Unit - I W

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Department of Chemistry,

Sri Sarada Niketan College of Arts & Science for Women Page 1

Kanavaipudur.

UNIT I

CARBOHYDRATES - I

All optically active polyhydroxy aldehydes, all polyhydroxy ketones and all compounds which

give the above said two classes of compounds on hydrolysis are called carbohydrates. Its molecular

formula is cn(H2O)n

e.g., glucose, fructose, maltose, cellobiose, starch, cellulose etc.,

Classification

Carbohydrates are classified as follows,

Monosaccharides Disaccharides Oligo saccharides and Polysaccharides.

Monosaccharides

Monosaccharides are defined as polyhydroxyl aldehydes or ketones, which cannot be further

hydrolysed to simple sugars.

Monosaccharides are divided into two groups according to their functional groups.

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They are also classified based on the number of carbon atoms present in the monosaccharides.

Aldoses Ketoses

Aldotriose e.g., Glycerose. Ketotriose e.g., Dihydroxy acetone.

Aldotetrose e.g., Erythrose. Ketotetrose e.g., Erythrulose.

Aldopentose e.g., Ribose. Ketopentose e.g., Ribulose.

Aldohexose e.g., Glucose Ketohexose e.g., Fructose.

Aldoses are sugars containing aldehyde group.e.g., glucose, galactose, mannose.

Ketoses are sugars containing ketone group.e.g., fructose and sorbose.

Monosaccharides may contain 4 to 10 carbon atoms. Monosaccharides containing 4 carbon atoms are called tetroses. Those containing 5, 6 and 7 carbon atoms are called pentoses, hexoses and heptoses

respectively.

Polysaccharides

They are complex in nature. They have high molecular weights. Most of them are non-crystalline (amorphous). They are not sweet. They are insoluble in water. E.g., starch, cellulose.

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Oligosaccharides

Sugars which yield 2 to 9 monosaccharide molecules on hydrolysis are called oligosaccharides. If sugar yields two monosaccharide molecules on hydrolysis it is called a disaccharide. E.g.,

sucrose and maltose.

GLUCOSE (Dextrose)

Glucose is found along with fructose in sweet fruits like grapes, banana and also in honey,

beetroote also contains glucose.

Preparation

1. From cane sugar (Laboratory method)

When an alcoholic solution of cane sugar is hydrolysed with a 4% solution of hydrochloric acid in

alcohol, we get glucose and fructose.

Glucose is insoluble in alcohol. It crystallizes on cooling fructose is more soluble in alcohol.

Therefore it remains in solution.

2. From starch (Manufacture)

When starch is hydrolysed with dilute mineral acid, we get pure glucose.

The starch material is mixed with three times its weight of water then dilute sulphuric acid isadded. The mixture is heated under pressure.

When the reaction is complete the excess acid is neutralized with calcium carbonate. Calciumsulphate is precipitated. It is filtered off. The filtrate is decolourised with animal charcoal. It is

concentrated in vacuum pans. Glucose monohydrate out. It is filtered.

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Physical Properties

It is a white crystalline solid. It is sweet. It is optically active. It is dextrorotatory. Therefore it is also called dextrose.

Chemical Properties

The structural formula of glucose is

Therefore it behaves as an aldehyde, a primary alcohol and a secondary alcohol.

As an alcohol

1. Acetylation

It reacts with acetic anhydride giving a penta acetyl derivative of glucose.

2. Formation of glucosides

With methanol in the presence of dry hydrochloric acid gas it gives - and -methyl glucosides.

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3. Formation of glucosates

Glucose reacts with metallic hydroxides giving glucosates. For example, with calcium hydroxide

it gives calcium giucosate (C6H12O6CaO)

As an Aldehyde

1. Reduction

When glucose is reduced with sodium amalgam inaqueous solution, we get sorbitol.

When glucose is reduced with conc.HCl and red phosphorous at 100oC we get 2-iodohexane.

Prolonged heating conc.HCl gives n-hexane.

2. Oxidation

Glucose on treatment with bromine water (mild oxidizing agent) is oxidized to glutonic acid. The

bromine water is decolourised.

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When glucose is treated with nitric acid (strong oxidizing agent) is oxidized to saccharic acid.

It reduces Tollens reagent (ammonical silver nitrate solution) giving silver mirror. It is oxidized

to gluconic acid.

It reduces Fehlings solution giving a red precipitate. It is oxidized to gluconic acid.

3. Addition Reactions

It adds HCN giving the cyanohydrin of the glucose.

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4. Condensation Reaction

With Hydroxylamine

Glucose condenses with hydroxylamine giving the oxime of glucose.

