrophile arises in one of two ways depending on which sort of acid
you are using. Concentrated sulphuric acid contains traces of SO3
due to slight dissociation of the acid.
The
reactive electrophilic species in oleum is SO3. Unlike nitronium
ion, the SO3 molecule carries no cationic charge. Nevertheless the
sulphur atom has a reasonable partial positive charge owing to the polarity of
the sulphur-oxygen bonds. The sulphur atom is hence the site of the
electrophilic reactivity of SO3 and this accounts for the formation
of carbon-sulphur bond when it accepts a pair of electrons from the aromatic
ring.
H2SO4 ⇌ H2O + SO3
🔥📘4. Friedel-Craft’s Reaction
The preparing
alkyl-substituted aromatic compound (alkylbenzene) and acyl-substituted
aromatic compound (acylbenzene i.e. aromatic ketone) by the introduction of an
alkyl group (–R) and an acyl group (R–CO–) respectively in the benzene ring in
presence of Lewis acid catalyst such as AlCl3 is called
Friedel-Craft’s Reaction (after a French chemist, Friedel and his American collaborator,
Craft).
🔥📘Friedel-Craft’s Alkylation
Friedel-Craft’s Alkylation is an electrophilic substitution reaction in which H atom of benzene or its derivatives is replaced by alkyl group (R– or R+) of alkyl halide.
💥Reagent used for Friedel-Craft’s Alkylation/Alkylating
agent
Alkylation of benzene is carried out by heating it with alkyl
halide in presence of trace of Lewis acid catalyst such as AlX3,
yielding alkylbenzene
e.g.
When benzene
reacts with methyl chloride in the presence of Lewis acid catalyst such as AlCl3,
methylbenzene/toluene is obtained.
💥Function of Lewis Acid Catalyst
The function of AlX3 is to provide powerful electrophilic alkyl carbonium ion from the alkylating agent.
💥Drawback
Friedel-Craft alkylation is less useful due to following serious objections:
▶ The reaction is not limited to mono-alkylation. Usually di or tri alkylated benzene is formed.
As alkyl group is activating group which increases the reactivity of benzene ring, therefore, the reaction does not stop and multi-substituents products is obtained.
Simple monoalkyl-benzene such as toluene cannot be prepared by FC alkylation. Since alkyl groups donate electrons to the aromatic system, the newly formed compound e.g. toluene, would react much faster than the starting material (benzene), and the reaction would lead to polymethylated benzenes rather than mainly toluene.
▶The alkyl group often rearrange. Thus the
possibility of rearrangement brings uncertainty about the actual products formed.
For example, treatment of benzene with n-propyl
chloride, gives isopropylbenzene rather than the
expected n-propylbenzene. This is because the reaction involves formation of
carbonium ion which can undergo rearrangement before attacking the benzene
ring.
▶ If a bulky alkyl halide is used such as tertiary-butyl chloride, the reaction will give rise to a simpler mixture of products since it is unlikely, on steric grounds, that one will obtain an o-di-ter-butylbenzene by this method.
💥alkylation of toluene
The alkylation of toluene with alkyl halide
gives a mixture of ortho, para and meta-dimethylbenzenes called xylenes.
🔥📘5. Friedel Craft’s Acylation
Friedel-Craft’s Acylation is an electrophilic substitution reaction in which H atom of benzene is replaced by acyl group (R–CO– or R–CO+) of acyl halide (RCOX) in the presence of Lewis acid catalyst AlCl3 to produce acylbenzene.
⚛️Reagent used for Friedel-Craft’s Acylation/Acylating agent
Acylation of benzene is carried out by heating it with acyl halide or acid halide, R–CO–X (or acid anhydride) in presence of trace of Lewis acid catalyst such as AlX3 (AlCl3), yielding acylbenzene or alkyl phenyl ketone or phenylalkanone (aryl ketone or aromatic ketone).
⚛Function of Lewis Acid Catalyst
The function of AlX3 is to provide powerful electrophilic acyl cation or acyl carbonium ion (RCO+) from the acylating agent produced by the reaction of acyl chloride with Lewis acid catalyst AlCl3.
⚛Acylation of toluene
Acylation of toluene gives
only the para product.
Normally, the methyl group in
methylbenzene being an ortho-para directing group, directs new groups into the
2- and 4- positions (assuming the methyl group is in the 1-position). In
acylation, though, virtually all the substitution happens in the 4-position.
The reason for the 2,4-directing
effect of the methyl group is attributed to the steric hindrance created by
newly entrant bulky acyl group. The reason that you get virtually none of the
2-isomer in this instance is because of the size of the incoming acyl group.
Everything gets too cluttered (and therefore less stable) if you try to put the
acyl group next door to the methyl group.
Addition Reactions
In spite of having unsaturated nature, benzene being an aromatic compound is highly resistant to addition due to hyper stability. Despite having six delocalized pi electron in its six p-orbitals, benzene is less susceptible to addition reaction compared to alkenes and alkynes. The reason for this is that in alkenes and alkynes pi electrons are more localized between the two carbon atoms making them more easily accessible for an electrophilic attack. Contrarily, the pi electrons in benzene are highly delocalized making it difficult for incoming electrophile to attack and add on the benzene ring. However, under drastic conditions, it can undergo addition reactions with hydrogen and chlorine.
