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| The carbonyl group in '''1''' abstracts a proton from ]. The protonated carbonyl group 2 is activated for nucleophilic addition of the alcohol. The structures '''2a''' and '''2b''' are ]s. After ] of '''3''' by water the ] or ] '''4''' is formed. The hydroxyl group in '''4''' is protonated leading to the ] '''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 ] media. | The carbonyl group in '''1''' abstracts a proton from ]. The protonated carbonyl group 2 is activated for nucleophilic addition of the alcohol. The structures '''2a''' and '''2b''' are ]s. After ] of '''3''' by water the ] or ] '''4''' is formed. The hydroxyl group in '''4''' is protonated leading to the ] '''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 ] media. | ||
| Typical reaction in organic chemistry: | |||
| ] | |||
| A solution of ] in ] is slowly added over a stirred mixture of ] ('''1''') and ] at ]. After workup 3-bromo-3-methyl-butan-2-one ('''2''') is dissolved in ] and ] ] and ] are added. A ] device is fitted to remove generated water and the reaction is ]ed for 4.5 h. The crude reaction is worked up by washing with saturated ] and ] and the organic layer dried over anhydrous ], after that, ] is added to evaporate the ] under reduced pressure. The residue is purified by ] (0.15 mmHg, 32 ºC) to yield the dioxolane compound '''3''' (62% from 1) as a colorless liquid. <sup></sup> | |||
| Examples: | Examples: | ||
| * acetalisation in ] | * acetalisation in ] | ||
| * acetalisation in ] | * acetalisation in ] | ||
| == References == | |||
| <sup></sup> 2-(1-Bromo-1-methyl-ethyl)-2-methyl-dioxolane Juan M. Castro, Pablo J. Linares-Palomino, Sofia Salido, Joaquan Altarejos Manuel Nogueras, Adolfo Sanchez ] 2004, M387 ] publication. | |||
| ] | ] | ||
Revision as of 16:31, 1 May 2005
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.
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.
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.
Typical reaction in organic chemistry:
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. 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 sulphate, 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.
Examples:
- acetalisation in mutarotation
- acetalisation in carbohydrate acetalisation
References
2-(1-Bromo-1-methyl-ethyl)-2-methyl-dioxolane Juan M. Castro, Pablo J. Linares-Palomino, Sofia Salido, Joaquan Altarejos Manuel Nogueras, Adolfo Sanchez Molbank 2004, M387 open access publication.
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