Glucose and Fructose — Revision Notes

⚡ 30-Second Revision

Glucose: — Aldohexose, $C_6H_{12}O_6$. Primary energy source. Predominantly $\alpha$/$\beta$-D-glucopyranose (6-membered ring). Reducing sugar. Forms sorbitol on reduction. Oxidized to gluconic acid by $Br_2/H_2O$.
Fructose: — Ketohexose, $C_6H_{12}O_6$. Sweetest natural sugar. Predominantly $\alpha$/$\beta$-D-fructopyranose/furanose (6- or 5-membered ring). Reducing sugar (due to isomerization). Forms sorbitol + mannitol on reduction. Gives positive Seliwanoff's test.
Both: — Isomers, exhibit mutarotation, form same osazone with phenylhydrazine.
Key Formulas:

* Glucose (open chain): $\text{CHO}-(\text{CHOH})_4-\text{CH}_2\text{OH}$ * Fructose (open chain): $\text{CH}_2\text{OH}-\text{CO}-(\text{CHOH})_3-\text{CH}_2\text{OH}$ * Osazone formation: Involves C-1 and C-2 for aldoses, C-1 and C-2 for ketoses.

2-Minute Revision

Glucose and fructose are essential monosaccharides, both with the molecular formula $C_6H_{12}O_6$, making them structural isomers. Glucose is an aldohexose, characterized by an aldehyde group at C-1, and is the body's primary energy source.

In solution, it mainly forms a six-membered pyranose ring, existing as $\alpha$- and $\beta$-anomers that interconvert via mutarotation. Fructose, a ketohexose, has a ketone group at C-2 and is the sweetest natural sugar.

It can form both five-membered furanose and six-membered pyranose rings, also exhibiting mutarotation. Both are reducing sugars; fructose's reducing ability comes from its isomerization to an aldose in alkaline conditions.

Key reactions to remember include oxidation (glucose to gluconic acid with bromine water), reduction (glucose to sorbitol; fructose to sorbitol + mannitol), and osazone formation (both form the same osazone).

Seliwanoff's test is specific for ketohexoses like fructose. Understanding their structural differences, cyclic forms, and characteristic reactions is paramount for NEET.

5-Minute Revision

Let's consolidate our understanding of glucose and fructose, focusing on high-yield NEET concepts. Both are $C_6H_{12}O_6$ monosaccharides, but their functional groups define their distinct chemistry. Glucose is an aldohexose (aldehyde at C-1), while fructose is a ketohexose (ketone at C-2).

Structures:

Glucose: — Open-chain Fischer projection shows the aldehyde at the top. In aqueous solution, it cyclizes to form a six-membered pyranose ring (hemiacetal formation between C-1 and C-5 -OH). This creates two anomers: $\alpha$-D-glucopyranose (C-1 -OH down in Haworth) and $\beta$-D-glucopyranose (C-1 -OH up). These interconvert via mutarotation.
Fructose: — Open-chain Fischer projection shows the ketone at C-2. It can cyclize to form either a five-membered furanose ring (hemiketal between C-2 and C-5 -OH) or a six-membered pyranose ring (hemiketal between C-2 and C-6 -OH). It also exhibits mutarotation between its anomeric forms.

Reactivity:

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Reducing Sugars: — Both are reducing sugars. Glucose directly reduces Tollens' and Fehling's reagents. Fructose, despite being a ketone, also gives positive tests because it isomerizes to aldoses (glucose and mannose) in alkaline conditions via an enediol intermediate.
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Oxidation:

* Mild Oxidation (Bromine water): Glucose (aldehyde) is oxidized to gluconic acid. Fructose (ketone) is not oxidized. This is a key distinguishing test. * **Strong Oxidation (Conc. $HNO_3$):** Glucose is oxidized to saccharic acid (glucaric acid, both aldehyde and primary alcohol oxidized).

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**Reduction ($NaBH_4$ or catalytic hydrogenation):**

* Glucose: Reduces to sorbitol (glucitol). The C-1 aldehyde becomes a primary alcohol. * Fructose: Reduces to a mixture of sorbitol and mannitol. The C-2 ketone becomes a secondary alcohol, creating a new chiral center, hence two diastereomers.

