Q16-68E - Organic Chemistry (Mcmurry)

Question

Propose a mechanism for the reaction of 1-chloroanthraquinone with methoxide ion to give the substitution product 1-methoxyanthraquinone. Use curved arrows to show the electron flow in each step.

Step-by-Step Solution

Answer

Mechanism followed as:

Step 1: Meisenheimer complex

It is the negatively charged complex formed the attack of the nucleophile on the aromatic ring structure. Generally, it is a stable intermediate due to the presence of the negative charge which helps in the delocalization of the 𝛑 electron in the aromatic ring and reaction followed by a nuclear aromatic substitution reaction (SNAr).

Step 2: Reaction of 1-chloroanthraquinone with methoxide ion

In the 1-chloro anthraquinone the chlorine containing is electron-poor because of the adjacent carbonyl which is a strong electron-withdrawing in nature and that’s why the nucleophilic attack of $-{\mathrm{OCH}}_3$ is occur from the chlorine side and the electron movement occurs from chlorine carbonyl and followed by meisenheimer complex formation occur and when again electron delocalization occurs from carbonyl to chlorine ring followed by chlorine removal desired product is obtained.

Step 3: Mechanism

1. Addition of nucleophile i.e methoxide ion
2. Formation of Meisenheimer complex
3. Elimination of chlorine 
4. Desired product formed

Mechanism