CBSE Class 12 Chemistry Biomolecules Notes Set C

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) OligosaccharidesThose 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 SugarsThose which reduce Fehling’s or Tollen’s reagent. They have free aldehydic groups, eg , glucose, fructose , galactose

(ii)Non Reducing SugarsThose 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)F mSucrose

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

(ii)From tarch

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

6.Structure

(i)Fischer structure –

CHO — (CHOH)₄ — CH₂OH

(+) Glucose has `D’ configuration as shown

`D’ means —OH group 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. 

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