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I believe that there are only two exocyclic bonds in this structure. But in some text its mentioned as three. can someone clarify.

  • $\begingroup$ IMHO there are total zero exocyclic bonds. $\endgroup$ – mykhal Dec 5 '18 at 10:28
  • $\begingroup$ I agree. Exocyclic means outside the ring structure; the double bonds are all within the ring structure $\endgroup$ – Waylander Dec 5 '18 at 10:39
  • $\begingroup$ but, ring B has two exocyclic bonds $\endgroup$ – RZZ Dec 5 '18 at 10:58
  • $\begingroup$ @RZZ ‚Ķrelative to that ring. yes, but they are still in another ring. It depends on the term exocyclic bond purpose and context. $\endgroup$ – mykhal Dec 5 '18 at 11:06
  • $\begingroup$ ok. so for the purpose of calculating the Lambda max using woodward fieser rule. how many do you think are there? $\endgroup$ – RZZ Dec 5 '18 at 11:15

By definition (in Woodward-Fieser rule), an exocyclic double bond is a double bond where one of the $sp^2$ carbon atoms of the bond is a part of a ring, while the other $sp^2$ carbon of the bond is not a part of same ring (it could be a part of another ring).

If both the $sp^2$ carbon atoms of the double bond is part of the same ring, then it is called endocyclic double bond. From these definitions, you can understand that exocyclic would stand for a double bond outside the ring and endocyclic would stand for a double bond within the ring. Following examples would illustrate the definition:

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  • $\begingroup$ yup!! agreed. so now according to this definition how many exocyclic double bonds are there in the structure i posted? $\endgroup$ – RZZ Dec 6 '18 at 6:07
  • $\begingroup$ You may count it by yourself. You'd learn by that. $\endgroup$ – Mathew Mahindaratne Dec 6 '18 at 22:28
  • $\begingroup$ Three is the correct answer. One double bond is exocyclic to both cyclopentyl and cyclohexyl rings. $\endgroup$ – Mathew Mahindaratne Dec 6 '18 at 23:08

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