Does the substituent on benzaldehyde affect the reaction rate or percent yield o

Question

Does the substituent on benzaldehyde affect the reaction rate or percent yield of the reduction of the aldehyde? For example, is the reduction of 4-methylbenzaldehyde expected to have a higher, lower, or the same percent yield than that of 4-hydroxy-3-methoxybenzaldehyde? Why? Does the substituent on benzaldehyde affect the reaction rate or percent yield of the reduction of the aldehyde? For example, is the reduction of 4-methylbenzaldehyde expected to have a higher, lower, or the same percent yield than that of 4-hydroxy-3-methoxybenzaldehyde? Why?

Explanation / Answer

Supposing that the reductor agent is an hydride as LiAlH4 or NaBH4, the substituent of the benzaldehyde does have an effect in the reaction rate because the slow step in the reaction is the transference of the H- to the carbonyl. If the carbonyl is poor in electrons, the more positive will be the Carbon atom and the faster the transference. The richer the carbonyl in electronic density the slower the transference and the reaction.

4-hydroxy-3-methoxybenzaldehyde has one donor group by resonance (4 hydroxy) which made the carbonyl rich in electronic density, thus the slower will be this reaction compared with 4-methylbenzaldehyde.

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