Question

What do L-, D-, R-, and S- (in enantiomers) stand for?

Answer 1

`L` and `D` refers to the direction of rotation of plane-polarized light. `R` and `S` refer to the absolute configuration possessed by the chiral centre.

Explanation:

`L` short for `"laevorotatory"` and `D` `"(dextrorotatory)"` refer to the direction, anticlockwise, or clockwise, that a solution of the molecule rotates plane-polarized light. It is a bit of an old-fashioned term, but it relates SOLELY to the experimental result. See here.

On the other hand, the terms `R` and `S` refers specifically to the absolute configuration of a chiral centre. And to determine this, specialized experimental techniques are necessary.

There are some `R` isomers that rotate plane-polarized light in an anticlockwise direction, and `L` isomers that rotate in the clockwise direction. Thus the terms `R` and `S` refer to absolute configuration, and not the polarized light experiment.