How Aldehydes and Ketones React

In Section 16.5, we saw that the carbonyl group of a carboxylic acid or a carboxylic acid derivative is attached to a group that can be replaced by another group. As a result, these compounds undergo nucleophilic addition-elimination reactions— the nucleophile adds to the carbonyl carbon and a group is eliminated from the tetrahedral intermediate. Overall, it is a substitution reaction in which Z substitutes for Y . 

Carboxylic acid derivatives undergo nucleophilic addition-elimination reactions with nucleophiles. 

In contrast, the carbonyl group of an aldehyde or a ketone is attached to a group that is too strong a base (H or R ) to be eliminated under normal conditions, so it cannot be replaced by another group. Aldehydes and ketones, therefore, do not form substitution products when they react with nucleophiles. 

The addition of a nucleophile to the carbonyl carbon of an aldehyde or a ketone forms a tetrahedral compound. If the nucleophile is a strong base, such as R or H , then the tetrahedral compound will be stable because it will not have a group that can be eliminated. Thus, the reaction will be an irreversible nucleophilic addition reaction. (Recall that a tetrahedral compound is unstable only if the sp carbon is attached to an oxygen and to another electronegative atom see Section 16.5). 

Aldehydes and ketones undergo irreversible nucleophilic addition reactions with nucleophiles that are strong bases. 

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