Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
2.3.2 OXIDATION
Commonly, primary alcohols (2.8) may be converted to aldehydes (2.9) and secondary
alcohols (2.10) to ketones (2.11).
RCH 2OH → RCHO and RCH(OH)R 1 → RCOR 1
2.8 2.9 2.10 2.11
The hydroxyl group can be easily oxidized with a number of strong oxidizing agents,
the most common being KMnO 4, Br 2, K 2Cr 2O 7, and Pb(CH 3COO) 4. Permanganate
and dichromate oxidize the primary hydroxyl group to the carboxyl group rather
than to the aldehyde group. Periodic acid, its salts, and lead tetracetate cleave 1,2glycols
(2.12) under mild conditions and with a good yield. Two molecules of either
aldehyde (2.13a) (R 1 = R 4 = H) or ketone (2.13b) of the structure dependent on the
substrate are the products of the oxidation.
R 1 R 2 C(OH)–C(OH)R 3 R 4 → R 1 COR 2 + R 3 COR 4
2.12 2.13 a, b
The reaction serves to identify the cis-diol structure.
Results of applying any of these oxidants to aldoses and ketoses are formally
the same, except when furanoses and pyranoses are oxidized either with periodates
or lead tetracetate, in which case one molecule of dialdehyde is formed as only the
C-2–C-3 bond in the ring is cleaved.
Halogen molecules and their oxy compounds are widely used to oxidize the
aldehyde group to the carboxylic group. It is a way to prepare aldonic acids from
aldoses and aldaric acids from glucuronic acids. Ketoses are usually resistant to
bromine. Cyclic aldoses (2.14) in acid solution are oxidized directly to aldonic acids
(2.15). In aqueous solutions, they reside in equilibrium with their lactones (2.16)
and (2.17). Pyranoses and furanoses produce 1,5-(2.16) and 1,4-(2.17) lactones,
respectively. The higher oxidation rate for β-D-glucopyranose is probably because
CH2OH CH2OH COOH CH2OH
OH
OH Br2
OH
HO
OH HO
OH
OH
OH
2.14
OH
OH
2.16
OH
OH
CH2OH
2.15
OH
2.17
O O + O
O
its equatorial anomeric hydroxyl group is sterically not hindered. Catalytic oxidation
of aldoses with oxygen leads to corresponding aldonic acids. It readily proceeds
over the platinum catalyst in an alkaline solution. Also, microbial oxidation of
reducing sugars provides a high yield of aldonic acids (Chapter 10). Aldonic acids
and lactones are stable toward acids. By treatment in alkaline solution, the
configuration at C-2 of aldonic acids can be altered. These acids easily form
O