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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?

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    $\begingroup$ If you reacted a carboxylic acid with an acyl chloride you would get an acid anhydride, not a lactone (which is an ester). $\endgroup$
    – orthocresol
    Commented Dec 31, 2015 at 4:17
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    $\begingroup$ 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
    Commented 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
    Commented Dec 31, 2015 at 19:55

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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.

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