Identifying Electron-Withdrawing Groups

The above reaction would more readily proceed if the electrophilicity of the carbonyl carbon were enhanced. This may be achieved through electron withdrawal via the R group.

The ether (-OMe), the methyl (-Me), and the hydroxyl (-OH), would all produce a electron-donating effect, and are thus incorrect answers.

The nitro group (-NO2), and the positively charged, tetra-substituted amino group (consider the structure once this trimethyl amino group is connected to the aryl ring) are both electron-withdrawing. As the trimethyl amino group will have an overall positive charge (and the nitro group is neutral overall), the trimethyl amino group is the stronger electron-withdrawing moiety, and is thus the correct answer.