Why does benzaldehyde not undergo Friedel-Crafts alkylation or acylation reactions in the presence of anhydrous aluminum chloride (AlCl3)?
Answer and explanation
Correct answer: C. The electron-withdrawing -CHO group and AlCl3 complexation strongly deactivate the ring
- Step 1: Friedel-Crafts reactions require an activated or moderately deactivated aromatic ring to undergo electrophilic substitution by carbocations or acylium ions.
- Step 2: The formyl group () is strongly electron-withdrawing through both (resonance) and (inductive) effects, heavily deactivating the aromatic -system.
- Step 3: The Lewis acid catalyst coordinates to the lone pair of electrons on the carbonyl oxygen atom (forming ), placing a positive charge adjacent to the ring and deactivating it even further.
- Step 4: Consequently, benzaldehyde is entirely inert toward Friedel-Crafts alkylation and acylation.
Answer
The strongly electron-withdrawing -CHO group deactivates the benzene ring, and AlCl3 complexes with the carbonyl oxygen lone pair to further deactivate the ring
Why not the others: 'Benzaldehyde is an aliphatic compound...' is false as it contains a benzene ring; 'Anhydrous AlCl3 reduces the aldehyde...' is chemically false as is a Lewis acid, not a reducing agent; 'Benzaldehyde reacts to form trinitrotoluene...' is absurd as no nitrogen source exists.
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