Dictionary of Chemistry [6th Ed.]

form constitute a pair of diastereoiso-

mers. See optical activity.

diatomaceous earthSee kiesel-

guhr.

diatomic moleculeA molecule

formed from two atoms (e.g. H 2 or

HCl).

diatomiteSee kieselguhr.

diazepamA *benzodiazepine used

medically to treat anxiety, convul-

sions, insomnia, and alcohol with-

drawal. It is widely used, and often

known under its tradename Valium.

diazineSee azine.

diazo compoundsOrganic com-

pounds containing two linked nitro-

gen compounds. The term includes

*azo compounds, diazonium com-

pounds, and also such compounds as

diazomethane, CH 2 N 2.

A

• Information about IUPAC nomenclature
  diazonium saltsUnstable salts
  containing the ion C 6 H 5 N 2 +(the dia-
  zonium ion: see formula). They are
  formed by *diazotization reactions.

diatomaceous earth 174

d

N

N

3 CH O

Cl

Diazepam

Diazonium salt

diazotizationThe formation of a

*diazonium salt by reaction of an

aromatic amine with nitrous acid at

low temperature (below 5°C). The ni-

trous acid is produced in the reaction

mixture from sodium nitrite and hy-

drochloric acid:

ArNH 2 + NaNO 2 + HCl →ArN+N +

Cl–+ Na++ OH–+ H 2 O

dibasic acidAn *acid that has two

acidic hydrogen atoms in its mol-

ecules. Sulphuric (H 2 SO 4 ) and car-

bonic (H 2 CO 3 ) acids are common

examples.

diboron trioxide See boron(iii)

oxide.

1,2-dibromoethane A colourless

liquid *haloalkane, BrCH 2 CH 2 Br; r.d.

2.2; m.p. 9.79°C; b.p. 131.36°C. It is

made by addition of bromine to

ethene and used as an additive in

petrol to remove lead during com-

bustion as the volatile lead bromide.

dicarbideSee carbide.

dicarboxylic acidA *carboxylic

acid having two carboxyl groups in

its molecules. In systematic chemical

nomenclature, dicarboxylic acids are

denoted by the sufÜx -dioic; e.g. hexa-

nedioic acid, HOOC(CH 2 ) 4 COOH.

dichlorine oxide (chlorine monox-

ide)A strongly oxidizing orange gas,

Cl 2 O, made by oxidation of chlorine

using mercury(II) oxide. It is the acid

anhydride of chloric(I) acid.

dichlorobenzene Any one of three

isomeric liquid aromatic compounds,

C 6 H 4 Cl 2. 1,2-Dichlorobenzene (b.p.

179 °C) and 1,4-dichlorobenzene (b.p.

174 °C) are made by chlorinating

benzene with an iron catalyst and

separating the mixed isomers by

fractional distillation; 1,3-dichloro-

benzene(b.p. 172°C) is made from

one of the other two by catalytic iso-

merization. The 1,2 isomer is used as

an insecticide and in making dyes;

the 1,4 compound is employed as a

deodorant and moth repellent.