Reactions of Monosaccharides...Water 16 •Bromine-water oxidizes the aldehyde group of an aldose to a carboxylic acid. Bromine water does not oxidize the alcohol groups or the ketoses

Reactions of Monosaccharides

112:44 PM

Reactions of MonosaccharidesIntroduction

2

• Even though, monosaccharide sugars are multifunctional

compounds, they undergo reactions typical of the functional

groups they contain, but with a few modifications brought

about by the co-existence of the functional groups in the

same molecule.

12:44 PM

H O

OHH

HHO

OHH

OHH

CH2OH

D-Glucose

O

OHHO

OH

OHHO

D-Glucopyranose

• Most monosaccharides exist in cyclic hemiacetals, yet in

solution they are in equilibrium with their open-chain

aldehyde or ketone forms.

• Thus, monosaccharides undergo most of the usual reactions

of aldehydes and ketones, alcohols and hemiacetals.

Epimerization of Monosaccharides

3

• One of the most serious limitations of carbohydrate chemistry

is the inability to transform monosaccharide sugars using

basic reagents because of the tendency of these reagents to

trigger base-catalysed epimerization to epimeric

monosaccharides or isomeric ketoses.

12:44 PM

H O

OHH

HHO

OHH

OHH

CH2OH

OHOH

H O

HHO

OHH

OHH

CH2OH

OHH-OH

H O

HHO

HHO

OHH

OHH

CH2OH

EnolateD-Glucose D-Mannose

Base-catalysed epimerization of glucose

• The proton a to the aldehyde group is reversibly

deprotonated resulting in an enolate. Since C-2 is no longer

chiral, its stereochemistry is lost. Reprotonation on either

face of the enolate, gives either configuration at this carbon.

Isomerization of Monosaccharides

4

• A base-catalysed enediol rearrangement culminates in the

migration of the carbonyl group up and down the

monosaccharide carbon chain.

• If the enolate ion formed by removal of a proton on C-2

reprotonates on the C-1 oxygen, an enediol intermediate

results.

• Keto-enol tautomerism of the enediol gives D-fructose, a 2-

ketose.

12:44 PM

Reduction of Monosaccharides

5

•Aldoses and ketoses can be reduced to the corresponding

alcohols (polyols), called sugar alcohols or alditols and typically

have a sweet taste.

•Glucitol, mannitol and xylitol are widely used as sweeteners

and moisturizers in a number of cosmetic products. They do

not promote tooth decay.

12:44 PM

• The reaction occurs by reduction of the small amount of

aldehyde that is in equilibrium with the cyclic hemiacetal.

• As the aldehyde is reduced, the equilibrium shifts to the right,

so that eventually all of the sugar is reduced.

Oxidation of Monosaccharides

6

• Since the cyclic hemiacetal forms of sugars are in equilibrium

with a small but finite amount of the open-chain aldehyde, they

can be easily oxidised to carboxylic acids.

• The products are called aldonic acids. Consequently,

monosaccharide sugars act as reducing agents. They are

often referred to as reducing sugars.

• The oxidation of aldoses is so easy that they react with such

mild oxidizing agents as:

(a)Tollens reagent (Ag+ in aqueous ammonia)

(b)Fehling’s reagent (Cu2+ complexed with tartrate ion)

(c)Benedict’s reagent (Cu2+ complexed with citrate ion)

(d)Oxidases (Enzymes that catalyse oxidation)

12:44 PM

Oxidation of Monosaccharides with TollensReagent

7

• The Tollens reagent (silver(1)ammonical hydroxide) oxidizes

aldehydes to carboxylate ions.

• The Ag(I) complex which is soluble in ammonium hydroxide is

reduced to metallic silver, which is insoluble in ammonium

hydroxide. This results in the formation of a silver mirror on the

inside of the test-tube.

12:44 PM

OR

H

+ Ag(NH3)2+OH-

Tollens reagent

OR

O–

+ Ag

Aldehyde Acid anion Silver mirror


812:44 PM

• In its open form, an aldose has an aldehyde group, which

reacts with the Tollens reagent to give an aldonic acid and a

silver mirror.

