Branched-chain alkanes

Branched-chain alkanes are alkanes that contain only C and H (with only C-C single bonds) but are not linear. They contain branches with groups such as methyl, ethyl,... coming off the main branch of the molecule. Consider the branched molecule below:

Figure: Example of a branched alkane.

which, for comparison, is shown together with butane (top panel). The molecule in the bottom panel is known as 2-methylpropane. It has the same number of carbons and hydrogens as butane but the bonding pattern is completely different. These two molecules are known as geometric isomers because the have the same number of carbons, hydrogens, and electrons but differ in the geometric arrangement and chemical bonding pattern between the atoms.

The name of the molecule is derived from the fact that the main chain of the molecule, i.e. the longest carbon chain, is that of propane. However, on the central carbon, one of the hydrogens is replaced by a methyl group. In general, the procedure for naming such compounds follows the IUPAC (International Union of Pure and Applied Chemistry) procedure, which is given below:

1.

Find the longest continuous carbon chain in the molecule. The molecule is named for this chain and will be a derivative of whatever this longest chain is.

2.

The hydrocarbon groups attached to the chain are called alkyl groups, such as methyl, ethyl, propyl, etc. The group name is determined by the number of carbons in the chain usually minus one hydrogen where it attaches to the main chain.

3.

Number the carbons along the main chain identified in step 1 and identify the attached groups. The carbon number where the group is attached appears in the name of the molecule. By convention, the numbering should start so as to give the shortest possible length of the main chain before the first group is encountered.

4.

If more than one alkyl group is attached to the main chain, use the prefixes di-, tri-, tetra- etc. for the number of times a given alkyl group appears.

5.

If several groups appear, then list them in alphabetical order in the name of the molecule.

The figure below provides a series of examples, showing how the geometric isomers of octane are named. Noting just the first group for now, which are all alkanes, one can see how the rules are applied.

Figure: Naming the geometric isomers of octane.

With branched alkanes and other organic molecules comes the intriguing possibility of having two molecules that are mirror images of each other (see figure below):

Figure: Example of chirality.

When two isomers of a molecule are mirror images, one cannot be converted into the other without breaking chemical bonds. These are called optical isomers or chiral isomers. These are of great interest to the pharmaceutical industry as it often happens that one of the isomers will have therapeutic activity in blocking the catalytic mechanism of an enzyme while the other isomer will be inactive and can even be toxic.