4.6 Handedness
A tetrahedral carbon atom has four sites where other groups can bond to, and if four different groups are attached to the same carbon atom, then different isomers are possible.
I'll start with a molecule we've already seen, alpha-amino propanoic acid a.k.a. alanine. Take a look at the two 3-dimensional models below, and see if you can rotate them so that they're the same:
L-alanine |
D-alanine |
They can't be rotated to become identical; these are two different molecules representing two different compounds, isomers, but ones whose molecular structures are mirror images. Alanine is chiral, from the Greek for "hand" - it has a "handedness" - and the asymmetrically-bonded carbon atom is called the chiral center. The left- and right-hand isomers are the enantiomers of alanine (I think it's fun to say "enantiomer"). They are named L-alanine and D-alanine for reasons which I will explain.
L-alanine |
D-alanine |
In the skeletal formulas, the solid wedge-shaped bond indicates the bond is coming out "at you" from the plane of the page or your screen. A dashed line or wedge indicates a bond that is plunging below or beyond that plane. This is the standard way to represent chirality in a molecular structure. If we want to indicate an equal mixture of both isomers, called a racemic mixture, we can use a squiggly line (not shown here because I don't know how to get this third party software to do that).
Some chiral cyclic compounds exhibit central chirality with a cis isomer and two trans isomers, such as 1,2-dimethyl cyclohexane:
cis-1,2-dimethyl cyclohexane |
(R,R)-1,2-dimethyl cyclohexane |
(S,S)-1,2-dimethyl cyclohexane |