Page 72 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

P1: LLL/LLL P2: FJU Final Pages
Encyclopedia of Physical Science and Technology EN002C-80 May 25, 2001 20:18

380 Carbohydrates

TABLE VI Stable conformation of D-Aldopyra- (which is less favored because it possesses two eclipsed
noses in Aqueous Solutions carbon atoms) to go to the next twist form. Because inter-
Aldose Conformation action between two eclipsed carbon atoms is greater than
between a carbon atom and an oxygen atom, the ring oxy-
Aldohexoses gen atom tends to occupy a position along the plane of the
α-D-Allose 4 C 1 ring and to leave the puckering to carbon atoms.
β-D-Allose 4 C 1
4
α-D-Altrose 4 C 1 , C 4
β-D-Altrose 4 C 1 B. Reactions of Monosaccharides
α-D-Galactose 4 C 1
β-D-Galactose 4 C 1 In reactions involving monosaccharides, it is important to
α-D-Glucose 4 C 1 remember that the functional groups found in the vari-
β-D-Glucose 4 C 1 ous cyclic and acyclic forms will be present side by side
α-D-Gulose 4 C 1 in the reaction mixture. Thus, in a reaction involving an
β-D-Gulose 4 C 1 aldohexose, a carbonyl group and two types of hydroxyl
1
α-D-Idose 4 C 1 , C 4 groups will be provided by the acyclic form. These are
β-D-Idose 4 C 1 the primary hydroxyl group attached to the terminal posi-
tion, and four (less reactive) secondary hydroxyl groups.
α-D-Mannose 4 C 1
In addition, each of the cyclic forms will contribute three
β-D-Mannose 4 C 1
types of hydroxyl groups: a hemiacetal hydroxyl function
α-D-Talose 4 C 1
at C-1, a terminal primary hydroxyl group, and three (less
β-D-Talose 4 C 1
reactive) secondary hydroxyl groups.
Aldopentoses
It is also important to remember that the course of a
α-D-Arabinose 1 C 4
reaction does not depend totally on the relative amount
1
β-D-Arabinose 4 C 1 , C 4
of a given species. Thus, free saccharides readily undergo
1
α-D-Lyxose 4 C 1 , C 4
nucleophilic additions, characteristic of carbonyl groups,
β-D-Lyxose 4 C 1
even though carbonyl groups are present only in their
1
α-D-Ribose 4 C 1 , C 4
acyclic form, which contributes little to the equilibrium
1
β-D-Ribose 4 C 1 , C 4
α-D-Xylose 4 C 1
β-D-Xylose 4 C 1
4
Using these values, it can be calculated that the C 1
form of β-D-glucopyranose has a conformational energy
1
of 8.38, considerably lower than that of the C 4 form with
a value of 33.5, leaving no doubt that the first is the more
stable conformer. Table VI shows the conformation of
D-aldopyranoses in aqueous solutions.
The principal conformers of the furanose ring are the
envelope (E) and the twist (T) forms, of which the latter
are the most stable. To designate a particular conforma-
tion, the method is the same as that used for pyranoses;
namely, the letter used to designate the form (E or T) is
superseded by the number of the ring atom situated above
the plane of the ring and is followed by the number of the
ring atom below the plane of the ring.
For furanose rings, the most stable conformers are the
envelope and twist forms; these can exist in 10 arrange-
ments each. Due to the low energy barriers between the
twist and envelope conformers, a sugar in the furanose
form is believed to undergo rapid interconversions be-
SCHEME 2 Representation of the anomeric configuration
tween them. The slightly more favored twist conformer D-glucose in a Fischer (top right) and a Haworth (bottom left)
would rapidly pass through an envelope conformation formula.