1. Hydrogenation/Reduction
(Addition of Hydrogen)
Benzene is reduced to cyclohexane by adding three molecules of hydrogen at high temperature (150-200°C) under 10 atm pressure in presence of Ni catalyst. It is a catalytic hydrogenation.
⚛Heat of Hydrogenation
The reduction of benzene evolves heat of 207 kJ/mol which is much less than expected. When cyclohexene is reduced to cyclohexane, the heat evolved is 119 kJ/mol. If benzene contained three alkene-like double bonds, we should expect that the heat evolved during its reduction would be three times that for cyclohexene i.e. 119 x 3 = 357 kJ/mol. The experimental value is 150 kJ/mol (357 – 207) less than this, from which we infer that benzene is more stable by this amount of bonding energy than would be expected if it possessed alkene-like double bonds.
⚛Mechanism
Benzene undergoes hydrogenation with greater
difficulty. However under vigorous conditions, benzene adds three molecules of
hydrogen one after one but addition of hydrogen usually cannot be stopped short
of total reaction. The addition of first molecule of H2 is slow
while second and third molecules add very rapidly as the various
non-aromatic intermediates react much faster than the benzene. That is why,
1,2-dihydrobenzene (1,3-cyclohexadiene) and 1,2,3,4-tetrahydrobenzene
(cyclohexene) are not prepared by this method, because once 1, 2-dihydro
benzene is formed, the molecule assumes a simple olefinic structure and further
addition of hydrogen becomes rapid.
⚛Heats of Hydrogenation as indicator of Stability
⚛Hydrogenation of Cyclohexene
Each molecule of
cyclohexene has one C=C double bond. The enthalpy change for the reaction of
cyclohexene with hydrogen is -120 kJmol-1.
⚛Hydrogenation of Benzene
The Kekule’s structure of benzene as
cyclohexa-1,3,5-triene has three double C=C bonds. It would be expected that
the enthalpy change for the hydrogenation of this structure would be three
times (-360 kJmol-1) the enthalpy change for the one C=C bond in
cyclohexene. When benzene reacts with
hydrogen, enthalpy changed is far less exothermic and is about -208 kJmol-1.
The difference between the thermochemical data for cyclohexa-1,3,5-triene and
benzene suggest that benzene has more stable bonding than the Kekule’s
structure.
The delocalization stability of benzene of
-152 kJmol-1 is the difference between the two enthalpy changes.
This is extra energy that must be provided to break the delocalized benzene
ring.
2. Halogenation in Sunlight (Addition of Halogen)
At 50°C, 400 atm. pressure and in the
presence of ultra-violet light (sunlight), benzene (in vapour phase) adds 3 molecules of Cl2 (or Br2)
to give an addition product; hexachlorocyclohexane or hexachlorobenzene or hexachlorophene or benzene hexachloride (BHC)
⚛Halogenation of Toluene in Sunlight
In the presence of sunlight (ultra-violet light), toluene (or any alkyl benzene) react with Cl2 or Br2 preferentially by a photochemical reaction occur via free radical mechanism to substitute hydrogen of methyl (alkyl) group rather than the H of benzene ring forming all the three possible products.
OR
Oxidation Reactions
1. Catalytic Air Oxidation into Maleic Anhydride
Benzene exhibits remarkable resistance to oxidation by alkaline KMnO4 or acidified K2Cr2O7. However, benzene may be oxidized to maleic anhydride (C4H2O3) when benzene is strongly heated at 450°C with
air in the
presence of vanadium pentaoxide (V2O5) as a catalyst.
Maleic anhydride is converted into maleic acid (C4H4O4)
on adding a water molecule.
Zinc is used to avoid
further oxidation of glyoxal to carboxylic acids by H2O2
3Zn + 3H2O2 → 3ZnO + 3H2O
⚛Oxidation of Toluene into
Benzoic acid (Formation of Benzoic Acid)
Alkyl benzenes like toluene are readily oxidized by alkaline KMnO4 or acidified K2Cr2O7. [An alkyl group, whatever its length is oxidized to one carboxylic group. Ethyl or higher alkyl groups are changed to one carboxylic group with the loss of CO2].
Substituent Effect/ Directive Influence/Orienting Effect /Orientation in Benzene (E.S.R. of Monosubstituted Benzene)
Isomerism in Monosubstituted Benzene
The substitution of any H atom of benzene by electrophilic reagent always gives a same monosubstituted product because all the six hydrogen atoms in benzene are equivalent and therefore only one monosubstitution of benzene is possible. e.g. chlorobenzene, nitrobenzene.
[Details: All the six hydrogen atoms of benzene are identical and equivalent. Thus replacement of any H atom of benzene always results in the same one monosubstituted product (C6H5–G) and thus only one monosubstitution benzene is possible (e.g. one nitrobenzene, one chlorobenzene etc.) and no isomerism evolves on monosubstitution. Thus an electrophilic substitution reaction of benzene results in the formation of only one monosubstituted product due to equivalent nature of all the six hydrogen atoms of benzene ring (and therefore the substituent group may occupy any of the six available positions on the ring].
