Aromatic Hydrocarbons

Which of the following contains 4n+2 π-electrons and is therefore aromatic?

1. Cyclobutadiene
2. Cyclooctatetraene
3. Cyclopentadienyl anion
4. Cyclopropyl cation

The C-C bond length in benzene is:

1. 1.20 Å (like acetylene)
2. 1.34 Å (like ethylene)
3. 1.39 Å (intermediate)
4. 1.54 Å (like ethane)

The resonance (delocalisation) energy of benzene is approximately:

1. 36 kJ/mol
2. 75 kJ/mol
3. 150 kJ/mol
4. 250 kJ/mol

Each carbon atom in benzene is:

1. sp hybridised, 180° bond angle
2. sp² hybridised, 120° bond angle
3. sp³ hybridised, 109.5° bond angle
4. sp²d hybridised, 90° bond angle

Cyclooctatetraene (C₈H₈) is not aromatic even though it is cyclic and fully conjugated. Why?

1. It has 4n electrons (n = 2)
2. It adopts a non-planar tub shape
3. Not all carbons are sp² hybridised
4. Both (a) and (b)

Which of the following is antiaromatic?

1. Benzene
2. Cyclobutadiene
3. Naphthalene
4. Pyridine

Tropylium cation (cycloheptatrienyl cation, C₇H₇⁺) is aromatic because it has:

1. 4 π-electrons
2. 6 π-electrons in a planar 7-ring
3. 8 π-electrons
4. 10 π-electrons

In pyridine, the lone pair on nitrogen:

1. Is part of the aromatic π-system
2. Occupies an sp² hybrid orbital in the ring plane, not part of the π-system
3. Is in a pure s-orbital
4. Is delocalised onto adjacent carbons

In pyrrole (C₄H₅N), the nitrogen lone pair:

1. Is in the ring plane, not part of aromaticity
2. Contributes 2 electrons to the aromatic π-system
3. Contributes 1 electron
4. Does not exist

Which of the following is non-aromatic?

1. Benzene
2. Naphthalene
3. Cyclohexane
4. Anthracene

Number of π-electrons in naphthalene:

1. 6
2. 8
3. 10
4. 12

The total number of σ- and π-bonds in benzene is:

1. 12 σ, 3 π
2. 6 σ, 6 π
3. 12 σ, 6 π
4. 15 σ, 3 π

Cyclopropenyl cation is aromatic. Its π-electron count is:

1. 0
2. 2
3. 4
4. 6

Benzene is represented as a resonance hybrid of how many Kekulé structures?

1. 1
2. 2
3. 3
4. 4

The C-C bond order in benzene is:

1. 1
2. 1.5
3. 2
4. 3

The IUPAC name of toluene is:

1. Benzene methane
2. Methylbenzene
3. Phenylmethane
4. Methanebenzene

1,4-dimethylbenzene is also called:

1. o-xylene
2. m-xylene
3. p-xylene
4. Ethylbenzene

Number of possible isomers of xylene (dimethylbenzene) is:

1. 2
2. 3
3. 4
4. 5

In naphthalene, positions labelled 1 and 2 are known as:

1. α and β
2. meta and para
3. ortho and meta
4. syn and anti

The number of trisubstituted isomers of benzene C₆H₃X₃ (X = same substituent) is:

1. 2
2. 3
3. 4
4. 6

Aniline is:

1. Benzoic acid
2. Aminobenzene / phenylamine
3. Nitrobenzene
4. Hydroxybenzene

The group C₆H₅-CH₂- is called:

1. Phenyl
2. Benzyl
3. Tolyl
4. Aryl

The correct IUPAC name of the compound with a -COOH and a -NO₂ group on benzene (with -NO₂ meta to -COOH) is:

1. 3-nitrobenzoic acid
2. Meta-nitrobenzoic acid
3. 3-carboxynitrobenzene
4. 1-nitro-3-carboxybenzene

3-methylphenol is also known as:

1. o-cresol
2. m-cresol
3. p-cresol
4. Anisole

General molecular formula of an aromatic hydrocarbon with one benzene ring and no side chain:

