Introduction:

Carbohydrates are a big class of naturally occurring polyhydroxy aldehydes and ketones or compounds which yield polyhydroxyl aldehydes or ketones on hydrolysis. They have just carbon, hydrogen and oxygen atoms. They encompass the general formula C_xH_2yO_z.

Simple carbohydrates are mostly synthesized via chlorophyll having plants via photosynthesis.

Classification of Carbohydrates:

Based on the size of the molecules, carbohydrates might be categorized into three groups, namely monosaccharides, disaccharides and Polysaccharides.

Monosaccharides are the simplest carbohydrates and they are termed as simple sugars. They encompass the general formula C₆H₁₂O₆. They can't be hydrolyzed to simpler compounds. They are the fundamental unit of carbohydrates. They encompass three to seven carbon atoms, and each consists of one aldehyde or one ketone functional group. The aldehyde group is for all time at the end of the carbon chain; the ketone group is always on the second carbon of the chain. In either case, there is a -CH₂OH group at the other end of chain.

There are hydroxyl groups on the entire carbon atom and the -CH₂OH at the other end, and on the end carbon subsequent to the ketone group.

Fig: Carbohydrates Classification

Disaccharides comprise of two monosaccharide units joined altogether via a glycosidic bond. Their general formula is C₁₂H₂₂O₁₁. Example: sucrose, maltose and lactose. Whenever one molecule of disaccharides is taken via acid hydrolysis, it results in two monosaccharide molecules.

Polysaccharides are the condensation polymers of monosaccharides; they have thousands of monosaccharide units in their structures. They encompass the general formula (C₆H₁₀O₅)n, here 'n' is a very large number. On the acid hydrolysis polysaccharides result a large number of monosaccharide molecules, example: cellulose, starch, insulin and glycogen. Polysaccharides are further sub-categorized into Homopolysaccharides and Heteropolysaccharides. The Homopolysaccharides on hydrolysis produce the similar molecule as Heteropolysaccharides produce various monosaccharides.  Starch and cellulose are Homopolysaccharides as insulin is a Heteropolysaccharides.

Carbohydrates which reduces Fehling's (or Benedict's) or Tollens' reagent are termed as reducing sugars. All the monosaccharides whether aldose or ketose are the reducing sugars. Most of the disaccharides are reducing sugars. Sucrose though is a non-reducing sugar.

Fig: Classes of Carbohydrates with examples

Monosaccharides:

Monosaccharides might be an aldose or a ketose. The aldose is a monosaccharide having an aldehyde carbonyl group whereas a ketose includes a ketone structure.

Naming Monosaccharides:

The family name ending '-ose' points out a carbohydrate, and simple sugars are termed by common names such as glucose, ribose, and fructose instead of systematic names. The number of carbon atoms in an aldose or ketose can be specified via one of the prefixes bi-, tri-, tetr-, pent-, hex-, hept-. Therefore, glucose is an aldohexose (aldo- = aldehyde; hex- = six carbons; -ose = sugar); fructose is a ketohexose (that is, a six-carbon ketone sugar); and ribose is an aldopentose (that is, a five-carbon aldehyde sugar).  Most of the naturally occurring simple sugars are aldehydes having either five or six carbons.

Fig: Naming Monosaccharides

i) Biose (C₂H₄O₂): Glycolaldehyde (CH₂OHCHO) is the simplest hydroxyl aldehyde. This is regarded as an aldobiose. It is not optically active as it doesn't have an asymmetric carbon.

ii) Trioses (C₃H₆O₃): Glyceraldehyde is the mere aldotriose known. It possesses an asymmetric carbon atom and exists in the two optically active forms.

Fig: Trioses

iii) Tetroses (C₄H₈O₂): The aldotetroroses has the formula CH₂OHCHOHCHOHCHO; it consists of two asymmetrical carbon atoms. It is thus capable of four optically active forms D- and L- erythrose and D- and L- threose.

A ketose consists of just one asymmetric carbon atom and therefore exists in only two optically active forms D- and L-erythrulose.

Fig: Tetroses

iv) Pentoses (C₅H₁₀O₅): Aldopentoses consists of 3 asymmetric carbon atoms and thus exists in 8 optically active forms D- and L- forms of ribose, arabinose, xylose and lyxose, D-ribose, L- arabinose, D-xylose and -lyxose take place in nature whereas the others are synthetic.

Fig: Pentoses

v) Hexoses: Aldo hexoses encompass four asymmetric carbon atoms. They encompass the formula CH₂OHCHOHCHOHCHOHCHOHCHO; therefore they exist in sixteen optically active forms- D- and L- forms of glucose, mannose, allose, altrose, gulose, idose, galactose, talose.

