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Chemistry · Glossary

What is Exo and endo cyclisations?

Also known as: exo cyclisation · endo cyclisation

Definition 27.3 University Chemistry — Year 3 · Chapter 27 — Radicals, Carbenes and Rearrangements

In an exo cyclisation the bond that is broken (or the π\pi bond attacked) ends up outside the new ring; in an endo cyclisation, inside it.

The 5-hexenyl radical closes by attack of C1 on C5 (5-exo, green): the new ring has five members and the radical ends outside it, on C6. Attack on C6 (6-endo, red dashed) would give the more stable secondary cyclohexyl radical but is much slower: the chain cannot place C1 on the line of approach to C6 that the π* orbital requires.
The 5-hexenyl radical closes by attack of C1 on C5 (5-exo, green): the new ring has five members and the radical ends outside it, on C6. Attack on C6 (6-endo, red dashed) would give the more stable secondary cyclohexyl radical but is much slower: the chain cannot place C1 on the line of approach to C6 that the π∗\pi^* orbital requires.
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