Chapter 24 Biomolecules Part 1- Chemistry free study material by TEACHING CARE online tuition and coaching classes

Chapter 24 Biomolecules Part 1- Chemistry free study material by TEACHING CARE online tuition and coaching classes

File name : Chapter-24-Biomolecules-Part-1.pdf

 

 

 

 

All living bodies are composed of several lifeless substances which are present in their cells in a very complex but highly organised form. These are called biomolecules. Some common examples are carbohydrates, proteins, enzymes, nucleic acids, lipids, amino acids, fats etc :

Living organisms ® Organs ® Tissues ® Cells ® Organelles ® Biomolecules.

The carbohydrates are naturally occurring organic substances. They are present in both plants and animals. The dry mass of plants is composed of 50 to 80% of the polymeric carbohydrate cellulose. Carbohydrates are formed in the plants by photosynthesis from carbon dioxide and water.

nCO2 + nH2O ¾¾Lig¾ht ®(CH2O)n  + nO2

Chlorophyll

Animals do not synthesise carbohydrates but depends on plants for their supply.

  • Defination : “Carbohydrates are defined as a class of compounds that include polyhydric aldehydes or polyhydric ketones and large polymeric compounds that can be broken down (hydrolysed) into polyhydric aldehydes or ”

 

Carbohydrates contain hemiacetal.

  • C = O

 

H

|

and

  • OH

groups. A carbonyl compound reacts with an alcohol to form

 

R C = O + HOR¢ ¾¾® R C OH

 

|

H

Aldehyde

Alcohol

|

OR¢

Hemiacetal

 

In carbohydrates, the carbonyl group combine with an alcoholic group of the same molecules to form an internal hemiacetal thus the correct defination of carbohydrates is as follows

“A polyhydroxy compound that has an aldehydic or a ketonic functional group either free or as hemiacetal or acetal.”

H                                                                             H         OH

 

 

 

 

O

CHOH

|

 

 

 

|

CH2OH

Glucose (C6H12O6)

|

CH2OH

An internal hemiacetal

 

In general, carbohydrates are white solids, sparingly soluble in organic solvents and (except certain polysaccharides) are soluble in water. Many carbohydrates of low molecular masses have a sweet taste.

  • Nomenclature : The name of simpler carbohydrates end is –ose. Carbohydrate with an aldehydic structure are known as aldoses and those with ketonic structure as ketoses. The number of carbon atom in the molecule is indicated by Greek

 

 

 

 

Number of carbon atoms in the

molecule

AldoseKetose
3AldotrioseKetotriose
4AldotetroseKetotetrose
5AldopentoseKetopentose
6AldohexoseKetohexose
7AldoheptoseKetoheptose
  • Classification : The complete classfication of carbohydrates may be depicted in short in the following

 

 

These are the simplest one unit non-hydrolysable sugars. They have the general formula

Cn H2nOn

where n

 

varies from 3 to 9 carbon atoms. About 20 monosaccharides occur in nature. The simplest are trioses (n=3)

 

 

 

C3 H6 O3   ;

Triose

H C = O

|

H C OH ;

|

CH2OH

Glyceraldehyde

CH2 OH

|

C = O

|

CH2 OH

Dihydroxyacetone

 

The most important naturally occurring monosaccharides are pentoses and hexoses. A common pentose is ribose and two common hexoses are glucose and fructose.

Except ketotriose {dihydroxyacetone}, all aldose and ketoses {monosaccharides} contain asymmetric carbon atoms and are optically active. Number of isomers depand upon the number of asymmetric carbon atom in the molecules of monosaccharide and is derived by the formula 2n where n is the number of asymmetric carbon atoms in the molecules.

 

 

 

 

 

ClassMolecular formulaStructural formulaExamples
  Aldoses 
AldotriosesC3H6O3CH2OHCHOHCHOGlyceraldehyde
AldotetrosesC4H8O4CH2OH(CHOH)2CHOErythrose, Threose
AldopentosesC5H10O5CH2OH(CHOH)3CHOArabinose, Ribose, Xylose, Lyxose
AldohexosesC6H12O6CH2OH(CHOH)4CHOGlucose, Galactose, Mannose,
   Allose, Talose, Gulose, Idose, etc.
  Ketoses 
KetotriosesC3H6O3CH2OHCO.CH2OHDihydroxyacetone
KetotetrosesC4H8O4CH2OH.CO.CHOH.CH2OHErythrulose
KetopentosesC5H10O5CH2OH.CO.(CHOH)2CH2O HRibulose, Xylulose
KetohexosesC6H12O6CH2OH.CO(CHOH)3CH2O HFructose, Sorbose, Tangatose, Psicose
  • D and L-designation : By convention, a molecule is assigned D-configuration if the –OH group attached to the carbon adjacent to the –CH2OH group (last chiral carbon) is on the right hand side irrespective of the position of other groups. On the other hand, the molecule is assigned L-configuration if the –OH group attached to the carbon adjacent to the –CH2OH group is on the

However, it may be noted that D- and L- do not represent dextrorotatory or laevorotatory. The optical activity of the molecule is represented by (+) and (–) which represent the direction of rotation of plane polarized light whether dextrorotatory or laevorotatory.

  • Configuration : Configuration of Monosaccharides

 

  • Aldotriose : CHOHCH2OH

CHO

|

H – C – OH

|

CH2OH

D-Glyceraldehyde

isomers (2)1 = 2

CHO

|

OH – C – H

|

CH2OH

L-Glyceraldehyde

 

 

 

 

 

  • Aldotetrose :

CHO.CHOH.CHOH.CH2OH

isomers (2)2=4

 

CHO

|

H – C – OH

|

H – C – OH

|

CH2OH

D-Erythrose

CHO

|

HO – C – H

|

H – C – OH

|

CH2OH

D-Threose

 

 

 

L-Erythrose      L-Threose

 

So it has four isomers, i.e., D, L-Erythrose and D, L-Threose.

