ACS O chem question

[alanan84]

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Which would form a racemic mixture upon hydrogenation of the double bond with H2/Pt?

A) C6H5-CH=C(CH3)C6H5
B) C6H5-CH=CH-C6H5
C) C6H5-CH=CH-CH3
D) 3-phenylcyclopentene

The answer is A and the only reasoning I can think of is that it creates a stereocenter on one of the carbons but I have no idea if that has anything to do with it... Confused.

 

[UCB05]

That is true. A racemic mixture has to have optically active constituents. A is the only molecule that results in the formation of a stereocenter, so that alone will lead you to A. The reason for racemic formation is because of the reaction. H2/Pt adds two hydrogens across a double bond by syn addition, but the syn addition can occur on either side, so adding the H's from the front will produce a chiral center, but adding both H's from the back will create its enantiomer. Both directions are equally likely so you get a racemic product.