Glucose and Fructose: Structure, Properties, and Applications

Carbohydrates

Definition and Classification

Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen, typically with the general formula Cn(H2O)m. They are essential biomolecules found in all living organisms and serve as a primary source of energy.

• Monosaccharides: Simple sugars with the formula C6H12O6 (e.g., glucose, fructose).

• Disaccharides: Composed of two monosaccharide units (e.g., sucrose, C12H22O11).

• Polysaccharides: Long chains of monosaccharide units (e.g., starch, (C6H10O5)n).

Not all compounds with the formula Cn(H2O)m are carbohydrates; classification depends on structural features such as the presence of polyhydroxy and carbonyl groups.

Key Structural Features

• Carbohydrates are classified based on the number of sugar units and the nature of their functional groups (aldehyde or ketone).

• Monosaccharides are further classified as aldoses (containing an aldehyde group) or ketoses (containing a ketone group).

Glucose and Fructose

Occurrence and Physical Properties

Glucose and fructose are two important monosaccharides found widely in nature.

• Glucose: A sweet, highly water-soluble solid found in fruits, honey, and plant tissues. In human blood, its concentration is about 4.4–7.2 mmol/L (80–130 mg/dL).

• Fructose: A sweet, highly water-soluble solid present in many fruits (pineapple, mango, apple, pomegranate, pear) and especially abundant in honey (about 40% fructose and 30% glucose by mass).

Example: Honey is sweeter than ripe fruits because it contains a higher proportion of fructose, which is sweeter than glucose.

Molecular Structure

Both glucose and fructose are polyhydroxy carbonyl compounds, meaning they contain multiple hydroxyl (−OH) groups and a carbonyl group (either aldehyde or ketone).

• Glucose: An aldohexose (contains an aldehyde group at C1).

• Fructose: A ketohexose (contains a ketone group at C2).

Both can exist in open-chain (acyclic) and cyclic (ring) forms. In solution, these forms interconvert.

Open-Chain Structures

• Both have unbranched carbon chains, multiple hydroxyl groups, and one carbonyl group.

• Glucose: Five consecutive hydroxyl groups and one terminal aldehyde group.

• Fructose: Four consecutive hydroxyl groups and one ketone group at C2.

Chemical Properties

The chemical reactivity of glucose and fructose is determined by their functional groups. Key reactions include:

1. Reaction with Copper(II) Hydroxide [Cu(OH)2]

Both glucose and fructose react with Cu(OH)2 due to their polyhydroxy structure, forming a deep blue solution.

• Glucose (and fructose) dissolve Cu(OH)2 due to the formation of a complex with the hydroxyl groups.

Equation: $\mathrm{C_6H_{12}O_6 + Cu(OH)_2 \rightarrow [C_6H_{11}O_6]Cu + 2H_2O}$

2. Reaction with Cu(OH)2 in Alkaline Solution (Heated)

When heated with Cu(OH)2 in NaOH, glucose (an aldehyde) reduces Cu(II) to Cu(I), forming a brick-red precipitate of Cu2O. Fructose, though a ketose, can isomerize under alkaline conditions and also gives a positive result.

Equation: $\mathrm{CH_2OH[CHOH]_4CHO + 2Cu(OH)_2 + NaOH \rightarrow CH_2OH[CHOH]_4COONa + Cu_2O \downarrow + 3H_2O}$

3. Reaction with Tollens' Reagent (Silver Mirror Test)

Glucose reacts with Tollens' reagent (ammoniacal AgNO3) to produce a silver mirror, indicating the presence of an aldehyde group. Fructose can also give a positive result after isomerization in basic medium.

Equation: $\mathrm{RCHO + 2[Ag(NH_3)_2]^+ + 3OH^- \rightarrow RCOO^- + 2Ag \downarrow + 4NH_3 + 2H_2O}$

4. Reaction with Bromine Water

Glucose reacts with bromine water, which oxidizes the aldehyde group to a carboxylic acid, forming gluconic acid. Fructose does not react directly with bromine water.

Equation: $\mathrm{CH_2OH[CHOH]_4CHO + Br_2 + H_2O \rightarrow CH_2OH[CHOH]_4COOH + 2HBr}$

5. Fermentation Reactions

Glucose can undergo fermentation to produce ethanol (alcoholic fermentation) or lactic acid (lactic fermentation), depending on the enzymes present.

• Alcoholic fermentation: $\mathrm{C_6H_{12}O_6 \xrightarrow{enzyme} 2C_2H_5OH + 2CO_2}$

• Lactic fermentation: $\mathrm{C_6H_{12}O_6 \xrightarrow{enzyme} 2C_3H_6O_3}$

Fermentation requires moderate temperatures; high temperatures can denature the enzymes involved.

6. Hemiacetal Hydroxyl Group

The hemiacetal −OH group in cyclic glucose is more reactive than other hydroxyl groups and can react with alcohols to form acetals (glycosides). In open-chain glucose, this group is absent.

Summary Table: Characteristic Reactions of Glucose and Fructose

Test	Glucose	Fructose
Cu(OH)2 (cold)	Blue solution (polyhydroxy)	Blue solution (polyhydroxy)
Cu(OH)2 + NaOH (hot)	Brick-red Cu2O (aldehyde)	Brick-red Cu2O (after isomerization)
Tollens' reagent	Silver mirror (aldehyde)	Silver mirror (after isomerization)
Bromine water	Oxidation to gluconic acid	No reaction
Fermentation	Alcoholic/lactic	Alcoholic/lactic

Applications of Glucose and Fructose

• Widely used in the food industry (sweets, syrups, jams, canned fruits).

• Glucose is used in silvering mirrors, making candy, vitamin C production, and as a medical intravenous solution for treating hypoglycemia.

• Fructose is used in syrups, candies, jams, and canned fruits due to its high sweetness.

Example: In medicine, 5% glucose solution is administered intravenously to quickly raise blood sugar levels in cases of hypoglycemia.