Chemistry class 12th Full Notes chapter 1
Answers
ChemistryCarbohydrates: Polyhydroxy aldehydes or polyhydroxy ketones or compounds on hydrolysis give carbohydrates.Classification of carbohydrates:
Monosaccharides
(a) Simplest carbohydrates
(b) It cannot be hydrolysed into simpler compounds
(c) Examples – Glucose, mannose
Oligosaccharides
(a) Carbohydrates which gives 2 to 10 monosaccharide units on hydrolysis
(b) Examples – Sucrose, Lactose, Maltose
Polysaccharides
(a) Carbohydrates which on hydrolysis give large number of monosaccharide units.
(b) Examples – Cellulose, starchAnomers: Pair of optical isomers which differ in configuration only around C1 atom are called anomers. Examples – -D-glucopyranose and -D-glucopyranose.Epimers: Pair of optical isomers which differ in configuration around any other C atom other than C1 atom are called epimers. E.g. D-glucose and D- mannose are C2epimers.
Preparation of glucose (also called dextrose, grape sugar):
From starch
Structure of glucose
Structure elucidation of glucose:
a) D – glucose with HI
b) D – glucose with HCN
c) D – glucose with NH2OH
d) D- glucose with Fehling’s reagent
e) D – glucose with Tollen’s reagent
f) D – glucose with nitric acid
g) D – glucose with (CH3CO)2O and ZnCl2
h) D – glucose with bromine water
i) Glucose with phenylhydrazine to form osazone
Glucose and fructose gives the same osazone because the reaction takes place at C1 and C2 only.
Other Reactions of Glucose (Presence of ring structure)
Glucose does not give Schiff’s test and does not react with sodium bisulphite and NH3. Pentaacetyl glucose does not react with hydroxyl amine. This shows the absence of –CHO group and hence the presence of ring structure.
Cyclic structure of glucose:
Haworth representation of glucose:
Cyclic structure of fructose:
Haworth representation of fructose
Glycosidic linkage: The oxide linkage formed by the loss of a water molecule when two monosaccharides are joined together through oxygen atom is called glycosidic linkage.Sucrose (invert sugar):
a) Sucrose is a non-reducing sugar because the two monosaccharide units are held together by a glycosidic linkage between C1 of -glucose and C2 of – fructose. Since the reducing groups of glucose and fructose are involved in glycosidic bond formation, sucrose is a non-reducing sugar.
b) Sucrose is dextrorotatory but on hydrolysis it gives dextrorotatory & laevorotatory and the mixture is laevorotatory.
Haworth Projection of Sucrose:
Maltose:Maltose is composed of two α-D-glucose units in which C1 of one glucose (I) is linked to C4 of another glucose unit (II).The free aldehyde group can be produced at C1 of second glucose in solution and it shows reducing properties so it is a reducing sugar.
Haworth