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While studying lactones I read that usually gamma and delta lactones are formed. Why aren't beta lactones formed. The first thought that came into my mind was angle strain. But then I thought of epoxides which have more angle strain and still are more stable. Please help.

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Now similarly in the above reaction why isn't beta lactone formed?

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    $\begingroup$ Two things. 1) the lactone has an additional sp2 carbons, whereas epoxides do not have sp2 carbons, so the analogy is not accurate. 2) 3-membered rings are not more thermodynamically stable than 4-membered rings - see the sections on ring strain in any (proper) org chem textbook e.g. Clayden or March. However, their formation is kinetically favoured - that means they are easy to form. $\endgroup$ – orthocresol Jan 1 '16 at 5:44
  • $\begingroup$ @orthocresol could u explain y 3 mannered ring is thermodynamically stable? $\endgroup$ – Aditya Kumar Jan 1 '16 at 6:25
  • $\begingroup$ I said it was not thermodynamically stable. That's mostly because of angle and torsional strain. $\endgroup$ – orthocresol Jan 1 '16 at 7:34
  • $\begingroup$ @AdityaKumar. Which book is this? Please, tell me book name and its writer. $\endgroup$ – solanki... Jan 1 '16 at 7:41
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    $\begingroup$ $\beta$-Lactones can be prepared by cycloaddition of ketenes with aldehydes $\endgroup$ – K_P Jan 2 '16 at 1:16
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It is all about angle strain. You may have heard about the Baeyer strain that consideres six-membered rings to be unstrained and all smaller ring sizes to have angle strain, getting more as the ring gets smaller. This is also relevant for the creation of rings — such as but not limited to lactones — five- and six-membered rings are kinetically possible, smaller rings are generally not formed by intramolecular substitution reactions. (Note the emphasis, of course the synthesis of smaller rings is possible due to other mechanisms, such as in the Prilezhaev reaction to synthesise epoxides.)

You also seem to have an incorrect assumption about the stability of epoxides. Epoxides are generally not stable with respect to nucleophilic subsitution. In fact, the Baeyer strain is so high that epoxides are commonly taught as the only aliphatic oxygen that can be displaced in an $\mathrm{S_N2}$ reaction to give the alcoholate anion and which does not need to be protonated first.

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  • $\begingroup$ But as far as I know there exists more epoxides than lactones. Am I right? If yes then why doesn't angle strain affect them? In fact I've rarely come across a 4 membered ring with an oxygen. $\endgroup$ – Aditya Kumar Jan 1 '16 at 16:33
  • $\begingroup$ @AdityaKumar Well, it is very easy to synthesise epoxides and much like primary halides (especially bromides and iodides) while they are generally susceptible for nucleophilic displacement a satisfactory nucleophile must actually be present for the substitution to happen. Oftentimes you are lucky and halides/epoxides survive a reaction. The oxetane rings do exist but they are less common because they don’t have as large a synthetic value in either way. $\endgroup$ – Jan Jan 1 '16 at 16:39

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