Expected and Experimental Proportions of Three Isomeric
Disubstituted Products
It might be expected that the three Isomeric Disubstituted Products would be formed in approximately equal proportion, in reality, the percentage of each disubstituted products is quite different.
In monosubstituted benzene (C6H5-G),
there are five available hydrogen atoms. Of these two are ortho, two meta and one
para to G. If substitution was random, on purely statistical basis, the
proportion of the disubstituted products would be
Ortho = 2/5 of
the total or 40%
meta
= 2/5 of the total or 40%
Para = 1/5 of the total or 20%
But, this distribution is never observed actually. The proportion of the products formed, in fact, is determined by the nature of the first substituent already present on the ring rather than on any mathematical probability.
Experimentally obtained
Disubstituted Products
MSBs have five replaceable hydrogens. A second substituent (Y) can occupy any of the remaining five positions to yield three isomeric disubstituted benzenes depending on whether the second group occupies ortho, meta or positions with respect to the first substituent (already attached group).
On disubstitution,
when one hydrogen atom of monosubstituted benzene (C6H5–G)
is replaced by second substituent Y (atom or group of atoms) to give
disubstituted benzene (C6H4GY), isomerism appears. The
second substituent (Y) may be situated on adjacent (2 and
6), alternate (3 and 5) or opposite (4) carbon atoms. [If a second substituent is introduced into the benzene ring, it can be
substituted at any of the remaining hydrogen atoms and occupy one of three isomeric positions (ortho, meta and para). The second
substituent (Y) may be situated on adjacent (2
and 6), alternate (3 and 5)
or opposite (4) carbon atoms]. Thus only three isomeric disubstituted products are possible as positions 1, 2 and 1, 6 are equivalent. Similarly 1, 3 and 1, 5 are equivalent.
OR
Effect
of Substituents on further substitution
Ortho position or compounds
The monosubstituted benzene (C6H5–G) having the second substituent at adjacent positions (2 or 6) are called ortho compounds abbreviated as –o.
Meta position or compounds
The monosubstituted benzene (C6H5–G) having the second substituent at alternate position (3, 5) are called meta compound abbreviated as –m.
Para position or compounds
The monosubstituted benzene (C6H5–G) having the second substituent at opposite position (4) are called Para compound abbreviated as –p.
For example;
Xylene exists in three isomeric forms:
Two types of influences of Substituents/Two Types of
Effects of Substituents
A substituent (G)
already present on the benzene ring exercises two types of influences on
disubstitution
1. Directive
effect or orientation effect (direct 2nd substituent to specific
carbon in the aromatic ring)
2. Activity
effect/Effect on reactivity of benzene ring (can activate or deactivate the
ring)
1. Orientation Effect or Substituent Effect/ Orienting Effect /
Directive Influence of Substituents
The difference in relative amount of three isomeric Disubstituted Products of MSB is due to the fact that first substituent influences the positions occupied by incoming electrophilic reagent.
The first substituent (G) may direct the next incoming substituent (Y+ or E+) to ortho, meta or para positions, depending on the nature of the first substituent. This is called directive effect or orientation effect (orient means to arrange).
The effect of the first substituent on the incoming
electrophilic reagent is known as Orientation Effect or Substitution Effect
OR
The influence of a substituent already present in the benzene ring which determines the position to be taken by the second entering group in the ring is called Directive Influence or Orienting Effect of the Substituents. It is the ability of a group already present in the benzene ring to direct the incoming group to a particular position.
Orientation
Orient means “To determine the position of”. The process of assigning the position of second incoming group in the monosubstituted benzene (C5H5–G) is called Orientation.
When mono substituted benzene undergoes an electrophilic attack, the
rate of reaction and the site of attack vary with the functional group already
attached to it. Some groups increase the reactivity of benzene ring and are
known as activating groups while others which decrease the reactivity are known
as deactivating groups. It has been experimentally observed that some groups
present in the benzene ring, direct the second incoming substituent to either
ortho and para positions or to the meta position. We further divide these
groups into two categories depending on the way they influence the orientation
of attack by the incoming electrophile. Those which increase the electron
density at ”ortho” and “para” positions are known as ortho-para directors while
those which increase the electron density at “meta” position are known as meta
directors. There are two types of substituents which produce directive effect
are:
(i) Those
which direct the incoming group to ortho- and para-positions simultaneously (neglecting
meta all together).
(ii) Those which direct the incoming group to meta-position only (neglecting ortho- and para-positions all together).
2. Activity effect
The substituent already present on a benzene ring not only directs the position of an incoming group, but also influences the rate of reaction. The substituent already attached to the benzene ring affects the reactivity of benzene ring in comparison with the unsubstituted benzene. The substituent already present may activate or deactivate the benzene ring towards further substitution (i.e. the substituent already present can increase or decrease the rate of further substitution). These effects are called the activity effects. These effects are called activity effects.
In general, it has been observed that ortho-para directors activate a
ring toward electrophilic substitution whereas meta-directing groups deactivate
a ring toward electrophilic substitution. The only exception to the above
generalization occurs in halobenzenes (C6H5X). Although –F, –Cl, –Br and –I are ortho-para directors, these substituents deactivate an
aromatic electrophilic substitution.
