Anhydrides

 Anhydrides are the chemical reaction of two carboxylic acids, which produces an acid anhydride and a water molecule.

CH₃CH₂COOH + CH₃CH₂COOH → CH₃CH₂COOCOCH₂CH₃ + H₂O
propanoic acid + propanoic acid → hexanoic anhydride (propanoic propanoic anhydride)

Acid anhydrides are significant in Organic Chemistry as acid anhydrides are involved in esterification with alcohol as a reagent without an catalyst to form an ester. [ N. Goaly https://chemrevise.files.wordpress.com/2018/11/6-1-3-revision-guide-carboxylic-acids-and-esters.pdf view online 05/12/2018 ]

(C₂H₅CO)₂O + CH₃ CH₂OH → CH₃CH₂COOH + CH₃CH₂COCH₂CH₃
hexanoic anhydride + ethanol → propanoic acid + ethyl propanoate
propanoic propanoic anhydride

Esters are used for industrial purposes as such as flavouring and perfumes.