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Reduction of Esters via Aldehydes to Alcohols

Reduction of Esters to Alcohols via Aldehydes: Mechanism

2D animation of ester reductions

The first step in the reduction of esters with metal hydrides is the addition of hydride ion to the ester creating a tetrahedral intermediate. This step is the slowest in the reaction sequence.

Fig.1

Elimination of alkoxide from the tetrahedral intermediate generates the aldehyde. This step proceeds much faster than the addition of hydride ion to the ester.

Fig.2

Being a much stronger electrophile than the ester, the newly formed aldehyde accepts another hydride ion from the aluminum alkoxide. Since the formation of the aldehyde is faster than the addition of hydride ion to the ester and the aldehyde is a stronger electrophile than the ester, the reaction does not stop at the aldehyde stage but proceeds to the alcohol.

Fig.3

The aluminum alkoxide formed in the last step can transfer two more hydride ions to reduce an additional ester molecule. Hydrolysis then yields the corresponding alcohols.

Fig.4
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