Model Test Questions on Chapter # 7, chemistry of Benzene Test # 6
Q1. How will you prepare a sample of benzene from petroleum (coal tar or n-hexane or n-heptane)?
Q2. How will you obtain benzene from the following? (i). Aliphatic sources (n-hexane or n-heptane or ethyne) (ii). Aromatic sources (phenol or sodium benzoate or toluene)
Q3.Define Aromaticity. How will you explain the stability of benzene?
Q4. Define Resonance. Draw all the possible structures of benzene.
Q5. Give two reactions in which benzene ring is not retained. (Hydrogenation and halogenation in sunlight)
Q6. Under what conditions benzene reacts with chlorine to form (i) Monochlorobenzene (ESR with chlorine in dark) (ii) hexachlorocyclohexane (Halogenation with chlorine in sunlight)
Q7. Starting from benzene or toluene how will you prepare the following?
(i) Acetophenone [acylation with acetyl chloride ]
(ii) Benzenesulphonic acid [sulphonation with oleum]
(iii) Cyclohexane [Hydrogenation]
(iv) Carbolic acid or Phenol [Chlorination, hydrolysis, acidification]
(v) Bromobenzene/Halobenzene [Bromination or halogenation in dark]
(vi) Cumene (isopropylbenzene) [alkylation or isopropylation with isopropyl chloride]
(vii) p-nitro benzoic acid [Alkylation (methylation) + Nitration + oxidation]
(viii) meta-nitrotoluene [Nitration + Alkylation(methylation)]
(ix) Ortho & para nitrotoluene [Alkylation + Nitration(methylation)]
(x) Chlorine derivatives of toluene [Halogenation in sunlight]
Q8. Benzene acts as Saturated as well as unsaturated compound. Support your answer by two chemical reactions for each type of behavior.
Q9. What is ESR? Why benzene undergoes ESR? Explain general pattern of mechanism of ESR.
Q10. Why does benzene give E.S.R? Give only one step equation of acylation of benzene. Explain 4 equations showing mechanism of isopropylation of benzene.
Q11. Explain Nitration & Acylation in Benzene with mechanism.
Model Test Questions on Chapter # 7, Chemistry of Benzene Test # 7
Q1. Indicate the main product in each of the following reactions (write only equation with proper condition)
i). Xylene + acidified potassium tetraoxomanganate(VII)
ii). Toluene+ oleum
iii). Toluene+ methyl chloride (catalyzed by aluminum (III) chloride)
iv). Toluene+ concentrated nitric acid
v). Toluene + chlorine in sunlight
Q2. Starting from benzene or phenol how the following compounds can be prepared:
i. Bakelite (Reaction of phenol with formaldehyde)
ii. m-nitrotoluene (Alkylation + Nitration)
iii. p-nitrobenzoic acid (Alkylation + Nitration + Oxidation)
iv). Cyclohexanol (Hydrogenation of phenol)
(v) Picric acid (Nitration of phenol with conc. Nitric acid)
(vi) Bromol (Bromination + Hydrolysis + Bromination)
Q3. How will you convert phenol into following?
(i) Cyclohexanol (Catalytic hydrogenation)
(ii) Benzene (Reduction)
(iii) picric acid (Successive nitration)
(iv) Phenoxide (Neutralization)
Q4. How do you convert ethanol into benzene?
Q5. What is meant by Orientation & orienting effects? Describe 3 different types of directing groups with examples.
Q6. Give 3 evidences in support of Kekule’s structures of benzene. What objection was raised to it & how was it defended?
Q7. Discuss the Molecular Orbital Treatment of benzene.
Q8. What are phenols and naphthols? Define their types with two examples each. Give the uses of Phenol.
Q9. How does Phenol react with nitric acid and sulphuric acid?
Q10. Starting from benzene how will you prepare the following?
