Formation of acid anhydride

Monocarboxylic acids, heated with a strong DEHYDRATING agent -- either P2O5 or concentrated H2SO4 -- give ACID ANHYDRIDES: 2 R-COOH, with P2O5 and heat, gives (R-CO)2O + H2O. This particular dehydration reaction is REVERSIBLE, and anhydrides made this way are indeed readily hydrolysed straight back to the parent acid simply on contact with water. A genuinely BETTER yield of the same acid anhydride, though, is obtained by a different route entirely, described already in section 12.5.2: heating a sodium carboxylate salt together with the corresponding acyl chloride (both of which are, in turn, prepared separately, straight from the parent acid, via sodium hydroxide and via thionyl chloride respectively) -- R-COONa + R-COCl, on heating, gives (R-CO)2O + NaCl -- a reaction the section flags as more efficient than the direct P2O5/H2SO4 dehydration route above. ('El …