CBSE Class 12 Chemistry Notes: Biomolecules – Important Reactions

(a) Kiliani – Fischer Synthesis: – This is a method of lengthening the carbon chain of an aldose. To illustrate, we take synthesis of D-threose and D-erythrose (Aldotetroses) from D-glyceraldehyde (an aldotriose).

Addition of HCN to glyceraldehyde produces two epimeric cyanohydrins because reaction creates a new stereocenter. The cyanohydrins can be separated easily (since they are diastereomers) and each can be converted to an aldose through hydrolysis, acidification and lactonisation, and reduction with Na—Hg in presence of H₂SO₄. One cyanohydrin ultimately yields D-erythrose and D-threose.

Here we can see that both sugars are D-sugars because starting compound is  D-glyceraldehyde and its stereocentrer is unaffected by its synthesis.

(b) Ruff Degradation: It is opposite to Kiliani Fischer synthesis that can be used to shorten the chain by a similar unit. The ruff degradation involves (i) Oxidation of the aldose to an aldonic acid using Bromine water. (ii)Oxidative decarboxylation of the aldonic acid to the next lower aldose using H₂O₂ and Fe₂(SO₄)₃. D-ribose for example can be reduced to D-erythrose.

Illustration 1. The open – chain glucose on oxidation with HIO₄ gives

                        (A)  5 HCOOH + H₂C = O                        (B)  4 HCOOH + 2H₂C = O

                        (C)  3 HCOOH + 3H₂C = O                      (D)  2HCOOH + 4H₂C = O

Solution: (A) C = O both CHO and CHOH form HCOOH. While HOCH₂ forms H₂C = O giving 5 HCOCH + H₂C = O.

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