  • Aldopentose : CHOH.CHOH.CHOH.CH2OH, isomers (2)3 = 8

D-Erythrose                                                              D-Threose

 

 

CHO

|

H – C – OH

|

||||
H – C – OHH – C – OHH – C – OHH – C – OH
||||
CH2OHCH2OHCH2OHCH2OH
D-RiboseD-ArabinoseD-XyloseD-Lyxose

 

H – C – OH

CHO

|

OH – C – H

|

H – C – OH

CHO

|

H – C – OH

|

HO – C – H

CHO

|

HO – C – H

|

HO – C – H

 

 

 

 

 

 

So aldopentoses has eight isomers, i.e., D– and L-Ribose, D– and L-Arabinose, D– and L-Xylose and D, L

Lyxose

  • Aldohexose : (CHOH)4CH2OH, isomers (2)4 = 16

 

D-Ribose

D-Arabinose

D-Xylose

D-Lyxose

 

 

CHO

|

CHO

|

CHO

|

CHO

|

CHO

|

CHO

|

CHO

|

CHO

|

H – C – OHHO – C – HH – C – OHHO – C – HH – C – OHHO – C – HH – C – OHHO – C – H
||||||||
H – C – OHH – C – OHHO – C – HHO – C – HH – C – OHH – C – OHHO – C – HHO – C – H
||||||||
H – C – OHH – C – OHH – C – OHH – C – OHHO – C – HHO – C – HHO – C – HHO – C – H
||||||||
H – C – OHH – C – OHH – C – OHH – C – OHH – C – OHH – C – OHH – C – OHH – C – OH
||||||||
CH2OHCH2OHCH2OHCH2OHCH2OHCH2OHCH2OHCH2OH

 

D-Allose

D-Altrose

D-Glucose

D-Mannose

D-Gulose

D-Idose

D-Glactose

D-Talose

 

 

  • Glucose; C6H12O6; Aldo-hexose : Glucose is known as dextrose because it occurs in nature as the optically active dextrorotatory isomer. It is also called grape sugar as it is found in most sweet fruits especially It is present in honey also. It is essential constituent of human blood. The blood normally contains 65 to 110 mg of glucose per 100 mL (hence named Blood sugar). In combined form, it occurs in cane sugar and polysaccharides such as starch and cellulose. It is also present in various glycosides like amygdalin and salicin.
  • Preparation
  • Laboratory method

 

 

 

 

C12

H22

O11

  • H2

O ¾¾H¾+  ® C

H12O6

  • C6

H12O6

 

Cane sugar (Sucrose)

Glucose

Fructose

 

6

Note : ®            HCl (dil.) is used for hydrolysis. Glucose being much less soluble in alcohol than fructose separates out by crystallising on cooling.

  • Manufacture : It is obtained on a large scale by the hydrolysis of starch (corn starch or potato starch) with dilute sulphuric acid or hydrochloric

 

(C6

H10

O5 )n

  • nH

O ¾¾H¾+  ® nC

H12O6

 

2
6

Starch                                               Glucose

A thin paste of starch is boiled with dilute acid till the hydrolysis is complete. The excess of acid is neutralised with chalk (calcium carbonate) and the filtrate containing glucose is decolourised with animal charcoal. The solution is concentrated and evaporated under reduced pressure. Glucose is obtained in crystalline form.

  • Physical properties : It is a colourless crystalline solid, melts at 146o C . It is readily soluble in

 

From aqueous solution, it separates as a crystalline monohydrate

(C6 H12O6 .H2O)

which melts at

86o C . It is

 

sparingly soluble in alcohol but insoluble in ether. It is less sweet (three-fourth) than cane sugar. It is optically active and the ordinary naturally occuring form is (+) glucose or dextro form. It shows mutarotation.

  • Chemical properties : Glucose is a polyhydroxy aldehyde e. aldohexose. It has five – OH groups and one aldehydic group. It shows characteristics of hydroxyl and aldehydic group. Important chemical reaction of the glucose are the following :
  • Alcoholic reaction (Reaction due to –OH group)
    • Reaction with acid chlorides and acid anhydride

 

CHO

|

CHO

|

 

(CHOH)4 + 5CH3COCl ¾¾ZnC¾l2  ®(CHOOCCH3 )4 + 5HCl

 

|

CH2OH

Glucose

Acetyl chloride

|

CH2OOCCH3

Glucose penta-acetate

 

This shows that a molecule of glucose contains 5 – OH groups.

  • Reaction with PCl5

 

CHO

|

CHO

|

 

(CHOH)4 + 5PCl 5 ¾¾® (CHCl)4 + 5POCl 3 + 5HCl

 

|

CH 2 OH

Glucose

|

CH 2 Cl

Penta-chloroglucose

(Glucose penta-chloride)

 

  • Reaction with metallic hydroxides :

C6 H11O5 — OH + H O Ca OH ¾¾® C6 H11O5 — O Ca OH+ H2O

 

Glucose

Calcium hydroxide

Calcium glucosate

 

Note : ®            Glucose behaves as a weak acid. Instead of

Ca(OH)2

we can take other metallic hydroxide

 

like

Ba(OH)2 , Sr(OH)2 ,Cu(OH)2

etc to form glucosate which is soluble in water.

