4.2 Monosaccharides 267( 4. 42 )formic acids in addition to formaldehyde andD-
arabinose (Formula 4.42). Depending on reaction
conditions, particularly the type of alkali present,
further hydroxyacids are also formed due to eno-
lization occurring along the molecule.
The nonstoichiometric sugar oxidation process in
the presence of alkali is used for both qualitative
and quantitative determination of reducing sug-
ars (Fehling’sreaction with alkaline cupric tar-
trate;Nylander’sreaction with alkaline trivalent
bismuth tartrate; or usingBenedict’ssolution, in
which cupric ion complexes with citrate ion).
Hydroxyaldehydes and hydroxyketones are
formed by chain cleavage due to retroaldol reac-
tion under nonoxidative conditions using dilute
alkali at elevated temperatures or concentrated
alkali even in the cold.
For example, fructose can yield glyceraldehyde
and dihydroxyacetone (Formula 4.43), and the
latter easily undergoes water elimination to give
2-oxopropanal (methylglyoxal). Starting from
1-deoxy-2,3-hexodiulose, several degradation
pathways leading to short-chain compounds are
possible (Formula 4.44). Among other com-
pounds, 2-oxopropanal, monohydroxyacetone,
acetic acid, glyceraldehyde or glyceric acid
can be formed by retro-aldol reactions (a),
α-dicarbonyl cleavages (b) and β-dicarbonyl
cleavages (c).
Since enolization is not restricted to any part of
the molecule and since water elimination and
redox reactions are not restricted in amount,
even the spectrum of primary cleavage prod-
ucts is great. These primary products are highly(4.43)