For example
(i) toluene (C6H5CH3)
is nitrated 25 times faster than benzene
itself.
(ii)
the
rate of nitration of chlorobenzene (C6H5Cl) is 30 times less than benzene.
(iii) In aromatic nitration, for instance, an –OH
substituent makes the ring 1000 times more reactive than benzene,
(iv) a
–NO2 substituent makes the ring more than 10 million times less reactive than benzene for nitration.
This means that
the presence of –CH3 and –OH groups on benzene ring activate it
towards the electrophilic substitution while the presence of –Cl and –NO2
groups
deactivates it.
A substituent which activates the aromatic electrophilic substitution by increasing the ring's electron density is called an activating substituent or ring activator. The ortho-para directing groups release electrons to the ring by resonance thereby making it electron-rich and facilitate the attack of a second electrophile. Thus the attack of a second electrophile would be faster as compared to the benzene ring itself. They are called ring activators or activating groups.
For example
When the substituents like –OH have an unshared pair of electrons, the resonance effect is stronger than the inductive effect which make these substituents stronger activators, since this resonance effect directs the electron toward the ring. In cases where the substituents are esters or amides, they are less activating because they form resonance structure that pull the electron density away from the ring. From the resonance structures of phenol shown below, we see how ortho and para positions become electron rich and electrophile can easily attack on that positions.
2. Deactivating groups
(meta directors)
A substituent which deactivates the aromatic ring to further electrophilic substitution by removing electron density from the ring is called a deactivating substituent or ring-deactivator. The meta-directing species decreases the reactivity of the benzene ring and they are called ring deactivators or deactivating groups. the meta-directing groups withdraw electrons from the ring making it electron-deficient thereby hindering the attack of a second electrophile. Thus the attack of a second electrophile would be slower as compared to the benzene ring. Halogens, though ortho-para directing are the exceptions to this and are ring deactivators.
The deactivating groups deactivate the ring by the inductive effect in
the presence of an electronegative atom that withdraws the electrons away from
the ring. From the resonance structures of nitrobenzene shown below, we see
that when there is an electron withdrawal from the ring that leaves the carbons
at the ortho, para positions with a positive charge which is unfavourable for
the electrophile, so the electrophile attacks the carbon at the meta positions.
Directing groups are of two types:
1. Ortho-para directors
2. Meta directors.
1. Ortho-Para Directing
Substituents
Definition
These are the
substituents when attacked to
benzene ring direct the second
substituent (incoming electrophilic substituents) to the ortho and para positions simultaneously.
Certain
substituents direct the 2nd entrant predominately to the ortho and
para positions simultaneously (the meta product is either not formed or formed
in less amount). They are called ortho-para directors.
The substituents that activate benzene ring and direct the next incoming entrant to ortho and para positions simultaneously are called o-p-directing activators groups.
Presence of non-bonding electron pair
All o,p-directors have non-bonding electron pair on the key atom except alkyl or aryl groups.
activating groups
These substituents enhance the reactivity of ring and they are activating groups (except halide groups).
electron-attracting
substituents
They are electron-attracting substituents (–I effect) but possess an unshared pair of electrons or lone pairs (–M effect, which is dominant over –I effect) except alkyl or aryl groups which are electron-releasing groups (+I effect).
Mechanism of Action
They increase the electron density on ortho
and para position through resonance effect. The increased electron density on
these positions make more attraction for the incoming electrophile to attack.
The most strongly activating members of this group are bonded to the ring by a nitrogen or oxygen atom that has an unshared electron pair. They stabilize the cyclohexadienyl carbocation intermediate formed in the rate determining step.
Examples
Cl–, Br–, I–,
F–,–R (–CH3, –C2H5), –OH, –NH2, –NHR
(Alkyl amino), –NR2 (dialkyl amino), –NHCOR, –OR, –OOCR
(Acyloxy), –Ar (– C6H5), –CH = CR2
(Alkenyl)
etc.
For example
(i) The –OH group is ortho-para director and
when react with chlorine in presence of ferric chloride catalyst, gives a
mixture of ortho-para chlorophenols.
(ii) Nitration of
toluene gives all the three disubstituted benzenes; o-nitrotoluene,
m-nitrotoulene and p-nitrotoluene (all are formed but not in equal amounts).
The product mixture comprises of 97% of the ortho and para-substituted isomers
and only 3% of meta isomer. Because substitution in toluene occurs primarily at
positions ortho and para to methyl, we say that a methyl group is an ortho-para
director.
(iii) Nitration of
phenol yields a product mixture of 53% ortho-nitrophenol and 47% of
p-nitrophenol without any meta isomer.
The substituent –OH is said to have directed the 2nd
substituent –NO2 to ortho and para position on the ring. Thus –OH
group is designated as o,p-director.
Ortho-para-Directing and Deactivating Groups
The halogens i.e. F, Cl, Br and I are moderate deactivating groups and direct the incoming groups to ortho-para positions.
2. Meta Directing Substituents
Definition
These are the substituents which direct the second incoming substituent primarily to meta position.
deactivating groups
These substituents reduce the activity of ring and they are deactivating groups.