 

  • Formation of glycosides : C6 H11O5 — OH + H OCH3  ¾¾H¾Cl ® C6 H11O5 OCH3  + H2O

β- and β-Methyl glucoside

 

 

 

 

 

H       OCH3                        CH3O       H

 

 

 

 

 

CH

|

CH2OH

a-Methyl glucoside

CH

|

CH2OH

b-Methyl glucoside

 

 

This reaction shows the presence of ring structure in glucose.

  • Reactions of carbonyl group (Aldehydic group)
    • Reduction : CH2OH(CHOH)4 CHO+ 2H ¾¾Na¾H¾g ® CH2OH(CHOH)4 CH2OH

 

Glucose

H2O

Sorbitol

 

On prolonged heating with concentrated HI and red phosphorus at iodohexane and n-hexane.

  • Oxidation

110o C , glucose forms a mixture of 2-

 

  • Reaction with Fehling solution : CH2OH(CHOH)4 CHO+ 2CuO ¾¾® CH2OH(CHOH)4 COOH + Cu2O .

 

Glucose

Gluconic acid

(red ppt.)

 

  • Reaction with Tollen’s reagent : CH 2 OH(CHOH)4 CHO + Ag 2 O ¾¾® CH 2 OH(CHOH)4 COOH +

2Ag      .

(Mirror) [or black ppt.]

 

  • Reaction with Bromine water : CH2OH(CHOH)4 CHO+ [O] ¾¾Br2 ¾/ HO ® CH2OH(CHOH)4 COOH .

Glucose                                                                        Gluconic acid

  • Reaction with Nitric acid : CH2OH(CHOH)4 CHO+ 3[O] ¾¾HN¾O¾3 ® COOH(CHOH)4 COOH+ H2O .

 

Glucose (C6 )

Saccharic acid (C6 )

CN

 

  • Reaction with HCN : CH 2 OH(CHOH)4 CHO + HCN ¾¾® CH 2 OH(CHOH)4 CH .

Glucose cyanohydrin                OH

  • Reaction with hydroxyl amine

CH 2 OH(CHOH)4 CHO + NH 2 OH  ® CH 2 OH(CHOH)4 CH  =  NOH + H 2 O .

Glucose oxime

  • Reaction with Phenyl hydrazine (Fischer’s mechanism) : When warmed with excess of phenyl hydrazine, glucose first forms phenylhydrazone by condensation with – CHO

 

CHO + H2NNHC6H5

CH = NNHC6H5

 

|

CHOH

|

Phenyl hydrazine

 

Warm

|

CHOH

|

 

(CHOH)3

|

CH2OH

Glucose

(CHOH)3

|

CH2OH

Glucose phenyl hydrazone

 

The adjacent – CHOH group is then oxidised by a second molecule of phenyl hydrazine.

 

 

 

 

 

 

CH = NNHC6H5

|

 

CH = NNHC6H5

|

 

CHOH + H2NNHC6H5

| (CHOH)3

|

CH2OH

C = O + C6H5NH2 + NH3

| (CHOH)3

|

CH2OH

Keto compound of Glucose phenyl hydrazone

 

 

The resulting carbonyl compounds reacts with a third molecule of phenyl hydrazine to yield glucosazone.

 

CH = NNHC6H5

|

CH = NNHC6H5

|

 

C = O + H2NNHC6H5

| (CHOH)3

|

CH2OH

  • Miscellaneous reactions

C = NNHC6H5 + H2O

| (CHOH)3

|

CH2OH

Glucosazone

 

  • Fermentation : C6 H12O6 ¾¾Zym¾a¾se ® 2C2 H5OH+ 2CO2

Glucose                                 Ethanol

  • Dehydration : When heated strongly or when treated with warn concentrated sulphuric acid, glucose is dehydrated to give a black mass (sugar carbon).
  • Reaction with alkalies : When warmed with concentrated alkali, glucose first turns yellow; then brown and finally gives a resinous

A dilute solution of glucose, when warmed with dilute solution of alkali, some glucose is converted into fructose and mannose. D-glucose and D-mannose are epimers.

 

CH = O

|

H — C — OH

 

Glucose

CH — OH

||

C — OH

 

Enol

CH2OH

|

C = O

 

Fructose

HO — CH

||

HO — C

 

Enol

CH = O

|

HO — C — H

 

Mannose

 

 

 

O         H

C

|

|

HO – C – H

|

H – C – OH

|

H – C – OH

|

CH2OH

D(+) Glucose

 

 

 

 

 

 

 

 

 

 

 

Epimers

O         H

C

|

|

HO – C – H

|

H – C – OH

|

H – C – OH

|

CH2OH

D(+) Mannose

 

 

 

 

  • Action of concentrated hydrochloric acid

C6 H12O6  ¾¾Con¾c.H¾Cl ® CH3 COCH2CH2COOH+ HCOOH + H2O

Laevulic acid

On treatment with conc. HCl, glucose can also form hydroxymethyl furfural.

 

 

C6H12O6                         CH — CH

||       ||

+ 3H2O

 

HOCH2 — C         C — CHO

O

Hydroxymethyl furfural

 

This on acid treatment gives laevulic acid

  • Uses
  • In the preservation of fruits and preparation of jams and
  • In the preparation of confectionary and as a sweetening
  • As a food for patients, invalids and
  • In the form of calcium glucosate as medicine in treatment of calcium deficiency.
  • As a reducing agent in silvering of
  • As a raw material for alcoholic
  • In industrial preparation of vitamin-C.
  • In the processing of
  • As an intravenous injection to the patients with lower glucose content in
  • Test of glucose
  • When heated in a dry test tube, it melts, turns brown and finally black, giving a characteristic smell of burnt

 

  • When warmed with a little

H 2 SO4 , it leaves a charred residue of carbon.

