MHT CET Chemistry Question (2026) — Solution

Step-by-step solution

Williamson's synthesis is a method for preparing ethers by the reaction of an alkyl halide with a sodium alkoxide. This reaction follows an S_N2 mechanism. For an S_N2 reaction to occur favorably, the halide should be primary or secondary (primary being the most reactive). Let us evaluate the given options: 1. C _2 H _5 Br (Ethyl bromide) is a primary alkyl halide and readily undergoes Williamson's synthesis. 2. CH _3- CH ( CH _3)- CH _2- Br (Isobutyl bromide) is a primary alkyl halide and undergoes Williamson's synthesis. 3. C _6 H _5- Br (Bromobenzene) is an aryl halide. In aryl halides, the carbon-halogen bond acquires a partial double bond character due to resonance with the benzene ring. Additionally, the sp^2 hybridized carbon is more electronegative, making the C - Br bond stronger and difficult to break. The bulky benzene ring also sterically hinders the backside attack required for an S_N2 mechanism. Therefore, aryl halides do not undergo Williamson's synthesis. 4. C _6 H _5- CH _2- Br (Benzyl bromide) is a primary benzylic halide, which is highly reactive towards S_N2 reactions and readily undergoes Williamson's synthesis. Thus, C _6 H _5- Br does not undergo Williamson's synthesis. Answer: C _6 H _5 - Br