| Revision as of 17:45, 26 November 2009 editRifleman 82 (talk | contribs)Extended confirmed users32,435 editsm tidy← Previous edit | Revision as of 11:59, 6 February 2011 edit undoDMacks (talk | contribs)Edit filter managers, Autopatrolled, Administrators186,439 edits →Examples: WP is not an instruction manual or cookbook; and one linked page does not appear to have acetal content (only hemiacetal)Next edit → | ||
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| The carbonyl group in '''1''' abstracts a proton from ]. The protonated carbonyl group 2 is activated for ] 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. 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. | The carbonyl group in '''1''' abstracts a proton from ]. The protonated carbonyl group 2 is activated for ] 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. 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. | ||
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== See also == | ||
| * ] | |||
| ] in ] is slowly added over a stirred mixture of ] ('''1''') and ] at ] in a ]. 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 °C) to yield the dioxolane compound '''3''' (62% from 1) as a colorless liquid.<ref>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.</ref>]] | |||
| Other acetalisation examples can be found in ] and ]. | |||
| ==References== | ==References== | ||
Revision as of 11:59, 6 February 2011
Acetalisation is an organic reaction that involves 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. 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.