 

  • When it is boiled with dilute NaOH solution, it first turns yellow and then
  • Molisch’s test : This is a general test for A drop or two of alcoholic solution of a-naphthol is

 

added to 2mL of glucose solution. 1 mL of concentrated

HSO4

is added carefully along the sides of the test tube.

 

The formation of a violet ring, at the junction of two liquids confirms the presence of a carbohydrate.

  • Silver mirror test : A mixture of glucose and ammonical silver nitrate is warmed in a test tube. Appearance of silver mirror on the inner walls confirms
  • Fehling’s solution test : A little glucose is warmed with Fehling’s A red precipitate of cuprous oxide is formed.
  • Osazone formation : Glucose on heating with excess of phenyl hydrazine in acetic acid gives a yellow crystalline compound, pt. 205o C .
  • Structure of glucose : The structure of glucose has been established as follows
  • Open chain structure : It is based on the following points :
    • Elemental analysis and molecular mass determination show that the molecular formula of glucose is

C6 H12O6 .

  • Glucose on complete reduction with HI and red phosphorus finally gives n- This indicates that it contains a straight chain of six carbon atoms.

 

 

 

  • It reacts with acetic anhydride and forms penta-acetate This shows the presence of five hydroxyl groups each linked to a separate carbon atom as the molecule is stable.
  • Glucose combines with hydroxyl amine to form a It also combines with one mole of HCN to

form a cyanohydrin. These reactions indicate the presence of a carbonyl group, > C = O , in the molecule.

  • Mild oxidation of glucose with bromine water gives gluconic This shows the presence of an aldehyde group.

On the basis of above observations, the following open chain structure can be written for glucose.

OH   OH  OH  OH  OH  H

|      *|     *|     *|     *|      |

H C C C C C C = O

|      |      |       |     |

H       H      H        H       H

There are four asymmetric carbon atoms marked by asterisks (*) in the molecule. This representation is incomplete, because a definite configuration to these asymmetric centres has not been assigned. The configuration of D-glucose was proved by Emil Fischer. The structure of D-glucose as elucidated by Emil Fischer is,

H         O

C1

|2

H – C – OH

|3

HO – C – H

|4

H – C – OH

|5

H – C – OH

|6

CH2OH

D-Glucose

 

Evidence against open chain structure : The open chain formula of glucose accounts for most of the reactions satisfactorily but fails to explain the following

 

  • Even though an aldehyde group is present, the glucose does not react with
  • Glucose does not give the Schiff’s test for
  • Glucose does not react with Grignard
  • Glucose penta-acetate does not react with hydroxyl-amine.

NaHSO3

and

NH3 .

 

  • Two isomeric methyl glucosides (a and b) are obtained by heating glucose with methyl alcohol in presence of dry HCl

 

  • Glucose exists in two stereoisomeric forms (a and b). a- glucose with specific rotation

+ 110o

is obtained

 

by crystallizing glucose from alcohol or acetic acid solution, whereas b-glucose with specific rotation obtained by crystallizing glucose from pyridine solution.

+ 19.7o is

 

  • An aqueous solution of glucose shows mutarotation, e., its specific rotation gradually decreases from

 

+ 110o

to + 52.5o

in case of a-glucose and increases from + 19.7o

to + 52.5o

in case of b-glucose.

 

All these observation indicate that free aldehydic group is not present in the molecule.

  • Cyclic structure of glucose : D-glucose exists in two optically active forms known as a-D-glucose and

b-D-glucose.

 

a-D-glucose has specific rotation of

+ 110o

and b-D-glucose has specific rotation of

  • 7o . The two

 

isomers are interconvertible in aqueous solution. The equilibrium rotation is + 52o . The equilibrium mixture has

 

 

 

36% a-glucose, 64% b-glucose. Glucose forms a stable cyclic hemiacetal (according to Fischer) between – CHO

 

group and the

  • OH

group of the fifth carbon atom in pyranose structure. In this process first carbon atom

 

becomes asymmetric giving two isomers (I) and (II) which differ only in the configuration of the first asymmetric carbon.

 

H                                                  H         OH

C

HO        H C

 

 

 

 

 

 

 

|6

CH2OH

D-Glucose

CH                O

|6

CH2OH

a-D-Glucose (I)

CH                O

|6

CH2OH

b-D-Glucose (II)

 

[a]D = + 52.5o

[a]D = + 110o

[a]D = + 19.7o

 

a-Glucose 36%

Open chain form 0.02%

b-Glucose 64%

 

Carbon-1 in both configuration (I) and (II) is called an anomeric carbon atom. Due to anomeric carbon, glucose exists in two forms. Both the forms have different physical properties and are called anomers.

The ring structure explains all the reactions of glucose. The objections against the open chain structure of glucose have also been satisfactory explained, e.g.,

 

  • a- and b-glucose on treatment with CH3 OH

respectively.

in presence of dry HCl gas forms a- and b-methyl glucosides

 

 

 

H – C – OH

|

H – C – OH

|

HO – C – H

|

H – C – OH

|

H – C

|

CH2OH

a-D-Glucose (I)

 

 

 

O                               + CH3OH

(Dry HCl gas)

H – C – OCH3

|

H – C – OH

|

HO – C – H

|                     O

H – C – OH

|

H – C

|

CH2OH

a-D-Methyl glucoside

 

 

 

 

HO – C – H

|

H – C – OH

|

HO – C – H

|

H – C – OH

|

H – C

|

CH2OH

b-D-Glucose (II)

 

 

 

O                         + CH3OH

(Dry HCl gas)

 

CH3O – C – H

|

H – C – OH

|

HO – C – H

|                     O

H – C – OH

|

H – C

|

CH2OH

b-D-Methyl glucoside

 

 

 

  • No reaction with NH3 and NaHSO3 : The glucose ring is not very stable. It is easily broken up by strong reagents like HCN, NH2OH and C6H5NHNH2, etc., to give the intermediate aldehyde form, which reacts with them just like an

But weak reagents like NH3 and NaHSO3 are unable to open the chain and cannot react with it. This explains the inability of glucose to form aldehyde ammonia and bisulphite compound.

