Question

Which of the following is least reactive towards S_n1 reactions?

• A. C₆H₅CH₂Cl
• B. C₆H₅Cl
• C. H₂C = CH - CH₂CL
• D. (C₆H₅)₂ CHCl

Answer 1

The Correct Option is B

The correct option is: (B): C₆H₅Cl.

SN1 reactions are characterized by a two-step mechanism where the leaving group (in this case, the chloride ion) first departs from the substrate to generate a carbocation intermediate. The nucleophile then attacks the carbocation to complete the reaction.

The reactivity of a compound in SN1 reactions is influenced by factors such as the stability of the carbocation intermediate and the leaving group ability. Here's why chlorobenzene (C6H5Cl) is least reactive towards SN1 reactions compared to the other options:

Carbocation Stability: The stability of the carbocation intermediate plays a significant role in SN1 reactions. The greater the stability of the carbocation, the faster the reaction. In chlorobenzene, the carbocation intermediate formed after the departure of the chloride ion is a tertiary carbocation (benzylic carbocation). Tertiary carbocations are relatively stable due to the resonance stabilization provided by the adjacent aromatic ring. This enhanced stability reduces the tendency of chlorobenzene to undergo SN1 reactions.

Leaving Group Ability: The leaving group ability also affects SN1 reactivity. In the provided options, all the compounds have chloride ions (Cl-) as leaving groups. Leaving group ability is generally affected by the polarity and bond strength of the leaving group. Since all the options have the same leaving group, this factor does not significantly differentiate their reactivity.

Aromatic Stabilization: Chlorobenzene is an aromatic compound, and its aromatic stabilization provides additional stability to the molecule as a whole. This aromatic stabilization further decreases the reactivity of chlorobenzene in undergoing SN1 reactions.

In comparison to the other options, such as alkyl chlorides (which lack the resonance stabilization of aromatic compounds) and alkyl bromides (which have a weaker C-Br bond), chlorobenzene's combination of aromatic stabilization and a stable carbocation intermediate makes it the least reactive towards SN1 reactions.