Presence of partial positive charge of a full positive charge on
the central atom of the group
All meta-directing groups have either a partial positive charge of a full positive charge on the atom directly attached to the ring.
electron-withdrawing
substituents with –I effect
They are electron-withdrawing, showing –I effect (electron-attracting) and destabilize carbocations. They withdraw electron density from all the ring positions by an inductive effect, but since the meta positions are deactivated less than the ortho and para, meta substitution is favoured.
Mechanism of Action
These groups decrease the electron density at ortho and para position due to inductive effect thereby increasing the chances of electrophilic attacks on meta position by default.
–SO3H,
–CN, –NO2, –CHO, –COR, –COOH etc.
For example
(i) The –COOH group of benzoic acid is meta directing. When benzoic reacts with nitric acid, it produces a meta product.
(ii) Nitration of
nitrobenzene gives 94% of m-dinitrobenzene and only 5% of ortho and 1% of
para-dinitrobenzene. Thus the substituent –NO2 group is said to have
directed the 2nd substituent –NO2 to the meta position on
the ring and –NO2 group is designated as meta-directing group.
(iii) Alkylation of nitrobenzene gives
m-alkylnitrobenzene as major product.
Mixed Classification of Directing Groups
For the sake of
convenience, directing groups are divided into three different classes with
regard to whether they are activating or deactivating and whether they are
o-p-directing or m-directing. We can further classify activating and deactivating groups
or atoms as strong, moderate, or weak in their directing influence.
(a) o-p-directing
and activating groups
(b) o-p-directing
and deactivating groups
(c) m-directing and deactivating groups
This table lists some typical activating and deactivating groups by the order of their strength.
Summary of Classification of Substituents of MSB for ESR
Preparation of Poly-substituted Benzene (PSB)
Benzene containing more than one substituents is called polysubstituted benzene. Poly-substituted Benzene are derivatives of benzene formed by the replacement of two or more hydrogen atoms of benzene ring with electrophilic reagents. When synthesizing PSB, it is important to consider the orientating effect of already attached electrophilic to the MSB, whether it is meta directing or ortho-para directing.
Since the position of electrophilic attack on a substituted benzene ring is controlled by the substituent already present rather than the approaching electrophile, the order of events in the synthesis of poly-substituted benzenes need careful planning to ensure success. The synthesis of polysubstituted product of benzene depends on the already present substituent. If the already present substituent is activator, the polysubstituted product is formed easily.
The two factors that need to be
monitored are:
(i)regiochemistry
(e.g. what are the substituent directing effects)
(ii)reactivity (e.g. Friedel-Crafts reactions are limited to halobenzenes and activated benzenes)
Preparation of
trinitrotoluene (TNT) from Benzene
“trinitrotoluene” (TNT; an
explosive organic compound) is synthesized from benzene by first converting it
into toluene by its alkylation followed by its nitration with hot concentrated
nitric acid. Since –CH3 group of toluene is ortho-para directing, it
invites the electrophilic NO2+ group of nitric acid
towards ortho and para positions.
[when nitration of toluene occurs in the presence of H2SO4 at 25°C, two isomeric ortho and para-nitrotoluene is obtained as methyl group is o-p-director. the further nitration of both isomers gives same trisubstituted product i.e. dinitrobenzene (as one ortho and one para are vacant). The further nitration gives trinitrotoluene (as one ortho is vacant)].
Preparation of m-nitrotoluene
In order to synthesize m-nitrotoluene from benzene, the first step is to convert benzene into nitrobenzene through nitration with conc. Nitric acid in the presence of conc. H2SO4 at high temperature. The second step is alkylation of nitrobenzene into m-nitrotoluene. Due to the meta directing nature of nitro group of nitrobenzene, it directs the incoming –CH3 group towards the meta position.
Preparation of trinitrobenzene
when
nitration of benzene occurs in the presence of H2SO4 at
60°C, nitrobenzene is obtained. But the further nitration will not occur easily
as NO2 group is deactivator. It needs high temperature (100°C) and
for dinitrobenzene and trinitrobenzene fuming nitric acid and sulphuric acid is
required. The second nitro group further deactivates the benzene ring, so more
drastic conditions are required for trinitration.
Phenols and Classification of Phenols
Definition
Phenols are the class of aromatic hydroxyl organic compounds containing one or more hydroxyl group (–OH) groups directly attached to the carbon atom of a benzene ring. Thus in phenols, the – OH group is linked to any aryl group and so they can be described by a general formula Ar–OH. The parent compound of this family is monohydroxy benzene which is also known as carbolic acid or benzenol or simply phenol.
Compounds that have a hydroxyl group attached to a polycyclic benzenoid ring are also phenols but they are called naphthols and phenanthrols.
Examples
Phenol(benzenol/benzophenol/carbolic acid/phenylic acid / Hydroxybenzene), cresols, xylenols, resorcinols, naphthols, phenanthrols etc.
Phenol as a Parent Member of Phenols Series
Phenol also known
as carbolic acid formulized as C6H5OH is the simplest
and the parent member of the series which was first obtained from coal tar by Runge in 1834. Phenol is a white crystalline solid
of deliquescent nature, corrosive and poisonous turning pink or red if impure
or in light with strong distinctive characteristic carbolic odour with sharp
burning taste (but in dilute weak solution has a sweetish taste).
