Chemistry Reference
In-Depth Information
The specific optical rotation of a liquid is obtained from equation
(4.2), where
d
relative density of the liquid.
a
[
a
]
--
(4.2)
ld
Compounds that rotate the plane of polarised light towards the right
(clockwise) are called
dextrorotatory
, while compounds that rotate the
plane to the left, or anticlockwise, are called
laevorotatory
. The direction of
rotation is often specified by the symbols (
) for
laevorotatory and the direction is considered with the operator facing the
light source.
If a sample cell in a polarimeter contains equal amounts of the (
) for dextrorotatory and (
)
and the (
) enantiomers, the angle of rotation due to one enantiomer will
be equal and opposite to the angle due to the other and the net observed
rotation will be zero. Such a mixture is called a
racemic mixture
or a
race-
mate
and is often encountered in the laboratory as a result of a non-chiral
organic synthesis. The common synthesis of adrenaline (epinephrine), the
'fight or flight' hormone, yields a racemic mixture, which has precisely 50%
of the biological activity of the natural hormone. Once the racemate is
resolved
into the two pure enantiomers, the (
R
)-(
)-adrenaline is found to
be identical to the natural hormone produced by the adrenal medulla, while
the other enantiomer, the (
S
)-(
) isomer, has little or no biological activity
(Figure 4.4). (The meaning and use of the (
R
) and (
S
) notation is described
later in this chapter.)
Occasionally, the specific rotation of a compound can change over
time. This phenomenon called
mutarotation
and is caused by a change in
the molecular structure of the chiral compound. A good example of this
can be seen with the monosaccharide glucose. a-
D
-(
)-Glucose has an [a]
value of
110
, while b-
D
-(
)-glucose has an [a] value of
19.7
. If freshly
prepared solutions of a-
D
-(
)-glucose are allowed to
stand, however, the [a] value of each compound slowly changes until an [a]
value of
)-glucose and b-
D
-(
is reached. This is the [a] value for the equilibrium mixture
of the two anomeric forms (which differ in configuration at carbon-1) of
glucose. Both the a- and the b-pyranose forms of
D
-glucose are in equilib-
rium with a common open-chain form and this allows interconversion
between the two cyclic forms. The equilibrium mixture obtained due to
mutarotation of
D
-glucose has the approximate composition 33% a, 66% b
and 1% open-chain aldehyde (Figure 4.5).
52.5