1. CₙH₂ₙ
2. CₙH₂ₙ₋₂
3. CₙH₂ₙ₋₆
4. CₙH₂ₙ₊₂

Benzene is obtained by cyclic polymerisation of:

1. Ethylene
2. Acetylene at 873 K on red-hot iron tube
3. Methane
4. Propyne

Sodium benzoate + soda lime (NaOH + CaO), Δ →

1. Benzene + Na₂CO₃
2. Toluene + CO₂
3. Phenol + NaOH
4. Benzoic acid + Na₂O

Reduction of phenol with Zn dust gives:

1. Cyclohexane
2. Benzene
3. Toluene
4. Benzyl alcohol

Wurtz-Fittig reaction between bromobenzene and methyl bromide (Na, dry ether) gives:

1. Benzene
2. Biphenyl
3. Toluene
4. Ethylbenzene

Benzene diazonium chloride + H₃PO₂ →

1. Chlorobenzene
2. Benzene
3. Aniline
4. Phenol

Which method gives the purest benzene?

1. Trimerisation of acetylene
2. Decarboxylation of sodium benzoate
3. Coal tar distillation
4. Reduction of phenol

Toluene is prepared by treating benzene with:

1. CH₃Cl, AlCl₃
2. CH₃COCl, AlCl₃
3. CH₃OH, H₂SO₄
4. CH₄, sunlight

n-propyl chloride + benzene + AlCl₃ gives mainly:

1. n-Propylbenzene
2. Isopropylbenzene (cumene)
3. Cyclopropylbenzene
4. Diphenylpropane

Benzene + acetyl chloride + AlCl₃ gives:

1. Toluene
2. Acetophenone
3. Benzaldehyde
4. Benzoic acid

Kolbe electrolysis of sodium benzoate yields:

1. Benzene
2. Biphenyl
3. Phenol
4. Diphenylmethane

The electrophile in the nitration of benzene by concentrated HNO₃ / H₂SO₄ mixture is:

1. NO⁺
2. NO₂⁺ (nitronium ion)
3. NO₂ radical
4. HNO₃

The electrophile in sulphonation of benzene by conc. H₂SO₄ or oleum is:

1. SO₃²⁻
2. SO₃ (or its protonated form H₃SO₃⁺)
3. HSO₄⁻
4. SO₂

The electrophile in bromination of benzene by Br₂ / FeBr₃ is:

1. Br•
2. Br⁻
3. Br⁺ (or Br-FeBr₃ complex generating Br⁺)
4. Br₂

The intermediate in EAS is called:

1. Arenium ion / σ-complex / Wheland intermediate
2. π-complex
3. Free radical
4. Carbanion

In EAS, the rate-determining step is:

1. Generation of the electrophile
2. Attack of the electrophile on benzene → arenium ion
3. Loss of H⁺ from the arenium ion
4. Neutralisation by the counterion

Benzene undergoes electrophilic substitution rather than addition because:

1. Benzene has no π-electrons
2. Substitution preserves the aromatic sextet, which is stabilising
3. Addition to benzene is impossible
4. The C-H bond is stronger than in alkanes

Which reaction on benzene is reversible?

1. Nitration
2. Halogenation
3. Sulphonation
4. Friedel-Crafts alkylation

The order of reactivity of halogens in EAS on benzene is:

1. F₂ > Cl₂ > Br₂ > I₂
2. I₂ > Br₂ > Cl₂ > F₂
3. Br₂ > F₂ > Cl₂ > I₂
4. All equally reactive

Direct iodination of benzene is difficult because:

1. I⁺ is too weak an electrophile · HI byproduct reduces the aryl iodide back to benzene
2. I₂ is a solid
3. Iodine forms addition products only
4. Benzene does not react with iodine

Which of the following is an intermediate in nucleophilic aromatic substitution (S_NAr) on activated aryl halides, NOT in EAS?

1. Wheland intermediate
2. Meisenheimer complex
3. Arenium ion
4. σ-complex

In EAS mechanism, which comes first?