• Sugars that reduce the Tollens reagent are called reducing

sugars.


9

• The Tollens test cannot distinguish between aldoses and

ketoses because the strongly basic solution in which

theTollens reagent is dissolved promotes enediol

rearrangements.

• Under the basic conditions, the open-chain form of a ketose is

converted to an aldose, which reacts to give a positive Tollens

test.

12:44 PMD-Fructose thus gives a positive test with the Tollens reagent

OH

D-Glucose

H OH

HHO

OHH

OHH

CH2OH

OH

CH2OH

HHO

OHH

OHH

CH2OH

O

D-Fructose

OH

H O

HHO

OHH

OHH

CH2OH

OHH Ag(NH3)2+OH-

Tollens reagent

COOH

OHH

HHO

OHH

CH2OH

OHH

+ Ag

Gluconic acid

Ketose Aldose

Positive TollensTest

Oxidation of Monosaccharides with FehlingsReagent

10

•Fehling’s solution, a tartrate complex of copper (II) sulphate,

has also been used as a test for reducing sugars.

12:44 PM

• Why does D-Fructose give a positive test with the Fehlings

reagent?

Oxidation of Monosaccharides with Benedict’s Reagent

11

•Benedict’s reagent, an alkaline solution of copper (II) sulphate

as its citrate complex oxidizes aliphatic aldehydes, aldoses and

ketoses to the corresponding carboxylic acid.

•In this test, the deep-blue colour of the solution is discharged to

give a red precipitate of cuprous oxide, Cu2O.

•A carbohydrate that gives a positive test with Benedict’s

reagent is termed a reducing sugar because the reduction of

the metal accompanies oxidation of the aldehyde group.

12:44 PM

Oxidation of Monosaccharides with Benedict’s Reagent

12

• When done quantitatively, this test can be used to estimate the

level of reducing sugar (i.e. glucose) in blood or urine.

• Diabetics tend to have unusually high glucose levels in their

urine and blood and must monitor their blood sugar carefully.

• A variety of over-the-counter diagnostic test kits utilizing this

reaction are available for those suffering from diabetes

mellitus.

• Benedict’s solution is the key reagent in the test kit available

from drugstores that permits individuals to monitor the glucose

levels in their urine.

12:44 PM

Enzymatic Oxidation of Monosaccharides

13

• Enzymes, being chiral catalysts, are very specific with respect

to the substrates they react with and do heavily discriminate

against any other close variants.

• For example, the enzyme glucose oxidase isolated from the

mould Penicillium notatum is known to catalyze the oxidation

of only b-D-glucopyranose to D-glucono-d-lactone.

• This enzyme is very specific to the oxidation of the b-anomer

of glucose and does not affect the a-anomer.

• In spite of this specificity, the reaction is commonly used in the

clinical assay for total blood glucose, containing both a- and b-

D-glucopyranose.

12:44 PM

How could this be heavenly possible?

Enzymatic Oxidation of Glucose with Glucose Oxidase

14

• The oxidation of the entire glucose content is possible due to

the fact that as b-D-glucopyranose is oxidised by glucose

oxidase, more of it is generated from the a-D-glucopyranose

component through the equilibrium shown below.

12:44 PM

O

H

HO

H

HO

H

HOH

HOH

O

H

HO

H

HO

H

OHOH

HH

b-D-Glucopyranose (64%)Open chain formof D-Glucose

OH

H

HO

H

HO

H

OOH

HH

OH

H

HO

H

HO

H

HOH

H

O

H+ H+

a-D-Glucopyranose (36%)

Glucose oxidase

Glucose oxidase

No reactionO

H

HO

H

HO

H

OHH

O

Glucono-d-lactone

OHOH

OHOH

OH

Enzymatic Oxidation of Glucose with Glucose Oxidase

15

• Glucose oxidase, coupled to a peroxidase reaction that

visualizes colorimetrically the formed H2O2, is widely used as a

diagnostic tool to quantify the amount of free glucose in sera

or blood plasma.

