Formation of anhydrides from acid halides and carboxylates

by James

Description: Addition of a carboxylic acid to an acid chloride results in the anhydride, displacing the chloride ion. Content available for Reactionguide members only. Not a member? Get access for about 30 cents / day!

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zwitterion

There’s this other reaction I learned in class for the formation of anhydrides–you can react two moles of carboxylic acid, with H2SO4 and heat, to form an anhydride. (We were not clearly taught the mechanism, however–correct me if I am wrong, but I assume that one of the OH groups nab the H off the other OH group and leave as H2O, so that an anhydride is formed.)

However, I have also read in a textbook (Solomons and Fryhle, specificially) that an anhydride can be formed using only heat, but only when the cyclic anhydride forms a 5- or 6-membered ring. What’s the significance of the H2SO4 from the first method, then?

In any case, using acyl halides and a carboxylate would be more effective, as mentioned above. But how so? I mean, does it give a faster reaction rate? Or is it more “efficient” (more yield)? Or a mixture of both?

Sorry if I’m asking too many questions, hehe. Curiosity killed the cat (but satisfaction brought it back).

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