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Chemistry

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

Carbohydrates are polyhydroxy or , or substances that yield such compounds on hydrolysis

The term “carbohydrate”comes from the fact that when we heat , we get carbon and water.

Classification of • contain a single polyhydroxy or unit (saccharo is Greek for “”) (e.g., , ).

• consist of two units linked together by a covalent bond (e.g., ).

• contain from 3 to 10 monosaccharide units (e.g., ).

• contain very long chains of hundreds or thousands of monosaccharide units, which may be either in straight or branched chains (e.g., , , ).

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

srrchemistrylessons - 1 - No. of Class of carbons Monosaccharide 3 4 tetrose 5 6

• The presence of an aldehyde is indicated by the prefix and a ketone by the prefix .

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

srrchemistrylessons - 2 - 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.

srrchemistrylessons - 3 - Evidence for Open chain Structure of D-Glucose

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

6-

CHO CH OH 2 CH3 H OH H OH H H HO H HO H H2/Pt HI/P H H H OH H OH H H H OH H OH H H CH OH CH OH 2 2 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

8- glucose react with HCN to give cyanohydrin which gives heptanoic acid on hydrolysis followed by heating with HI CN CHO CHOH COOH H OH H OH CHOH HO H HO H + H OH HCN H3O H OH H OH HO H H OH H OH H OH H OH CH2OH CH2OH CH2OH D-Glucose Glucose cyanohydrin

COOH

CH2 H H HI H H  H H H H

CH3 Heptanoic acid

srrchemistrylessons - 4 - 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

7- Glucose + Acetic anhydride D-Glucose pentaacetate

CHO CHO H OH H OAc Ac O HO H 2 AcO H H OH H OAc H OH H OAc

CH2OH CH2OAc D-Glucose D-Glucose pentaacetate

The reaction shows that glucose contains five –OH groups

srrchemistrylessons - 5 -

Gluconic Acid

srrchemistrylessons - 6 -

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 () on reaction with CH3OH/HCl 3. It exhibits when dissolved in water

srrchemistrylessons - 7 -

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

srrchemistrylessons - 8 - Evidence for ring structure Howarth –Hirst methylation method:

Pyranose structure contains link between C1 –C5 carbons

srrchemistrylessons - 9 -

Structure of Fructose

srrchemistrylessons - 10 -

Reaction of Glucose with Phenyl hydrazine: Formation of Osazone:

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

1. Osazone formation from Glucose

srrchemistrylessons - 11 -

1. Osazone formation from Fructose

srrchemistrylessons - 12 -

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.

srrchemistrylessons - 13 -

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 )

srrchemistrylessons - 14 -

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

srrchemistrylessons - 15 -

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

srrchemistrylessons - 16 -