Phenols may be considered the enol form of cyclohexadienones
Phenols may be
considered the enol form of cyclohexadienones, in which the
equilibrium lies almost entirely on the enol side. The driving force for the
predominance of this enol form is the formation of the six p-electrons aromatic system.
1). Monohydric Phenols
Monohydric phenols contain one –OH group in the ring. Phenol itself is the parent member of the class e.g.
2). Dihydric PhenolsThey have two –OH groups.
e.g.
3). Trihydric Phenols
They have 3 –OH groups.
Structure of Phenol
In phenol, the oxygen atom in the –OH group is sp3-hybridized while the carbon atoms in the aromatic ring are sp2-hybridized. The oxygen atom of –OH group forms sigma bond with carbon atom of aromatic ring through sp3-sp3 linear overlapping. The C–O–H bond angle is set at 109o which is almost the same as the bond angle of tetrahedral geometry (109.5o) but due to two non-bonded electrons pairs of oxygen atom, the geometry is slightly distorted from a normal tetrahedral and acquire bent shape.
Physical Properties of Phenol
Uses of Phenol
1. It is used as an antiseptic and disinfectant.
2. It is used in the manufacturing of soaps, plastics,
ointments and lozenges
3. It is used in the preparation of picric acid and
phenolphthalein.
4. It is used as ink preservative.
Methods of Preparation of Phenols
Sodium benzene sulphonate when fused with solid sodium hydroxide at 300-350°C gives sodium phenoxide, which on acidification with HCl gives phenol.
OR
Details
Benzene sulphonic acid, derived from the
reversible Sulphonation of benzene with sulphuric acid or oleum, can be
converted to its sodium salt; Sodium benzenesulphonate by treatment with dilute
NaOH. Fusion of sodium benzenesulphonate with molten sodium hydroxide at
250-300°C yielding the
salt of phenol, sodium phenoxide, which on acidification with dilute acids like
HCl liberates phenol from cooled reaction mixture.
Chlorobenzene is hydrolyzed with 10% aqueous NaOH at 350°C and under 150 atmospheric pressure to form sodium phenoxide which on acidification with dilute HCl gives phenol.
Details
Chlorobenzene, derived from the Lewis acid
catalyzed chlorination of benzene in dark, is hydrolyzed with 10% hot
concentrated NaOH at 300°C and under 200
atmospheric pressure yielding the salt of phenol called sodium phenoxide from
which phenol is released on acidification with dilute acids like HCl or H2SO4.
Diazonium salt is a class of organic compounds that contains a functional group –N2Cl attached with alkyl or aryl carbon chain.
Benzene diazonium chloride on heating with water undergoes hydrolysis forming phenol with the releases of nitrogen gas and HCl. It is a laboratory method for the preparation of phenol.
Much phenol is now made by the oxidation of
iso-propylbenzene. Benzene is alkylated with propene (obtained by the cracking
of petroleum) at high pressure and temperature with catalyst like AlCl3
or H3PO4 yielding Cumene which is then oxidized by air at
high pressure to form its hydroperoxide which is finally decomposed with warm,
dilute sulphuric acid into phenol and acetone.
Mechanism
The conversion of peroxide to final product first involves rearrangement; as the O–O bond in the protonated hydroperoxide breaks, allowing the phenyl group to migrate from carbon to oxygen giving carbonium ion which reacts with water to form final product.
Chemical
Reactions of Phenols
🌟⚛️ Arenes (Aromatic Compounds) Complete Nomenclature High-Yield Master Guide from Basics to Advanced | IUPAC, Common & Trivial Names Explained by Inam Jazbi
🧪🔥1. Naming Monosubstituted Benzene (MSB)
🧪🔥2. Naming Disubstituted Benzene (DSB)
(i) In DSB, benzene ring is considered to be parent if the substituents do not give a special name to the molecule. DSB is named by prefixing benzene to the substituent name. The relative positions of the substituents in the benzene ring are indicated by the numbers or by using the prefixes ortho (o–), meta (m–) and para (p–) for the 1,2, 1,3 and 1,4–substituents respectively. If both substituents are different, both are used as prefixes successively in alphabetical order. In this case, the last named substituent is understood to be at position number 1.
🧪🔥3. Naming Polysubstituted Benzene (PSB)
🌈🧬 1. SUMMARY OF NAMING RULES
🔷⚛️ IUPAC Naming Rules🔶🧿 Common Naming Rules
🔥 Ortho (o-) for 1,2 positions
💥 Meta (m-) for 1,3 positions
🌈 Para (p-) for 1,4 positions
⭐ Very popular for exam MCQs
🟡 Used commonly with toluene, nitrobenzene, aniline etc.
🟣💠 Trivial Naming Rules
🧪📘 2. TABLE OF IMPORTANT TRIVIAL NAMES
🌸 Naming Arenes – Challenging MCQs
1️⃣ What is the IUPAC name of C₆H₅CH₃?🟥 Methylbenzene
🟩 Toluene
🟦 Benzyl
🟨 Phenylmethane
2️⃣ The compound C₆H₅OH is commonly known as:
🟥 Phenol
🟩 Hydroxybenzene
🟦 Carbolic acid
🟨 All of the above
3️⃣ What is the correct IUPAC name of CH₃C₆H₄NO₂ (with NO₂ at meta position)?
