Back to chemistry, and the product from the reaction of an aldehyde with cyanide. We explained above that this compound, being chiral, can exist as two enantiomers. Enantiomers are clearly isomers; they consist of the same parts joined together in a different way. In particular, enantiomers are a type of isomer called stereoisomers because the isomers differ not in the connectivity of the atoms, but only in the overall shape of the molecule.

● Stereoisomers and constitutional isomers

 Isomers are compounds that contain the same atoms bonded together in different ways. If the connectivity of the atoms in the two isomers is different, they are constitutional isomers. If the connectivity of the atoms in the two isomers is the same, they are stereoisomers. Enantiomers are stereoisomers, and so are E and Z double bonds. We shall meet other types of stereoisomers shortly.

We should also introduce you briefly to another pair of concepts here, which you will meet again in more detail in Chapter 16: configuration and conformation. Two stereoisomers really are different molecules: they cannot be interconverted without breaking a bond somewhere. We therefore say that they have different configurations. But any molecule can exist in a number of conformations: two conformations differ only in the temporary way the molecule happens to arrange itself, and can easily be interconverted just by rotating around bonds. Humans all have the same configuration: two arms joined to the shoulders. We may have different conformations: arms folded, arms raised, pointing, waving, etc.

● Configuration and conformation •

 Changing the configuration of a molecule always means that bonds are broken.

 • A different configuration is a different molecule.

 • Changing the conformation of a molecule means rotating about bonds, but not breaking them.

• Conformations of a molecule are readily interconvertible, and are all the same molecule.

The aldehyde cyanohydrin is chiral because it does not have a plane of symmetry. In fact, it cannot have a plane of symmetry because it contains a tetrahedral carbon atom carrying four different groups: OH, CN, RCH₂, and H. Such a carbon atom is known as a stereogenic or chiral centre. The product of cyanide and acetone is not chiral; it has a plane of symmetry and no chiral centre because two of the groups on the central carbon atom are the same.

●If a molecule contains one carbon atom carrying four different groups it will not have a plane of symmetry and must therefore be chiral. A carbon atom carrying four different groups is a stereogenic or chiral centre.

We saw how the two enantiomers of the aldehyde cyanohydrin arose by attack of cyanide on the two faces of the carbonyl group of the aldehyde. We said that there was nothing to favour one face over the other, so the enantiomers must be formed in equal quantities. A mix ture of equal quantities of a pair of enantiomers is called a racemic mixture.

●A racemic mixture is a mixture of two enantiomers in equal proportions. This principle is very important. If all the starting materials and reagents in a reaction are achiral and the products are chiral, they will be formed as a racemic mixture of two enantiomers.

Here are some more reactions you have come across that make chiral products from achiral starting materials. In each case, the principle must hold—equal amounts of the two enantiomers (racemic mixtures) are formed.

مواضيع ذات صلة

Significance of the SN2 rate equation

Mechanisms for nucleophilic substitution

Resolutions using diastereoisomeric salts

Separating enantiomers is called resolution

Chiral compounds with no stereogenic centres

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