Pharmaceuticals - Fundamentals of Industrial Chemistry: Pharmaceuticals, Polymers, and Business - page 172

waste-disposal needs. Diethyl ether can be used safely in the laboratory, but
its low boiling point and flammability make it problematic in larger reactions.
A process with chromium, mercury, or copper waste, for example, might
be rejected because of the difficulties and costs in properly disposing of the
waste. Things that a laboratory chemist may not consider, such as how fast
something filters, can make a tremendous difference when the reaction is done
on a large scale.

10.4 CHIRALITY

The term chiral comes from the Greek word for hand. A chiral carbon has four
different groups bonded to the carbon and the mirror image is not superim-
posable and therefore a different compound. This is true for our hands - our
left hand is the mirror image of the right and our hands are not superimpos-
able. Our shoes are another example of nonsuperimposable mirror images.
They have the same physical properties such as weight and density. However
they can be distinguished by a chiral material - in this example by our eyes
or by putting them on our feet which are also chiral. You can only put the left
shoe on the left foot. Contrast this with socks which are achiral. Socks are
superimposable mirror images and there is no left sock and right sock. They
are the same. Lactic acid is an example of a compound with a chiral center.

CO 2 H

CO 2 H

H 3 C

CH 3
H

HO

H OH

R-lactic acid S-lactic acid

The mirror image of R-lactic acid is not superimposable and is a different
compound. Non-superimposable mirror images are called enantiomers. Enan-
tiomers have the same physical properties. For example, the boiling point,
melting point and solubility are the same. They differ in the rotation of plane
polarized light. One enantiomer rotates plane polarized light in a clockwise
direction; the other enantiomer in a counterclockwise direction. The ability
to rotate plane polarized light is called optical activity. A 50 - 50 mixture of
enantiomers is called a racemic mixture or a racemate. Because each enan-
tiomer rotates light in an equal and opposite direction, in a racemic mixture
the rotations cancel each other out and there is no net rotation. A racemic mix-
ture is said to be optically inactive. Separation of the enantiomers in a racemic
mixture is called resolution. This can be done analytically by HPLC using a
chiral stationary phase. If there are x number of chiral centers, there can be