1. π-complex, then σ-complex
2. σ-complex, then π-complex
3. Only σ-complex is formed
4. Only π-complex is formed

The arenium ion is:

1. Aromatic
2. Non-aromatic (sp³ carbon disrupts the ring's π-system)
3. Antiaromatic
4. A biradical

Standard conditions for mono-nitration of benzene:

1. Dilute HNO₃, room temp
2. Conc. HNO₃ + conc. H₂SO₄, 50-60 °C
3. Fuming HNO₃, 100 °C
4. HNO₃ alone, boil

D₂SO₄ + benzene (D = deuterium) gives:

1. C₆H₆ (no reaction)
2. C₆H₅D and C₆D₆ eventually, via EAS-type H/D exchange
3. C₆D₆ only
4. Benzene-2H (only at one position)

In step 3 of EAS, aromaticity is restored by:

1. Rearrangement of atoms
2. Loss of H⁺ from the sp³ carbon that was attacked
3. Loss of an electron
4. Addition of a nucleophile

Nitration of phenol gives predominantly:

1. m-nitrophenol
2. o- and p-nitrophenol
3. 3,5-dinitrophenol
4. No reaction

Nitration of nitrobenzene gives:

1. o-dinitrobenzene
2. p-dinitrobenzene
3. m-dinitrobenzene (major)
4. No further reaction

Chlorobenzene undergoes nitration to give:

1. m-chloronitrobenzene mainly
2. o- and p-chloronitrobenzene mainly, slower than benzene
3. o- and p-chloronitrobenzene mainly, faster than benzene
4. No reaction

Bromination of toluene gives predominantly:

1. m-bromotoluene
2. o- and p-bromotoluene, faster than benzene
3. Benzyl bromide
4. 3,5-dibromotoluene

The most powerfully ring-activating group among the following is:

1. -CH₃
2. -Cl
3. -NH₂
4. -OCH₃

Nitration of benzoic acid gives mainly:

1. o-nitrobenzoic acid
2. p-nitrobenzoic acid
3. m-nitrobenzoic acid
4. 3,5-dinitrobenzoic acid

Sulphonation of benzaldehyde gives mainly:

1. o-sulphonic acid
2. p-sulphonic acid
3. m-sulphonic acid
4. Both o and p in equal amounts

Direct nitration of aniline with conc. HNO₃/H₂SO₄ gives a large amount of m-nitroaniline. Why?

1. -NH₂ is naturally m-directing
2. In strongly acidic medium, -NH₂ is protonated to -NH₃⁺, a strong deactivator and m-director
3. Aniline is oxidised
4. Nitration prefers positions with more space

In p-nitrotoluene, further nitration goes to which position?

1. 2 (ortho to methyl, meta to nitro)
2. 3 (meta to methyl, meta to nitro)
3. Both directors send the electrophile to position 2 — one product
4. No reaction

In p-hydroxyacetophenone (-OH at C1, -COCH₃ at C4), further nitration goes to:

1. Ortho to -COCH₃
2. Ortho to -OH (position 2 or 6)
3. Meta to both
4. Doesn't react

Nitration of tert-butylbenzene gives:

1. Only o-product
2. Only p-product
3. Mainly p-product (steric hindrance blocks ortho)
4. Mainly m-product

Aniline + Br₂ (excess, in water) gives:

1. Bromobenzene
2. o- and p-bromoaniline
3. 2,4,6-tribromoaniline
4. No reaction

Order of reactivity of the following in electrophilic bromination:

1. Nitrobenzene > benzene > toluene > aniline
2. Aniline > toluene > benzene > nitrobenzene
3. Benzene > toluene > aniline > nitrobenzene
4. Toluene > benzene > aniline > nitrobenzene

Nitrobenzene does NOT undergo Friedel-Crafts reactions because:

1. Nitro group forms complex with AlCl₃
2. Ring is too deactivated for the electrophile to attack
3. The reaction is thermodynamically unfavourable
4. Both (a) and (b)

Rate of electrophilic substitution: phenol vs anisole. Which is faster and why?