  • It explains mutarotation : Ordinary glucose is a-glucose, with a fresh aqueous solution has specific rotation, [a ]D+ 110o. On keeping the solution for some time; a-glucose slowly changes into an equilibrium mixutre of a-glucose (36%) and b-glucose (64%) and the mixture has specific rotation + 5o.

 

Similarly a fresh aqueous solution of b-glucose having specific rotation, gradually changes into the same equilibrium mixutre (having, specific rotation

[a]D + 19.7o , on keeping (standing)

  • 7o ). So an aqueous solution of

 

glucose shows a physical property, known as mutarotation, i.e., a change in the value of specific rotation

(muta=change; rotation = specific rotation) is called mutarotation.

  • Methods for determining the size of rings : Fischer and Tollen’s proposed that the ring or the internal

 

hemiacetal is formed between C1

Furanose strucutre,

and

C4 . It means the ring is Furan type or 5-membered ring; this is called

 

4           3

CH — CH

 

|| 5

|| 2

 

CH        CH

O

Furan

However according to Haworth and Hirst the ring is formed between C1

type or 6-membered ring, this is called Pyranose structure.

CH2

5          3

HC        CH

 

and

 

C5 . It means the ring is Pyran

 

|| 6

|| 2

 

HC        CH

O

Pyran

The two forms of D-glucose are also shown by Haworth projection formula which are given below;

 

 

 

 

4                                                                                                                    4

 

 

 

H                  OH

H                  OH

 

a-D glucose                                                                        b-D glucose

The above projection formulae show that the six membered ring is planar but actually the ring has a chain structure similar to cyclohexane.

In Haworth formula all the OH groups on the right in Fischer’s formula are directed below the plane of the

 

ring while those on the left go above the plane. The terminal CH 2 OH

projects above the plane of the ring.

 

  • Fructose, fruit sugar C6H12O6, Ketohexose : It is present in abundance in fruits and hence is called

fruit sugar. It is also present in cane sugar and honey alongwith glucose in combined form. The polysaccharide

 

 

 

inulin is a polymer of fructose an gives only fructose on hydrolysis. Since naturally occurring fructose is laevorotatory, it is also known as laevulose.

  • Preparation

 

  • Hydrolysis of cane sugar :

C12 H22O11 + H2O ¾¾H2S¾O4 ¾(d¾il.) ® C6 H12O6 + C6 H12O6

 

Cane sugar

Warm

D-Glucose

D-Fructose

 

The solution having equal molecules of D-glucose and D-fructose is termed invert sugar and the process is known as inversion.

Note : ®            The excess of sulphuric acid is neutralised by adding milk of lime. A little more of lime is added which converts both glucose and fructose into calcium glucosate and calcium fructose respectively.

C6 H11O5 – O CaOH+ CO2 ¾¾® C6 H12O6 + CaCO3

Calcium fructose                                                Fructose

  • Hydrolysis of Inulin with dilute sulphuric acid : (C6 H10 O5 )n + nH2O ¾¾H2S¾O4 ¾(d¾il.) ® nC6 H12O6

Inulin                                                           Fructose

  • Properties : The anhydrous fructose is a colourless crystalline compounds. It melts at 102o C. It is soluble in water but insoluble in benzene and ether. It is less soluble in water than glucose. It is the sweetest* of all sugars and its solution is Like glucose, it also shows mutarotation.

Fructose is a pentahydroxy ketone and its open-chain and closed-chain structures can be represented as :

 

CH2OH

|

C= O

|

 

 

CH2OH

HOH2C OH  C

|

OH        CH2OH C

|

 

HO – C – H

|

H – C – OH

|

H – C – OH

|

CH2OH

D-Fructose

|

C= O

or           |

(CHOH)3

|

CH2OH

HO – C

|

H – C – OH                O

|

H – C – OH

|

CH2

a– D- Fructose

HO – C

|

H – C – OH                O

|

H – C – OH

|

CH2

b– D- Fructose

 

[a]D = – 92o

 

(5)  Comparison between glucose and fructose

[a]D

= – 21o

[a]D = – 133o

 

 

PropertyGlucoseFructose
Molecular formulaC6H12O6C6H12O6
NaturePolyhydroxy aldehyde.Polyhydroxy ketone
Melting point146oC102oC
Optical activity of natural formDextrorotatoryLaevorotatory
With ethyl alcoholAlmost insolubleMore soluble
Oxidation  
(a) With bromine waterGluconic acidNo reaction
(b) With nitric acidSaccharic acid (Glucaric acid)Mixture of glycollic acid, tartaric acid and trihydroxy glutaric acid
ReductionSorbitolMixture of sorbitol and mannitol
Calcium hydroxideForms calcium glucosate, soluble in waterForms calcium fructosate, insoluble in water
Molisch’s reagentForms a violet ringForms a violet ring
Fehling’s solutionGives red precipitateGives red precipitate
Tollen’s reagentForms silver mirrorForms silver mirror

 

 

 

 

Note : ®            Fructose gives reactions similar to glucose. The difference in properties is due to the fact that it contains a ketonic group while glucose contains an aldehydic group.

