An acetal is a functional group with the following connectivity RC(H)(OR')2, where R, R' are organic fragments. They are saturated and tetrahedral. The two R'O groups can be equivalent or not. Acetals are formed from and convertible to aldehydes. The related functional group, ketals, are correspondingly obtained from and convertible to ketones. Cellulose is a ubiquitous example of an acetal.
Formation of an acetal occurs when the hydroxyl group of a hemiacetal becomes protonated and is lost as water. The carbocation ion that is produced is then rapidly attacked by a molecule of alcohol. Loss of the proton from the attached alcohol gives the acetal.
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. 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.
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. 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 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.
Examples of acetals 
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "ketals".
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "glycosides".
- Volatile Compounds in Foods and Beverages, ISBN 0-8247-8390-5, http://books.google.com/books?id=_OvXjhLUz-oC, p.554