An acetal is a
molecule with two single bonded oxygens attached to the same carbon atom. Traditional usages distinguish
ketal from acetals; where the acetal has one R group as H-. Current accepted terminology classifies ketals as a subset of acetals. For engineering applications, "acetal" is shorthand for the
plastic polyoxymethylene, which is a polyacetal.Formation of an acetal occurs when the
hydroxyl group of a
hemiacetal becomes protonated and is lost as water. The
oxonium 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 reversible as with
esters. As a reaction to create an acetal proceeds, water must be removed from the reaction mixture or it will
hydrolyse the product. 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.
See more at Wikipedia.org...
Free Download Now!
