A few compounds are chiral, yet have no stereogenic centres. Try making a model of the allene in the margin. It has no stereogenic centre, but these mirror images are not superimposable and so the allene is chiral: the structures shown are enantiomers. Similarly, some biaryl compounds, such as the important bisphosphine below, known as BINAP, exist as two separate enantiomers because rotation about the green bond is restricted. If you were to look at this molecule straight down along the green bond, you would see that the two flat rings are at right angles to each other and so the molecule has a twist in it rather like the 90° twist in the allene. Compounds that are chiral because of restricted rotation about a single bond are called atrop isomers (from the Greek for ‘won’t turn’).

These two examples rely on the rigidity of π systems but this simple saturated system is also chiral. These two rings have to be orthogonal because of the tetrahedral nature of the central carbon atom. There is no chiral centre, but there is no plane of symmetry. Cyclic compounds like this with rings joined at a single C atom are called spiro compounds. Spiro compounds are often chiral even when at a first glance they look quite symmetrical, and you should look particularly carefully for planes of symmetry when you think about their stereochemistry.

مواضيع ذات صلة

Significance of the SN2 rate equation

Mechanisms for nucleophilic substitution

Resolutions using diastereoisomeric salts

Separating enantiomers is called resolution

Diastereoisomers can arise when structures have more than one stereogenic centre

Absolute and relative stereochemistry

Diastereoisomers can be chiral or achiral

Dia stereoisomers are stereoisomers that are not enantiomers

The rotation of plane-polarized light is known as optical activity

R and S can be used to describe the configuration of a chiral centre

Many chiral molecules are present in nature as single enantiomers

Stereogenic centres