With Phenyl Hydrazine(Osazone formation)

Glucose with excess phenyl hydrazine gives osazone.

Mechanism

1. When glucose is treated with excess of phenyl hydrazine, one molecule of phenyl hydrazinecondenses with the aldehyde group of glucose and forms the corresponding phenyl hydrazone.

2. Second molecule of phenyl hydrazine oxidises the secondary alcoholic group on the carbonatom next to the aldehyde group. The corresponding ketone is formed.

3. Third molecule of phenyl hydrazine condenses with this newly produced keto group giving thefinal product glucosazone.

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5. Fermentation

When a solution of glucose is treated with yeast we get ethanol. The enzyme zymase present in

yeast converts glucose into ethanol and carbondioxide.

6. With alkalis

When glucose is warmed with alkali, it becomes yellow and turns brown and finally gives a

resinous product.

7. Epimerization

Two sugars which differ from one another only in configuration around a single carbon atom aretermed epimers. e.g., Glucose and mannose are epimers in respect of C2. Glucose and galactose

differ only with respect to C4.

The process by which one epimer is converted to other is called as epimerization.

Muta Rotation

When a monosaccharide is dissolved in water, the specific rotation of the solution graduallychanges and reaches a constant value. This change in the value of specific rotation is known as

mutarotation.

For example, a solution of glucose has a specific rotation of +1100 when it is freshly prepared.

On standing, the specific rotation decreases slowly and reaches the constant value of +52.50

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Mechanism (Lowry)

Mutarotation takes place only in the presence of a solvent like water which behaves both as anacid and a base.

According to Lowry, in the presence of water, the ring structure of glucose opens. Then it isrecloses either in the original position or in the inverted position via an intermediate product

aldehydrol.

Uses

It is used as a sweetening agent in confectionery. It is used in the preparation of vinegar. It is used in silvering mirror. It is used to reduce Indigo blue to Indigo white. It is used as a food for infants.

Tests

It reduces Fehlings solution and gives a red precipitate. It reduces Tollens reagent and forms a silver mirror. It gives an yellow precipitate of glucosazone with excess phenyl hydrazine.

Structural Elucidation of Glucose

Open Chain structure for Glucose

1. Molecular formula of glucose is C6H12O62. On treatment with acetic anhydride it gives a penta acetyl derivative. Therefore it contains five

OH groups.

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3. It is not easily dehydrated. This shows that each hydroxyl group must be linked to differentcarbon atoms.

4. Glucose forms a cyanohydrin with HCN and an oxime with hydroxylamine. This shows thepresence of a carbonyl group (-CO) in glucose.

5. Glucose decolourises bromine water. It is oxidized to gluconic acid. Glucose and gluconic acidhave the same number of carbon atoms. Therefore the carbonyl group present is an aldehydic

group.

6. When glucose is treated with conc.hydriodic acid and red phosphorous at 100 0C we get amixture of 2-iodohexane and hexane. Thus all the six carbon atoms in glucose are in a straight

chain.

From the above evidences Bayer suggested an open chain structural formula for glucose.

There are four asymmetric carbon atoms in glucose. Hence it must have 24

= 16 optically active

forms.

Cyclic Structure for Glucose

The open chain structure of glucose does not explain the following reactions of glucose.

1. It does not add sodium bisulphite and NH3. This raises a doubt whether the aldehyde group isactually present or not.

2. When glucose is crystallised from an alcoholic solution or from acetic acid solution we getglucose with specific rotation +110

0.

3. But when glucose is crystallised from pyridine solution we get glucose with specific rotation+19.7

0. This shows that glucose exist in two stereo isomeric forms.

4. When an aqueous solution of glucose is allowed to stand, its specific rotation slowly decreasesfrom 1100 to +52.50. This is called mutarotation.

5. When glucose is treated with methyl alcohol and dry HCl gas, we get two stereo isomeric methylglucosides.

These facts lead to the new structure of glucose. That is,

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1. Furanose structure2. Pyranose structure.

Furanose Structure for Glucose (Tollens Structure)

Tollens assigned the furanose structure for glucose i.e., five membered ring structure. During the formation of the ring a new asymmetric carbon would be produced.

This can exist in two configurations. They are,

1. -D(+)-glucofuranose2. - D(+)-glucofuranose.

Pyranose Structure for glucose (Haworth Structure)

Haworth assigned the pyranose structure for glucose i.e., six membered ring structure. This isshown by the following methylation studies.

When glucose is heated with dimethyl sulphate in the presence of dilute sodium hydroxidesolution it forms a penta methyl derivative.

