Class notes chemistry

HANDOUT OF ORGANIC CHEMISTRY
TOPIC : CARBOHYDRATES


CARBOHYDRATES
They were considered as hydrates of carbon having general formula Cx(H2O)y.
CGH
Ex. Glucose, C6H12O6 C6(H2O)6
CGH2O6
However, a number of compounds have been found, which are carbohydrates by chemical behaviour but do not
conform to the formula Cx(H2O)y. E.g : Rhamnose (C6H12O5)
Also all the compounds which fits into the formula are not carbohydrates. E.g: Formaldehyde, C(H2O); acetic
acid, C2(H2O)2 etc.
HCHO
crycoon optically active NCERT
Carbohydrates can be defined as polyhydroxy aldehyde / ketone or compounds which produces such units on
hydrolysis.
(a) There should be more than one –OH groups.
(b) There should be one –CHO or one –Keto group (Keto group should be at 2nd carbon).
I (III)
CH2–OH
CH2–CH–CHO CHz OH
– –




C=O ketotriose
–

–




4CH Ctb
Ex.
OH OH CH2–OH These are simplest carbohydrates 0
C3(H2O)3
C3(H2O)3


Note: (i) All the carbohydrates have chiral center and are optically active except Keto-triose.
HzOH
(ii) Carbohydrates are also called as Saccharides (sugar in greek).

Classification of Carbohydrate
saccharon
(A) Classification on the basis of number of hydrolysed products
(i) Monosaccharides :
A carbohydrate that can not be hydrolysed to simpler unit of polyhydroxy aldehyde/ ketone is called monosaccharide.
Ex. Glucose, Fructose, Mannose,Galactose etc.

H2O Glucose fructose
Sucrose
(ii) Oligosaccharides : Carbohydrates that yield 2 to 10 monosaccharide units on hydrolysis, are called
oligosaccharides. They are further classified as disaccharides, trisaccharides, tetrasaccharide, etc., depending
upon the number of monosaccharides, they provide on hydrolysis.
Ex. Sucrose, Maltose, Lactose etc. All these are disaccharides

(iii) Polysaccharides.
Polymers
A carbohydrate that give large no of monosaccharide units on hydrolysis, is called a polysaccharide.
Ex : Starch, Cellulose, Gums etc.
H2O
Note:
starch nauscose
Polysaccharides are not sweet in taste hence they are also called as non-sugars.

(B) Classification on the basis of reducing property:
Reducing sugars Non Reducing sugars
(i). Which reduces Tollen’s reagent, Fehling’s solution (i) Which do not reduces Tollen’s, Fehling’s solution &
& Benedicts’s solution. Benedict’s solution.

T.R.ly
fs
(ii) They have free aldehyde f.g. or hemiacetal group (ii) They do not have free aldehyde f.g. or hemiacetal
group

BS (iii) Ex. All Monosaccharides and Oligosaccharides
except Sucrose
(iii) Ex. All Polysaccharides and Sucrose
.


Nyc OH Hgc
C n C O 1 CHz OH
H OCHz H

,Monosaccharides
Nomenclature of monosaccharides
(i) If f.g. is aldehyde Aldose
If f.g. is ketone Ketose

S .No. Ca rb on a to m s Ge ne ra l te rm Ald e hyd e Ke to ne
1 3 Trios e A ldotrios e K etotrios e
2 4 Tetros e A ldotetros e K etotetrose
3 5 P entose A ldopentos e K etopentos e
4 6 Hex os e A ldohexos e K etohex os e
5 7 Heptose A ldoheptos e K etoheptos e



Note 1. Simplest aldose is Glyceraldehyde and simplest Ketose is Dihydroxyacetone.

I CH=O 1 CH2OH HIC OH
H 2 OH 2 C=O
zd o Ketotetrose
zCH2OH 3 CH OH 3dHEoH optically
2


(D-Glyceraldehyde) (Dihydroxyacetone)

Optically active Optically Inactive 4kHzOH active
All higher ketose are optically active.


2. The monosaccharides are chiral and may have D or L configuration. Glyceraldehyde contains one asymmetric
carbon atom and exists in two enantiomeric forms as shown below.
1 CHO I CHO
H 2 OH OH 2 H

3CH2OH 3CH2OH
LDKnomenclature
D – Glyceraldehyde L – Glyceraldehyde

3. All naturally occuring carbohydrates have D configuration. Some naturally occuring monosacharides are :
(a) Pentoses E.g. D-Ribose (present in RNA) and 2-Deoxyribose (present in DNA)
(b) Hexoses E.g. D-Glucose, D-Mannose, D-Allose, D-Galactose and D-Fructose




(A) Glucose (C6H12O6): It is aldohexose. Glucose is the most common monosaccharide.


ICHO i i
µ 2 OH 2 2
3
Ho 3 µ 3
4 4
µ 4 OH
5 5
µ 5 OH
6 6
6CHEON
D (+) Glucose Enantiomers L (-) Glucose
D Glucose Enantiomer
Dextrorotatory
(i) Glucose is dextrorotatory and also called as ‘Dextrose’ and it is reducing sugar.
of DHglucos

, Cyclic structure of glucose
Open chain structure of glucose explained most of its properties but could not explain following facts:
(i) Despite having aldehyde group, glucose do not give 2,4-DNP test (test for carbonyl compounds) and do not
form bisulphite addition product with NaHSO3.
(ii) Glucose do not give positive test with Schiff’s reagent.
(iii) The pentaacetate of glucose doesn’t react with NH2OH.

All these observation indicates the absence of free -CHO group in glucose.
Open chain structure of glucose exist in equilibrium with 2 cyclic structure due to hemiacetal formation between
C1 & -OH group at C5.


r
T ay


I II

C1- Symmetric (sp2)
I 1
onrightCCH 021 OH left I
Due to hemiacetal formation, C1 becomes assymetric so two different cyclic structure are possible which only differ
in configuration at C1 (hemiacetal carbon), they are called as and form.

Haworth projection Anomeric carbon
The six membered cyclic structure of glucose is called pyranose structure ( - or -), in analogy with pyran.
and five membered cyclic structure of monosaccharides is called furanose structure ( or ) in analogy with furan

ICHO H
2 G .
H OH H S CH H
no 3 µ Coiled 4 OH CHO
4 H
H OH
right down no 2 3
H 5 OH Left Up
H OH
CHEOH CqCs Anti
G clockwise
GCHzOH

us Hh
4 H c f
OH H
-D(+) Glucopyranose ( 36%)
Ho 2 -D(+) Glucopyranose( 64%)
3
(-OH at C1 is below) - alpha (-OH at C1 is above) - beta
H OH
no 21





Axial