Acetal 89092 214822275 2008-05-25T12:45:35Z Wizard191 935746 added for template and shortened sentence reference to the plastic {{For|the plastic|Polyoxymethylene plastic}} [[Image:Generic Acetal.png|thumb|right|Structure of a generic acetal]] 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. The plastic known as acetal is a polyacetal. 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 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. * [[Hemiacetal]] + alcohol + [[acid]] (catalyst) ↔ acetal + [[water]] Acetals are important in nature, for example in solution the most stable form of [[glucose]] is its cyclic hemiacetal and [[maltose]] is an acetal made from two glucose units. Acetaldehyde diethyl acetal is an important flavouring compound in [[distilled beverage]]s.<ref>Volatile Compounds in Foods and Beverages, ISBN 0824783905, http://books.google.com/books?id=_OvXjhLUz-oC, p.554</ref> Acetals are sometimes used as [[protecting group]]s for [[carbonyl]] groups in organic synthesis as they are stable with respect to hydrolysis by bases. == Plastics == Advantages of acetal based plastics: * Chemically resistant * Very low water absorption * Resistant to [[hydrolysis]] by [[base (chemistry)|base]] Disadvantages of acetal based plastics: * Low impact strength * Low melting temperature * More rigid than [[nylon]]s == Examples == * [[Dimethoxymethane]] (DME), a solvent * [[Dioxolane]] * [[Methylenedioxy]] group * [[Metaldehyde]] * [[Paraldehyde]] * [[1,3,5-Trioxane]] == References == <references/> [[Category:Acetals|*]] [[Category:Functional groups]] [[Category:Protecting groups]] [[de:Acetale]] [[et:Atsetaalid]] [[fa:استال]] [[fr:Acétal et cétal]] [[it:Acetali]] [[mk:Ацетал]] [[nl:Acetaal]] [[nn:Acetal]] [[ja:アセタール]] [[pl:Acetale]] [[pt:Acetal]] [[ru:Ацетали]] [[sv:Acetal]] [[uk:Ацеталі]] [[zh:缩醛]]