The optical behaviour of L.-benzoin, L-benzoin
methyl ether and methyl L-mandelate, with change of
solvent, concentrat ion, and temperature has been examined
for a range of wavelengths over the visible region of
the spectrum. The results may be summarised as
(a) On the whole the negative rotation of
L-benzoin varies with the polarity of the solvent in
such a manner that solvents of high polarity depress
the negative rotatory power. This depression is
attributed to the greater degree of association existing
between the active, solute and solvents of higher
(b) In agreement with (a), an increase in
the concentration of L-benzoin in a non-polar solvent
also lowers the negative rotation, owing to a rise in
the degree of association of the solute molecules with
(c) Among non-polar solvents the solutions
with cyclohexane, mesitylene, toluene and benzene gave
abnormally low negative rotations as compared with their
non-polar character. These displacements were assumed
to be due to the association of L-benzoin in such media,
and this view is supported by molecular weight measurements .
(d) On rise of temperature the T-R curves for
L-benzoin in bromobenzene and. nitrobenzene respectively
(typical of weakly and strongly polar solvents) tend to
converge. This is in agreement with the dipole theory.
(e) For L-benzoin methyl ether, molecular
weight determinations now show that very little association
at low concentrations occurs even in non-polar solvents.
In agreement with this difference, the rotatory powers
of the ether are in much closer agreement with the
polarity of the solvents than was found for L-benzoin.
Cyclohexane, mesitylene, and other non-polar solvents
fall into position at one end of the series to give
dextro-rotatory solutions. Strongly polar solvents
On the other hand yield laevo-rotatory solutions.
The typical effect of polar and non-polar
solvents corresponds to the remarkable variation in
rotatory power of L-benzoin methyl ether with
change of concentration in benzene solution. With
increase in concentration, the dextro-rotation changes
into a laevo-rotation.
With rise of temperature the T-R curves for
mesitylene, bromobenzene and nitrobenzene converge
strongly. These solvents were selected as being
typical of non-polar, weakly polar and strongly polar
(f) Methyl L-mandelate, as was expected,
resembles L-benzoin in many of its optical properties.
The laevo-rotation tends to fall with increase in the
dipole moment of the solvent, although certain non-
polar media yield abnormally low values. The latter
may be explained by the fact that the ester exists in
these solvents in the associated state. The changes
with rise of concentration and temperature are also in
agreement with the predictions of the theory of dipoles.
(g) The dispersions of L-benzoin and methyl
L-mandelate are in every case normal and complex. When is plotted against λ² , the deviation of the
graph from the line joining the extreme points is in
the same direction.
This statement also holds for L-benzoin
methyl ether in non-polar and strongly polar solvents
respectively. In solvents of medium polarity, or in
the homogeneous state the deviation of the graph may be
in the opposite direction to the above, or anomalous
dispersion may be exhibited.