Question

What is an achiral carbon atom?

Answer 1

An achiral carbon atom is an atom INCAPABLE of generating optical isomerism. In practice, this means a carbon, `CR_4`, with at least 2 of the substituents the same.

Explanation:

A carbon which bears 4 different substituents, `CR_1R_2R_3R_4`, is potentially chiral, and can exist as 1 of 2 optical isomers, depending on the arrangement of the `R` groups. Should carbon bear at least 2 of the same substituents, optical isomerism is insupportable, and the carbon is ACHIRAL.

Most of the time, such carbon centres are present AS BOTH OPTICAL ISOMERS. These so-called racemic mixtures still contain chiral carbons, but because both isomers are present (so-called optical antipodes), the mixture will not rotate plane-polarized light.