(i) Cyclohexane [Hydrogenation]
(ii) Carbolic acid or Phenol [Chlorination, hydrolysis, acidification]
(iii) p-nitro benzoic acid [Alkylation (methylation) + Nitration + oxidation]
(iv) meta-nitrotoluene [Nitration + Alkylation(methylation)]
(v) Ortho & para nitrotoluene [Alkylation + Nitration(methylation)]
Q11 Attempt the following conversions:
(i) Benzene into o-nitro benzoic acid & p-nitro benzoic acid (Alkylation followed by nitration & then oxidation)
(ii) Benzene into m-nitro benzoic acid (Alkylation followed by oxidation and then nitration)
(iii) Toluene into T.N.T. (Successive nitration of toluene thrice)
(iv) Benzene into benzoic acid (Alkylation followed by oxidation)
(v) Toluene into benzene (Hydro-demethylation of toluene with H2, CH4 is also obtained)
Q. Choose the right best answer from given options:
Q1. Indicate the main product in each of the following reactions (write only equation with proper condition)
i). Xylene + acidified potassium tetraoxomanganate(VII)
ii). Toluene+ oleum
iii). Toluene+ methyl chloride (catalyzed by aluminum (III) chloride)
iv). Toluene+ concentrated nitric acid
v). Toluene + chlorine in sunlight
Answer
i). Xylene + acidified potassium tetraoxomanganate(VII)
Xylene is chemically dimethylbenzene having three positional isomers namely ortho, meta and para xylene. acidified potassium tetraoxomanganate(VII) or KMnO4 being an oxidizing agent always oxidizes alkyl group of any length into one carboxylic group. Since there are two methyl groups on adjacent, alternate and opposite carbons, they are oxidized to give two carboxylic groups on respective positions forming phthalic acid, isophtalic acid and Terephthalic acid respectively.
ii). Toluene+ oleum
Toluene is methylbenzene containing methyl group which is o,p-directing group. Toluene reacts with oleum or fuming sulphuric acid containing SO3 as a Sulphonating agent to form ortho and para toluenesulphonic acid due to the presence of ortho-para directing CH3- group.
iii). Toluene+ methyl chloride (catalyzed by aluminum (III) chloride)
Toluene is methylbenzene containing methyl group which is o,p-directing group. Toluene reacts with methyl chloride in the presence of Lewis acid catalyst aluminum (III) chloride (AlCl3) undergoing Friedel-Craft alkylation (methylation) to form two isomeric dimethylbenzenes called ortho and para xylene.
iv). Toluene+ concentrated nitric acid
Toluene is a benzene with a methyl substituent. The methyl group activates the ortho and para positions for electrophilic aromatic substitution (EAS). In order to obtain TNT from toluene, we must add NO2 groups to these three positions which are already activated via nitration using nitrating mixture of HNO3 and H2SO4. However, each NO2 that is added greatly increases the energy needed for the next substitution to occur. The third substitution requires a massive input of energy to occur.
TNT is synthesized in a three-steps process. First, toluene is nitrated with a mixture of sulfuric and nitric acid to produce mono-nitrotoluene (MNT) which is renitrated to produce dinitrotoluene (DNT) which is further nitrated to tironitrotoluene (NT). At low temperatures, the mononitrotoluenes are made, and as the reaction temperature is increased the dinitro compounds are formed, until eventually TNT is formed.
TNT is a yellow, odorless material that does not occur naturally in nature. TNT is a kind of explosive that is extensively employed in military shells, bombs, and grenades, as well as industrial and undersea blasting. TNT is an abbreviation for trinitrotoluene. The IUPAC name is 2-methyl-1,3,5-trinitrobenzene. The International Union of Pure and Applied Chemistry (IUPAC) nomenclature of organic chemistry is a system of naming organic chemical compounds. Other names include 2,4,6-trinitromethylbenzene, as it contains a benzene ring with methyl group, 2,4,6-trinitrotoluol TNT, trinitrotoluol, tritolol, trinol, triton, tritolo, trotol, and tritone.
v). Toluene + chlorine in sunlight
In the presence of sunlight, toluene (or any alkyl benzene) react with Cl2 or Br2 by a free radical photochemical substitution reaction to substitute hydrogen of methyl (alkyl) group rather than the H of benzene ring forming all the three possible products. In this reaction, hydrogen atoms from side chain get replaced by chlorine giving benzyl chloride on monochlorination, benzal chloride on dichlorination and benzochloride on trichlorination.
Q2. Starting from benzene or phenol how the following compounds can be prepared:
i. Bakelite (Reaction of phenol with formaldehyde)
ii. m-nitrotoluene (Alkylation + Nitration)
iii. p-nitrobenzoic acid (Alkylation + Nitration + Oxidation)
iv). cyclohexanol (Hydrogenation of phenol)
(v) Picric acid (Nitration of phenol with conc. Nitric acid)
(vi) Bromol (Bromination + Hydrolysis + Bromination)
Answer
i. Bakelite
In presence of dilute base, phenol reacts with formaldehyde to produce a mixture of ortho and para-methylol phenols which are polymerized into a condensation polymer, Bakelite (first synthetic plastic), a thermo-setting plastic, once moulded, it cannot be remoulded and remelted.