Ketohexoses encompass 3 asymmetric carbon atoms and they encompass the formula CH₂OHCOCHOHCHOHCHOHCH₂OH. They exist in eight optically active forms of which just six- D- and L-fructose, D- and L-sorbose, D-tagalose and L- psicose- takes place in nature.

As well there are sugars having heptoses (7-carbon atoms), octoses (8 carbon atoms) and so forth.

Configuration of Monosaccharides:

Glyceraldehyde includes a central Chiral carbon atom. For each of its optical isomers the (+) and (-) signs pointing out dextrorotatory and laevorotatory correspondingly, specifying the direction in which each one rotates plane polarized light, give no indication of the way in which the groups are linked to the Chiral carbon. The absolute configuration of the two is thus illustrated through placing a D or L in front of the name.

Fig: Configuration of Monosaccharides

As all the monosaccharides are in effect higher homologue of one of the enantiomers of glyceraldehydes, they are termed as D-compound if they are associated to D-glyceraldehydes and L- compounds if associated to L-glyceraldehydes. The comparism to glyceraldehydes is done via considering the lowest Chiral carbon atom.

Occurrence of Monosaccharides:

Most of the monosaccharides exist in nature and most of the others have been synthetically prepared.

General physical characteristics:

a) They are colourless compounds.

b) They are soluble in water however insoluble in ether and other organic compounds.

c) They encompass sweet taste.

d) They char on heating providing a characteristic smell.

e) They are optically active.

Two significant monosaccharides are glucose and fructose. They are hexoses and generally found in fruit juice and honey and are the main constituents of most of the disaccharides and polysaccharides.

Fig: Glucose and Fructose

Glucose:

Glucose is the most significant and the richest monosaccharide. It is the unit of which starch, cellulose and glycogen are made up and it consists of special role in the biological procedures. It can exist in the acyclic and cyclic forms. This is an aldohexose as it contains an aldehyde in its open chain structure.

Fig: Glucose-monosaccharide

Such monosaccharides form rings via the addition of an -OH group of one of their Chiral carbons to the carbonyl group of aldose. 

Six-membered rings are chemically more stable than the straight chains of the monosaccharides. The direction from which the -OH adds on to the carbonyl group finds out which one of the two possible cyclic structures forms (α- and β-glucose).

Physical Properties of Glucose:

a) Glucose posses a sweet taste however this is not quite so instantly distinctive in that of fructose and sucrose.

b) Glucose is extremely soluble in water.

c) It is not soluble in alcohol.

d) Whenever crystallized from warm solution at 98^oC, it results anhydrous crystals having melting point 146^oC, however from cold solution, it makes a monohydrate having melting point 86^oC.

Reactions of Glucose:

Glucose goes through numerous reactions typical of carbonyl compounds. As a reducing sugar it reacts by Fehling solution and Tollen's reagent and the formation of osazones gives a helpful means of characterization and identification.

Usually the reactions of sugars give a considerable amount of information regarding the structure of the molecules and this clearly illustrated by glucose in its capability to form α- and β- cyclic glucosides.

a) Oxidation:

Glucose, similar to aldehydes and ketones on warming readily reduces Fehling solution, ammoniacal silver (i) nitrate and Tollen's reagent. It can as well be oxidized through bromine water to Gluconic acid. Though a more powerful oxidizing agent such as trioxonitrate (v) acid will oxidize it to the dicarboxylic acid, glycaric acid.

Fig: Glucose-Oxidation reaction

b) Reduction:

Glucose react by hydrogen and nickel catalyst or sodium tetrahydridoborate (III) or sodium amalgam and water to results hexahydric alcohol, sorbitol.

Fig: Glucose-reduction reaction

Glucose can as well be reduced via hydrogen iodide and red phosphorus at 100^oC to give hexane. This reaction proves the existence of a straight chain structure for glucose.

c) Acylation:

Glucose reacts by surplus ethanoic anhydride or ethanoyl chloride in the presence of an anhydrous zinc chloride catalyst to result pentaethanoyl derivative. This reaction points out the presence of five hydroxyl groups in the glucose.

Fig: Glucose-Acylation reaction

d) Addition Reaction of the carbonyl group:

i) Reaction with HCN:

Glucose reacts by HCN to provide hydroxynitrile (cyanohydrins). However do not react by sodium hydrogen sulphite. Whenever the hydroxylnitrile formed is taken via acid hydrolysis followed through reduction with hydriodic acid, heptanoic acid is made. This reaction as well points out the existence of a straight chain structure.

