Q.What happens when D-glucose is heated with HI? (Give equation only)

Bromine water is a mild oxidising agent that selectively oxidises the aldehyde group of open-chain glucose to a carboxylic acid, without touching the -OH groups; the product is gluconic acid.

Open-chain glucose has a -CHO group at C1. Bromine water (a mild oxidant, milder than nitric acid) selectively oxidises this aldehyde group:

CH2OH-(CHOH)4-CHO (glucose) --(Br2 water)--> CH2OH-(CHOH)4-COOH (gluconic acid)

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Hydroxylamine reacts with the free aldehyde group of D-glucose in a typical carbonyl condensation, replacing the $C=O$ oxygen with $=N-OH$ to give the oxime.

D-glucose exists predominantly in its cyclic hemiacetal form but is in equilibrium with a small amount of the open-chain form, which carries a free aldehyde group at C1:

$D\text{-glucose (open chain)}: \ CHO-(CHOH)_4-CH_2OH$

Hydroxylamine, $NH_2OH$, is a classic carbonyl-reactive reagent: its nucleophilic nitrogen attacks the aldehyde carbon, and after loss of a water molecule, the $C=O$ is converted into a $C=N-OH$ (oxime) linkage:

$CHO-(CHOH)_4-CH_2OH + NH_2OH \longrightarrow CH{=}NOH-(CHOH)_4-CH_2OH + H_2O$

i.e. glucose oxime: $CH_2OH-(CHOH)_4-CH=N-OH$.

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Glucose + HI (Δ) → n-hexane; strong reduction removes every oxygen function, proving a straight six-carbon chain.

Glucose is an open-chain aldohexose: OHCH2-(CHOH)4-CHO. On heating with hydroiodic acid (HI), a powerful reducing agent, all the hydroxyl (-OH) groups and the aldehyde (-CHO) group are reduced (oxygen functions replaced by hydrogen).

The carbon skeleton is left intact as a straight chain of six carbons:

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Prolonged heating of glucose with concentrated HI removes all the oxygen-bearing functional groups (reducing the carbonyl and every hydroxyl), leaving a straight-chain saturated hydrocarbon — evidence that glucose's six carbon atoms are unbranched.

On strong/prolonged treatment with concentrated hydroiodic acid (HI) and red phosphorus on heating, D-glucose undergoes complete reduction of the carbonyl group and all five hydroxyl groups (a total deoxygenation), converting the entire six-carbon skeleton into the saturated straight-chain alkane n-hexane:

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