116 HYDROGENATOMIC HYDROGENIf a high voltage electric discharge is passed through hydrogen at
low pressure, a small fraction of the hydrogen molecules are disso-
ciated into atoms, which are highly reactive and unite with many
elements to give hydrides. If a metal such as zinc is dissolved in
acid, hydrogen gas is evolved, and thus the dissolving metal is a
good reducing agent: Zn2+(aq) + 2 e~ -> Zn(s): £^ = —0.76V.
Here, therefore, hydrogen is being formed as a reduction product of
the proton: H 3 O+ + e' -~> |H 2 (g) + H 2 O: E^ = 0 V, and it is not
itself the reducing agent, (As we have seen, the kinetic stability of
the hydrogen molecule makes it a poor reducing agent in practice.)
However, it is probable that hydrogen atoms can be produced by
proton reduction (i.e. by the process H+ +e~->H); these will all
usually unite with each other to give molecular hydrogen, but can
attack other species present. Thus in the reduction of an arsenic-con-
taining compound to arsine (AsH 3 ) or of an alkyl halide (C 2 H 5 C1) to
an alkane (C 2 H 6 ) by a metal couple (Al-Zn-Cu) in aqueous acid,
hydrogen atoms may participate in the reaction.
Deuterium, the isotope of hydrogen fH, is made by prolonged
electrolysis of water, during which hydrogen is evolved preferentially
to deuterium at the cathode. Consequently the residual water is
enriched in deuterium oxide, D 2 O, ('heavy water). The D 2 O finally
obtained has a b.p. 374.2 K and a density at 293 K of 1.106 gcm"^3
(water, 0.998 g cm ~^3 ); electrolysis of D 2 O gives deuterium which
again has physical properties slightly different from those of hydro-
gen (for example b.p. 24 K). Ordinary hydrogen contains about 1
part in 6000 of deuterium.
The slightly different physical properties of deuterium allow its
concentration in ordinary hydrogen (or the concentration of a
deuterium-containing compound in a hydrogen compound) to be
determined. Exchange of deuterium and hydrogen occurs and can
be used to elucidate the mechanism of reactions (i.e. the deuterium
is a non-radioactive tracer). Methanol exchanges with deuterium
oxide thus:
CH 3 OH + D 2 O ^ CH 3 OD + HDOThe hydroxyl hydrogen exchanges but the hydrogen atoms of the
CH 3 (methyl) group do not.
This method produces an endothermic hydride by indirect means.