# How do you write anhydride formulas?

$R C \left(= O\right) O H + H O \left(O =\right) C R \rightarrow R C \left(= O\right) O \left(O =\right) C R + {H}_{2} O$
The carboxylic acids have lost a water molecule in forming a $C \left(= O\right) O \left(O =\right) C$ bond. Of course, we could use different carboxylic acids to make non-symmetric anhydrides, $R C \left(= O\right) O \left(O =\right) C R '$.