🟥 3-Nitrotoluene
🟩 2-Nitrotoluene
🟦 4-Nitrotoluene
🟨 Nitrotoluene
4️⃣ Which of the following is the correct name for C₆H₄Cl₂ with Cl atoms at 1,3 positions?
🟥 1,2-Dichlorobenzene
🟩 1,3-Dichlorobenzene
🟦 1,4-Dichlorobenzene
🟨 Meta-Dichlorobenzene
5️⃣ The substituent –CH₂CH₃ on benzene is named:
🟥 Methyl
🟩 Ethyl
🟦 Propyl
🟨 Phenyl
6️⃣ C₆H₅CH₂Cl is named as:
🟥 Benzyl chloride
🟩 Chlorobenzene
🟦 Phenylmethyl chloride
🟨 Toluyl chloride
7️⃣ Which of the following is an ortho-substituted compound?
🟥 1,2-Dibromobenzene
🟩 1,3-Dibromobenzene
🟦 1,4-Dibromobenzene
🟨 1,5-Dibromobenzene
8️⃣ What is the IUPAC name of C₆H₅COOH?
🟥 Benzoic acid
🟩 Phenylcarboxylic acid
🟦 Carboxybenzene
🟨 Benzene carboxylic acid
9️⃣ How do you name C₆H₄(NO₂)(CH₃) with CH₃ at position 1 and NO₂ at 3?
🟥 3-Nitrotoluene
🟩 2-Nitrotoluene
🟦 4-Nitrotoluene
🟨 1-Nitro-3-methylbenzene
🔟 Which of these is para-xylene?
🟥 1,2-Dimethylbenzene
🟩 1,3-Dimethylbenzene
🟦 1,4-Dimethylbenzene
🟨 1,5-Dimethylbenzene
1️⃣1️⃣ The compound C₆H₄ClBr (Cl at 1 and Br at 4) is named:
🟥 1-Chloro-4-bromobenzene
🟩 4-Bromo-1-chlorobenzene
🟦 Para-bromochlorobenzene
🟨 1-Bromo-4-chlorobenzene
1️⃣2️⃣ What is the common name of C₆H₅NH₂?
🟥 Aniline
🟩 Aminobenzene
🟦 Phenylamine
🟨 Benzenamine
1️⃣3️⃣ The correct IUPAC name of C₆H₄(OH)₂ with OH groups at 1,3 positions is:
🟥 Catechol
🟩 Resorcinol
🟦 Hydroquinone
🟨 Phloroglucinol
1️⃣4️⃣ Which of these compounds is named m-nitrophenol?
🟥 NO₂ at ortho to OH
🟩 NO₂ at meta to OH
🟦 NO₂ at para to OH
🟨 NO₂ at any position
1️⃣5️⃣ What is the substituent name of –CH₂OH on benzene?
🟥 Hydroxymethyl
🟩 Methylol
🟦 Benzyl alcohol
🟨 Hydroxybenzyl
1️⃣6️⃣ How do you name C₆H₄CH₃Cl with CH₃ at 1 and Cl at 3?
🟥 3-Chlorotoluene
🟩 2-Chlorotoluene
🟦 4-Chlorotoluene
🟨 1-Chloromethylbenzene
1️⃣7️⃣ The compound C₆H₅CHO is commonly called:
🟥 Benzaldehyde
🟩 Phenylmethanal
🟦 Benzenecarbaldehyde
🟨 All of the above
1️⃣8️⃣ What is the IUPAC name of C₆H₄(CH₃)₂ with CH₃ at 1,3 positions?
🟥 Ortho-xylene
🟩 Meta-xylene
🟦 Para-xylene
🟨 Dimethylbenzene
1️⃣9️⃣ Which is the correct name of C₆H₄(CN)(CH₃) with CH₃ at 1 and CN at 2?
🟥 2-Cyanotoluene
🟩 3-Cyanotoluene
🟦 4-Cyanotoluene
🟨 Tolunitrile
2️⃣0️⃣ The common name of C₆H₄SO₃H is:
🟥 Benzenesulfonic acid
🟩 Sulfobenzene
🟦 Phenylsulfonic acid
🟨 Benzosulfonic acid
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🎯 🌸 IUPAC Rules for Naming Phenols Quick MDCAT Summary💥
🎯 1. Parent Name in IUPAC⚡The base name for phenols in IUPAC is benzenol.
⚡"Phenol" is also accepted as a name because it is widely used.
⚡The logic follows aliphatic alcohol naming, but attached to a benzene ring.
🔢 2. Numbering the Ring
⚡The OH group always gets position 1 by default.
⚡All other substituents are numbered after assigning OH as 1.
⚡Numbering proceeds in the direction that gives the lowest numbers to substituents.
🌿 3. Multiple –OH Groups
⚡When more than one hydroxyl group is present, use:
di-, tri-, tetra- (as needed).
⚡The parent becomes benzene-x,y-diol or benzene-x,y,z-triol, etc.
⚡Example:benzene-1,2,3-triol
🧭 4. Ortho / Meta / Para Notation
⚡For common naming, positions may be shown as:
ortho (o–) → 1,2
meta (m–) → 1,3
para (p–) → 1,4
⚡These prefixes are commonly used with substituents on phenol.
