Aldehydes and ketones are part of a class of compounds known as carbonyls, carbon-oxygen double bonds. The majority of reactions these compounds undergo are nucleophilic additions. Nucleophilic addition reactions involve a nucleophilic attack on a carbonyl carbon either following or preceding a proton transfer step. These nucleophilic reactions can involve a variety of nucleophiles such as oxygen, nitrogen, sulfur, or carbon, but the reactions all tend to take place in either acidic or basic conditions. Nucleophiles are chemical species that donate an electron pair to an electrophile, an electron-deficient species. On rare occasions, these reactions will take place under neutral conditions. Additionally, nucleophilic addition reactions tend to be reversible, and isolation of the desired product can sometimes be difficult.
At A Glance
Nucleophilic addition to carbonyls is one of the main reactions of carbonyls. This reaction involves a nucleophilic attack on a carbonyl carbon either following or preceding a proton transfer step. The nucleophile can be a wide range of compounds with oxygen, nitrogen, sulfur, carbon, or hydrogen as the nucleophilic atom.
- Use of oxygen nucleophiles involves the attack of water, hydroxide, or an alcohol on a carbonyl carbon of a ketone or aldehyde. Hydrates are formed from the addition of water or hydroxide, while hemiacetals and acetals are formed from the addition of alcohols.
- Addition of a thiol to a carbonyl will form a thioacetal, the sulfur analog of an acetal. Removal of the thioacetal with Raney nickel will also remove the carbonyl, providing a way to remove a carbonyl using neutral conditions.
- Addition of a primary amine or ammonia with an acid catalyst to a ketone or aldehyde leads to the formation of an imine. If a secondary amine is used instead of a primary amine, the reaction will lead to the formation of an enamine.
Carbon nucleophiles include organometallic reagents, cyanide or nitriles, and phosphorus ylides, or phosphonate ester carbanions. These can be used to convert aldehydes and ketones into a variety of alcohols, carboxylic acids, or alkenes.
- Reduction of aldehydes with lithium aluminum hydride or sodium borohydride will yield primary alcohols. Reduction of ketones with lithium aluminum hydride or sodium borohydride will yield secondary alcohols.