Chirality
It is the symmetry of a molecule (or any other object) that determines whether it is chiral
or not. Technically, a molecule is achiral (not chiral) if and only if it has an axis of im-
proper rotation; that is, an n-fold rotation (rotation by 360°/n) followed by a reflection in
the plane perpendicular to this axis which maps the molecule onto itself. A chiral molecule
is not necessarily dissymmetric (completely devoid of symmetry) as it can have, e.g., rota-
tional symmetry. A simplified rule applies to tetrahedrally-bonded carbon, as shown in the
illustration: if all four substituents are different, the molecule is chiral.
It is important to keep in mind that molecules which are dissolved in solution or are in the
gas phase usually have considerable flexibility and thus may adopt a variety of different
conformations. These various conformations are themselves almost always chiral. However,
when assessing chirality, one must use a structural picture of the molecule which corresponds
to just one chemical conformation - the one of lowest energy.
32.2 Chiral Compounds With Stereocenters.
Most commonly, chiral molecules have point chirality, centering around a single atom,
usually carbon, which has four different substituents. The two enantiomers of such com-
pounds are said to have different absolute configurations at this center. This center is thus
stereogenic (i.e., a grouping within a molecular entity that may be considered a focus of
stereoisomerism), and is exemplified by the α-carbon of amino acids.
The special nature of carbon, its ability to form four bonds to different substituents^1 , means
that a mirror image of the carbon with four different bonds will not be the same as the
original compound, no matter how you try to rotate it. Understanding this is vital because
the goal of organic chemistry is understanding how to use tools to synthesize a compound
with the desired chirality, because a different arrangement may have no effect, or even an
undesired one.
A carbon atom is chiral if it has four different items bonded to it at the same time. Most
often this refers to a carbon with three heteroatoms and a hydrogen, or two heteroatoms
plus a bond to another carbon plus a bond to a hydrogen atom. It can also refer to a
nitrogen atom bonded to four different types of molecules, if the nitrogen atom is utilizing
its lone pair as a nucleophile. If the nitrogen has only three bonds it isnotchiral, because
the lone pair of electrons can flip from one side of the atom to the other spontaneously.
Any atom in an organic molecule that is bonded to four different types of atoms or chains
of atoms can be considered ”chiral”.
If a carbon atom (or other type of atom) has four different substituents, that carbon atom
forms achiral center(also known as astereocenter). Chiral molecules often have one or
more stereocenters. When drawing molecules, stereocenters are usually indicated with an
asterisk near the carbon.
Example:
Figure 81 Which of the indicated carbon atoms is a stereocenter?