Prepare for exams with solved exercises on Aromatic Hydrocarbons. Cover key topics like structure, properties, reactions, and applications to reinforce your understanding and improve exam performance.
Q. 4
What are aromatic hydrocarbons? How are they classified?
- Aromatic hydrocarbons are organic compounds that contain one or more benzene rings or similar structures with alternating double bonds. These hydrocarbons are highly unsaturated, and their electrons are delocalized across the ring, giving them extra stability. Classification:
- Benzenoid Aromatic Hydrocarbons: Contain one or more benzene rings (e.g., benzene, toluene, xylene).
- Non-benzenoid Aromatic Hydrocarbons: Do not contain a benzene ring but still have delocalized π-electrons that give them aromatic character (e.g., tropolone, azulene).
Q. 5
What happens when:
(a) Benzene is heated with conc. H₂SO₄ at 250°C?
- When benzene is heated with concentrated sulfuric acid at a high temperature, sulfonation occurs, resulting in benzene sulfonic acid (C₆H₅SO₃H). This is an electrophilic substitution reaction.
(b) Chlorine is passed through benzene in sunlight?
- In the presence of sunlight, chlorine reacts with benzene to form hexachlorocyclohexane (C₆H₆Cl₆) via a free radical substitution reaction.
(c) A mixture of benzene vapors and air is passed over heated vanadium pentoxide?
- This leads to the formation of maleic anhydride (C₄H₂O₃) through oxidation of benzene.
(d) Benzene is burnt in a free supply of air?
- When benzene is burnt in an excess of air, it combusts completely to form carbon dioxide (CO₂) and water (H₂O), releasing a large amount of heat.
Q. 6
What is meant by the terms:
i) Aromatic: Refers to organic compounds that contain a conjugated π-electron system within a closed loop, typically a benzene ring, that follows Huckel’s rule (4n + 2 π-electrons).
ii) Oxidation: A chemical reaction that involves the loss of electrons or the addition of oxygen or removal of hydrogen from a molecule.
iii) Sulphonation: The introduction of a sulfonic acid group (-SO₃H) into an organic compound, typically via reaction with sulfuric acid.
iv) Nitration: The introduction of a nitro group (-NO₂) into an organic molecule, usually via reaction with nitric acid in the presence of sulfuric acid.
v) Halogenation: The addition or substitution of a halogen atom (Cl, Br, I) into an organic molecule, typically through an electrophilic substitution or free radical mechanism.
Q. 7
(a) Draw structural formulas for the following compounds:
- m-Chlorobenzoic acid:
A benzene ring with a carboxylic acid group (-COOH) at position 1 and a chlorine atom at position 3. - p-Hydroxybenzoic acid:
A benzene ring with a hydroxyl group (-OH) at position 4 and a carboxylic acid group (-COOH) at position 1. - m-Bromonitrobenzene:
A benzene ring with a nitro group (-NO₂) at position 1 and a bromine atom at position 3. - o-Ethyltoluene:
A benzene ring with a methyl group (-CH₃) at position 1 and an ethyl group (-C₂H₅) at position 2. - p-Nitroaniline:
A benzene ring with an amino group (-NH₂) at position 1 and a nitro group (-NO₂) at position 4. - 2,4,6-Trinitrotoluene (TNT):
A benzene ring with a methyl group (-CH₃) at position 1 and nitro groups (-NO₂) at positions 2, 4, and 6. - m-Nitrophenol:
A benzene ring with a hydroxyl group (-OH) at position 1 and a nitro group (-NO₂) at position 3. - p-Dibenzylbenzene:
A benzene ring attached to two benzyl groups (-CH₂Ph) at positions 1 and 4. - 2-Amino-5-bromo-3-nitrobenzenesulphonic acid:
A benzene ring with an amino group (-NH₂) at position 2, a bromine atom at position 5, a nitro group (-NO₂) at position 3, and a sulfonic acid group (-SO₃H) at position 1.
(b) Give names and possible isomeric structures of:
- Xylenes:
- Ortho-xylene, Meta-xylene, Para-xylene (isomers of dimethylbenzene).
