Arenes EPQ Guiding Questions

Based on the molecular formula C₈H₁₀, what structural feature is likely to be present in compound B?

• An alkene group

• A cycloalkane

• A benzene ring

Answer explanation

B has a high C:H ratio of close to 1:1, therefore it likely contains a benzene ring.

What type of reaction occurs when B reacts with Cl₂ in the presence of a suitable catalyst and in the absence of UV light?

• Free radical substitution

• Nucleophilic substitution

• Electrophilic substitution

• Electrophilic addition

Answer explanation

The reaction of compound B with Cl₂ and a suitable catalyst is an electrophilic substitution.

In this process, the catalyst reacts with Cl₂ to form Cl⁺. Cl⁺ is the electrophile which reacts with the benzene ring in compound B, by substituting a H atom.

What structural feature of compound B can be deduced from its reaction with Cl₂ in the presence of a suitable catalyst and in the absence of UV light?

• A double bond (C=C)

•  A hydroxyl group (-OH)

• An alkane

• A benzene ring

Answer explanation

The reaction of compound B with Cl₂ in the absence of UV light suggests the presence of a benzene ring, as benzene undergoes electrophilic substitution with chlorine without requiring light, unlike alkenes would not require a catalyst for electrophilic addition to occur.

What type of reaction occurs when compound B reacts with hot acidified KMnO₄?

• Side-chain oxidation.

• Electrophilic substitution

• Electrophilic addition

• Free radical substitution

Answer explanation

KMnO₄ is a strong oxidising agent. When compound B reacts with hot acidified KMnO₄, it undergoes side-chain oxidation, where the alkyl side chains are oxidized to form carboxylic acid groups.

What structural feature can be deduced about compound B from its reaction with hot acidified KMnO₄?

• Compound B contains an alkene functional group

• Compound B contains an alkyl side chain attached to a benzene ring.

• Compound B contains a cycloalkane. 

• Compound B contains a hydroxyl group (-OH).

Answer explanation

The reaction of compound B with hot acidified KMnO₄ indicates it can be oxidized, suggesting the presence of an alkyl side chain attached to a benzene ring, as this structure can undergo oxidation to form carboxylic acids.

There is no alkene functional group. From earlier questions, we have already established that Compound B contains a benzene ring. If there is an alkene functional group present with a benzene ring, it will be C₆H₅CH=CH₂, and its molecular formula will be C₈H₈ and not C₈H₁₀.

What type of reaction occurs between compound C and NaOH?

• Electrophilic Substitution

• Electrophilic addition

• Free radical substitution

• Acid-base reaction

Answer explanation

The reaction between compound C and NaOH is an acid-base reaction, where NaOH acts as a base and neutralizes the acidic component of compound C, resulting in the formation of water and a salt.

What structural feature of compound C can be deduced from its reaction with NaOH?

• Compound C contains an alkene group

•  Compound C contains a carboxylic acid group (-COOH)

• Compound C contains an alkyl group

•  Compound C contains a carbonyl group (C=O)

Answer explanation

The reaction of compound C with NaOH indicates it contains a carboxylic acid group (-COOH), as carboxylic acids react with bases to form salts and water, confirming the presence of this functional group.

One mole of C is exactly neutralised by one mole of NaOH. How many -COOH group(s) does compound C have?

• 1

• 2

• 3

Answer explanation

Since C reacts with NaOH in a 1:1 ratio, this suggests that C only contains only one
-COOH group (it is a monobasic acid). Hence, C is benzoic acid.