Question

Write chemical equations of two methods of preparation of glucose. How will you obtain saccharic acid and gluconic acid from glucose?

Hint:

Mild oxidant (Br$_2$/H$_2$O) stops at the aldonic acid (gluconic acid), while hot conc.\ HNO$_3$ over-oxidises both ends of glucose to the dicarboxylic saccharic (glucaric) acid.

Answer 1

Two preparations of glucose:
(1) Hydrolysis of starch
\[ (\mathrm{C_6H_{10}O_5})_n + n\,\mathrm{H_2O} \xrightarrow[\text{steam}]{\text{dil.\ H$_2$SO$_4$}} n\,\mathrm{C_6H_{12}O_6} \]
(2) Hydrolysis of sucrose
\[ \mathrm{C_{12}H_{22}O_{11}} + \mathrm{H_2O} \xrightarrow[\text{invertase}]{\text{dil.\ acid}} \mathrm{C_6H_{12}O_6}\ (\text{glucose}) + \mathrm{C_6H_{12}O_6}\ (\text{fructose}) \]
From glucose to saccharic acid and gluconic acid:
(a) Gluconic acid (oxidise \(-\)CHO only)
\[ \mathrm{C_6H_{12}O_6} + \mathrm{Br_2} + \mathrm{H_2O} \rightarrow \mathrm{C_6H_{12}O_7}\ (\text{gluconic\ acid}) + 2\,\mathrm{HBr} \]
(b) Saccharic (glucaric) acid (oxidise \(-\)CHO and \(-\)CH$_2$OH)
\[ \mathrm{C_6H_{12}O_6} + 2\,[O] \xrightarrow{\mathrm{conc.\ HNO_3,\ \Delta}} \mathrm{HOOC\!-\!(CHOH)_4\!-\!COOH}\ (\text{saccharic\ acid}) + \mathrm{H_2O} \]