Chapter 7 : Stereochemistry

Different compounds with the same molecular formula but different chemical structures are called isomers.
Since (in principle at least), different chemical structures have different properties such as melting point or boiling point, then they can be separated from each other.

Remember isomerism is a property between a pair (or more) of molecules, i.e. a molecule is an isomer of another molecule and one molecule can have different isomeric relationships to two different molecules (i.e. A is a constitutional isomer of X and stereoisomer of Y.
Similar relationships occur within families, for example, you can only be a brother or sister to someone else while a person can be both a brother and a son to different people.

Depending on the nature of the difference between the structures, it is possible to classify the isomeric relationship into various sub-types. The following tree diagram should help you identify the most appropriate term to describe the relationship between a pair of structures based on a series of simple YES / NO questions.  If you "click" on the named boxes there is a link to a definition and an example.

Isomers are different compounds with the same molecular formula but that are structurally different in some way. It is important to be able to recognise isomers because they can have different chemical, physical properties and biological properties.

Constitutional isomers differ in the order in which the atoms are connected together. As a result they can contain different functional groups (functional) and / or bonding patterns (e.g. functional group location (positional) or hydrocarbon branching (skeletal)). The terms for each of these subclass variations (functional, positional and skeletal) are not particularly commonly used. example: 1-propanol, 2-propanol and ethyl methyl ether (C3H8O)

Stereoisomers have the same functional groups and connectivity order, they differ only in the arrangement of atoms and bonds in space.

Conformational isomers (or conformers or rotational isomers or rotamers) are stereoisomers produced by rotation about s bonds, and are often rapidly interconverting at room temperature (review Chapter 3 ?) example 1: butane : anti (left) and syn (center). The C2-C3 s bond rotation is animated (right). Try rotating the model to look along the C-C to see the two forms.     example 2: cyclohexane : chair (left) and boat (right).These two forms can be interconverted by twisting the ring structure.

Configurational isomers are stereoisomers that do not readily interconvert at room temperature and can (in principle at least) be separated. Interconversion of configurational isomers requires bond breaking and bond making.

Geometric isomers (an obselete term that should be replaced by cis-trans isomers) are a subtype of configurational isomers that differ in the spatial position around a bond with restricted rotation (e.g. a double bond or across a ring system): E-2-butene and Z-2-butene

Optical isomers are configurational isomers that differ in the 3D relationship of the substituents about one or more atoms.

Enantiomers are optical isomers that are non-superimposable mirror images. (R)- or (S)-2-chlorobutane.

Diastereomers is the term that students are most likely to get incorrect. This is likely because of the ways in which they can be defined. Diastereomers can be defined as any stereoisomer that is not an enantiomer. This definition means that for configurational isomers with multiple chirality centres, the R- and S- designations can be used to make the isomer classification. However, in the broader sense of the definition, since geometric isomers and conformational isomers are types of stereisomer (they are below that branch in the tree diagram) then they are also technically a type of diastereomer but those terms (geometric or conformational) would be better because they are more precise. (S,R)- or (R,R)-2-bromo-3-chlorobutane E-2-butene and Z-2-butene

© Dr. Ian Hunt, Department of Chemistry