Food Chemistry

4.2 Monosaccharides 253

The cis-arrangement of the two adjacent
HO-groups in positions C-1 and C-2 ofα-D-
glucopyranose, unlike itsβ-anomer, increases the
conductivity of boric acid. A borate complex is
formed which is a stronger acid than boric acid
itself (cf. Formula 4.10).
InTollensring formula, in the D-series, theα-an-
omer has the hydroxyl group at C-1 on the right
and theβ-anomer has this OH-group on the left.
In Haworth projections the HO-group of
α/β-anomers of the D-series usually occurs
below/above the pyranose or furanose ring
planes, while in theL-series the reverse is true
(cf. Formula 4.11).

(4.10)

(4.11)

Each monosaccharide can exist in solution
together with its open chain molecule in a total
of five forms. Due to the strong tendency towards
cyclization, the amount of the open chain form is
greatly reduced. The contribution of the different
cyclic forms to the equilibrium state in a solution
depends on the conformation. An aqueous
D-glucose solution is nearly exclusively the two
pyranoses, with 36% α- and 64% β-anomer,
while the furanose ring form is less than 1%. The

equilibrium state varies greatly among sugars (cf.

Table 4.6).

The transition into the different hemiacetal

forms, called mutarotation, proceeds via the

open-chain carbonyl compound. The acid- and

base-catalyzed ring opening is the rate limiting

step of the reaction:

(4.12)

2,3,4,6-Tetramethyl-D-glucose reaches equilib-

rium in benzene rapidly through the concerted

action of cresol and pyridine as acid–base

catalysts (Table 4.3). Bifunctional reagents, like

2-pyridone and benzoic acid, are especially

efficient acid–base catalysts in both polar and

nonpolar solvents:

(4.13)

Water can also be a bifunctional catalyst:

(4.14)

Table 4.3.Mutarotation rate of 2,3,4,6-tetramethyl-D-

glucose (0.09 mol/l) in benzene

Catalyst (kmin−^1 )krel

–7. 8 × 10 −^51. 0

Pyridine (0.1mol/l) 3. 7 × 10 −^44. 7

p-Cresol (0.1mol/l) 4. 2 × 10 −^45. 4

Pyridine + p-cresol (0.1mol/l) 7. 9 × 10 −^3101

2-Pyridone (0.1mol/l) 1. 8 × 10 −^12307

Benzoic acid (0.1mol/l) 2. 228 , 205