Can you tell me why ketones and aldehydes don't bond with hydroxyl groups?
Certainly! Ketones and aldehydes do not typically bond with hydroxyl groups because the reaction between them is not thermodynamically favorable. The process of forming a bond between a ketone or aldehyde and a hydroxyl group involves the formation of a new carbon-carbon bond, while breaking the existing carbon-oxygen bond in the ketone or aldehyde. This process requires a significant amount of energy.
To understand why this reaction doesn't occur, we need to consider the electronic properties of carbonyl compounds (ketones and aldehydes) and hydroxyl groups. In a carbonyl compound, the carbon atom of the carbonyl group is electron-deficient, meaning that it has a partial positive charge. On the other hand, the oxygen atom in the hydroxyl group of an alcohol is electron-rich, with a partial negative charge.
When attempting to form a bond between a carbonyl compound and a hydroxyl group, the partial positive charge on the carbon atom in the carbonyl compound will repel the partial negative charge on the oxygen atom in the hydroxyl group. Additionally, the reaction requires breaking the existing carbon-oxygen bond in the carbonyl compound, which is relatively strong.
To overcome these energy barriers and allow the reaction to occur, a catalyst or a more reactive nucleophile is usually needed. For example, in the presence of a strong acid catalyst, carbonyl compounds can undergo acid-catalyzed reactions with alcohols, leading to the formation of hemiacetals or acetals. In these reactions, the acidic conditions and the formation of intermediate species help to facilitate the reaction and make it more thermodynamically favorable.
In summary, the lack of reactivity between ketones/aldehydes and hydroxyl groups is due to the electronic and steric factors that make the formation of a carbon-oxygen bond energetically unfavorable.