Predicting the product of a hydrogenation reaction


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> [![Reaction](https://i.stack.imgur.com/LVHzt.png)](https://i.stack.imgur.com/LVHzt.png)
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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.




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 (2*S*) isomer the *syn*-product could be called (1*R*,2*S*) and the *anti*-isomer would be (1*S*,2*S*). From the (2*R*) isomer, another *syn* (1*S*,2*R*) and *anti* (1*R*,2*R*) 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.


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