Fig: Glucose reaction with HCN

ii) Condensation reactions of the carbonyl group:

Hydroxylamine and hydrazine condenses by glucose to make an oxime and a hydrazone correspondingly.

Fig: Condensation reactions of the carbonyl group

e) Glucoside Formation:

Glucose reacts by methanol to make a crystalline solid that includes only one methyl group. This compound formed doesn't have aldehyde group as it doesn't reduce Fehling solution or form oxazone with the phenylhydrazine and it exist in two anomeric forms.

This behavior of glucose can merely be accounted for in terms of the α- and β- cyclic structures being in the dynamic equilibrium.

Fig: Glucoside Formation

f) Fermentation:

Glucose, throughout fermentation react by the enzyme zymase from yeast at 15^oc to make ethanol and carbon (iv) oxide.

C₆H₁₂O₆ + (zymase at 15^oC) → 2CH₃CH₂OH + 2CO₂

g) Dehydration:

Glucose, whenever strongly heated chars and leave a black carbon residue. Alternatively if treated by concentrated tetraoxosulphate (vi) acid, it sugar charcoal.

C₆H₁₂O₆ + (conc.H₂SO₄/- H₂O) → 6C

Glucose                              Sugar charcoal

Fructose:

Fructose occurs broadly in fruits and combined with glucose, in the disaccharide, sucrose. It is often termed as fruit sugar.

Physical Properties of Fructose:

a) This is more soluble in water than glucose.

b) It is the sweetest among all sugars, around two times as sweet as glucose.

c) It is reasonably soluble in alcohol dissimilar glucose. This property can be used in separating it from glucose.

d) It is a colourless crystal having the melting point of 95^oC

Structures of Fructose:

Fructose can exist by an acyclic and also with cyclic structures of six-membered ring and five membered rings. This is a ketohexose as it includes a keto group in its open chain structure.

Fig: Structures of Fructose

In the crystalline form, just one form of fructose is known- the six membered, cyclic, β- isomer.  

In solution, the β-isomer exists in equilibrium by the cyclic α-isomer, with the open-chain structure and as well having a five membered ring structure. The α- and β- isomer varies in configuration of the C-2 atom, that is, at the carbonyl atom of the acyclic structure. This phenomenon is termed as epimerism and the isomers are termed as epimers.

The six membered ring structures are termed as a fructopyranose whereas the five membered rings are termed as a fructofuranose. Usually, sugars that have a six-membered ring structure comprising the oxygen atom termed to as pyranose compounds and those having a five-membered ring structure are termed as furanose compounds.

Reactions of Fructose:

Fructose goes through reactions which are similar to those of glucose.

a) Oxidation: 

Fructose similar to glucose reduces the Fehling's solution, ammoniacal silver (I) nitrate and Tollen's reagent. It is not however not oxidized via bromine water pointing out the absence of an aldehyde group.

b) Reduction:

Fructose such as glucose is as well reduced to sorbitol.

c) Acylation:

Fructose undergoes acylation as illustrated in glucose to result pentaethanoyl derivative, pointing out the presence of five hydroxyl groups in the molecule.

d) Addition and Condensation Reactions of the carbonyl group:

Similar to glucose, fructose forms hydroxylnitrile (that is, cyanohydrin) having hydrogen cyanide and condenses by hydroxylamine and hydrazine. Whenever Fructose is warmed with the excess phenylhydrazine, it results an osazone, fructose phenyl osazone that is identical to the glucose derivative.

e) Fermentation and Dehydration:

Fermentation and dehydration of fructose is identical to that of glucose.

Disaccharides:

Disaccharides are carbohydrates which are made up of two monosaccharide units. They encompass the molecular formula C₁₂H₂₂O₁₁. On hydrolysis a molecule of disaccharide results in two molecules of monosaccharide. Common illustrations are sucrose (like cane or beet sugar), maltose (like malt sugar) and lactose (like milk sugar).

Sucrose is prepared of a glucose molecule and a fructose molecule, a maltose comprise of two glucose molecules whereas lactose is prepared of one glucose molecule and one galactose molecule.

Natural Sources of Disaccharides:

Sucrose occurs in as much as around 15% in cane and beet sugar that is the principal source. Apart from cane and beet sugar, it takes place naturally in fruit and plants. Maltose is made up as an intermediate in the fermentation of starch whereas lactose is present in the milk of mammal in around 5%.