- Trimethylbenzene:
- 1,2,3-Trimethylbenzene, 1,2,4-Trimethylbenzene, 1,3,5-Trimethylbenzene.
- Bromonitrotoluene:
- Isomers where bromine, nitro, and methyl groups are positioned differently on the benzene ring.
Q. 8
Write IUPAC names of the following molecules:
- 4-Chlorobenzaldehyde
- 3-Bromo-2-methylpropanoic acid
- 3,4-Dibromophenol
Q. 9
General mechanism of electrophilic aromatic substitution reactions:
- Formation of the electrophile: The reaction begins with the generation of an electrophile (e.g., Br⁺, NO₂⁺) via a catalyst like FeBr₃ or H₂SO₄.
- Electrophilic attack: The π-electrons of the aromatic ring attack the electrophile, forming a non-aromatic carbocation (an arenium ion).
- Deprotonation: A proton (H⁺) is removed from the carbocation, restoring the aromaticity of the ring, resulting in the substitution of a hydrogen atom with the electrophile.
Q. 10
(a) Describe the structure of benzene on the basis of:
i) Atomic orbital treatment:
- Benzene’s carbon atoms are sp² hybridized, and each carbon forms three sigma bonds: two with neighboring carbon atoms and one with a hydrogen atom. The unhybridized p orbitals overlap sideways to form a delocalized π-electron cloud above and below the plane of the carbon atoms.
ii) Resonance method:
- Benzene is represented as a resonance hybrid of two structures where the double bonds alternate between the carbon atoms. This delocalization of π-electrons provides extra stability to the benzene molecule.
(b) Prove that benzene has a cyclic structure:
- Experimental evidence shows that all six C–C bond lengths in benzene are identical and shorter than a typical single bond but longer than a double bond, which is consistent with a resonance-stabilized cyclic structure. Additionally, the molecular formula (C₆H₆) and aromatic behavior (like undergoing substitution reactions rather than addition reactions) further confirm its cyclic nature.
Q. 11
Predict the major products of bromination of the following compounds:
(a) Toluene
- Major product: o-Bromotoluene and p-Bromotoluene
- Toluene (methylbenzene) undergoes electrophilic substitution in the ortho- and para-positions relative to the methyl group due to the electron-donating nature of the methyl group.
(b) Nitrobenzene
- Major product: m-Bromonitrobenzene
- The nitro group is electron-withdrawing and directs incoming electrophiles to the meta-position, leading to the formation of m-bromonitrobenzene.
(c) Bromobenzene
- Major product: o-Dibromobenzene and p-Dibromobenzene
- Bromine is an ortho/para-directing group, so bromination will occur in the ortho and para positions relative to the already present bromine atom.
(d) Benzoic acid
- Major product: m-Bromobenzoic acid
- The carboxyl group (-COOH) is an electron-withdrawing group, so bromination occurs at the meta-position.
(e) Benzaldehyde
- Major product: m-Bromobenzaldehyde
- The formyl group (-CHO) is an electron-withdrawing group, directing the bromine to the meta-position.
(f) Phenol
- Major product: o-Bromophenol and p-Bromophenol
- The hydroxyl group (-OH) is an electron-donating group, leading to substitution in the ortho and para positions.
Q. 12
How will you prepare the following compounds from benzene in two steps:
(a) m-Chloronitrobenzene
- Nitration of benzene:
Benzene reacts with concentrated nitric acid and sulfuric acid to form nitrobenzene (C₆H₅NO₂). - Chlorination of nitrobenzene:
Nitrobenzene is treated with chlorine in the presence of a Lewis acid catalyst (FeCl₃), directing substitution at the meta position to give m-chloronitrobenzene (C₆H₄ClNO₂).
(b) p-Chloronitrobenzene
- Chlorination of benzene:
Benzene is chlorinated using chlorine in the presence of FeCl₃, yielding chlorobenzene (C₆H₅Cl). - Nitration of chlorobenzene:
Chlorobenzene undergoes nitration with HNO₃ and H₂SO₄, giving p-chloronitrobenzene (C₆H₄ClNO₂) as the major product.