(6)  Interconversions

  • Chain Lengthening of Aldoses (Killiani-Fischer synthesis) : The conversion of an aldose to the next higher member involves the following steps :
  • Formation of a
  • Hydrolysis of – CN to – COOH forming aldonic
  • Conversion of aldonic acid into lactone by
  • The lactone is finally reduced with sodium amalgam or sodium borohydride to give the higher

 

 

CHO

|

(CHOH)

 

HCN

CN

|

CHOH

 

H2O/H+

COOH

|

CHOH

O = C

|

CHOH

O = C – H

|

CHOH

 

3                  |                             |

|                                           Ba(OH)2

heat      |

Na – Hg                |

 

CH2OH

Arabinose (Aldopentose)

(CHOH)3

|

CH2OH

(CHOH)3

|

CH2OH

Gluconic acid

–H2O      CHOH

|

CH

|

CHOH

|

CH2OH

g-Lactone

O      in acid solution

(CHOH)3

|

CH2OH

Glucose (Aldohexose)

 

  • Chain Shortening of Aldoses (Ruff Degradation)
  • An aldose can be converted to the next lower member by Ruff It involves two steps:
  • Oxidation of the aldose to aldonic acid by using bromine

 

  • The aldonic acid is treated with CaCO3

to give the calcium salt which is then oxidised by Fenton’s reagent

 

( H2O2 + ferric sulphate) to form the next lower aldose.

 

 

CHO

|

CHOH

|

(CHOH)3

|

CH2OH

Aldohexose (D-Glucose)

 

 

Br2 H2O

COOH

|

CHOH

|

(CHOH)3

|

CH2OH

Aldonic acid

 

 

Ca- salt H2O2+Fe3+

 

CHO

| (CHOH)3

|

CH2OH

Aldopentose (D-Arabinose)

 

  • By Wohl’s method : It involves the following steps
  • Formation of oxime with hydroxyl

 

 

 

  • Heating of oxime with acetic anhydride undergoes dehydration into cyano compound, whereas the hydroxyl groups get
  • The acetyl derivative is warmed with ammonical silver nitrate which removes the acetyl group by hydrolysis and eliminates a molecule of HCN.

CH = O

 

|

CHOH

CH = NOH                          CN

|                                   |

CHO

 

|               H NOH

CHOH

(CH CO) O

CHO.COCH3

AgOH

– HCN       |

 

(CHOH)3          2

|

CH2OH

Glucose (Aldohexose)

| (CHOH)3

|

CH2OH

Oxime

3        2          | (CHO.COCH3)3

|

CH2O.COCH3

warm

| (CHOH)3

|

CH2OH

(CHOH)3

|

CH2OH

Aldopentose

 

 

  • Conversion of an Aldose to the isomeric Ketose : Three steps are involved :
  • Treatment of aldose with excess of phenyl hydrazine to form
  • Hydrolysis of osazone with HCl to form osone.
  • Reduction of osone with zinc and acetic acid to form

 

 

CHO

|

CHOH

|

 

 

C H NHNH

HC=NNHC6H5

|

C=NNHC6H5

|

 

 

2H O/H+

HC=O

|

C=O

|

CH2OH

|

C=O

2H                      |

 

(CHOH)3           6   5             2

(CHOH)3

2                         (CHOH)3

Zn/CH COOH

(CHOH)3

 

|

CH2OH

Glucose

(Excess)

|

CH2OH

Osazone

(–2C6H5NHNH2)

|                         3

CH2OH

Osone

|

CH2OH

Fructose

 

  • Conversion of a Ketose to the isomeric Aldose : Two steps are involved,

 

  • Reduction of a ketose with

H 2 / Ni to form polyhydric alcohol.

 

  • Oxidation with Fenton’s reagent to form

 

CH2OH

|

C=O

|

 

 

H /Ni

CH2OH

|

CHOH

|

[O]

CHO

|

CHOH

|

 

(CHOH)3               2

(CHOH)3

H O  + Fe3+

(CHOH)3

 

|

CH2OH

Fructose

|                       2  2

CH2OH

|

CH2OH

Glucose

 

 

The disaccharides yield on hydrolysis two monosaccharides. Those disaccharides which yield two hexoses on hydrolysis have a general formula C12 H22O11. The hexoses obtained on hydrolysis may be same or different.

C12 H22O11 ¾¾H¾2O ® C6 H12O6 + C6 H12O6

 

Sucrose

H +                  Glucose

Fructose

 

Lactose

¾¾H¾2O ®

H +

Glucose + Galactose

 

 

 

 

Maltose

¾¾H¾2O ® Glucose + Glucose

H +

 

The hydrolysis is done by dilute acids or enzymes. The enzymes which bring hydrolysis of sucrose, lactose and maltose are invertase, lactase and maltase, respectively. Out of the three disacchrides, sucrose (cane-sugar) is the most important as it is an essential constituent of our diet.

In disaccharides, the two monosaccharides are joined together by glycoside linkage. A glycoside bond is formed when hydroxy group of the hemiacetal carbon of one monosaccharide condenses with a hydroxy group of another monosaccharide giving – O– bond.

  • Sucrose; Cane-sugar [C12H22O11] : It is our common table sugar. It is obtained from sugar cane and It is actually found in all photosynthetic plants.
  • Properties : It is a colourless, odourless, crystalline compound. It melts at 185 – 186oC. It is very soluble in water, slightly soluble in alcohol and insoluble in It is dextrorotatory but does not show mutarotation. It is a non-reducing sugar as it does not reduce Tollen’s or Fehling’s reagent. Sucrose, on heating slowly and carefully, melts and then if allowed to cool, it solidifies to pale yellow glassy mass called ‘Barley sugar’. When heated to 200oC, it loses water to form brown amorphous mass called Caramel. On strong heating, it chars to almost pure carbon giving smell of burnt sugar. It is composed of a-D-glucopyranose unit and a b-D-fructofuranose unit. These units are joined by a-b-glycosidic linkage between C –1 of the glucose unit and C – 2 of the fructose unit.