1. Phenol; -OH is stronger donor than -OCH₃
2. Anisole; -OCH₃ is more activating due to +M and +I
3. Both equal
4. Anisole; -OH is deactivating

Sulphonation of phenol at low temperature (~15°C) gives:

1. p-hydroxybenzenesulphonic acid (kinetic)
2. o-hydroxybenzenesulphonic acid (kinetic)
3. m-hydroxybenzenesulphonic acid
4. None

Chlorination of o-nitroanisole (-OCH₃ at C1, -NO₂ at C2) with Cl₂/FeCl₃ gives predominantly Cl at:

1. C3
2. C4
3. C5
4. C6

To synthesise m-bromoacetophenone from benzene, correct order:

1. Bromination, then Friedel-Crafts acylation
2. Friedel-Crafts acylation, then bromination
3. Nitration, then bromination
4. Two brominations, then oxidation

The -CF₃ group in trifluoromethylbenzene (benzotrifluoride) is:

1. Activating, o/p director
2. Deactivating, o/p director
3. Deactivating, m-director
4. Activating, m-director

The -CH=CH₂ (vinyl) group on benzene is:

1. Deactivating m-director
2. Activating o/p director
3. Non-directing
4. Deactivating o/p director

The catalyst used in Friedel-Crafts alkylation and acylation is:

1. NaOH
2. Anhydrous AlCl₃ (Lewis acid)
3. Conc. H₂SO₄
4. Fe powder

The electrophile in Friedel-Crafts acylation (RCOCl + AlCl₃) is:

1. R⁺
2. R-COO⁻
3. Acylium ion RCO⁺
4. H⁺

Friedel-Crafts alkylation of benzene with excess ethyl chloride gives:

1. Ethylbenzene mostly
2. Polyethylbenzenes (di-, tri-, tetra-) because product is more reactive than benzene
3. Cyclohexane
4. Only para-diethylbenzene

Why doesn't Friedel-Crafts acylation give rearranged products (unlike alkylation)?

1. Because acyl chlorides are more reactive
2. Because the acylium ion RCO⁺ is stabilised by resonance (R-C≡O⁺) — no need to rearrange
3. Because AlCl₃ prevents rearrangement
4. Because acyl groups are always small

Benzene + n-propyl chloride + AlCl₃ gives cumene as the major product because:

1. n-propyl chloride is less reactive than isopropyl chloride
2. The n-propyl carbocation (1°) rearranges by 1,2-hydride shift to the more stable isopropyl carbocation (2°)
3. Cumene is more thermodynamically stable
4. AlCl₃ selectively activates 2° halides

To convert acetophenone (C₆H₅COCH₃) to ethylbenzene (C₆H₅CH₂CH₃), the reagent is:

1. NaBH₄, then H⁺
2. Zn-Hg, HCl (Clemmensen reduction) or NH₂NH₂, KOH (Wolff-Kishner)
3. LiAlH₄
4. H₂, Pd

Friedel-Crafts alkylation of aniline fails because:

1. Aniline's -NH₂ is basic and reacts with AlCl₃ to form a stable complex, poisoning the catalyst
2. Aniline is not aromatic
3. Aniline is too reactive
4. -NH₂ is a meta director

Benzene + CO + HCl + AlCl₃/CuCl (Gattermann-Koch reaction) gives:

1. Chlorobenzene
2. Benzoyl chloride
3. Benzaldehyde
4. Phenol

To make ethylbenzene without polysubstitution, best method:

1. Use excess benzene relative to ethyl chloride
2. Use ethyl chloride in excess
3. Use AlCl₃ in excess
4. Add water

Benzene + propene + H₃PO₄ (acid catalyst) →

1. n-propylbenzene
2. Cumene (isopropylbenzene)
3. 1-phenyl-2-propanol
4. No reaction

Toluene + hot alkaline KMnO₄ →

1. Benzyl alcohol
2. Benzaldehyde
3. Benzoic acid
4. Cyclohexanoic acid

tert-Butylbenzene + hot KMnO₄ →

1. Benzoic acid + acetone
2. Benzoic acid + tert-butanol
3. No reaction — no benzylic H
4. Cumene

Toluene + Cl₂ + FeCl₃ (dark) gives mainly:

