Write a note on the preparation and structure of glucose.

(N/A) Glucose occurs freely in nature as well as in a combined form. It is present in sweet fruits and honey. Ripe grapes contain glucose in large amounts and thus it is called "Grape Sugar".
Preparation of Glucose:
$(i)$ From cane sugar: The glucose and fructose are obtained in equal amounts when an alcoholic solution of cane sugar is boiled with dilute $HCl$ or $H_{2}SO_{4}$.
$C_{12}H_{22}O_{11} + H_{2}O \xrightarrow{H^{+}} C_{6}H_{12}O_{6} + C_{6}H_{12}O_{6}$
After completion of this reaction,alcohol is added in excess. Glucose,being insoluble in alcohol,precipitates and is filtered out,whereas fructose remains soluble.
$(ii)$ From Starch: Commercially,glucose is manufactured by the hydrolysis of starch by boiling it with dilute $H_{2}SO_{4}$ at $393 \ K$ and $2-3 \ atm$ pressure.
$(C_{6}H_{10}O_{5})_{n} + nH_{2}O \xrightarrow{H^{+}, 393 \ K, 2-3 \ atm} nC_{6}H_{12}O_{6}$
After hydrolysis,calcium carbonate is added to neutralize excess acid,forming $CaSO_{4}$ precipitates which are filtered. The filtrate is decolorized with charcoal and concentrated to obtain glucose crystals.
Structure of Glucose:
Glucose is an aldohexose with the open-chain structure: $CHO-(CHOH)_{4}-CH_{2}OH$.
Evidence for the structure includes the reaction with $HI$ to form $n$-hexane,indicating a straight chain of $6$ carbon atoms:
$CHO-(CHOH)_{4}-CH_{2}OH \xrightarrow{HI, \Delta} CH_{3}-CH_{2}-CH_{2}-CH_{2}-CH_{2}-CH_{3}$