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  • Write the reactions of D-glucose which cannot be explained by its open-chain structure. How can cyclic structure of glucose explain these reactions?

Write the reactions of D-glucose which cannot be explained by its open-chain structure. How can cyclic structure of glucose explain these reactions?

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Some of the short comings of the open chain structure of glucose are mentioned below, these are the facts unexplained by the open chain structure –                                                                                                                                          

(i)Even after the presence of (-CHO) an aldehydic group, it is unable to react some of the characteristic reactions of aldehydes such as,                                                                                                                                              

(a) Non-reactive with ammonia.
(b) No addition product formation when added in sodium bisulphite (NaHSO_3).
(c) The group indicator tests like Schiff’s test and 2, 4-DNP test are not valid
(ii) Due to the absence of -CHO groups in glucose pentaacetate, it does not react with hydroxylamine (-NH_2OH) but the reaction with hydroxylamine is feasible for glucose.                                                                            

  (iii) The isomerism shown by D (+) – Glucose is, \alpha-D-glucose and \beta-D-glucose. Both forms are crystalline and differ in melting points and optical rotations.
(v) Its reaction with methanol in the presence of dry hydrogen chloride gas, results into two isomers namely methyl \alpha -D-glucoside (m.p. = 438 K or 165 ^{\circ}C) and methyl \beta-D-glucoside (m.p. = 380 K or 107 ^{\circ}C).

All the above points clarify that glucose does not have an open chain structure.

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