AQA A-Level Chemistry Paper 3, 2023: Question 29

Which is not a possible product of a reaction between KOH and the given organic bromide?

• A: Cyclohexanol
• B: 2-Cyclohexanone
• C: Benzene
• D: Cyclohexene

The correct answer is B: 2-Cyclohexanone, as this product cannot be formed through the reaction of KOH with the given bromide compound. The reaction conditions provided by KOH (either aqueous or alcoholic) favour nucleophilic substitution or elimination reactions, not oxidation to form a ketone.

The reaction between KOH and the organic bromide typically proceeds via two mechanisms:

1. Substitution Reaction (Aqueous KOH): This reaction replaces the bromine atom with a hydroxyl group, forming cyclohexanol (Option A).
2. Elimination Reaction (Alcoholic KOH): This reaction eliminates HBr, forming cyclohexene (Option D).

However, formation of a ketone (Option B) would require an oxidation reaction, which is not facilitated under the reaction conditions of KOH.

Option C, benzene, is not a direct product of this reaction but is included for context in complex elimination reactions.

• Option A: Cyclohexanol is formed through nucleophilic substitution with aqueous KOH.
• Option C: While less common, benzene may arise through complex side reactions in harsh elimination conditions.
• Option D: Cyclohexene is the product of elimination with alcoholic KOH.

Tip: Remember the distinction between substitution (aqueous KOH) and elimination (alcoholic KOH). Also, oxidation reactions require specific oxidising agents, which are not present here.