Structure and constitution of Disaccharides:

The bond made between two monosaccharides is termed as a glycosidic linkage. This is formed from a condensation reaction among the two hydroxyl groups.

Disaccharides are glycosides in which the alcohol substituent is replace by the other monosaccharide that is attached through one of its hydroxyl groups. Sucrose is prepared by glucose molecule and a fructose molecule by the elimination of a water molecule-a pyranose and a furanose ring. The maltose molecule is built up from the two glucose molecules by the removal of a water molecule-two hexose (pyranose).

Maltose and lactose molecules comprise of two hexose (or pyranose) units whereas sucrose consists of a hexose-pyranose, that is, pyranose-furanose structure.

Fig: Structure and constitution of Disaccharides

Maltose and lactose exist in the α- and β- forms and they go through mutarotation in solution, whereas sucrose doesn't exist in anomeric forms. Maltose and lactose are able of reducing the Fehling solution and forming osazones pointing out that the structures of the molecules have a potentially 'free' carbonyl group and a hemiacetal ring structure whereas sucrose is a non-reducing sugar, pointing out the absence of a 'free' carbonyl group.

Physical properties of Disaccharides:

a) Sucrose, maltose and lactose are readily soluble in the water.

b) They are virtually insoluble in alcohol.

c) Sucrose, maltose and lactose are dextrorotatory and are employed as food-stuffs.

d) If sucrose is heated above 160^oC (its melting point), and cooled, it re-solidify to make a solid mass termed as 'barley sugar'. On increasing the temperature to around 200^oC, it lost some water molecule to make 'caramel'- a softer, brownish substance.

Reactions of Disaccharides:

Considering the common illustrations, most of their reactions are useful means of differentiating between them. 

a) Hydrolysis:

All the disaccharides are hydrolyzed through dilute mineral acid to the constituent monosaccharides. Maltose only provides glucose; lactose provides glucose and galactose whereas sucrose provides glucose and fructose.

C₁₂H₂₂O₁₁ + H₂O + (dil. acid or maltase) → 2C₆H₁₂O₆

Maltose                                                       Glucose

C₁₂H₂₂O₁₁ + H₂O + (dil. acid or lactase) → C₆H₁₂O₆ + C₆H₁₂O₆

Lactose                                                     Glucose    Galactose

C₁₂H₂₂O₁₁ + H₂O + (dil. acid or invertase) → C₆H₁₂O₆ + C₆H₁₂O₆

Sucrose                                                       Glucose     Fructose 

Sucrose that is dextrorotary on hydrolysis gives an equivalent proportion of (+) -glucose and (-) - fructose having specific rotations + 52.7^o and -92.4^o correspondingly. As the laevorotatory power of fructose is more than the dextrorotatory power of glucose, the overall mixture formed is thus laevorotatory. Therefore the rotation of plane polarized light changes from positive to negative whenever sucrose is hydrolyzed. This is termed as the inversion of (+) -sucrose.

(+) -Sucrose → (H+ or invertase) → (+)-glucose + (-)-fructose

[α]_D = +66.5^o                                  [α]_D  = +52.7^o    [α]_D = -92.4^o

                                                            [α]_D = -19.9^o

b) Oxidation:

Maltose and lactose are influenced through Fehling's solution; ammoniacal silver (I) nitrate and bromine water whereas sucrose is not. This is as sucrose consists of no 'free' carbonyl group. Bromine water oxidizes maltose and lactose to maltobiotic acid and lactobiotic acid correspondingly.

Though, whenever sucrose is warmed by more powerful oxidising agent such as dilute trioxonitrate (V) acid, it is oxidized to ethanedioic acid (that is, oxalic acid).

c) Acylation:

Sucrose, maltose and lactose react by ethanoic anhtdride on heating to give an octaethanoyl derivative. This reaction proves the presence of eight hydroxyl groups.

d) Condensation:

Maltose and lactose reacts by phenylhydrazine to form phenylhydrazone. Sucrose doesn't go through this reaction because it lacks reducing power and consists of no carbonyl group.

e) Dehydration:

All disaccharides results sugar charcoal whenever heated above their melting points or whenever they are warmed by concentrated tetraoxosulphate (VI) acid.

C₁₂H₂₂O₁₁ + (conc.H₂SO₄/-11H₂O) → 12C

Polysaccharides:

Polysaccharides are high relative molecular mass condensation polymers of monosaccharides. They encompass the general formula (C₆H₁₀O₅)n, here 'n' is a large number. They include a large number of monosaccharide units linked altogether in the similar manner as disaccharides by means of the common linking oxygen atom.

