Just as with the ester formation and hydrolysis reactions we discussed in Chapter 10, every step in the formation of an acetal is reversible. To make acetals, therefore, we must use an excess of alcohol or remove the water from the reaction mixture as it forms, by distillation for example.

In fact, acetal formation is even more difficult than ester formation: while the equilibrium constant for acid-catalysed formation of ester from carboxylic acid plus alcohol is usually about 1, for acetal formation from an aldehyde and ethanol (shown above), the equilibrium constant is K = 0.0125. For ketones, the value is even lower: in fact, it is often very difficult to make the acetals of ketones (sometimes called ketals) unless they are cyclic (we consider cyclic acetals later in the chapter). However, there are several techniques that can be used to prevent the water produced in the reaction from hydrolysing the product.

para-Toluenesulfonic acid

para-Toluenesulfonic acid is commonly used to catalyse reactions of this sort. It is a stable solid, yet is as strong an acid as sulfuric acid. It is widely available and cheap because it is produced as a by-product in the synthesis of saccharin (for more details, see Chapter 21).

With the more reactive aldehyde, it was sufficient just to have an excess of one of the rea gents (acetaldehyde) to drive the reaction to completion. Dry HCl gas can work too. With a less reactive ketone, molecular sieves (zeolite) were used to remove water from the reaction as it proceeded.

مواضيع ذات صلة

Cyanide will attack iminium ions: the Strecker synthesis of amino acids

Lithium aluminium hydride reduces amides to amines

Iminium ions can react as electrophilic intermediates

Secondary amines react with carbonyl compounds to form enamines

Iminium ions and oxonium ions

Some imines are stable

Imines are usually unstable and are easily hydrolysed

Imines are the nitrogen analogues of carbonyl compounds

Amines react with carbonyl compounds

Modifying reactivity using acetals

Cyclic acetals are more stable than acyclic acetals

Acetals hydrolyse only in the presence of acid