Chemistry, Third edition

CARBONYL COMPOUNDS 337

An aldehyde or ketone is converted to the corresponding dinitrophenyl-

hydrazone (DNP). The DNP is purified by recrystallization and its melting point

compared with those reported in data books. In this way the original carbonyl

compound can be identified.

Reactions of aldehydes and ketones

Draw the structural formulae of the organic products of the following reactions:

(i) The reaction of butanal on heating with Fehling’s solution.

(ii)The reaction of propanone with 2,4-dinitrophenylhydrazine.

(iii)The reaction of propanal with hydrazine.

(iv)The reaction of propanal on warming with Tollens’ reagent.

Exercise 18H

BOX 18.1

Carbohydrates

Carbohydrates are naturally occurring

compounds of carbon, hydrogen and oxygen,

with hydrogen and oxygen atoms in the ratio

2:1 as in water. They are polyhydroxy(they

contain many –OH groups) aldehydes and

ketones, or compounds that can be

hydrolysed to make polyhydroxy aldehydes or

ketones. The chemical names of many

carbohydrates end in ‘ose’. Starch, table

sugar (sucrose) and cotton and paper

(cellulose) are all composed of

carbohydrates.

The simplest carbohydrates are the

monosaccharides(or simple sugars). They

cannot be split up into simpler carbohydrate

units. Glucose (C 6 H 12 O 6 ) and fructose

(C 6 H 12 O 6 ) are monosaccharides:

Glucose is a sugar found in grapes, whereas

fructose is the very sweet-tasting sugar found

in honey and fruit. Because simple carbohy-

drates have a large number of –OH groups they

are able to hydrogen bond and are therefore

very soluble in water. Sugars also undergo

some reactions that are typical of carbonyl

compounds; for example, glucose reacts with

DNP to give a crystalline derivative.

In addition to the ‘open chain’ form described

above, glucose can exist in a ring form:

The–OH group on carbon atom 5, reacts with

the carbonyl group 1, and a ring is formed. In

aqueous solution an equilibrium exists

between the two forms.

Monosaccharide units can join together by

eliminating water: in twos (disaccharides),

between two and eight units

(oligosaccharides) and more than eight units

(polysaccharides). Sucrose is a disaccharide

that can be broken up into glucose and

fructose units, whereas starch (from flour)

and cellulose (from plants) are both

polysaccharides. Our bodies convert sucrose

and starch to glucose, which we either use for

energy or store as glycogen(another

polysaccharide). Glycogen is reconverted to

glucose when energy is needed.