Of course you want us to prove it. Well, in this example the starting material has three acetals: an ordinary acetal formed from methanol (in black), a five-membered cyclic acetal, and a dithioacetal. Only the black acetal hydrolyses under these mild conditions.

The acetals you have met so far were formed by reaction of two molecules of alcohol with one of carbonyl compound. Cyclic acetals, formed by reaction of a single molecule of a diol, a compound containing two hydroxyl groups, are also important. When the diol is ethylene glycol (as in this example) the five-membered cyclic acetal is known as a dioxolane.

Before looking at the answer below, try to write a mechanism for this reaction. If you need it, use the mechanism we gave for the formation of acyclic acetals.

Water is still generated, and needs to be got rid of: in the example above, you can see that water was distilled out of the reaction mixture. This is possible with these diols because they have a boiling point above that of water (the boiling point of ethylene glycol is 197 °C). You can’t distil water from a reaction mixture containing methanol or ethanol because the alcohols distil too! One very useful piece of equipment for removing water from reaction mixtures containing only reagents that boil at higher temperatures than water is called a Dean Stark head.

مواضيع ذات صلة

Amines react with carbonyl compounds

Modifying reactivity using acetals

Acetals hydrolyse only in the presence of acid

Amines from imines: reductive amination

Making acetals

Acetals are formed from aldehydes or ketones plus alcohols in the presence of acid

Acid or base catalysts increase the rate of equilibration of hemiacetals with their aldehyde and alcohol parents

Aldehydes can react with alcohols to form hemiacetals

Making the products less reactive

Making the starting materials more reactive

A bit of shorthand

Making alcohols instead of ketones