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