Isomers

Isomers are molecules with the same molecular formula (i.e. the same number and type of atoms) but in which the atoms are arranged in a different way.

Structural isomers have different structural formulae. Three types of structural isomerism are chain isomerism, positional isomerism and functional group isomerism.

Stereoisomers have the same structural formula, but the 3D arrangement of atoms is different (enantiomers). Two types are cis–trans isomerism and optical isomerism.

In chain isomers, the carbon chain is arranged differently.

For example, hexane has several chain isomers, all with the molecular formula C₆H₁₄:

In positional isomers, the functional group is attached to a different carbon atom.

For example, chloropentane has several positional isomers, all with the molecular formula C₅H₁₁Cl:

Positional isomerism also exists in alkenes with four or more carbon atoms.

For example, hexene has several positional isomers, all with the molecular formula C₆H₁₂:

Functional group isomers contain different functional groups and so are members of different homologous series.

For example, both alcohols and ethers have the general formula CnH2n+2O so they may be functional group isomers:

Molecules can rotate freely around single C-C covalent bonds, but not around C=C double bonds.

Isomers differ in the arrangement of the groups attached to the carbons in the double bonds.

These isomers cannot be superimposed on each other because the arrangement of the methyl groups is different.

If an alkyl group or atom other than hydrogen is attached to each carbon then the isomers can be named either cis (‘on the same side’) or trans (‘on the opposite side’).

For example, is it possible to identify which of these halogenoalkanes is the cis isomer and which is the trans isomer?

Instead, a different system is used for these type of molecules: E–Z notation.

In more complex organic compounds, in which multiple hydrogens have been substituted by different groups, isomers cannot be defined using the cis–trans notation.

A molecule can exist as two isomers that are non-superimposable, mirror images of each other, just like a left hand and right hand.

Optical isomers have the same physical properties, but they rotate polarized light in opposite directions.