Water as a nucleophile hydrates

as the simplest alcohol, should also be able to act as a nucleophile towards aldehydes and ketones and produce a gem-d o, sometimes termed a hydrate. The prefix gem is an abbreviation for geininal (Latin gemini twins) we use it to indicate two like groups on the same carbon. However, for most aldehydes and ketones, the equilibrium is unfavourable, and the reaction is not important. 

Formaldehyde is normally a gas at room temperature, but dissolves in water. In aqueous solution, formaldehyde exists almost entirely as the 

If there is a suitable electron-withdrawing substituent, hydrate formation may be favoured. Such a situation exists with trichloroacetaldehyde (chloral). Three chlorine substituents set up a powerful negative inductive effect, thereby increasing the 8- - charge on the carbonyl carbon and favouring nucleophilic attack. Hydrate formation is favoured, to the extent that chloral hydrate is a stable solid, with a history of use as a sedative. 

These observations emphasize the fact that gem-diols are usually unstable and decompose to carbonyl compounds. However, it can be demonstrated that hydrate formation does occur by exchange labelling of simple aldehyde or ketone substrates with 0-labelled water. Thus, after equilibrating acetone with labelled water, isotopic oxygen can be detected in the ketone s carbonyl group. 

The reaction of thiols with aldehydes and ketones parallels that of alcohols. However, the reactions are more favourable because sulfur is a better nucleophile than oxygen (see Section 6.1.2). Electrons in larger atoms are more easily polarizable and it becomes easier for them to be donated to an electrophile. 

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