Usually polysaccharides are tasteless and amorphous compounds. They are usually insoluble in the water and organic solvents. Most of them form colloidal solution. Whenever they are hydrolyzed by dilute acids or enzymes they result in monosaccharides.

Common illustrations of polysaccharides are starch and cellulose. Insulin and glycogen are as renowned polysaccharides.

Starch:

Starch is a polyglucose found out in potatoes, rice, wheat, maize, barley and usually in green plants. In nature it is converted into complex polysaccharides such as gums and cellulose. This can as well be transformed into simpler mono and disaccharides via enzymic actions.

The correct chemical nature of starch differs from source to source. Starch acquired from the similar source has been found to comprise of two fractions: amylase and amylopectin.

Amylose is a linear polymer having α-D-glucopyranose units joined via 1, 4-α-glycosidic linkages whereas amylopectin is a highly branched polymer. The branches of amylopectin comprise of 20 to 25 glucose units joined via 1,4-α- linkages and joined to one other via 1,6-α- glycosidic linkages.

Fig: Starch-amylase and amylopectin

A) General Properties of starch:

a) Physical Properties

• Starch is a white amorphous substance.
• It consists of no taste or odor.
• It is insoluble in water.
• Whenever added to boiling water, the starch granules swells and burst making colloidal translucent suspension.

b) Chemical Properties:

• Action of heat: Whenever starch is heated to a temperature between 200 and 250^oc, it changes to dextrin- a gum like substance. At higher temperature, charring occur.
• Hydrolysis: Whenever starch is boiled with dilute acid, all the glycosidic linkages are broken and it ultimately results glucose.
• Action of iodine solution: If a drop of iodine solution is added to starch solution, it provides a blue colour. The blue colour disappears on heating and reappears on cooling. This reaction serves as the test for starch.
• Action of a mixture of concentrated tetraoxosulphate (VI) acid and trioxonitrate (V) acid: If starch is heated by a mixture of concentrated tetraoxosulphate (VI) acid and trioxonitrate (V) acid, it provides nitro starch.

=> Uses of starch:

• Use as food. It makes the significant part of daily diet such as bread, rice, potatoes and so on.
• This is used in the manufacture of starch jelly and adhesives.
• This is a valuable source of glucose and alcohol.
• Used in laundries.
• Used in the textile and paper industries.
• Used in the formation of starch acetate, nitro-starch and soluble starch.
• Used as an indicator for the iodometric titration.
• Dextrins obtained from the starch are employed in sizing paper and in adhesives.

Cellulose:

This is the principal structural component of the cell walls of plant. Similar to starch, it is a polyglucose having 1000-1500 glucose units. It is obtained from the cotton plant, wood, straw and so on. Cellulose plays a very important role in the polymer industry.

Fig: Cellulose

Similar to starch, the structure of cellulose differs by the source.

Cellulose is a linear polymer containing β-D-glucopyranose units joined through 1,4-β-linkages.

On complete hydrolysis it results in glucose however unlike starch it is present in cellulose as β-D- glucopyranose unit. Enzymatic hydrolysis results in a disaccharide cellobiose having the glucose linked via 1,4-β-glycosidic linkages.

A) Properties of Cellulose:

• Cellulose is a colourless amorphous substance having organized fibrous structure.
• This is insoluble in water however dissolves in ammoniacal solution of copper (II) hydroxide. It as well dissolves in a solution of zinc chloride in the hydrochloric acid.
• Cellulose makes a starch like substance amyloid. It is treated by concentrated tetraoxosulphate (VI) acid in cold to form a solution. The solution made then dilute by water to form amyloid.
• Whenever boiled with dilute teraoxosulphate (VI) acid, it is fully hydrolyzed to glucose.
• If treated with 20% caustic acids, it becomes smooth and lustrous in look.
• If treated by a mixture of concentrated tetraoxosulphate (VI) acid and trioxonitrate acid, it forms mono-, di-, and tri-nitrates of cellulose. Cellulose of nitrates is utilized in the preparation of explosives.
• Whenever treated with a mixture of acetic acid and acetic anhydride, cellulose provides a mixture of di- and tri-acetates. Cellulose acetates are employed in the manufacture of synthetic fibres and paints.

B) Uses of Cellulose:

• Utililized in the manufacture of paper and cloth.
• Cellulose nitrates are employed in the manufacture of explosives.
• Cellulose nitrates with camphor give celluloid that is employed in the manufacture of toys.
• Cellulose acetate is employed in the production of rayon and plastics.

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