Dictionary of Chemistry [6th Ed.]

industrial processes, such as the

reduction of oxide ores, the reÜning

of petroleum, the production of

hydrocarbons from coal, and the hy-

drogenation of vegetable oils. Consid-

erable interest has also been shown

in its potential use in a ‘hydrogen

fuel economy’ in which primary en-

ergy sources not based on fossil fuels

(e.g. nuclear, solar, or geothermal en-

ergy) are used to produce electricity,

which is employed in electrolysing

water. The hydrogen formed is

stored as liquid hydrogen or as metal

hydrides. Chemically, hydrogen re-

acts with most elements. It was dis-

covered by Henry *Cavendish in

1766.

A

• Information from the WebElements site

hydrogen acceptor See hydrogen

carrier.

hydrogenation 1.A chemical re-

action with hydrogen; in particular,

an addition reaction in which hydro-

gen adds to an unsaturated com-

pound. Nickel is a good catalyst for

such reactions. 2.The process of

converting coal to oil by making the

carbon in the coal combine with hy-

drogen to form hydrocarbons. See fis-

cher–tropsch process; bergius

process.

hydrogen azide (hydrazoic acid;

azoimide)A colourless liquid, HN 3 ;

r.d. 1.09; m.p. –80°C; b.p. 37°C. It is

highly toxic and a powerful reduc-

tant, which explodes in the presence

of oxygen and other oxidizing

agents. It may be prepared by the re-

action of sodium amide and sodium

nitrate at 175°C followed by distilla-

tion of a mixture of the resulting

sodium azide and a dilute acid. See

also azides.

hydrogen bond A type of electro-

static interaction between molecules

occurring in molecules that have

hydrogen atoms bound to electro-

negative atoms (F, N, O). It can be

regarded as a strong dipole–dipole

attraction caused by the electron-

withdrawing properties of the elec-

tronegative atom. Thus, in the water

molecule the oxygen atom attracts

the electrons in the O–H bonds. The

hydrogen atom has no inner shells of

electrons to shield the nucleus, and

there is an electrostatic interaction

between the hydrogen proton and a

lone pair of electrons on an oxygen

atom in a neighbouring molecule.

Each oxygen atom has two lone pairs

and can make hydrogen bonds to

two different hydrogen atoms. The

strengths of hydrogen bonds are

about one tenth of the strengths of

normal covalent bonds. Hydrogen

bonding does, however, have signiÜ-

cant effects on physical properties.

Thus it accounts for the unusual

properties of *water and for the rela-

tively high boiling points of H 2 O, HF,

and NH 3 (compared with H 2 S, HCl,

and PH 3 ). It is also of great impor-

tance in living organisms. Hydrogen

bonding occurs between bases in the

chains of DNA. It also occurs be-

tween the C=O and N–H groups in

proteins, and is responsible for main-

taining the secondary structure.

hydrogen acceptor 276

h

oxygen hydrogen

0.177nm

Hydrogen bond