• Glucose oxidase converts glucose to gluconic acid and

hydrogen peroxide. In the presence of peroxidase and o-

dianisidine, a yellow color is generated that can be quantified

colorimetrically by spectrophotometry. This forms the basis for

the measurement of urinary and blood glucose.

12:44 PM

Oxidation of Monosaccharides with Bromine-Water

16

•Bromine-water oxidizes the aldehyde group of an aldose to a

carboxylic acid. Bromine water does not oxidize the alcohol

groups or the ketoses.

•Bromine-water is also acidic and does not cause epimerization

or movement of the carbonyl group.

•In the acidic media, the sugar exists as cyclic hemiacetals and

reactions proceed through the cyclic hemiacetals.

12:44 PM

C

(CHOH)n

CH2OH

HO Br2

H2O

C

(CHOH)n

CH2OH

OHO

Aldose Aldonic acid(glyconic acid)

Aldehyde Acid

CHO

OHH

CH2OH

HO H

H OH

H OH

Br2

H2O

COHO

OHH

CH2OH

HO H

H OH

H OH

D-Glucose Gluconic acid

Example

Mechanism of Oxidation of Aldoses with Bromine-Water

17

• Bromine reacts with water to form a mixture of bromic acid and

hypobromous acid (a weak acid).

• The formation of hypobromous acid proceeds through an

electrophilic bromonium species.

12:44 PM

Mechanism of Oxidation of Aldoses with Bromine-Water

18

•In the acidic media, the sugar exists and reacts through the

cyclic hemiacetals.

•The bromonium ion then reacts with the cyclic hemiacetal

leading to the formation of the lactone.

12:44 PM


1912:44 PM

O

OHHO OH

HO

COOH

OHH

HHO

CH2OH

OHHb-D-Xylopyranose

open-chain form

D-Xylonic acid

Br2

H2O

O

OHHO O

HOH

CH2OH

H OH

OH HO O

D-Xylono--lactoneD-Xylono-d-lactone

or or

D-Xylono-1,5-lactone D-Xylono-1,4-lactone

ab

ab

d

•Since the product of bromine-water oxidation is an aldonic acid

and no epimerization occurs under these conditions, bromine-

water serves as a convenient reagent for the conversion of

aldoses to aldonic acids.


20

• Because bromine-water oxidizes aldoses but not ketoses, it

serves as a useful test reagent for distinguishing aldoses from

ketoses.

• This difference is conveniently observed in the colour changes

that accompany these oxidation reactions. Bromine-water is

red in colour, but the product of its reduction is colourless.

Consequently, aldoses decolourize bromine-water, while

ketoses do not decolourize bromine-water.

12:44 PM

Oxidation of Monosaccharides with Nitric Acid

21

•Nitric acid is a stronger oxidizing agent than bromine water,

oxidizing both the aldehyde group and the terminal –CH2OH

group of an aldose to carboxylic acid groups. The resulting

dicarboxylic acid is called an aldaric acid.

12:44 PM

•This oxidation proceeds through nitrate ester intermediates

formed from the reactive aldehyde group and also the terminal

CH2OH group which is easily accessible to engage in a

nucleophilic attack to an activated nitric acid molecule.

C

(CHOH)n

CH2OH

HO C

(CHOH)n

C

OHO

Aldose Aldaric acid(glycaric acid or saccharic acid)

Aldehyde Acid

O OH

O

OH

OH

OHHOHNO3

CO2H

OHHO2C

OH

HO

Oxidation of Monosaccharides with Nitric Acid

22

Examples

12:44 PM

• Note that the oxidation of altrose and talose yield the same

aldaric acid (altraric acid), and so does the oxidation of

glucose and gulose, both of which provide glucaric acid.

• Note that the aldaric acid assumes, as its derived name, the

name of the aldose that comes first alphabetically.

Reaction of Monosaccharides with Phenylhydrazine

23

• One of the best methods of derivatizing ketones and

aldehydes is conversion to hydrazones, especially

phenylhydrazones and 2,4-dinitrophenylhydrazones.

