Chemistry Reference
In-Depth Information
Although recently prepared aqueous solutions of glucose present the above speciic rota-
tion(
[
α
20
)values,aftersometime,thesolutionsofbothformstendtoreachthesamethreshold
[
= +
. In the equilibrium state, the two forms of glucose (α and β) are encountered at
ratesof38%and62%,respectively.Theexistenceofthesetwoformsofglucosecanexplainthe
mutarotationofitsaqueoussolutions,asduringsustentionofthesolution,theoneformconvertsto
theotheruntiltheequilibriumisattained(PigmanandAnet1972;ShallenbergerandBirch1975;
Coultate2002;Izydorczyk2005;Kuszmann2006).Thetautomer/isomerformsofglucose(αform
hastheOH-oftheanomericcarboninaxialpositionandβforminequatorialposition,respectively)
inanaqueoussolutionatroomtemperaturearepresentedinFigure2.6.
Infructose,theequilibriumofthecyclicformsisreachedatthethresholdof
[
α
D
]
20
52 5
.
°
α
D
20
= −
92
°
,and
therespectivetautomers/isomersattheequilibriumarepresentedinFigure2.7.
2.3.2 Monosaccharides' reactions
Oxidation
:Bymeansofmildoxidizingagents(Br
2
inwater,dilutenitricacid),aldosescanbe
converted to the corresponding acids (aldehyde group to carboxylic acid group forming aldonic
acids),whereasketosesundercommonconditionsdonotreact.Basedonthisreactivity,itiseasy
todistinguishaldosesfromketones.Bymeansofstrongoxidizingagents(concentratednitricacid),
thetwoterminalgroupsofthemoleculeareoxidized,anddibasicacids(aldaricacids)areformed.
Byprotectingthealdehydegroup,themoleculeisoxidizedattheterminalgrouptouronicacids.
By using very strong oxidizing agents as HIO4, sugar molecules are broken down, and several
decompositionproducts(aldehydes/ketonesandformicacid)areproduced.Thepotentialoxidation
reactionsofglucose,asshownbelow(Figure2.8),leadtogluconicacid,glucaricacid(saccharic
acid),andglucuronicacid.Theoxidationreactiontosaccharicacidsiscommonlyusedforquantita-
tivedeterminationofsugars;Fehling'ssolutionisthenused,andCu(II)ionscontainedarereduced
toCu(I)formingacopperoxideredprecipitate(Multon1997;Stick2001;Tomasik2002;Stickand
Williams2009).
Reduction
: Both aldoses and ketoses with reducing agents, that is, by NaBH
4
or by catalytic
hydrogenation(H
2
/Ni),canbereducedatthecarbonylgrouptopolyvalentalcohols(alditols).These
polyolsgenerallyhaveasweettasteandareusedasalternativesweeteners,suchaspentitolsfrom
pentoses and hexitols from hexoses (BeMiller and Whistler 1996; Stick 2001; Izydorczyk 2005).
Speciicallyd-glucoseandtheresultingd-sorbitolarepresentedasfollows:
HO
OH
HO
Br
2
/H
2
O or dilute
HNO
3
Bacterial, catalytic, or
electrolytic oxidation
O
HO
OH
glucose
Conc. HNO
3
HO
OH
HO
HO
OH
O
HO
OH
OH
O
HO
HO
HO
O
HO
OH
O
HO
OH
O
HO
OH
Gluconic acid
Saccharic acid
Glucuronic acid
Figure 2.8
Oxidation products of glucose.
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