4.10 : 氮、磷和硫的手性

Recall that any tetrahedral center with four different functional groups is chiral.

While this property is commonly observed in carbon, it holds true for other elements in the same group, such as silicon and germanium, or in nearby groups like nitrogen, phosphorus, and sulfur. As such, they form enantiomers that are resolvable.

Replacing one of the four different functional groups by an electron pair also forms a chiral tetrahedral center. For instance, an sp³ nitrogen in ethylmethylamine is a chiral tetrahedral center, as its mirror image is non-superposable.

In principle, while the enantiomers of chiral amines are resolvable, they cannot be separated due to pyramidal inversion. As with an umbrella exposed to a storm, the chiral amine turns itself inside out spontaneously, resulting in a racemic mixture.

However, pyramidal inversion is not possible in quaternary ammonium salts, such as allylethylmethylphenyl ammonium chloride, allowing easy resolution of R- and S- enantiomers.

Similarly, trivalent sp³ phosphorus compounds and sulfur compounds with lone pairs do not undergo pyramidal inversion as readily because it is less energetically favorable. As such, they are easily resolvable.

Naming the enantiomers of non-carbon chiral centers uses the same procedure as for carbon chiral centers. Recall that naming enantiomers as R- or S- involves three steps:  assignment of priorities to the substituent groups, the orientation of the lowest-priority substituent away from the observer, and determining whether the priority sequence of the other three groups at the chiral center is clockwise or counterclockwise.

In molecules with a lone pair such as methylsulfinylbenzene, the substituents are assigned priorities, with the lone pair as the lowest. Next, the molecule is rotated such that the lone pair points away. 

Here, the one-two-three sequence is clockwise. Accordingly, the prefix R is assigned to the chiral center, and the enantiomer is labeled as (R)-methylsulfinylbenzene. Similarly, the other enantiomer is labeled (S)-methylsulfinylbenzene.