[NewtoAtoms]

A mixture of stereoisomers is formed in the reaction shown.
What is the composition of this mixture?  the starting material is a single enantiomer.

**  This is a very perplexing problem for me, as the unit I just finished studying didn't address stereoisomerism,therefore  I am at a bit of a loss.

My instinct would say that both compound W and compound X would be the correct answer, however I have no evidence which would suggest that the methyl, ethyl and lone H, located to the right would rotate. 

Can anyone help me on the stereoisomerism of such a molecule?

Thank you!

[azmanam]

You are correct.  W and X will be the exclusive products of this reaction.  For Y and/or Z to form, the stereocenter alpha to the carbonyl would have to scramble.  This could happen under acidic or basic conditions, but that's not what's happening here.  All that's happening here is hydride reduction of the ketone to a secondary alcohol.

If you want to impress your instructor, look up some information about the Felkin Ahn model and the Burgi-Dunitz angle and tell your instructor which of the two (W or X) would form preferentially.

[Mitsunobo]

I agree with the previous poster. There will be no rearrangement on the alpha carbon to the carbonyl. The action is only in the carbonyl group ( a reduction reaction ).So you are left with W and X as the  products for this reaction. Based on either Felkin-Ahn or Cram model, W will be the major product.