The question is to predict the nature of the product(s) of the above reaction, that is if the product(s) form a racemic mixture, meso, diasteromers, or are structural isomers.

I am aware of the fact that addition of $\ce{H2}$ is a syn addition. So it should form a meso compound. However the answer key gives it as a mixture of diastereomers. I am not quite sure if the reaction is strictly selective and some anti products are also formed.

  • $\begingroup$ Ask yourself first if the methyl group is up or down.... $\endgroup$
    – Zhe
    Jan 2, 2017 at 14:34

1 Answer 1


To answer and understand this, let’s first consider the products of the hydrogenation ($\ce{H2; Pd/C}$) of 1-methylidene-4-methylcyclohexane (the same reactant except the methyl group is in para-position). This compound is symmetric and therefore achiral. We can draw the methyl group pointing upwards iwthout loss of generalisation. The hydrogenation is syn-selective and it can occur from the top or the bottom. We expect a $1:1$ mixture of the former methylidene group pointing upwards (the syn-isomer) and pointing downwards (the anti-isomer). Both are achiral (the plane of symmetry is retained in the reaction) but not identical; therefore, they must be diastereomers.

Now let’s move the methyl group back into the 2-position. By moving the methyl group, the plane of symmetry is lost; it suddenly makes a difference whether the methyl group points upwards (S) or downwards (R). For each of the two isomers, we can again hydrogenate from both above and below; we can expect two anti and two syn isomers at first approximation: we enter one stereocentre into the reaction and we form an additional one making two stereocentres in total and thus 4 theoretically possible isomers.

In explicit forms, starting from the (2S) isomer the syn-product could be called (1R,2S) and the anti-isomer would be (1S,2S). From the (2R) isomer, another syn (1S,2R) and anti (1R,2R) isomer is expected. Only when taking a closer look at the products, it becomes obvious that the two syn isomers are in fact identical because they can be transformed into each other by rotation; and that they are achiral because they generated a new plane of symmetry. Thus, the syn isomer is a meso isomer.

Putting this together, the hydrogenation products are two enantiomeric anti products and a meso (syn) product. These two classes are diastereomers of each other, thus ‘a mixture of diastereomers’ is the only correct answers.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.