CARBOHYDRATE
Reactions ofReactions ofMonosaccharidesMonosaccharides
Reactions of Reactions of MonosaccharidesMonosaccharides sugars are multifunctional compounds; most exist
as cyclic hemiacetals
in solution, sugars are in equilibrium with their open-chain aldehyde or ketone forms
undergo most of the usual reactions of ketones,aldehydes and alcohols
Side rxns: 2 common base-catalyzed side reactions:
; Epimerization
; Enediol Rearrangement
EpimerizationEpimerization proton alpha to carbonyl group (in aldehyde or ketone)
is reversibly removed; form an enolate - C2 no longerchiral.
reprotonation can occur on either side of the enolate togive either configuration
products are mixture of original sugar and its C2 epimer
EpimerizationEpimerization
Enediol Rearrangement moves the carbonyl group up and down the chain conversion of aldose to ketose
Reduction of Reduction of MonosaccharidesMonosaccharides
C=O of aldoses or ketoses can be reduced to C-OH by NaBH4 or H2/Ni.
Name the sugar alcohol by adding -itol to the root name of the sugar; commonly known as an alditol.
Reduction of D-glucose produces D-glucitol, commonly called D-sorbitol.
Reduction of D-fructose produces a mixture of D- glucitol and D-mannitol.
HOOH
HC O
HO
OH
H
CH2OH
H
HH
OHO
OHHO
CH2OH
H
HH
H
H
OHHO
CH2OH
OH
H
CH2OH
H
HH
OH
OHH2, Ni
-D-glucopyranose D-glucitol(D-sorbitol)an alditol
open-chain aldehyde
O CH2OHHOCH2
OHHO
HOH
H
HO
C O
OH
H
CH2OH
HH OH
CH2OH
NaBH4
HO
CH2OH
OH
H
CH2OH
H
HH
OH
OHHO
CH2OH
OH
H
CH2OH
H
HH OH
HO
+
-D-fructofuranoseopen-chain keton
mixture of mixture of alditolalditol
D-glucitol D-mannitol
Oxidation by Bromine
Oxidation by Nitric Acid
Oxidation by Tollens Reagent
Ag(NH3)2+ -OH (strong basic condition) --- give Ag (silver mirror) promotes epimerization & enediol rearrangements
- therefore, cannot distinguish aldoses & ketoses
Sugars that reduce Tollens reagent are called Reducing sugars
Formation of Glycosides: Sugar acetals
Aldoses & ketoses are converted to acetals by treatmentwith alcohol in the presence of acid.
methyl -D-glucopyranoside
methyl -D-glucopyranoside
-glycosidic bond
(acetal)
(acetal)
hemiacetal
-glycosidic bond
OHHO
O
OHHO
CH2OH
H
HH
H
H
CH3OH, H+
OCH3HO
O
OHHO
CH2OH
H
HH
H
H
OCH3
HOO
OHHO
CH2OH
HH
H
H
H
+
H2O, H+
Glycosides
Aglycone is the term used for the group bonded to the anomeric carbon.
Glycosides
Nonreducing Sugars
Glycosides are acetals, stable in base, so they do not react with Tollens reagent.
Disaccharides and polysaccharides are also acetals, nonreducing sugars.
Ether Formation
Osazone Formation
Ester Formation
Ruff Degradation Aldose chain is shortened by oxidizing the
aldehyde to -COOH, then decarboxylation.
Kiliani-Fischer Synthesis
This process lengthens the aldose chain.
A mixture of C2 epimers is formed.
two epimericcyanohydrins
* Pd/BaSO4 to avoid over reduction