🧪 5. Common Names Used in IUPAC Context
⚡Some substituted phenols are still known by traditional names:
Cresols → methyl phenols
⚡Hydroxy-substituted phenols also have traditional names used alongside IUPAC.
⚡Typically, the first name is the common name, and the name inside parentheses is the IUPAC name.
📌 6. Key Points for MDCAT/ECAT
✨ OH always gets position 1
✨ Benzenol is the parent IUPAC name
✨ Use prefixes (di-, tri-) for multiple OH groups
✨ o-, m-, p- notation still acceptable
✨ Many substituted phenols retain well-known common names
🧠🌸 Naming Phenols – Challenging MCQs🔥
1️⃣ The strict IUPAC name of phenol is:
🟥 Benzenol
🟩 Hydroxybenzene
🟦 Phenyl hydroxide
🟨 Benzyl alcohol
2️⃣ In naming phenols, the –OH group is assigned:
🟥 Position giving lowest locant
🟩 Position 1 always
🟦 Position depending on substituent priority
🟨 Alphabetical priority
3️⃣ The correct IUPAC name of o-cresol is:
🟥 1-Methyl-2-hydroxybenzene
🟩 2-Methylbenzenol
🟦 2-Hydroxy-1-methylbenzene
🟨 1-Hydroxy-2-methylbenzene
4️⃣ A benzene ring with OH groups at positions 1 and 4 is named:
🟥 para-dihydroxybenzene
🟩 benzene-1,4-diol
🟦 benzenol-1,4-diol
🟨 1,4-phenyl diol
5️⃣ The compound m-nitrophenol has NO₂ at:
🟥 Position 2
🟩 Position 4
🟦 Position 3
🟨 Position 5
6️⃣ What is the correct IUPAC name of resorcinol?
🟥 Benzene-1,2-diol
🟩 Benzene-1,3-diol
🟦 Benzene-1,4-diol
🟨 Dihydroxybenzene
7️⃣ A phenol with –CH₃ at position 3 is named:
🟥 m-methylphenol
🟩 3-methylbenzenol
🟦 3-hydroxy-toluene
🟨 1-hydroxy-3-methylbenzene
8️⃣ In naming phenols with multiple substituents, numbering starts from OH because:
🟥 OH has highest alphabetical priority
🟩 OH is considered principal functional group
🟦 OH is lowest priority but traditional
🟨 OH is avoided in common names
9️⃣ The name benzene-1,2,3-triol indicates:
🟥 Three OH groups in a row
🟩 Three substituents including OH
🟦 Three OH groups on positions 1,3,5
🟨 Only two OH groups
🔟 Which is a correct IUPAC name?
🟥 1-hydroxy-4-nitrobenzene
🟩 4-nitrobenzenol
🟦 para-nitrophenol
🟨 nitrohydroxybenzene
1️⃣1️⃣ The common name “cresol” refers to:
🟥 Any hydroxybenzene
🟩 Any methyl phenol
🟦 Any dihydroxybenzene
🟨 Any hydroxy-toluene
1️⃣2️⃣ The IUPAC name of p-ethylphenol is:
🟥 benzene-1-ethyl-4-ol
🟩 4-ethylbenzenol
🟦 1-ethyl-4-hydroxybenzene
🟨 para-ethylbenzenol
1️⃣3️⃣ Which of the following retains its common name in IUPAC tables?
🟥 Catechol
🟩 Resorcinol
🟦 Hydroquinone
🟨 All of them
1️⃣4️⃣ The substituent –OH gets position 1 even if a nitro group is present because:
🟥 NO₂ has lower electronegativity
🟩 OH defines the parent compound
🟦 Alphabetical order demands it
🟨 Tradition overrides rules
1️⃣5️⃣ The correct IUPAC name of hydroquinone is:
🟥 benzene-1,2-diol
🟩 benzene-1,3-diol
🟦 benzene-1,4-diol
🟨 dihydroxybenzene
1️⃣6️⃣ Which is the correct common–IUPAC pair?
🟥 Phenol — benzenol
🟩 Catechol — benzene-1,3-diol
🟦 Cresol — benzene-1,4-diol
🟨 Hydroquinone — benzene-1,3-diol
1️⃣7️⃣ A phenol substituted at position 4 by Br is:
🟥 p-bromophenol
🟩 4-bromobenzenol
🟦 1-bromo-4-hydroxybenzene
🟨 All of these (depending on system)
1️⃣8️⃣ The compound 2,4-dinitrophenol contains:
🟥 Two nitro groups and one OH
🟩 One nitro group and two OH
🟦 Two NO₂ groups at 2 & 3
🟨 One OH at 4 and NO₂ at 2
1️⃣9️⃣ Which is incorrect?
🟥 OH always gets position 1
🟩 Cresols are methyl phenols
🟦 Benzene-1,2-diol = catechol
🟨 m-cresol = benzene-1,4-diol
2️⃣0️⃣ The correct IUPAC name for 3,5-dimethylphenol is:
🟥 benzene-1,3,5-triol
🟩 3,5-dimethylbenzenol
🟦 dimethylhydroxybenzene
🟨 meta-dimethylphenol
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