molecules have nonsuperimposable mirror images. And as a general
rule of thumb, chiral molecules must have one or more chiral
centers -- that is, carbons that have four non-identical
substituents around it. (There are, of course, exceptions
to this rule). A classic case of a simple chiral molecule
is the following halogenated methane derivative:
atom has four non-identical substituents around it, making
this carbon a chiral center, and as proof of its chirality
the molecule has a non-superimposable mirror image. A fancy
term used in textbooks and in the literature to describe molecules
that are mirror images of each other is enantiomers,
as in "the enantiomer of the left molecule above is the
molecule on the right, its mirror image."
between enantiomers, chemists use the R and S classification
system. Stereocenters, (sometimes called chiral centers, or
stereogenic centers) are carbons that have four non-identical
substituents on them, and are designated as either of R stereochemistry
or S stereochemistry. If a molecule has one stereocenter of
R configuration, then in the mirror image of that molecule,
the stereocenter would be of S configuration, and vice-versa.
of R or S configuration can be applied in three steps:
1. Order the substituents coming off the stereogenic
carbon atom using the Cahn-Ingold-Prelog rules.
2. Rotate the molecule until the lowest priority (number
4) substituent is in the back
3. Draw a curve from number 1 to number 2 to number
3 substituent. If the curve is clockwise, the stereocenter
is of R configuration. If the curve is counterclockwise, the
stereocenter is of S configuration.
1. Order the substituents: Order the substituents coming
off the carbon stereocenter from 1 to 4, with 1 being the
highest priority substituent and 4 being the lowest priority
substituent. To assign priority using the Cahn-Ingold-Prelog
rules, compare the first atoms of the substituents.
those substituents with higher molecular weight atoms a
higher priority number. In our example below, iodine
would be 1, bromine 2, chlorine 3, and fluorine 4, because
iodine has the highest molecular weight (and is therefore
highest priority) and fluorine has the lowest molecular
weight (and is therefore the lowest priority).
the first atom of two substituents happen to be identical
identical in molecular weight, go to the next atom and make
the molecular weight comparison (eg. an ethyl group
would have higher priority over a methyl group).
priorities for double bonds becomes a bit more challenging
(this applies in the same fashion to carbonyls, C=O, and
imines, C=N). A carbon with a double bond to another carbon
is treated as a carbon singly bonded to two carbons, as
shown below. This means that, for example, an ethylene substituent,
R-CH=CH2, will have a higher priority then an ethyl substituent
Shown here is an ethylene substituent (often called an allyl
substituent). By the Cahn-Ingold-Prelog rules for assigning
R and S nomenclature, this allyl group can be redrawn with
each double bond carbon singly bonded to an additional carbon
with three "phantom ligands," that are ignored.
molecule can now be numbered as follows:
Rotate the molecule. There are a couple of different ways
to go from the priority numbering to determining R and S confiuguration.
One of the best methods taught by a lot of undergraduate textbooks
is to rotate the molecule until the number 4 priority subsituent
is in the back, as shown below. (Remember that dashed lines
mean a bond going into the computer screen, and a solid wedged
line indicates a bond coming out of the screen). This takes
some practice, especially if you are like most people and
have difficulty visualizing molecules in three dimensions.
Draw the curve. A curve is then drawn from the 1 to 2
to 3 priority substituents, ignoring the 4th priority subsituent
(as shown below). If that curve goes clockwise then that stereocenter
is of the R configuration. If the curve goes in a counterclockwise
direction, then that stereocenter is of S configuration. In
our example below, the curve goes counterclockwise so the
stereocenter is of S configuration.
of the compound above, then, would be (S)-bromo, chloro, fluoro,
iodomethane, and the name of its enantiomer, or its mirror
image, would be (R)-bromo, chloro, fluoro, iodomethane.
Click here for a short quiz
on assigning R or S configuration.