8 min readConcept sheet1-mark MCQ and assertion-reason, and 2-mark 'define and give examples' questions

Carbohydrates and their classification

Carbohydrates are optically active polyhydroxy aldehydes or ketones, or compounds that give them on hydrolysis. They are grouped by how many sugar units hydrolysis gives.

DefinitionOptically active polyhydroxy aldehydes or ketones, or compounds that give these units on hydrolysis

General formulaMany fit Cx(H₂O)y, but not all: rhamnose, C₆H₁₂O₅, is a carbohydrate; acetic acid, C₂H₄O₂, is not

Reducing sugarsReduce Fehling's solution and Tollens' reagent: all monosaccharides, maltose and lactose. Sucrose is non-reducing

Aldose and ketoseAldehyde group (glucose, an aldohexose) or keto group (fructose, a ketohexose)

What carbohydrates are

Carbohydrates are made by plants and form a large part of our food. Many have the formula Cx(H₂O)y, so they were once called 'hydrates of carbon'. The formula is not a good test. Rhamnose, C₆H₁₂O₅, is a carbohydrate but does not fit it. Acetic acid, CH₃COOH (C₂H₄O₂), fits it but is not a carbohydrate. So chemists define carbohydrates by structure: optically active polyhydroxy aldehydes or ketones, or compounds that give such units on hydrolysis (splitting by water). Sweet carbohydrates are called sugars. Carbohydrates are also called saccharides, from a Greek word for sugar.

Classification by behaviour on hydrolysis

Class	Hydrolysis gives	Examples
Monosaccharides	Cannot be hydrolysed further	Glucose, fructose, ribose
Oligosaccharides	2 to 10 monosaccharide units	Disaccharides: sucrose (glucose + fructose), maltose (two glucose), lactose (glucose + galactose); also tri- and tetrasaccharides
Polysaccharides	A very large number of monosaccharide units	Starch, cellulose, glycogen, gums

Classifying monosaccharides

Carbon atoms	General term	With an aldehyde group	With a keto group
3	Triose	Aldotriose (glyceraldehyde)	Ketotriose (dihydroxyacetone)
4	Tetrose	Aldotetrose	Ketotetrose
5	Pentose	Aldopentose (ribose)	Ketopentose
6	Hexose	Aldohexose (glucose)	Ketohexose (fructose)
7	Heptose	Aldoheptose	Ketoheptose

Reducing vs non-reducing sugars

Reducing sugarsHave a free aldehyde or keto group, or can form one in solution. They reduce Fehling's solution (orange-red Cu₂O) and Tollens' reagent (silver mirror). Examples: all monosaccharides, maltose and lactose.

Non-reducing sugarsThe reducing groups of both units are used up in the bond joining them, so no free group is left. Sucrose is the example to learn.

Ketoses such as fructose also reduce Fehling's and Tollens' reagents, although simple ketones do not.

Sugars vs non-sugars

SugarsSweet, crystalline and soluble in water: monosaccharides and oligosaccharides such as glucose and sucrose.

Non-sugarsNot sweet: polysaccharides such as starch and cellulose.

In real life

The old urine sugar test for diabetes, still seen in some Indian clinics, boils urine with Benedict's solution. A green, orange or red precipitate shows a reducing sugar, glucose.

Exam angle

2 marks: “Define a reducing sugar. Is sucrose one? Give a reason.” Sugars that reduce Fehling's solution and Tollens' reagent. Sucrose is non-reducing, because the reducing groups of glucose (C1) and fructose (C2) are used in the glycosidic bond.

1 marks: “Classify glucose and fructose as aldose or ketose, with the number of carbon atoms.” Glucose: aldohexose. Fructose: ketohexose.

2 marks: “Why is the formula Cx(H₂O)y not a good definition of a carbohydrate? Give two examples.” Rhamnose, C₆H₁₂O₅, is a carbohydrate but does not fit it; acetic acid, C₂H₄O₂, fits it but is not a carbohydrate.

Remember it

Mono = 1 (cannot split), oligo = 2 to 10, poly = many. Sucrose is the sweet one that will not reduce.