 

6CH2OH

Glycoside linkage

 

 

Glycoside linkage                                                                                     1

4

 

H                      O

1

C

|

(CHOH)3

5|

HC                 O

|

6

CH2OH

1CH2OH

2|

C

|

(CHOH)2           or

5|

HC                 O

6|

CH2OH

 

 

 

6CH2OH

 

5

4

 

 

 

O

 

O

 

2

OH

3       1CH2OH

 

  • Uses

Structure of sucrose              OH

 

  • As a sweetening agent for various food preparations, jams, syrups sweets, et
  • In the manufacture of sucrose octa-acetate required to denature alcohol, to make paper transparent and to make anhydrous
  • Inversion of cane-sugar : The hydrolysis of sucrose by boiling with a mineral acid or by enzyme invertase, produces a mixture of equal molecules of D-glucose and D-fructose.

 

C12 H 22 O11

  • H 2

O ¾¾H¾+  ®

C6 H12 O6

+ C6 H12 O6

 

Sucrose

D-Glucose

D-Fructose

 

(This mixture is laevorotatory)

Sucrose solution is dextrorotatory. Its specific rotation is

  • 5o. But on hydrolysis, it becomes laevorotatory.

 

The specific rotation of D-glucose is

+ 52o

and of D-fructose is

– 92o.

Therefore, the net specific rotation of an

 

equimolar mixture of D-glucose and D-fructose is.

 

+ 52o – 92o

2

= -20o

 

 

 

 

Thus, in the process of hydrolysis of sucrose, the specific rotation changes from

+ 66.5o

to – 20o , i.e., from

 

dextro it becomes laevo and it is said that inversion has taken place. The process of hydrolysis of sucrose is thus termed as inversion of sugar and the hydrolysed mixture having equal molar quantities of D-glucose and D- fructose is called invert sugar. The enzyme that brings the inversion is named as invertase.

(3)  Distinction between glucose and sucrose

TestGlucose Sucrose
With conc. H2SO4 in coldNo effect Charring occurs and turns black
Molisch’s reagentViolet ring is formed Violet ring is formed
With NaOHTurns yellow No effect
With Tollen’s SolutionGives silver mirror No effect
With Fehling’s solutionGives red precipitate of Cu2O No effect
On heating with phenyl hydrazineGives     yellow     precipitateofNo effect, i.e., does not form
 glucosazone osazone
Aqueous solution + resorcinol +No effect Reddish-brown precipitate which
HCl (conc.)  dissolves in ethanol.

Polysaccharides are polymers of monosaccharides. The most important polysaccharides are starch and

 

cellulose. They have a general formula

(CH10O5 )n.

Starch (Amylum) is most widely distributed in vegetable

 

kingdom. It is found in the leaves, stems, fruits, roots and seeds. Concentrated form of starch is present in wheat, corn, barley, rice, potatoes, nuts, etc. It is the most important food source of carbohydrates.

  • Starch and its derivatives : Starch is a white amorphous substance with no taste or When heated

 

to a temperature between

200 – 250o C,

it changes into dextrin. At higher temperature charring occurs. When

 

boiled with dilute acid, starch ultimately yields glucose.

1

(C6 H10O5 )n  ¾¾®(C6 H10O5 )n    ¾¾® C12 H22O11 ¾¾® C6 H12O6

 

Starch

Dextrin

Maltose

Glucose

 

Both n and n1, are unknown, but n is believed to be greater than n1 .

 

When treated with enzyme, diastase, it yields maltose.

2(CH10O5 )n + nH2O ¾¾® nC12 H22O11

Maltose

Starch solution gives a blue colour with a drop of iodine which disappears on heating to

 

75 – 80o C

 

 

and

 

reappears on cooling. The exact chemical nature of starch varies from source to source. Even the starch obtained from same source consists of two fractions (i) amylose and (ii) amylopectin.

Amylose is a linear polymer while amylopectin is a highly branched polymer. Both are composed of a-D– glucose units linked by glycosidic linkages. The number of D-glucose units in amylose range from 60 – 300. It is soluble in hot water, Amylopectin consists of D-glucose units from 300 – 600. It is insoluble in water.

 

CH2OH                               CH2OH

 

 

 

 

O

 

 

a-1, 4-Glycoside bonds

Structure of amylose

O

 

n

Repeating monomer

 

 

 

 

 

 

CH2OH

O

OH

 

OH

O

Repeating monomer              a-1, 6-Glyoside bonds

 

 

 

 

 

 

 

a-1, 4-Glycoside bonds                       Repeating monomer

Structure of amylopectin

Uses : Starch and its derivatives are used

  • As the most valuable constituent of food as rice, bread, potato and corn-flour,
  • In the manufacture of glucose, dextrin and adhesives (starch paste).
  • In paper and textile
  • In calico printing as a thickening agent for
  • Nitro starch is used as an
  • Starch-acetate is a transparent gelatin like mass and is used mainly for making

Distinction between glucose, sucrose, starch

 

TestGlucoseSucroseStarch
With iodine solutionNo effectNo effectBlue colour
With Fehling’s solutionGives red precipitateNo effectNo effect
With Tollen’s reagentGives silver mirrorNo effectNo effect
With phenyl hydrazineForms yellow osazoneNo effectNo effect
Solubility in waterSolubleSolubleInsoluble
TasteSweetSweetNo taste
  • Cellulose and its uses : It is found in all plants and so is the most abundant of all It is the material used to form cell walls and other structural features of the plants. Wood is about 50% cellulose and the rest is lignin. Cotton and paper are largely composed of cellulose.

