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Carbohydrate Chemistry

The term ‘carbohydrate’ is derived from the Cn(H2O)n general chemical formula

Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis The term “carbohydrate”comes from the fact that when we heat sugars, we get carbon and water.

Classification of Carbohydrates • Monosaccharides contain a single polyhydroxy aldehyde or ketone unit (saccharo is Greek for “sugar”) (e.g., glucose, fructose). • Disaccharides consist of two monosaccharide units linked together by a covalent bond (e.g., sucrose). • Oligosaccharides contain from 3 to 10 monosaccharide units (e.g., raffinose). • Polysaccharides contain very long chains of hundreds or thousands of monosaccharide units, which may be either in straight or branched chains (e.g., cellulose, glycogen, starch).

• Monosaccharides are classified according to the number of carbon atoms they contain:

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No. of Class of carbons Monosaccharide 3 triose 4 tetrose 5 pentose 6 hexose • The presence of an aldehyde is indicated by the prefix aldose and a ketone by the prefix ketose.

Fischer Projections • Fischer projections are a convenient way to represent mirror images in two

dimensions.

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For carbohydrates, the chiral carbon furthest from the carbonyl is used to determine the D or L nomenclature.15 The structure below is a D-aldohexose (recall that “hex-” means “six”), because it has the D configuration at the fifth carbon.

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Evidence for Open chain Structure of D-Glucose

1. Glucose forms n-hexane by reduction with HI (evidence of 6 straight chain C-link)

66--

CHO

OHH

HHO

OHH

OHH

CH2OH

H2/Pt

CH2OH

OHH

HHO

OHH

OHH

CH2OH

HI/P

CH3

HH

HH

HH

HH

CH3

D-Glucose D-Sorbitol n- Hexane

The above reaction showed that glucose is not branched

2. Glucose forms n-heptanoic acid by reduction with HCN, reduction by HI/P followed by hydrolysis

88-- glucose react with HCN to give glucose react with HCN to give cyanohydrincyanohydrin which which

gives gives heptanoicheptanoic acid on hydrolysis followed by acid on hydrolysis followed by

heating with HI heating with HI

CHO

OHH

HHO

OHH

OHH

CH2OH

CHOH

OHH

HHO

OHH

OHH

CH2OH

D-Glucose Glucose cyanohydrin

HCN

CN

H3O+

CHOH

OHH

HHO

OHH

OHH

CH2OH

COOH

HI

CH2

HH

HH

HH

HH

CH3

COOH

Heptanoic acid

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3. Evidence for -CHO group in Glucose Glucose reacts with HCN, NH2OH, Tollen’s reagent, Fehling’s reagents

4. Evidence for Five -HO groups in Glucose

77--

CHO

OHH

HHO

OHH

OHH

CH2OH

D-Glucose D-Glucose pentaacetate

CHO

OAcH

HAcO

OAcH

OAcH

CH2OAc

Glucose + Acetic anhydride D-Glucose pentaacetate

Ac2O

The reaction shows that glucose contains five –OH groups

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Gluconic Acid

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Ring structure for Glucose Open chain structure of Glucose can’t explain the following reactions :

1. Glucose does not react with Sod. Bisulphate NaHSO3

2. It gives two isomeric compounds Methyl- α-D- Glucoside and Methyl- β-

D-Glucoside (Anomers) on reaction with CH3OH/HCl

3. It exhibits mutarotation when dissolved in water

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Mutarotation : Change in the specific rotation [α]D of aqueous solution of either α-

D-glucopyranose ( + 1120) or β-D-glucopyranose ( + 190) to a equilibrium

value of +52.50 is called mutaotation. It is favoured by amphiprotic solvent like

water. At equilibrium the mixture contains 36% of -D-glucopyranose and 64%

of β-D-glucopyranose.

Methyl- α-D- Glucoside and Methyl- β-D-Glucoside (Anomers) formation

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Evidence for Pyranose ring structure Howarth –Hirst methylation method:

Pyranose structure contains link between C1 –C5 carbons

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Structure of Fructose

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Reaction of Glucose with Phenyl hydrazine: Formation of Osazone:

Glucose, Fructose and Mannose all give same Osazone on reaction with Phenyhadrazine

1. Osazone formation from Glucose

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1. Osazone formation from Fructose

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Lobry De Bruyn Van Ekenstein Rearrangement:

Anomers.

The pair of diastereomers that differ in configuration only at C1 are called Anomers. The difference at C1 configuration is due to cyclisation of hemiacetals.

For example, anomers of glucose are alpha-glucose and beta-glucose.

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Epimers.

The pair of diastereomers that differ in configuration at only a single stereo centre (not the anomeric carbon ) are called Epimeres.

Example : Dlucose and Mannose ( they differ at C2 )

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Conversion of to Glucose ( Aldohexose) to Arabinose ( Aldopentose ) Ascending order : Ruff’s degradation

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Conversion of Arabinose ( Aldopentose ) to Glucose ( Aldohexose) Ascending order : Kiliani-Fischer Synthesis