I came across a reaction whereby a 1-5 dicarboxylic acid is converted to lactone with $\ce{SOCl2}$. I was wondering what the mechanism of this was.

I know that $\ce{SOCl2}$ converts carboxylic acids to acyl chlorides. Is it the case that one carboxylic acid undergoes this transformation, then the oxygen from the $\ce{OH}$ of the other carboxylic acid attacks the acyl chloride, making the $\ce{Cl}$ a leaving group?

Is this the case or am I missing something?

  • 2
    $\begingroup$ If you reacted a carboxylic acid with an acyl chloride you would get an acid anhydride, not a lactone (which is an ester). $\endgroup$ Dec 31, 2015 at 4:17
  • 5
    $\begingroup$ Are you sure it was a lactone and not a cyclic anhydride? I could see the latter as more likely. Could you provide a source or a link? $\endgroup$
    – Ben Norris
    Dec 31, 2015 at 4:17
  • $\begingroup$ Thank you so much, turns out it was a cyclic anhydride. I was happy with the mechanism but getting the naming of the functional group wrong made it very hard to look up if I was correct. Thanks again! $\endgroup$
    – Daniel
    Dec 31, 2015 at 19:55

1 Answer 1


The simplest 1,5-dicarboxylic acid is glutaric acid (pentanedioic acid in a more systematical naming). If you react it with $\ce{SOCl2}$, you will indeed intermediately arrive at a compound that has one carboxylic group and one acid chloride group. Then, as you suggest, the other carboxylic acid can attack the acid chloride and $\ce{HCl}$ is eleminated.

However, the resulting product is not a lactone but rather glutaric anhydride; a cyclic acid anhydride. To convert it to a lactone you would need to remove exactly one carbonyl group. Whether this is done before or after the reaction with $\ce{SOCl2}$ does not matter, but before sounds easier to me.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.