Pure cellulose is obtained by successively treating cotton, wool, flax or paper with dilute alkali, dilute HCl or HF . This treatment removes mineral matter, water, alcohol and ether. Cellulose is left behind as a white amorphous powder.

 

 

 

Cellulose is insoluble in water and in most of the organic solvents. It decomposes on heating but does not melt. It dissolves in ammonical copper hydroxide solution (Schwitzer’s reagent). Cellulose also dissolves in a solution of zinc chloride in hydrochloric acid.

 

When it is treated with concentrated

HSO4

in cold, it slowly passes into solution. The solution when diluted

 

with water, a starch like substance amyloid is precipitated and is called parchment paper. When boiled with dilute

H 2 SO4 , it is completely hydrolysed into D-glucose.

(C6 H10 O5 )n + nH 2 O ¾¾® nC6 H12 O6

Cellulose                                            Glucose

The cattle, goats and other ruminants can feed directly cellulose (grass, straw, etc.) as they have digestive enzymes (celluloses) capable of hydrolysing cellulose into glucose. Man and many other mammals lack the necessary enzymes in their digestive tract and thus cannot use cellulose as food stuff.

Cellulose is a straight chain polysaccharide composed of D-glucose units which are joined by B-glycosidic linkages between C-1 of one glucose unit and C-4 of the next glucose unit. The number of D-glucose units in cellulose ranges from 300 to 50000.

 

 

O                                            O                                                                                                                                                                                                                  O

 

 

H             OH

 

Uses : Cellulose is used

Structure of celluose

 

  • As such in the manufacture of cloth (cotton), canvas and gunny bags (jute) and paper (wood, bamboo, straw, )
  • In the form of cellulose nitrates for the manufacture of explosives (gun-powder), medicines, paints and The cellulose nitrates with camphor yield celluloid which is used in the manufacture of toys, decorative articles and photographic films.
  • In the form of cellulose acetate for the manufacture of rayon (artificial silk) and

Proteins : Proteins are a class of biologically important compounds. They are crucial to virtually all processes in living systems. Some of them are hormones which serve as chemical messengers that coordinate certain biochemical activities. Insulin, for example, controls the level of sugar in the blood stream. Some proteins serve to transport the substances through the organism. Haemoglobin, for instance, carries oxygen in blood stream and delivers to different parts of the body. a-keratin, serves as a major constituent of hairs, nails and skin, while collegen is the prime constituent of tendons. Proteins are also found in toxins (poisonous materials) as well as in antibiotics.

Amino acids : An amino acid is a bifunctional organic molecule that contains both a carboxyl group,

COOH, as well as an amine group, –NH2. They are classified as acidic basic or neutral according to number of amine and carboxyl groups in a molecule. Neutral amino acids contain only one amine and one carboxyl group. They are further classified according to the position of amine group in relation to carboxyl group into a-, b-, g-and

damino acids. Out of these a-amino acids are most important as they are building blocks of bio-proteins.

In an a-amino acid, the amine group is located on the carbon atom adjacent to the carboxyl group (the a– carbon atom). The general structure of the a-amino acids is represented as

 

 

 

 

H               Carboxyl group

|

 

 

 

 

R may be alkyl, aryl or any other group.

 

a-Carbon atom

R – C – COOH

|

NH2 ¬ Amine group

 

The proteins differ in the nature of R-group bonded to a-carbon atom. The nature of R-group determines the properties of proteins. There are about 20 amino acids which make up the bio-proteins. Out of these 10 amino acids (non-essential) are synthesised by our bodies and rest are essential in the diet (essential amino acids) and supplied to our bodies by food which we take because they cannot be synthesised in the body. The a-amino acids are classified into the following four types.

Amino acids with non polar side chain : Examples are :

 

NameStructureThree letter symbolOne letter code
 NH2

CH2

COOH

 

 

NH2

CH3CH

COOH

NH2

(CH3)2CHCH

COOH

(Essential)

NH2

(CH3)2CHCH2CH

COOH

(Essential)

NH2

C2H5CH–CH

|                       COOH

CH3

(Essential)

NH2

C6H5CH2CH

(Essential)     COOH

GlyG
Glycine  
  

Ala

 

A

Alanine  
 

Valine

 

Val

 

V

 

 

Leucine

 

 

Leu

 

 

L

 

Isoleucine

 

ILE

 

I

 

 

Phenyl alanine

 

 

PHE

 

 

F

 

 

 

 

Proline

H2C          CH2

H2C           CHCOOH

Pro                            P

 

 

N

 

H

Amino acids with polar but neutral side chain: Examples are

Name                                                 Structure                               Three letter symbol        One letter code Tryptophan                                                                                                Trp.                          W

 

H

| N

CH NH2

||   |

 

 

 

Serine

C – CH2 – COOH

(Essential)

NH2

HO–CH2–CH

 

Ser                            S

 

 

Threonine

 

CH3CHOH–CH

COOH

NH2

 

Thr                            T

 

 

 

Tyrosine

COOH

(Essential)

NH2

 

 

Tyr                           Y

 

 

 

 

Cysteine

|

HO                    CH2CH–COOH

 

NH2

HS–CH2–CH

 

 

Cys                            C

 

COOH

Methionine                                                                                              Met                           M

NH2

 

CH3·S·CH2·CH2·CH

(Essential)

COOH

 

Aspargine                                                                                                Asn                           N

 

H2N

 

O

C·CH2·CH

NH2 COOH

 

 

 

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