Lianyungang Klinechem Co.,Ltd
call us on: +86-518-88103268
English中文
Knowledge
Home > Knowledge > Content

Acetalisation

Lianyungang Klinechem Co.,Ltd | Updated: Nov 27, 2018

Formation of an acetal occurs when the hydroxyl group of a hemiacetal becomes protonated and is lost as water. The carbocation that is produced is then rapidly attacked by a molecule of alcohol. Loss of the proton from the attached alcohol gives the acetal.

400

Aldehyde to acetal conversion

Ketone to ketal conversion

Acetals are stable compared to hemiacetals but their formation is a reversible equilibrium as with esters. As a reaction to create an acetal proceeds, water must be removed from the reaction mixture, for example, with a Dean-Stark apparatus, lest it will hydrolyse the product back to the hemiacetal. The formation of acetals reduces the total number of molecules present and therefore is not favourable with regards to entropy, unless one uses a diol rather than two discrete alcohol molecules. A way to improve this is to use an orthoester as a source of alcohol. Aldehydes and ketones undergo a process called acetal exchange with orthoesters to give acetals. Water produced along with the acetal product is used up in hydrolysing the orthoester and producing more alcohol to be used in the reaction.

Acetals are used as protecting groups for carbonyl groups in organic synthesis because they are stable with respect to hydrolysis by bases and with respect to many oxidizing and reducing agents. They can either protect the carbonyl in a molecule (by temporarily reacting it with an alcohol) or a diol (by temporarily reacting it with a carbonyl). That is, either the carbonyl, or the alcohols, or both could be part of the molecule whose reactivity is to be controlled.

Various specific carbonyl compounds have special names for their acetal forms. For example, an acetal formed from formaldehyde is sometimes called a "formal"[2] or the methylenedioxy group. The acetal formed from acetone is sometimes called an acetonide.

Acetalisation

Acetalisation is the organic reaction that involves the formation of an acetal (or ketals). 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 is acid catalysed with elimination of water; acetals do not form under basic conditions. 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 carbonyl group in 1 takes a proton from acidic hydronium. 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.

401

Acetalisation mechanism

Examples

·         benzylidene acetal, a protecting group

·         Dimethoxymethane, a solvent, a.k.a. methylal, a.k.a. formal [ambiguous]

·         Dioxolane

·         Metaldehyde

·         Paraldehyde

·         1,3,5-Trioxane

·         Phenylsulfonylethylidene (PSE) acetal is an example of arylsulfonyl acetal possessing atypical properties, like resistance to acid hydrolysis which leads to selective introduction and removal of the protective group.[3]

·         Most glycosidic bonds in carbohydrates and other polysaccharides are acetal linkages.[4]

·         Cellulose is a ubiquitous example of a polyacetal.

Although many compounds contain an acetal functional group, at least two acetal compounds are called "acetal" for short:

·         Polyoxymethylene (POM) plastic, also known as "acetal" or "polyacetal", is a polyacetal (and a polyether), and a polymer of formaldehyde.

·         1,1-Diethoxyethane (acetaldehyde diethyl acetal), sometimes called simply "acetal", is an important flavouring compound in distilled beverages.[5]

References

1.    Jump up^ IUPACCompendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "ketals".

2.    Jump up^ Morrison, Robert T. and Boyd, Robert N., "Organic Chemistry (6th ed)". p683. Prentice-Hall Inc (1992).

3.    Jump up^ Chéry, Florence; Rollin, Patrick; De Lucchi, Ottorino; Cossu, Sergio (2000). "Phenylsulfonylethylidene (PSE) acetals as atypical carbohydrate-protective groups". Tetrahedron Letters. 41 (14): 2357–2360. doi:10.1016/s0040-4039(00)00199-4ISSN 0040-4039.

4.    Jump up^ IUPACCompendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "glycosides".

5.    Jump up^ Volatile Compounds in Foods and Beverages, ISBN 0-8247-8390-5https://books.google.com/books?id=_OvXjhLUz-oC, p.554


Contact Us
Address: Jingqi Road, Weiwu Road, Coastal Industrial Park, Guanyun County, Lianyungang City, Jiangsu Province
Tel:+86-518-88103268
Fax:+86-518-88103267
E-mail:sales@klinechem.com

© Lianyungang Klinechem Co.,Ltd