Chirality at Nitrogen, Phosphorus, and Sulfur

The most common cause of chirality is the presence of four different substituents bonded to a tetrahedral atom, but that atom doesn t necessarily have to be carbon. Nitrogen, phosphorus, and sulfur are all commonly encountered in organic molecules, and all can be chirality centers. We know, for instance, that trivalent nitrogen is tetrahedral, with its lone pair of electrons acting as the fourth substituent (Section 1.10). Is trivalent nitrogen chiral Does a compound such as ethylmethylamine exist as a pair of enantiomers  

The answer is both yes and no. Yes in principle, but no in practice. Trivalent nitrogen compounds undergo a rapid umbrella-like inversion that inter-converts enantiomers. We therefore can t isolate individual enantiomers except in special cases. 

A similar situation occurs in trivalent phosphorus compounds, or phosphines. It turns out, though, that inversion at phosphorus is substantially slower than Inversion at nitrogen, so stable chiral phosphines can be isolated. (R)- and (5)-metbylpropylphenylphosphine, for example, are configurationally stable for several hours at 100 °C. We ll see the Importance of phosphine chirality in Section 26.7 in connection with the synthesis of chiral amino adds. 

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The spatial relationships that exist for the human body also exist at the molecular level because the molecules of nature exist as three-dimensional symmetrical and asymmetrical figures. One of the most common asymmetric molecules is a tetravalent carbon atom with four different ligands attached to it. The spatial arrangement of the atoms in this molecule is shown in Fig. 2, The carbon atom depicted in Fig. 2 is the asymmetric center of the molecule, and the molecule is a chiral stereoisomer. If the molecule and its mirror image are nonsuperimposable, the relationship between the two molecules is enantiomeric, and the two stereoisomers are enantiomers. Carbon is not the only atom that can act as an asymmetric center. Phosphorus, sulfur, and nitrogen are among some of the other atoms that can form chiral molecules. 

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