CBSE Class 12 Chemistry Biomolecules Notes and Questions

Bio molecule

Gist Of The Chapter

1.Carbohydrates- These are optically active polyhydroxy  aldehydes or ketones due to presence of chiral `C’ or the compounds which produce these on hydrolysis except dihydroxy acetone is not optically active.

2. Classification-

(i) Monosaccharide’s – Those carbohydrates which cannot get hydrolysed e.g. glucose ,fructose, galactose etc.

(ii) Oligosaccharides- Those carbohydrates which give to or more monosaccharide’s on hydrolysis e.g. sucrose on hydrolysis gives glucose and fructose. Raffinose on hydrolysis gives glucose, fructose and galactose.

(iii) Polysaccharides- Those carbohydrates which on hydrolysis give large number of monosaccharide’s hydrolysis.eg  starch, cellulose, glycogen.

3. Sugar-

(i) Reducing Sugars- Those which reduce Fehling’s or Tollen’s reagent. They have free aldehydic groups, eg , glucose, fructose , galactose

(ii) Non Reducing Sugars- Those which do not reduce Fehling’s or Tollen’s reagent. They do not have free functional group ,e.g.,  sucrose

4. Glucose- It is a monosaccharide’s with molecular formula C₆H₁₂O₆

5. Preparation

(i) From Sucrose

     C₁₂H₂₂O₁₁ + H₂O ------->  C₆H₁₂O₆ + C₆H₁₂O₆ ( Only from sucrose)                                        

(ii)From Starch

(C₆H₁₀O₅)_n + nH₂O -------> C₁₂H₂₂0₁₁ + H₂0 ------> 2C₆H₁₂O₆

6.Structure

(i)Fischer structure –

CHO — [CHOH] — CH2OH

[+] Glucose has `D’ configuration as shown

`D’ means —OHgroup on first chiral `C’ from the bottom is on right hand and + means it is dextro rotator, i.e, it rotates plane polarized light towards right.

(ii) Cyclic Structure OF Glucose: the straight chain is unable to explain the following reactions.

(a) It does not give the 2, 4‐DNP test, Schiff’s Test and does not form the hydrogensulphide product with NaHSO3 .

(b) The pentacetate of glucose does not react with NH2OH, indicating the absence of free aldehydic group.

(iii) Glucose exist in 2 different crystalline forms α and β forms. These are called anomers. They differ in optical rotation, they also differ in melting point.

Anomers are isomers which have a different configuration across C‐1 (first chiral ‘C’ atom).

7. Glycosidic Linkage:The linkage between two monosaccharide units through oxygen is called the glycosidic linkage.