Question

What is an achiral carbon?

Answer 1

This is a carbon centre that is not of the form `CR_1R_2R_3R_4`... In other words it is of the form `C(R_1)_2R_2R_3`...

Explanation:

`CR_1R_2R_3R_4` substitution has a geometric dimension. Given a tetrahedral array, the carbon centre can be left-handed or right-handed with respect to the substitution. Such a carbon centre is CHIRAL...

The so-called stereoisomer on the LEFT (left-handed with respect to carbon) is geometrically different to the stereoisomer on the RIGHT. The two isomers are optical isomers that CANNOT be superimposed. With a model (and you should be using a model!) the interchange of any two groups results in the enantiomer, interchange again and you get the enantiomer of an enantiomer, i.e. the original isomer.

See here and links for more explanation and examples....

For a more practical example, consider the pair of shoes that you put on for work. Can you put your left shoe on your right hoof? Will it be comfortable? But how is your left foot different from your right hoof. They are (usually) identical structurally...