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Osazone Formation (with excess phenylhydrazine): — Both glucose and fructose (along with mannose) form the same osazone (glucosazone). This is because the reaction involves C-1 and C-2, and their configurations from C-3 onwards are identical.
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Distinguishing Test (Seliwanoff's Test): — Fructose (ketohexose) gives a rapid red color with resorcinol and HCl, while glucose (aldohexose) reacts much slower or not at all.

Biological Role: Glucose is the primary energy source. Fructose is 'fruit sugar' and a component of sucrose, metabolized primarily in the liver.

Worked Example: A student is given an unknown monosaccharide. It gives a positive Seliwanoff's test and, upon reduction, yields a mixture of two alditols. Identify the monosaccharide.

Step 1: — Positive Seliwanoff's test indicates a ketohexose. This points to fructose.
Step 2: — Reduction yielding a mixture of two alditols (sorbitol and mannitol) is characteristic of fructose, as its C-2 ketone reduction creates a new chiral center. Glucose would yield only one alditol (sorbitol).
Conclusion: — The monosaccharide is fructose.

Prelims Revision Notes

Glucose and Fructose: NEET Quick Recall

1. Basic Classification & Formula:

Both are monosaccharides (cannot be hydrolyzed).
Molecular Formula: $C_6H_{12}O_6$ (structural isomers).
Glucose: Aldohexose (aldehyde at C-1).
Fructose: Ketohexose (ketone at C-2).

2. Open-Chain Structures (Fischer Projections):

Glucose: — CHO at C-1, CH$_2$OH at C-6. -OH on right at C-2, C-4, C-5 for D-glucose. -OH on left at C-3.
Fructose: — CH$_2$OH at C-1, C=O at C-2, CH$_2$OH at C-6. -OH on left at C-3. -OH on right at C-4, C-5 for D-fructose.

3. Cyclic Structures (Haworth Projections):

Glucose: — Predominantly forms a 6-membered pyranose ring (hemiacetal from C-1 aldehyde + C-5 -OH).

* Anomers: $\alpha$-D-glucopyranose (C-1 -OH down), $\beta$-D-glucopyranose (C-1 -OH up). * Mutarotation: Interconversion between $\alpha$, $\beta$ and open-chain forms in solution.

Fructose: — Can form 5-membered furanose ring (hemiketal from C-2 ketone + C-5 -OH) or 6-membered pyranose ring (C-2 ketone + C-6 -OH).

* Also exhibits mutarotation.

4. Key Chemical Reactions & Tests:

Reducing Sugars: — Both are reducing sugars.

* Glucose: Free aldehyde group reduces Tollens' ($Ag^+$ to $Ag$) and Fehling's ($Cu^{2+}$ to $Cu_2O$). * Fructose: Isomerizes to aldoses (glucose, mannose) in alkaline medium via enediol, then reduces reagents.

Oxidation:

* **Bromine Water ($Br_2/H_2O$, mild): Oxidizes glucose to gluconic acid. No reaction with fructose** (distinguishing test). * **Conc. $HNO_3$ (strong):** Oxidizes glucose to saccharic acid.

**Reduction ($NaBH_4$ or $H_2/Ni$):**

* Glucose $\rightarrow$ Sorbitol (glucitol, one product). * Fructose $\rightarrow$ Mixture of Sorbitol and Mannitol (two products due to new chiral center at C-2).

Osazone Formation (excess phenylhydrazine): — Both glucose and fructose (and mannose) form the same osazone (glucosazone). Reaction involves C-1 and C-2.
Seliwanoff's Test: — Specific for ketohexoses. Fructose gives a rapid red color. Glucose reacts slowly or not at all (distinguishing test).

5. Biological Significance:

Glucose: Primary energy source ('blood sugar').
Fructose: 'Fruit sugar', sweetest natural sugar, component of sucrose, metabolized in liver.

Vyyuha Quick Recall

Glucose is an Aldo-sugar, Fructose is a Keto-sugar. Bromine water for Glucose, Seliwanoff for Fructose. Sorbitol from Glucose, Sorbitol and Mannitol from Fructose. Osazones are the Same for Glucose and Fructose.