Dr. Peter Wipf
Chemistry 0320 - Organic Chemistry 2
Carbohydrates
Carbohydrates are polyhydroxy aldehydes (aldoses) or ketones (ketoses) with the general
formula Cx(H2O)y (x=4, tetroses; x=5, pentoses; x=6, hexoses). A carbohydrate that cannot be
broken down into simpler units by hydrolysis is a monosaccharide. Oligosaccharides contain from
two to ten monosaccharide units, and polysaccharides contain even more.
CHO
Aldotriose
OH
CH2OH
D-Glyceraldehyde
CHO
CHO
OH
HO
Aldotetroses
OH
OH
CH2OH
CH2OH
D-Erythrose
D-Threose
CHO
CHO
OH
CHO
HO
CHO
OH
Aldopentoses
HO
OH
OH
OH
OH
OH
OH
CH2OH
CH2OH
CH2OH
CH2OH
D-Ribose
D-Arabinose
HO
HO
D-Lyxose
D-Xylose
Aldohexoses
CHO
OH
CHO
OH
OH
OH
OH
OH
OH
CH2OH
D-Allose
CHO
HO
OH
CH2OH
D-Altrose
HO
CHO
CHO
OH
HO
OH
CH2OH
D-Glucose
OH
OH
CH2OH
D-Mannose
HO
OH
HO
CH2OH
HO
OH
CH2OH
D-Idose
CHO
HO
HO
HO
OH
D-Gulose
CHO
OH
OH
HO
OH
CHO
HO
HO
OH
CH2OH
D-Galactose
OH
CH2OH
D-Talose
Glucose is the primary source of energy for humans. Its stereochemistry can be conveniently
compared to other monosaccharides by representation with Fischer projection formula. In
aqueous solution, two cyclic hemiacetals coexist in equilibrium with the open-cyclic aldehyde.
This effect, which results in a change in the optical rotation of the solution, is called mutarotation.
α-Glucose and β-glucose are anomers, e.g. diastereomer that differ at the one stereocenter
derived from the carbonyl group. Conversion of carbohydrates hemiacetals to acetals by reaction
with alcohol under acid catalysis is called glycosylation. Glycosides often are thermodynamically
more stable in the α-form due to the anomeric effect.
CHO
OH
HOH2C
HO
HO
O
HO
HOCH2
HO
OH
!-D-glucopyranose
OH O
H
HO
OH
CH2OH
HOH2C
O
HO
HO
H
H
OH
OH
H
OH
OH
OH
H
"-D-glucopyranose
D-glucose in aldehyde form
For fructose, a ketohexose, the furanose form is favored in solution.
CH2OH
HOCH2
O
HO
O
OH
HO
OH
CH2OH
OH
OH
!-D-fructofuranose
CH2OH
D-fructose in ketone form
As part of Fischer’s proof of the configuration of glucose, the stereochemistry of the natural
carbohydrates was arbitrarily assigned to be in the D-series, “D” referring to the position of the
C(2)-hydroxyl group in the Fischer projection of glyceraldehyde (dexter=right). “D” accordingly
also refers to the stereocenter at C(5) of hexoses, C(4) of pentoses, and C(3) of tetroses. “L”
marks the enantiomeric (unnatural) series.
Due to the presence of both alcohol and carbonyl (hemiacetal)functions, carbohydrates undergo
reactions typical for both functionalities. Besides glycosylation, etherification of the alcohol
functions is performed with aqueous acid and dimethylsulfate, and esterification occurs with acid
anhydrides and pyridine. Oxidation of aldoses with HNO3 provides aldaric acids, bromine in
water gives aldonic acids, and periodate cleavage between vicinal hydroxy functions breaks the
linking C,C-bond. Reduction with sodium borohydride reduces the hemiacetal to the alcohol, and
treatment with phenylhydrazine yields osazones.
The Kiliani-Fischer synthesis extends
aldoses by a carbon unit, and the Ruff
and Wohl degradations cleave one
carbon atom off the chain.
CHO
OH
CH2OH
1. HCN
2. Ba(OH)2
+
3. H3O
4. Na-Hg, H2O, pH 3-5
CHO
OH
OH
CH2OH
CHO
+
HO
OH
CH2OH
1. Br2, H 2O
2. H2O2, Fe2(SO4)3
CO2
Sucrose (2↔1 β−fructofuranosyl-α−glucopyranoside), maltose (α1→4 glucopyranosylglucopyranoside, building block of starch), cellobiose (β1→4 glucopyranosylglucopyranoside,
building block of cellulose), and lactose (β1→4 glucopyranosylgalactopyranoside) are important
examples of disaccharides.