I came across this question regarding hydrogenation of alkenes

Reaction A and Reaction B were carried out using the two different substrates as shown I have depicted how i thought the reaction would occur.This gives me the correct answer for Reaction A but not for Reaction B.

Apparently instead of b,the compound c would be formed,implying simple hydrogenation of the exocyclic bonds without the formation of a transition state as in reaction a .

I cannot figure out the exact cause for this. Does this have anything to do with the two double bonds being on the opposite sides in coumpound Y ,and therefore not being able to form the same transition state as X?

P.S: No catalysts were mentioned.Only H2(1 mole) and heat were mentioned in the original question.

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  • $\begingroup$ It has to be catalytic hydrogenation following the standard mechanism. I can't see how they expect you to predict the product of the top one unless it has been taught specifically in class. $\endgroup$
    – K_P
    Oct 10, 2015 at 9:32
  • $\begingroup$ Isn't that how hydrogenation occurs?(in absence of a catalyst) And no, we weren't taught non-catalytic hydrogenation in class, but this isnt a classroom material. I came across this question while going through a problem book. $\endgroup$ Oct 10, 2015 at 9:35
  • $\begingroup$ I don't know any examples of non catalytic hydrogenation. For the (catalytic) mechanism see: link $\endgroup$
    – K_P
    Oct 10, 2015 at 9:43
  • $\begingroup$ I know the mechanism for catalytic hydrogenation.But i dont see how, if we were using a heterogenous catalyst,the correct products of the above reactions can be predicted $\endgroup$ Oct 10, 2015 at 9:54
  • $\begingroup$ I agree. The bottom one is exocyclic disubst vs endocyclic trisub. But for the top one I can' t see how you can predict the product they ask unless it has been specifically mentioned in theory. $\endgroup$
    – K_P
    Oct 10, 2015 at 10:17

1 Answer 1


The main point of these exercises is that not all C=C bonds are created equal. Their reactivity towards hydrogenation can be influenced by steric effects and specifically applying here, the degree of substitution of the double bond. So for the bottom example, the exocyclic disubstituted double bond will react but not the endocyclic one which is much more stable. Now here there is some uncertainty in this exercise since the actual result is dependent on the exact conditions which are not provided. For example hydrogenations are well known to be very capricious on the selection of metal catalyst and $\ce{Pd}$ can give a different result than using $\ce{Pt}$ or $\ce{Rh}$. And this only an example as there are so many different set of conditions possible with variable results.

The top one is an even worse exercise in the sense that the outcome is very specific on the exact conditions. Now, knowing the exact outcome we can rationalise as follows: Here the electronics also play a role and this diene because of conjugation will be more stable and thus more resistant towards hydrogenation. When one of the exocyclic bonds gets reduced (even if it only partially does so) then in the intermediate formed the other unreacted double bond will isomerise (much much faster than the reduction of the exocyclic conjugated bond in the starting material) to the more stable product with the endocyclic double bond i.e. Structure a. This structure will be unreactive towards further reduction and eventually will be the sole product of the reaction.


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