Saturday, 28 May 2016

spectroscopy - What is an exocyclic double bond?


I have been tasked to find the λmax of 3,4-dimethyl-1,2,3,4,4a,5-hexahydronaphthalene.


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This was supposed to be fairly easy. I used the base value for homoannular diene (253 nm) and added the increments for the three ring residues attached to the diene (+( 3×5)=+15nm). This should get me a value of λmax=268 nm. However, checking the solution this question, they have also added the increment for an exocyclic double bond, and have got the answer to be 273 nm instead.


As far as I know, an exocyclic double bond means exo+cyclic, where exo means "outside" and cyclic means, well "cyclic", or more simply, a double bond that is outside the ring. I'm looking at this compound, and I dont see any exocyclic double bond at all.


Am I dealing with a key mistake here or is there something that I'm not aware of?



Answer



Surely you are aware that the calculation of this sort is but a very rough estimate, yet still I feel the urge to stress that once more. Having said that, let's look at the double bond in the first cycle: isn't it exocyclic with respect to the other cycle?



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