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I have been tasked to find the $\lambda_{\text{max}}$ of 3,4-dimethyl-1,2,3,4,4a,5-hexahydronaphthalene.

enter image description here

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

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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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    $\begingroup$ I like your comments a lot, as they are often very helpful. In this case I don't think that you answer it properly, as in my eyes you simply repeat the obvious but why is it this way? I don't use this increment method often so its not very obvious for me on which ring to choose and my analytical chemistry courses are quite some time ago. $\endgroup$ – pH13 - Yet another Philipp Apr 22 '18 at 9:47
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    $\begingroup$ What's obvious for us may be less than obvious for the others, so the OP and possibly the later readers can benefit from reading it; isn't that what the answers are for? Come to think of it, most of my answers on Chem.SE simply restate the obvious. $\endgroup$ – Ivan Neretin Apr 22 '18 at 10:06
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enter image description here

The double bond B is exocyclic to ring 2 as it is attached to an atom which is shared between ring 1 and ring 2 while the double bond A is not connected to any ring 2 atoms and is within just one ring, hence making it endocyclic

for further references visit https://pharmaxchange.info/2012/08/ultraviolet-visible-uv-vis-spectroscopy-%E2%80%93-woodward-fieser-rules-to-calculate-wavelength-of-maximum-absorption-lambda-max-of-conjugated-dienes-and-polyenes/

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