The reaction may be summarized as: (according to book)
ii. m-nitrotoluene
To prepare m-nitrotoluene, we first perform nitration of benzene using nitrating mixture of HNO3 and H2SO4 to produce nitrobenzene which is then alkylated using methyl chloride in the presence of Lewis acid catalyst AlCl3 to produce the required compound. Nitro group being meta-directing group, direct the second group i.e. methyl group to meta position.
iii. p-nitrobenzoic acid
p-nitrobenzoic acid can be produced (along with its ortho isomer; o-nitrobenzoic acid) from nitration of toluene using nitrating mixture of HNO3 and H2SO4 to produce two isomeric nitrotoluenes followed by their oxidation using acidified potassium permanganate to get the required compound (along with o-nitrobenzoic acid)
iv). Cyclohexanol
Phenol is reduced to cyclohexanol when hydrogen gas is bubbled through phenol at 150-200°C in the presence of finely divided nickel catalyst. Cyclohexanol is used in the manufacture of nylon.
(v) Picric acid
Phenol undergoes nitration with concentrated nitric acid on heating in such a way that all the active position have been substituted by nitro group yielding 2,4,6-trinitrophenol (TNP) or picric acid which is quite a strong acid (Ka = 10–1 ) and can liberate CO2 from Na2CO3 solution, as the acidity of a phenol is increased when electron-attracting substituents (like –NO2) are introduced into the aromatic ring. [The dry solid picric acid is highly explosive compound but solutions have been used as an antiseptic and as yellow dye, and it was probably the first artificial dye (1849)].
(vi) Bromol
Phenol reacts with bromine water (or chlorine water) very rapidly in such a way that substitution in all available ortho and para-positions take place to yield white precipitate of 2,4,6-tribromophenol or TBP or bromol (or 2,4,6-trichlorophenol). This reaction is used as a test for phenol. tribromophenol also called bromol is a soft-white needles like solid with a sweet taste and penetrating bromine odour. trichlorophenol simply called 2,4,6-T is a yellow flakes with strong phenolic odour used as fungicide and herbicide.
Q3. How will you convert phenol into following?
(i) Cyclohexanol (Catalytic hydrogenation)
(ii) Benzene (Reduction)
(iii) picric acid (Successive nitration)
(iv) Phenoxide (Neutralization)
Answer
(i) Cyclohexanol (Catalytic hydrogenation)
Phenol is reduced to cyclohexanol when hydrogen gas is bubbled through phenol at 150-200°C in the presence of finely divided nickel catalyst. Cyclohexanol is used in the manufacture of nylon.
(ii) Benzene from Phenol (Reduction)
The removal of oxygen from compound is called reduction. Phenol is reduced to benzene when its vapours are passed over red hot with zinc dust (as zinc being a reducing agent removes oxygen from phenol). The new hydrogen atom in benzene is derived from – OH group of phenol.
(iv) Phenoxide (Neutralization)
Phenol forms salt called Phenoxide or phenate or Phenolate with aqueous strong alkalis like sodium hydroxide. Phenol is fairly acidic in nature and it is stronger acid than alcohols and hence it can neutralize alkalis.
Phenoxide may be prepared from phenol either by its neutralization with NaOH or by displacement reaction with Na in which hydroxylic hydrogen of phenol is displaced by sodium to form sodium phenoxide liberating H2 gas
(iii) Picric acid (Successive nitration)
Phenol undergoes nitration with concentrated nitric acid on heating in such a way that all the active position have been substituted by nitro group yielding 2,4,6-trinitrophenol (TNP) or picric acid which is quite a strong acid (Ka = 10–1 ) and can liberate CO2 from Na2CO3 solution, as the acidity of a phenol is increased when electron-attracting substituents (like –NO2) are introduced into the aromatic ring. [The dry solid picric acid is highly explosive compound but solutions have been used as an antiseptic and as yellow dye, and it was probably the first artificial dye (1849)].
Q4. How do you convert ethanol into benzene?
Q5. How do you convert ethene into benzene?
Ethene is first converted into 1,2-dibromoethane by the action of bromine, tetrachloromethane. Then it is treated with alc. KOH to form bromoethene which will further react with NaNH2/liq.NH3 to form ethyne. After that ethyne is heated in the red-hot iron tube at 873 K, benzene is obtained.
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