Acetalisation

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Acetalisation is an organic reaction that involves is the formation of an acetal or ketal. One way of acetal formation is the nucleophilic addition of an alcohol to a ketone or an aldehyde. Acetalisation is often used in organic synthesis to create a protecting group because it is a reversible reaction.

[edit] Acetalisation of carbonyl groups by alcohols

Acetalisation is acid catalysed with elimination of water. The reaction can be driven to the acetal when water is removed from the reaction system either by azeotropic distillation or trapping water with molecular sieves or aluminium oxide. The general reaction mechanism for acetalisation of a carbonyl group is shown below.

Acetalisation

The carbonyl group in 1 abstracts a proton from hydrochloric acid. The protonated carbonyl group 2 is activated for nucleophilic addition of the alcohol. The structures 2a and 2b are mesomers. After deprotonation of 3 by water the hemiacetal or hemiketal 4 is formed. The hydroxyl group in 4 is protonated leading to the oxonium ion 6 which accepts a second alcohol group to 7 with a final deprotonation to the acetal 8. The reverse reaction takes place by adding water in the same acidic medium. Acetals are stable towards basic media. In a transacetalisation or crossacetalisation a diol reacts with an acetal or two different acetals react with each other. Again this is possible because all the reaction steps are equilibria.

[edit] Examples

One typical example is the acetalisation of 3-bromo-3-methyl-butan-2-one:

A solution of bromine in tetrachloromethane is slowly added over a stirred mixture of 3-methyl-butan-2-one (1) and acetic acid at room temperature in a ketone halogenation. After workup 3-bromo-3-methyl-butan-2-one (2) is dissolved in benzene and anhydrous p-toluenesulfonic acid and ethylene glycol are added. A Dean-Stark trap device is fitted to remove generated water and the reaction is refluxed for 4.5 h. The crude reaction is worked up by washing with saturated sodium bicarbonate and brine and the organic layer dried over anhydrous sodium sulfate, after that, methanol is added to evaporate the azeotrope under reduced pressure. The residue is purified by vacuum distillation (0.15 mmHg, 32 ºC) to yield the dioxolane compound 3 (62% from 1) as a colorless liquid. [1]
A solution of bromine in tetrachloromethane is slowly added over a stirred mixture of 3-methyl-butan-2-one (1) and acetic acid at room temperature in a ketone halogenation. After workup 3-bromo-3-methyl-butan-2-one (2) is dissolved in benzene and anhydrous p-toluenesulfonic acid and ethylene glycol are added. A Dean-Stark trap device is fitted to remove generated water and the reaction is refluxed for 4.5 h. The crude reaction is worked up by washing with saturated sodium bicarbonate and brine and the organic layer dried over anhydrous sodium sulfate, after that, methanol is added to evaporate the azeotrope under reduced pressure. The residue is purified by vacuum distillation (0.15 mmHg, 32 ºC) to yield the dioxolane compound 3 (62% from 1) as a colorless liquid. [1]

Other acetalisation examples can be found in mutarotation and carbohydrate acetalisation.

[edit] References

[1] 2-(1-Bromo-1-methyl-ethyl)-2-methyl-[1,3]dioxolane Juan M. Castro, Pablo J. Linares-Palomino, Sofia Salido, Joaquan Altarejos Manuel Nogueras, Adolfo Sanchez Molbank 2004, M387 [1] open access publication.

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