Which of the following compounds produces an optically inactive compound on hydrogenation?

Solution

Chemistry ma’am · 3 steps

Watch the stereocentre: after adding H2, does the alkene arm become a copy of a group already there?

1. What hydrogenation changes. H₂ adds across the C=C only. The stereocentre is not touched, but one of its four groups turns from an alkenyl group into an alkyl group. If that new alkyl group matches another group on the same carbon, the carbon stops being chiral.

   Option	Groups on the stereocentre (besides H)	After hydrogenation	Product
   (a)	propyl, methyl, -CH=C(CH₃)₂	propyl, methyl, isobutyl	2,4-dimethylheptane, chiral
   (b)	ethyl, methyl, -CH=CH-CH₃	ethyl, methyl, propyl	3-methylhexane, chiral
   (c)	isopropyl, methyl, -CH=CH-CH₃	isopropyl, methyl, propyl	2,3-dimethylhexane, chiral
   (d)	isopropyl, methyl, -C(CH₃)=CH₂	isopropyl, methyl, isopropyl	2,3,4-trimethylpentane, achiral

2. Why (d) loses chirality. The isopropenyl group -C(CH₃)=CH₂ picks up two H atoms and becomes -CH(CH₃)₂, an isopropyl group. That carbon now carries two identical isopropyl groups, so it is no longer a chiral centre and the molecule has a mirror plane through it.
3. So. Only (d) gives an optically inactive product, 2,3,4-trimethylpentane. In (a), (b) and (c) all four groups stay different, so the products remain optically active.

The idea to keep. For any reaction near a stereocentre, rewrite its four groups after the reaction. Chirality survives only if all four are still different, so check whether the changed group now duplicates a neighbour.

Answer from the NTA final answer key. Solution by PrepWiser.