Complete Class 12 Chemistry Notes of Benzene and Phenols according to the Federal and Punjab Board Syllabus 2025. Includes structure, resonance, chemical properties, electrophilic substitution reactions, and preparation methods with examples.
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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
-
Benzene is aromatic and highly stable.
-
Phenol is acidic and reacts with active metals and alkalis.
-
Bromination and nitration of phenol occur faster than benzene.
-
Benzene undergoes electrophilic substitution, not addition reactions.
📘 Suggested Reading
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.
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
Important Electrophilic Substitution Reactions
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
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)