Q. 13
Complete the following reactions. Also mention the conditions needed to carry out these reactions:
- Benzene + H₂ → Cyclohexane
- Condition: High pressure and nickel (Ni) catalyst.
- This is the hydrogenation of benzene, where three equivalents of hydrogen add to the benzene ring to form cyclohexane.
- Benzene + O₂ → Carbon dioxide (CO₂) and water (H₂O)
- Condition: Combustion in the presence of oxygen.
- Complete combustion of benzene yields carbon dioxide and water.
- Phenol + Zn → Benzene
- Condition: Heated with zinc dust.
- Zinc reduces phenol (C₆H₅OH) to benzene (C₆H₆).
- Benzene + SO₃ → Benzene sulfonic acid (C₆H₅SO₃H)
- Condition: Sulfonation with concentrated sulfuric acid or SO₃.
- Sulfonation of benzene introduces a sulfonic acid group (-SO₃H) onto the ring.
- Benzene + HOH (hydration) → No reaction
- Condition: Hydration requires more drastic conditions, and benzene does not react with water directly under normal conditions.
- Benzene + H₂SO₄ → Benzene sulfonic acid
- Condition: Sulfonation occurs when benzene is treated with concentrated sulfuric acid or oleum.
- This introduces a sulfonic acid group (-SO₃H) into the benzene ring.
Q. 14
Detail out three reactions in which benzene behaves as if it is a saturated hydrocarbon and three reactions in which it behaves as if it is unsaturated:
As a saturated hydrocarbon:
- Hydrogenation:
Benzene can undergo hydrogenation in the presence of a nickel catalyst under high pressure to form cyclohexane (C₆H₁₂), similar to a saturated hydrocarbon. - Halogenation (under UV light):
Benzene reacts with halogens (e.g., chlorine) under UV light to form hexachlorocyclohexane (C₆H₆Cl₆), indicating a saturated character. - Combustion:
Like saturated hydrocarbons, benzene combusts completely in air to form carbon dioxide and water.
As an unsaturated hydrocarbon:
- Bromination (in the presence of FeBr₃):
Benzene undergoes electrophilic substitution rather than addition with bromine, acting like an unsaturated hydrocarbon. - Nitration:
In the presence of concentrated H₂SO₄ and HNO₃, benzene undergoes nitration, an electrophilic substitution reaction, indicative of unsaturation. - Friedel-Crafts alkylation/acylation:
Benzene reacts with alkyl halides or acyl chlorides in the presence of AlCl₃, undergoing electrophilic substitution, which is a feature of unsaturation.
Q. 15
What are Friedel-Crafts reactions? Give the mechanism with an example of the following reactions:
(i) Friedel-Crafts alkylation reaction:
- Definition: It is the alkylation of an aromatic ring using an alkyl halide (e.g., CH₃Cl) and a Lewis acid catalyst (e.g., AlCl₃). Mechanism:
- Formation of the electrophile: The alkyl halide reacts with AlCl₃, forming a carbocation (CH₃⁺).
- Electrophilic attack: The benzene ring donates its π-electrons to the carbocation, forming an arenium ion (carbocation intermediate).
- Deprotonation: A proton is lost from the arenium ion, restoring the aromaticity and forming the alkylated product. Example:
Benzene + CH₃Cl (in the presence of AlCl₃) → Toluene (methylbenzene)
(ii) Friedel-Crafts acylation reaction:
- Definition: This involves the acylation of benzene using an acyl chloride (e.g., CH₃COCl) in the presence of a Lewis acid catalyst (e.g., AlCl₃). Mechanism:
- Formation of the acylium ion: The acyl chloride reacts with AlCl₃, forming an acylium ion (CH₃CO⁺).
- Electrophilic attack: The acylium ion is attacked by the π-electrons of benzene, forming an arenium ion.
- Deprotonation: A proton is lost, and the aromaticity is restored, giving the acylated benzene. Example:
Benzene + CH₃COCl (in the presence of AlCl₃) → Acetophenone (C₆H₅COCH₃)