When the penta methyl derivative is heated with bromite solution in the presence of dilutesulphuric acid, we get 2,3,4,6-tetramethyl gluconic acid. This proves that the C1 and C5 are

involved in the ring structure in glucose. In other words the ring in glucose is a pyranose ring.

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It is clear that OCH3 (methoxy group) on C1 in penta methyl derivative of glucose has beenhydolysed and then oxidized. This reaction provides an evidence for the presence of a pyranose

ring structure in glucose.

This can exist in two configurations. They are,

1. -D(+)-glucopyranose2. - D(+)-glucopyranose.

Configuration of Glucose

Glucose on degradation gives a triose i.e., D-glyceraldehyde. This is the molecule having -OH

group on the right side. So glucose has gotOH group on the right hand side of the C5 carbon atom.

With this in mind various possible configurations for glucose may be given as follows,

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Glucose is oxidized to dicarboxylic acid and the substance is optically active. Therefore structure(1) and (6) are ruled out, because they have a plane of symmetry and hence would give only the

meso form.

Glucose on degradation to a pentose and then converted into a dicarboxylic acid gives an activecompound. Structure (2), (7) and (8) are ruled out, because they will give only meso forms.

When glucose is converted into a heptose and then converted into a dicarboxylic acid, it givesone meso and one active compound. Structure (3) gives both in active form. Hence structure (3)

is ruled out.

Glucose is degraded into a tetrose and then converted into a dicarboxylic acid, we get mesotartaric acid. This is possible only with structure (5). Hence glucose has the configuration (5).

Plane Diagram of Glucose

Perspective formula of Glucose

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Conformation of Glucose

FRUCTOSE (Laevulose)

Fructose is found along with glucose in sweet fruits like grapes, banana and also in honey. It is

also found in inulin.

Preparation

1. From Cane Sugar

When an alcoholic solution of cane sugar is hydrolysed with a 4% solution of hydrochloric acid in

alcohol, we get fructose and glucose.

When the hydrolysis is over, the solution is treated with lime. We get calcium fructosate,C6H12O6.CaO as precipitate.

Calcium glucosate remains in solution. The precipitate is removed and suspended in water. Carbondioxide is passed through the

solution, calcium carbonate is precipitated. It is removed and the solution is concentrated.

Fructose crystallizes out.

2. From Inulin (Manufacture)

When inulin, a polysaccharide is hydrolysed with dil.sulphuric acid we get fructose.

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When the hydrolysis is over, the solution is treated with barium carbonate, barium sulphate is

precipitated. It is removed and the solution is concentrated. Fructose crystallizes out.

Physical Properties

It is a white crystalline solid. It is sweeter than cane sugar. It is optically active. It is laevo-rotatory. Therefore it is also called laevulose.

Chemical Properties

The structural formula of fructose is

It contains a keto group, two primary alcoholic groups and three secondary alcoholic groups. Therefore it behaves as a ketone, a primary alcohol and a secondary alcohol.

As an Alcohol

1. Acetylation

Furanose reacts with acetic anhydride giving penta acetyl derivative of fructose.

2. Formation of Fructosides

Fructose with monohydric alcohol in the presence of dry hydrochloric acid gas, gives fructoside.

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3. Formation of Fructosates

Fructose reacts with metallic hydroxides giving fructosates. For example, with calcium hydroxide

it gives calcium fructosate, C6H12O6.CaO

As a Ketone

1. Reduction

When fructose is reduced with sodium amalgam in aqueous or alcoholic solution (OR) using

catalysts (OR) even electrolytically, we get a mixture of sorbitol and mannitol.

2. Oxidation

It resists oxidation. On strong oxidation with nitric acid, fructose gives a mixture of trihydroxy

glutaric acid, tartaric acid and glycollic acid.

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It reduces Tollens reagent and Fehlings solution. This can be explained as follows,

A keto group containing an -CHOH group undergoes tautomeric change to form an ene-diol.

This ene-diol can easily get oxidized. This is why fructose shows the above reducing properties.

3. Addition Reactions

Fructose adds HCN giving the cyanohydrin of the fructose.

4. Condensation Reactions

With Hydroxylamine

Fructose condenses with hydroxylamine giving the oxime of fructose.

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With Phenylhydrazine (Osazone formation)

Fructose with excess phenyl hydrazine gives osazone.

Mechanism

1. When fructose is treated with excess of phenyl hydrazine, one molecule of phenyl hydrazinecondenses with the keto group of fructose and forms the corresponding phenyl hydrazone.

