Final answer:
Recrystallization of anhydrides in water or alcohol should be avoided as these compounds can react with these solvents, leading to hydrolysis or esterification, which yields impure products.
Step-by-step explanation:
Recrystallization of anhydrides in water or alcohol is typically not advised due to the reactive nature of acid anhydrides. These compounds can undergo hydrolysis when exposed to water, yielding two equivalents of carboxylic acids, or they can react with alcohols to form one equivalent of an ester and one equivalent of carboxylic acid. The reaction can be driven forward by removing water from the products. Alternatively, if a reverse reaction is required, excess water can be added. However, for the purpose of recrystallization, where a pure product is desired, the use of solvents that react should be avoided to prevent the formation of carboxylic acids or esters, leading to an impure sample.
For example, in the conversion of ethylene glycol to acetaldehyde, the product initially formed is a hydrate which is usually unstable and can rapidly lose water, thus undergoing dehydration synthesis to yield the desired alcohol. Similarly, during the synthesis of ethanol from ethylene, water is added in the presence of a strong acid catalyst. In both cases, proper solvent choice is crucial for the success of the recrystallization step without unwanted side reactions.