Dictionary of Chemistry [6th Ed.]

are formed by oxidation of primary
alcohols; further oxidation yields
carboxylic acids. They are reducing
agents and tests for aldehydes in-
clude Fehling’s test and Tollens
reagent. Aldehydes have certain char-
acteristic addition and condensation
reactions. With sodium hydrogensul-
phate(IV) they form addition com-
pounds of the type [RCOH(SO 3 )H]–
Na+. Formerly these were known as
bisulphite addition compounds. They
also form addition compounds with
hydrogen cyanide to give cyanohy-
drins and with alcohols to give ac-
etals and undergo condensation
reactions to yield oximes, hydra-
zones, and semicarbazones. Aldehy-
des readily polymerize. See also
ketones.

A

• Information about IUPAC nomenclature

17 alicyclic compound

a

RC

O

H

aldehyde group

Aldehyde

aldohexoseSee monosaccharide.

aldol See aldol reaction.

aldol reaction A reaction of alde-
hydes of the type

2RCH 2 CHO ˆ

RCH 2 CH(OH)CHRCHO

where R is a hydrocarbon group. The
resulting compound is a hydroxy-
aldehyde, i.e. an aldehyde–alcohol or
aldol, containing alcohol (–OH) and
aldehyde (–CHO) groups on adjacent
carbon atoms. The reaction is base-
catalysed, theÜrst step being the for-
mation of a carbanion of the type
RHC–CHO, which adds to the car-
bonyl group of the other aldehyde
molecule. For the carbanion to form,
the aldehyde must have a hydrogen

atom on the carbon next to the car-

bonyl group.

Aldols can be further converted to

other products; in particular, they

are a source of unsaturated aldehy-

des. For example, the reaction of

ethanal gives 3-hydroxybutenal (ac-

etaldol):

2CH 3 CHO ˆCH 3 CH(OH)CH 2 CHO

This can be further dehydrated to 2-

butenal (crotonaldehyde):

CH 3 CH(OH)CH 2 CHO →H 2 O +

CH 3 CH:CHCHO

aldoseSee monosaccharide.

aldosterone A hormone produced

by the adrenal glands that controls

excretion of sodium by the kidneys

and thereby maintains the balance of

salt and water in the bodyÛuids.

aldoximesCompounds formed by

reaction between hydroxylamine and

an aldehyde

RCOH + H 2 NOH →RCH:NOH + H 2 O

If R is an aliphatic group, the al-

doxime is generally a liquid or low-

melting solid. If R is aromatic, the

aldoxime is a crystalline solid. Al-

doximes have a planar structure and

can exist in two isomeric forms. In

the syn form, the OH group is on the

same side of the double bond as the

H. In the anti form the OH and H are

on opposite sides. Typically, aliphatic

aldehydes give anti aldoximes; aro-

matic aldehydes give syn aldoximes.

algin(alginic acid)A complex poly-

saccharide occurring in the cell walls

of the brown algae (Phaeophyta).

Algin strongly absorbs water to form

a viscous gel. It is produced commer-

cially from a variety of species of

Laminaria and from Macrocystis pyrifera

in the form of alginates, which are

used mainly as a stabilizer and tex-

turing agent in the food industry.

alicyclic compoundA compound