2. Second molecule of phenyl hydrazine oxidises the adjacent primary alcoholic group into analdehyde group.

3. Third molecule of phenyl hydrazine condenses with this newly produced aldehyde group givingthe final product osazone.

5. Fermentation

When a solution of fructose is treated with yeast we get ethanol. The enzyme zymase present in

yeast converts fructose into ethanol and carbondioxide.

6. With Alkalis

When fructose is warmed with dilute alkalis no resin is formed.

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Uses

Fructose is used as sweetening agent. It is used by diabetic patients as a substitute for cane sugar.

Tests

1. Pinoffs test: A solution of fructose is treated with a solution of ammonium molybdate and afew drops of acetic acid. A blue colour is produced.

2. Selivanoffs test: Fructose is treated with a dilute solution of resorcinol in dilute hydrochloricacid. A reddish brown precipitate is formed.

3. It reduces Fehlings solution and gives a red precipitate.4. It reduces Tollens reagent and forms a silver mirror.5. It gives a yellow precipitate of osazone with excess phenyl hydrazine.6. Furfural test: To 1 ml of fructose solution 1 ml of 1% alcoholic solution of-naphthol is added

followed by 6 ml of hydrochloric acid and the mixture is boiled. A violet colour is produced.

Structural Elucidation of Fructose

Open Chain structure for Fructose

1. Molecular formula of fructose is C6H12O62. On treatment with acetic anhydride it gives a pentaacetyl derivative. Therefore it contains five

OH groups.

3. It is not easily dehydrated. This shows that each hydroxyl group must be linked to differentcarbon atoms.

4. Fructose forms a cyanohydrin with HCN and an oxime with hydroxylamine. This shows thepresence of a carbonyl group (-CO) in fructose.

5. On treatment with HNO3 fructose is oxidized to a mixture of trihydroxy glutaric acid, tartaric acidand glycollic acid. All the above acids have fewer number of carbon atoms than fructose.

Therefore the carbonyl group in fructose is a ketonic group.

6. On treatment with HCN, fructose gives cyanohydrin, which is then reduced with hydriodic acidwe get 2-methyl hexanoic acid. This shows that the ketonic group is adjacent to one of the

terminal carbon atoms.

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7. Fructose on reduction gives a mixture of sorbitol and mannitol. These on reduction withconc.hydriodic acid and red phosphorous at 100

0C we get a mixture of 2-iodohexane and

hexane. Thus all the six carbon atoms in fructose are in a straight chain.

From the above evidences Bayer suggested an open chain structural formula for fructose.

There are three asymmetric carbon atoms in fructose. Hence it must have 23

= 8 optically active

forms.

Cyclic Structure for Fructose

The open chain structure of fructose does not explain the following reactions of glucose.

1. It does not add sodium bisulphite and NH3. This raises a doubt whether the keto group isactually present or not.

2. An aqueous solution of fructose shows mutarotation. This suggests that it exists in two forms.3. When fructose is treated with methyl alcohol and dry HCl gas we get two stereo isomeric methyl

fructosides.

4. To account for the above observations the following ring structures have been assigned tofructose.

1. Furanose structure2. Pyranose structure

Furanose structure for fructose

This structure is confirmed by the following methylation studies.

When fructose is heated with dimethyl sulphate, it forms a penta acetyl derivative (I). When the penta acetyl derivative is heated with bromite solution in the presence of dil.sulphuric

acid, (II) is obtained.

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The formation of the freeOH group in C5 indicates the furanose ring.

Pyranose structure for fructose

Configuration of fructose

Both glucose and fructose give the same osazone. This reaction involves only C1 and C2. Thus theconfigurations of fructose with respect to C3, C4 and C5 are the same as in glucose.

Glucose and fructose can be easily interconverted. This also confirms that they differ only in C1and C2. Thus fructose is,

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Determination of the size of sugar Rings

When glucose is refluxed with excess of methanol in the presence of a small amount ofhydrocholoric acid we get a white crystalline solid of-methyl glucoside and -methyl glucoside.

Fischer explained the existence of these two isomers by a ring structure for them, and followedTollens in believing that they were five membered rings.

But there was no experimental evidence for the existence of a five-membered ring. Thus the above glucosides were six-membered rings i.e., glucopyranosides.This is proved by the following studies.

It include, methylate the glucoside (I), hydrolyse the product, methyl tetramethyl--D-glucoside(II) to tetramethyl--D-glucose (III), oxidise this with bromine water at 90 0C to the

lactone (IV), and then to oxidise this with nitric acid. The product obtained was shown to be

xylotrimethoxyglutaric acid (V).

The most reasonable interpretation of these results is in accordance with the assumption thatmethyl glucoside is a six-membered ring and not a five-membered ring.

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