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
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
|
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 |
Aldose | Ketose |
3 | Aldotriose | Ketotriose |
4 | Aldotetrose | Ketotetrose |
5 | Aldopentose | Ketopentose |
6 | Aldohexose | Ketohexose |
7 | Aldoheptose | Ketoheptose |
- 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.
Class | Molecular formula | Structural formula | Examples |
Aldoses | |||
Aldotrioses | C3H6O3 | CH2OHCHOHCHO | Glyceraldehyde |
Aldotetroses | C4H8O4 | CH2OH(CHOH)2CHO | Erythrose, Threose |
Aldopentoses | C5H10O5 | CH2OH(CHOH)3CHO | Arabinose, Ribose, Xylose, Lyxose |
Aldohexoses | C6H12O6 | CH2OH(CHOH)4CHO | Glucose, Galactose, Mannose, |
Allose, Talose, Gulose, Idose, etc. | |||
Ketoses | |||
Ketotrioses | C3H6O3 | CH2OHCO.CH2OH | Dihydroxyacetone |
Ketotetroses | C4H8O4 | CH2OH.CO.CHOH.CH2OH | Erythrulose |
Ketopentoses | C5H10O5 | CH2OH.CO.(CHOH)2CH2O H | Ribulose, Xylulose |
Ketohexoses | C6H12O6 | CH2OH.CO(CHOH)3CH2O H | Fructose, 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 – 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 – OH | HO – C – H | H – C – OH | HO – C – H | H – C – OH | HO – C – H | H – C – OH | HO – C – H |
| | | | | | | | | | | | | | | |
H – C – OH | H – C – OH | HO – C – H | HO – C – H | H – C – OH | H – C – OH | HO – C – H | HO – C – H |
| | | | | | | | | | | | | | | |
H – C – OH | H – C – OH | H – C – OH | H – C – OH | HO – C – H | HO – C – H | HO – C – H | HO – C – H |
| | | | | | | | | | | | | | | |
H – C – OH | H – C – OH | H – C – OH | H – C – OH | H – C – OH | H – C – OH | H – C – OH | H – C – OH |
| | | | | | | | | | | | | | | |
CH2OH | CH2OH | CH2OH | CH2OH | CH2OH | CH2OH | CH2OH | CH2OH |
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
|
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
|
|
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 ¾¾Na–¾H¾g ® 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/ H2¾O ® 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
H 2 SO4
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
Property | Glucose | Fructose |
Molecular formula | C6H12O6 | C6H12O6 |
Nature | Polyhydroxy aldehyde. | Polyhydroxy ketone |
Melting point | 146oC | 102oC |
Optical activity of natural form | Dextrorotatory | Laevorotatory |
With ethyl alcohol | Almost insoluble | More soluble |
Oxidation | ||
(a) With bromine water | Gluconic acid | No reaction |
(b) With nitric acid | Saccharic acid (Glucaric acid) | Mixture of glycollic acid, tartaric acid and trihydroxy glutaric acid |
Reduction | Sorbitol | Mixture of sorbitol and mannitol |
Calcium hydroxide | Forms calcium glucosate, soluble in water | Forms calcium fructosate, insoluble in water |
Molisch’s reagent | Forms a violet ring | Forms a violet ring |
Fehling’s solution | Gives red precipitate | Gives red precipitate |
Tollen’s reagent | Forms silver mirror | Forms 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
Test | Glucose | Sucrose | |
With conc. H2SO4 in cold | No effect | Charring occurs and turns black | |
Molisch’s reagent | Violet ring is formed | Violet ring is formed | |
With NaOH | Turns yellow | No effect | |
With Tollen’s Solution | Gives silver mirror | No effect | |
With Fehling’s solution | Gives red precipitate of Cu2O | No effect | |
On heating with phenyl hydrazine | Gives yellow precipitate | of | No 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
(C6 H10O5 )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.
|
(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(C6 H10O5 )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
Test | Glucose | Sucrose | Starch |
With iodine solution | No effect | No effect | Blue colour |
With Fehling’s solution | Gives red precipitate | No effect | No effect |
With Tollen’s reagent | Gives silver mirror | No effect | No effect |
With phenyl hydrazine | Forms yellow osazone | No effect | No effect |
Solubility in water | Soluble | Soluble | Insoluble |
Taste | Sweet | Sweet | No 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
H 2 SO4
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
d–amino 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 :
Name | Structure | Three letter symbol | One letter code |
NH2
CH2 COOH
NH2 CH3CH COOH NH2 (CH3)2CH–CH COOH (Essential) NH2 (CH3)2CH–CH2CH COOH (Essential) NH2 C2H5–CH–CH | COOH CH3 (Essential) NH2 C6H5CH2CH (Essential) COOH |
Gly | G | |
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 CH2–CH–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