12:44 PM

•Aldoses and ketoses react with excess phenylhydrazine to

form products known as osazones, which contain two

phenylhydrazine residues at C-1 and C-2; a third molecule of

the reagent is turned into aniline and ammonia.

+H2O

R1

O

R

Aldehyde or ketone

H2NHN

Phenylhydrazine

NHN

R1

R

Phenylhydrazone

+

Reaction of Monosaccharides with Phenylhydrazine

24

• The term osazone is derived from the –ose suffix of a sugar

and the suffix of the word hydrazone.

12:44 PM

+

Aldose

H2NHN

Phenylhydrazine

N NH PhC+

CH O

OHH

H

R

OH

3C

H

N

OHH

R

NH PhNH3 H2N+

Osazone

+

Ketose

H2NHN

Phenylhydrazine

N NH PhC+

CH2OH

O

H

R

OH

3C

H

N

OHH

R

NH PhNH3 H2N+

Osazone

Phenylamine(Aniline)

•Sugars that are epimeric at C-2 yield the same osazone.

Consequently, the melting points of osazone derivatives are

valuable clues for identification and comparison of sugars.

Mechanism of Osazone Formation

2512:44 PM

Mechanism of Osazone Formation

2612:44 PM

Chain Shortening of Monosaccharides: Ruff Degradation

2712:44 PM

• The most commonly used method of shortening sugar chains

is the Ruff degradation, developed by Otto Ruff, a prominent

German chemist.

• The Ruff degradation is a two-step process that begins with

oxidation of the aldose to its aldonic acid.

• Treatment of the aldonic acid with hydrogen peroxide and

ferric sulphate oxidizes the carboxyl group to CO2 and gives an

aldose with one less carbon.

• The Ruff degradation is used both for structure determination

and synthesis of new sugars.

Structure Determination of Monosaccharides: Ruff Degradation

2812:44 PM

Structure Determination of Monosaccharides: Ruff Degradation

2912:44 PM

•L-sugars are relatively rarer in nature and can only be obtained

via synthesis from L-arabinose, the most abundant L-sugar in

nature. It possesses the correct configuration at its three chiral

centres for elaboration to the relatively rare L-erythrose and L-

glyceraldehyde.

Chain Extension of Monosaccharides: Kiliani-Fischer Synthesis

30

•In 1886, Heinrich Kiliani (at the Technische Hochshule in

Munich) showed that an aldose can be converted into two

diastereomeric cyanohydrins of the next higher carbon number

by addition of HCN.

•The resulting diastereomeric cyanohydrins can be partially

reduced to imines and then hydrolysed to diastereomeric

sugars.

•The Kiliani-Fischer synthesis therefore extends an aldose

carbon chain by adding one carbon atom at a time.

•This synthesis is useful both for determining the structure of

existing sugars and for synthesizing new sugars.

12:44 PM

Chain Extension of Monosaccharides: Kiliani-Fischer Synthesis

31

•L-Arabinose is abundant in nature and possesses the correct

configuration at its three chiral centres for elaboration to the

relatively rare L-glucose and L-mannose.

12:44 PM

HO H

CH2OH

HO H

H OH

L-(+)-Arabinose

HO H

CH2OH

HO H

H OH

CN

HO H

L-Glucononitrile

Epimeric cyanohydrins

CHO

HCN

HO H

CH2OH

HO H

H OH

CN

H OH

L-Mannononitrile

H2O

HO H

CH2OH

HO H

H OH

CHO

HO H

HO H

CH2OH

HO H

H OH

CHO

H OH

L-(-)-Glucose

L-(+)-Mannose

Epimers

HO H

CH2OH

HO H

H OH

C

HO H

HO H

CH2OH

HO H

H OH

C

H OH

H2O

H

H

NH

NH

Imines

H2

Pd/BaSO4

H2

Pd/BaSO4

Documents

Reactions of Monosaccharides...Water 16 •Bromine-water oxidizes the aldehyde group of an aldose to a carboxylic acid. Bromine water does not oxidize the alcohol groups or the ketoses