racemic mixture

A racemic mixture is the 50:50 mixture of the enantiomers of a compound.

The enantiomers are different in symmetry, because they are the mirror image to each other. The enantiomer molecules in solid racemic mixture are thus packed less closely than in either racemic, and less energy is needed to break the lattice of the solid racemic mixture. Therefore, the melting point of racemic mixture is lower than either enantiomer.

In liquid racemic mixture, when a molecule escapes from the liquid, the molecule is "blocked" by the identical molecules are the enantiomer molecules. The molecule and the enantiomer molecule are different in symmetry, and thus the molecules is "blocked" to a great extent by enantiomer molecules than by identical molecules. Therefore, the molecule needs more energy escape from the enantiomer molecules than identical molecules, and thus the boiling point is higher than that of either enantiomer.

A simpler way of thinking is that racemic mixture can be thought as either enantiomer with impurity of another enantiomer. The impure substance has a lower melting point and a higher boiling point than the pure substance.