section 9.1
Classification
137
F I G U R E 9 - 6
The two chair conformational formulas for /i-D-glucopyranose.
However, two chair forms can be drawn for each of the
two anomers of D-glucopyranose, as shown in Figure 9-6
for /3-D-glucopyranose. The structure on top is designated
the 4Ci form because it contains carbon 4 and carbon 1
above and below the plane of the molecule, respectively.
The designation of *C
4
for the structure on the bottom
conveys the opposite sense. In the
4C\
conformation, all
the large substituent groups are in equatorial positions,
whereas in the *C
4
conformation, they are in axial posi-
tions. 4Cj is the more stable and therefore the preferred
chair conformation.
The
conformational
formulas
for
a-
and
/3-D-
glucopyranose are shown in Figure 9-7. In aqueous so-
lutions, the /3-anomer of glucose is better solvated, more
stable, and therefore the predominant form. However,
for other aldohexoses (e.g., mannose and galactose), the
a-anomer is more stable than the /3-anomer because of the
dipole effect. The dipole effect involves the Ci -hydroxyl
group and the ring oxygen. In the a-anomer, the dipoles
of the axial Ci -hydroxyl group and the ring oxygen are
F I G U R E 9 -7
Conformational formulas for
a -
and /J-D-glucopyranose.
T h e a -a n o m e r : T h e axial hydroxyl
g ro u p is in a fav o rab le d ip o le-
d ip o le in teractio n w ith th e ring
o x y g en a n d resu lts in a m o re
s ta b le co n fo rm atio n .
T h e p -a n o m e r: T h e eq u ato rial
hydroxyl g ro u p is in a n u n fav o rab le
d ip ole-dipole rep u lsiv e interaction
with th e ring o x y g en a n d yields a
le s s s ta b le con fo rm atio n .
F I G U R E 9 -8
Dipole-dipole interaction between the Ci -hydroxyl group and the ring
oxygen in aldohexoses. The arrows indicate the direction of the dipole
moment.
nearly antiparallel, whereas in the /3-anomer, the dipoles
of the equatorial Ci -hydroxyl group and the ring oxygen
are nearly parallel (Figure 9-8). The dipoles impart sta-
bility to a structure when they are antiparallel. Although
the dipole effect is also applicable to a- and /3-anomers
of glucose, the solvation effect overrides the dipole
effect.
Interconversion of
a-
and /
6
-forms can be followed in a
polarimeter by measuring the optical rotation. Crystalliza-
tion of D-glucose from water yields a-D-glucopyranose,
a form which is least soluble and has a specific rota-
tion of +112.2°. Ordinary crystalline glucose is in the
a-form, but if the crystallization of D-glucose takes place
in pyridine, /
6
-D-glucopyranose, which has a specific rota-
tion o f+18.7°, is obtained. Freshly prepared solutions of
the
a-
and /
6
-forms show specific rotations of +
1 1 2
.
2
°
and +18.7°, respectively. However, over a period of a
few hours at room temperature, the specific rotation of
both forms in aqueous solution changes and attains a
stable value of +52.7°. This change in optical rotation,
mutarotation,
is characteristic of sugars that form cyclic
structures. It represents the interconversion of the
a-
and
/
6
-forms, yielding an equilibrium mixture consisting of
about two thirds /
6
-form, one third a-form, and a very
small amount of the noncyclic form (Figure 9-9). Thus, the
change in structure can occur in solution and attain equi-
librium, which favors the formation of more stable (lowest
energy) forms. However, the less stable forms may occur
under certain conditions, such as during the formation of
