Discuss the structure of glucose on the basis of its chemical properties.

Explanation

1. The molecular formula of glucose is found to be `C_6H_12O_6` . It can be written as `CHO-(CHOH)_4-CH_2OH`
2. Acylation of glucose with acetic anydride gives glucose pentacetate. Hence glucose molucule has five - OH groups
`underset("glucose")(CHO-(CHOH)_4-CH_2OH ) overset((CH_3CO)_2O)(to) (CHO) -underset("glucose pentacetate")( (CHO COCH_3)_4-CH_2O. COCH_3)`
3. Glucose reacts with hydroxylamine and hydrogen cyanide forming corresponding oxime and cyno hydrin respectively . Hence glucose molecule has carbonyl group.

4. Glucose on oxidation with tollen's reagent and fehling's solution gives gluconic acid. Hence carbonyl group present in glucose is aldehydic group.
`underset("glucose")(CH_2-OH-(CHOH)_4-CHO) underset("Tollen's reagent")overset((O))(to)underset("gluconic acid")((CH_2OH)-(CHOH)_4-COOH)`
5. Glucose on oxidation with nitric acid gives glucaric (saccharic) acid. This suggests that glucose molecule has primary alcoholic group `(-CH_2OH)`.
`underset("glucose")(CH_2OH-(CHOH)_4-CHO) underset(NHO_3)overset((O))(to) underset("glucaric acid")(COOH-(CHOH)_4-COOH)`
6. On reduction with HI, glucose gives n - hexane.
`underset("glucose")(CH_2-OH-(CHOH)_4-CHO) underset((H))overset(HI) (to) underset("n-hexane")(CH_3-(CH_2)_4-CH_3)`
This reaction suggest that all six carbon in glucose are linearly linked .
7. Open chain structure of glucose:
Based on above observations, glucose is given the open chain structure.

Objections against open chain structure of glucose : Open chain structure of glucose could not explain
i) Why glucose does not respond to schiff's test.
ii) why is does not react with `NaH SO_3 " and " NH_3`
ii) Why there is mutarotation.
Mutarotation :
Change in specific rotation of a compound from certain value to fixed value is known as Mutarotation.
The specific rotation of `alpha` - glucose is + `110^@` and that of `beta` - form is + `19.7^@` . If each of them is saparately dissolved in water and allowed to stand, their specific rotation gradually change and each to a specific constant value + `52.5^@`
9. Cyclic structure of glucose :

These two structures have difference in configuration at `C_1` only and are called Anomers.
Cyclic structure of glucose explains all the properties of glucose .
10. Pyranose structure of glucose :