Question

How many stereoisomers are possible for 3,4-dimethylheptane? which are pairs of enantiomers and which are meso compounds?

Answer 1

There are four possible stereoisomers for 3,4-dimethylheptane. There are two pairs of enantiomers and no meso compounds.

Explanation:

The formula of 3,4-dimethylheptane is

`"C-3"` and `"C-4"` are chiral centres, so the maximum number of stereoisomers is `2^2=4`.

The easiest way to see these isomers is to draw their Fischer projections.

Start by drawing four templates. `"C-1"` is at the top, and `"C-3"` and `"C-4"` are the two atoms with the horizontal bonds.

Now add the `"H"` atoms in all possible positions on the horizontal lines:

• two to the left;
• two to the right;
• one to the left and one to the right;
• one to the right and one to the left.

There are no internal mirror planes of symmetry, so there are no meso compounds.

The two isomers on the left are a pair of enantiomers, and the two isomers on the right are a pair of enantiomers.