Carbohydrates — Part 1: Classification and the structure of glucose: visual lesson, ISC Class 12 Chemistry

Lesson

Sugars, starch and fibre

Part 1 — what makes a compound a carbohydrate, and how chemists proved the structure of glucose using nothing but test-tube reactions.

In this lesson you will

Classify carbohydrates by hydrolysis and by reducing power
Explain how six reactions of glucose establish its open-chain structure
Explain why glucose must also exist as a six-membered ring, and what anomers are
Assign D or L configuration to a sugar from its Fischer projection

Table of everyday materials and the carbohydrate each mainly contains
Everyday material	Main carbohydrate
Honey, grapes	Glucose and fructose
Jaggery, table sugar	Sucrose
Milk	Lactose
Rice, wheat, potatoes	Starch
Cotton, paper, jute	Cellulose

Concept

Definition and classification

Carbohydrates are optically active polyhydroxy aldehydes or ketones, or substances that give them on hydrolysis. Many fit Cₓ(H₂O)ᵧ, but the formula is not a definition: rhamnose C₆H₁₂O₅ is a carbohydrate, while ethanoic acid C₂H₄O₂ is not. By hydrolysis: monosaccharides (no simpler sugar), oligosaccharides (2–10 units) and polysaccharides (many units). Monosaccharides are aldoses (–CHO) or ketoses (C=O), named by carbon count: aldopentose, ketohexose.

Table classifying carbohydrates by the number of units obtained on hydrolysis, with examples, taste and solubility
Class	Units on hydrolysis	Examples	Taste and solubility
Monosaccharide	Does not hydrolyse	Glucose, fructose, ribose	Sweet, soluble (sugars)
Disaccharide	2	Sucrose, maltose, lactose	Sweet, soluble (sugars)
Trisaccharide	3	Raffinose	Sweet, soluble
Polysaccharide	Hundreds to thousands	Starch, cellulose, glycogen	Not sweet, mostly insoluble (non-sugars)

Concept

Reducing and non-reducing sugars; importance

A sugar that reduces Tollens’ reagent (silver mirror) or Fehling’s solution (red Cu₂O) is a reducing sugar: it has a free aldehyde or α-hydroxy ketone group, or a ring that can open to one. All monosaccharides are reducing. A disaccharide is non-reducing if both anomeric carbons are tied up in the glycosidic link (sucrose). Importance: energy source (glucose, starch, glycogen), structure (cellulose in plants), raw materials (cotton, paper, ethanol by fermentation), and the sugars ribose and deoxyribose in nucleic acids.

Table of six carbohydrates with their response to Tollens’ and Fehling’s reagents and their classification as reducing or non-reducing
Sugar	Tollens’ / Fehling’s	Classification
Glucose	Positive	Reducing
Fructose	Positive	Reducing
Maltose	Positive	Reducing
Lactose	Positive	Reducing
Sucrose	Negative	Non-reducing
Starch	Negative (practically)	Non-reducing

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