1. Benzyl chloride (C₆H₅CH₂Cl)
2. o- and p-chlorotoluene
3. m-chlorotoluene
4. Trichlorotoluene

Toluene + Cl₂ + sunlight (or UV) gives:

1. o- and p-chlorotoluene
2. Benzyl chloride (side-chain product), further chlorination gives benzal chloride and benzotrichloride
3. m-chlorotoluene
4. Hexachlorocyclohexane

Order of stability of the following radicals:

1. Methyl > benzyl > ethyl
2. Benzyl > allyl > tertiary > secondary > primary > methyl
3. Primary > secondary > tertiary
4. All equally stable

Toluene + CrO₂Cl₂ (Étard reaction) after hydrolysis gives:

1. Benzoic acid
2. Benzaldehyde
3. Benzyl alcohol
4. Benzyl chloride

Toluene + N-bromosuccinimide (NBS) + CCl₄, peroxide, heat →

1. o- and p-bromotoluene
2. Benzyl bromide
3. 2,4,6-tribromotoluene
4. Benzoic acid

Benzyl chloride + aq. NaOH →

1. Benzyl alcohol (S_N2 or S_N1)
2. Phenol
3. Toluene
4. Benzaldehyde

In the cumene process, cumene is oxidised by O₂ and then hydrolysed with dilute acid to give:

1. Benzoic acid + methanol
2. Phenol + acetone
3. Benzaldehyde + ethanol
4. Cumene hydroperoxide only

Bromobenzene + ethyl bromide + 2 Na (dry ether) →

1. Biphenyl only
2. n-butane only
3. Ethylbenzene (main) + biphenyl + n-butane (side products)
4. Styrene

Which of the following is TRUE for benzene?

1. Colourless liquid, characteristic smell, immiscible in water, density < water
2. Yellow crystalline solid, water-soluble
3. Colourless gas, sweet-smelling
4. Blue liquid, denser than water

Benzene burns in air with:

1. Blue flame (like methane)
2. Sooty flame (yellow, smoky, because high C:H ratio favours unburnt carbon)
3. No visible flame
4. Green flame

Benzene + H₂ / Ni, 200 °C →

1. No reaction
2. Cyclohexane (all three double bonds hydrogenated)
3. Cyclohexene
4. 1,3-cyclohexadiene

Benzene + 3 Cl₂ + UV light →

1. 1,3,5-trichlorobenzene (EAS)
2. Chlorobenzene
3. 1,2,3,4,5,6-hexachlorocyclohexane (BHC / Lindane) — addition
4. Cl₃CH benzene

Ozonolysis of benzene followed by reduction with Zn/H₂O gives:

1. One molecule of glyoxal
2. Three molecules of glyoxal (OHC-CHO)
3. Six molecules of formaldehyde
4. Only CO₂

The most notorious carcinogenic polycyclic aromatic hydrocarbon (PAH), found in tobacco smoke and grilled meat, is:

1. Naphthalene
2. Anthracene
3. Benzo[a]pyrene
4. Phenol

Nitration of benzene at 100 °C with excess conc. HNO₃/H₂SO₄ gives:

1. Mononitrobenzene only
2. m-dinitrobenzene (mainly)
3. 1,3,5-trinitrobenzene (TNB)
4. 2,4,6-trinitrotoluene (TNT)

Electrophilic substitution on naphthalene occurs preferentially at:

1. Position 2 (β)
2. Position 1 (α) at low temperature
3. Position 4a (bridgehead)
4. Position 5

Benzene + Br₂/CCl₄ (no FeBr₃, dark) →

1. Bromobenzene rapidly
2. No reaction / very slow — Br₂ colour persists
3. Cyclohexane
4. Addition product 1,2-dibromocyclohexadiene

The number of possible isomers for the disubstituted benzene C₆H₄XY (X ≠ Y) is:

1. 2
2. 3
3. 4
4. 6