Question

The reaction most likely occurs by which of the following mechanism?

• A. Addition-elimination
• B. Addition only
• C. Elimination-addition
• D. Neither of these

Answer 1

Answer: (A)

Addition-elimination

Answer 2

The given reaction is a nucleophilic aromatic substitution. The substrate is 1-chloro-2,4-dinitrobenzene, which has two strong electron-withdrawing nitro (-NO₂) groups at the ortho and para positions relative to the chlorine atom. The reagent is methoxide ion (CH₃O⁻), a strong nucleophile.

Aryl halides generally do not undergo nucleophilic substitution easily. However, the presence of strong electron-withdrawing groups at ortho or para positions activates the benzene ring towards nucleophilic attack. This reaction proceeds via the addition-elimination mechanism, also known as the SNAr (nucleophilic aromatic substitution) mechanism.

The mechanism involves two main steps:

1. Addition: The nucleophile (CH₃O⁻) attacks the carbon atom bearing the leaving group (Cl). This attack is facilitated by the electron-withdrawing nitro groups, which delocalize and stabilize the negative charge that develops on the ring in the intermediate. This forms a resonance-stabilized intermediate called a Meisenheimer complex (or sigma complex). In the Meisenheimer complex, the carbon atom that was attacked becomes sp³ hybridized, temporarily breaking the aromaticity. The negative charge is delocalized onto the ortho and para carbons, and effectively onto the oxygen atoms of the nitro groups via resonance.

2. Elimination: The leaving group (Cl⁻) departs from the Meisenheimer complex, which restores aromaticity and forms the substituted product.

The elimination-addition mechanism (benzyne mechanism) typically occurs with aryl halides that lack strong electron-withdrawing groups or under very harsh conditions (e.g., NaNH₂ in liquid NH₃), and often leads to isomeric products. Given the presence of strong activating groups, the addition-elimination mechanism is overwhelmingly favored here.