Question

Among the following, the suitable route(s) for the conversion of benzaldehyde to acetophenone is/are

• A. $CH_3COCI$, anhydrous $AICI_3$
• B. (i) $HS(CH_2)_3SH, F_3B.OEt_3$; (ii) $n-BuLi$; (iii) $Mel$; (iv) $HgCl_2$, $CdCO_3$, $H_2O$
• C. $NaNH_2, Mel$
• D. (i) $MeMgBr$; (ii) aq. acid; (iii) pyridinium chlorochromate ($PCC$)

Answer 1

The Correct Option is B, D

The conversion of benzaldehyde to acetophenone involves changing an aldehyde group to a ketone group with a methyl substitution. Let's analyze the given routes to determine which ones are suitable for this transformation.

1. \(CH_3COCl\), anhydrous \(AlCl_3\):
   • This option represents the Friedel-Crafts acylation reaction. However, Friedel-Crafts acylation of benzaldehyde is not feasible due to the deactivating nature of the aldehyde group. Thus, this route is not suitable for the transformation.
2. (i) \(HS(CH_2)_3SH, F_3B.OEt_3\); (ii) \(n-BuLi\); (iii) \(MeI\); (iv) \(HgCl_2, CdCO_3, H_2O\):
   • This is a multi-step process involving protection of the carbonyl group, followed by deprotonation and alkylation, and finally deprotection.
   • Step (i): The carbonyl group of benzaldehyde is protected using a dithiol linkage with \(HS(CH_2)_3SH\).
   • Step (ii): Use of \(n-BuLi\) generates a carbanion intermediate that can later react with methyl iodide (\(MeI\)) in step (iii) to introduce a methyl group.
   • Step (iv): The deprotection step involves breaking the protective thioketal group to yield the ketone, thus forming acetophenone.
   • This route is suitable for the conversion of benzaldehyde to acetophenone.
3. \(NaNH_2, MeI\):
   • This option represents a nucleophilic substitution reaction where the aldehyde could potentially be converted to an imine. However, \(NaNH_2\), a strong base, can generate a carbanion which, when treated with \(MeI\), is more likely to lead to side reactions rather than the desired methyl ketone formation. Thus, this route is not suitable for the transformation.
4. (i) \(MeMgBr\); (ii) aq. acid; (iii) pyridinium chlorochromate (\(PCC\)):
   • Step (i): Grignard reagent, \(MeMgBr\), will add methyl and form a secondary alcohol with benzaldehyde.
   • Step (ii): Aqueous acidification converts the Grignard addition product into a secondary alcohol.
   • Step (iii): Oxidation of this secondary alcohol using \(PCC\) yields acetophenone.
   • This route effectively converts benzaldehyde to acetophenone.

Thus, the suitable routes for the conversion of benzaldehyde to acetophenone are:

• (i) \(HS(CH_2)_3SH, F_3B.OEt_3\); (ii) \(n-BuLi\); (iii) \(MeI\); (iv) \(HgCl_2, CdCO_3, H_2O\)
• (i) \(MeMgBr\); (ii) aq. acid; (iii) pyridinium chlorochromate (\(PCC\))