Acid chlorides and acid anhydrides, like esters and amides are both derivatives of carboxylic acids.
Acid chlorides are more reactive than acid anhydrides and acid anhydrides are actually formed from the carboxylic acid via the acid chloride.
Acid chlorides are formed from carboxylic acids by addition of SOCl2. To convert an acid chloride into an acid anhydride, RCO2- is used, where R is an alkyl group.
Both acid chlorides and acid anhydrides are used in acylation reactions. They both contain very electronegative elements, (ie chlorine, oxygen) which results in the carbon atom being more positive and therefore susceptible to nucleophilic attack. The type of reaction is called nucleophilic addition-elimination and with different nucleophiles it can be used to produce carboxylic acids, esters and amides. An example mechanism is shown below for each of these conversions, where R and R' are alkyl groups or aromatic rings:
This reaction produces a high yield of ester because it is irreversible.
When ammonia is used (ie R'=H) then a normal amide is produced.
Acylation reactions are very useful and are used in the manufacture of cellulose acetate (for OHP transparancies) and for the manufacture of aspirin.
Author: Jo Lynn (document modification date: 05 May 2002)