In the racemic mixture below, if it is reacted with Mosher's Acid Chloride, it would form X and Y. Can I ask why my book state that X and Y are not enantiomers and thus can based on differences in their boiling points? I thought X and Y are enantiomers since they are non-superimposable mirror images of each other.
In this case, You now have two chiral centers ( in X and Y). The stereo of the Mosher's Acid Chloride did not change during the reaction while, regarding the other chiral centre: one is R and other is S. So say Mosher's Acid Chloride has a stereo S, then X has SR and Y has SS configuration. These are not enantiomers and instead are diastereomers.
For example, RR and SS or RS and SR are enantiomers but SR and SS or RR and RS are not enantiomers, these are diastereomers.