with a chemical structure that is sim-
ilar to that of an existing drug. De-
signer drugs are produced either to
increase potency or to avoid detec-
tion. An example is ecstasy (MDMA),
which is a variant of MDA.desorptionThe removal of ad-
sorbed atoms, molecules, or ions
from a surface.destructive distillationThe
process of heating complex organic
substances in the absence of air so
that they break down into a mixture
of volatile products, which are con-
densed and collected. At one time
the destructive distillation of coal (to
give coke, coal tar, and coal gas) was
the principal source of industrial or-
ganic chemicals.desulphurationThe removal of
sulphur from a compound. Desulphu-
ration has been implicated in the
toxicity of a number of sulphur-
containing organic compounds: it is
postulated that the atomic sulphur
released into a cell by desulphura-
tion, which is highly electrophilic,
can bind to proteins and thereby
alter their function.detailed balanceThe cancellation
of the effect of one process by an-
other process that operates at the
same time with the opposite effect.
An example of detailed balance is
provided by a chemical reaction be-
tween two molecular species A and
B, which results in the formation of
the molecular species C and D. De-
tailed balance for this chemical reac-
tion occurs if the rate at which the
reaction A + B →C + D occurs is
equal to the rate at which the reac-
tion C + D →A + B occurs. The equi-
librium state in thermodynamics is
characterized by detailed balance.
detergentA substance added to
water to improve its cleaning proper-
ties. Although water is a powerfulsolvent for many compounds, it will
not dissolve grease and natural oils.
Detergents are compounds that cause
such nonpolar substances to go into
solution in water. *Soap is the origi-
nal example, owing its action to
the presence of ions formed from
long-chain fatty acids (e.g. the
octadecanoate (stearate) ion,
CH 3 (CH 2 ) 16 COO–). These have two
parts: a nonpolar part (the hydro-
carbon chain), which attaches to the
grease; and a polar part (the –COO–
group), which is attracted to the
water. A disadvantage of soap is that
it forms a scum with hard water (see
hardness of water) and is relatively
expensive to make. Various synthetic
(‘soapless’) detergents have been de-
veloped from petrochemicals. The
commonest, used in washing pow-
ders, is sodium dodecylbenzene-
sulphonate, which contains
CH 3 (CH 2 ) 11 C 6 H 4 SO 2 O–ions. This, like
soap, is an example of an anionic de-
tergent, i.e. one in which the active
part is a negative ion. Cationic deter-
gentshave a long hydrocarbon
chain connected to a positive ion.
Usually they are amine salts, as in
CH 3 (CH 2 ) 15 N(CH 3 ) 3 +Br–, in which the
polar part is the –N(CH 3 ) 3 +group.
Nonionic detergents have non-
ionic polar groups of the type
–C 2 H 4 –O–C 2 H 4 –OH, which form
hydrogen bonds with the water.
Synthetic detergents are also used as
wetting agents, emulsiÜers, and sta-
bilizers for foam.detonating gasSee electrolytic
gas.deuterated compoundA com-
pound in which some or all of the
hydrogen–1 atoms have been re-
placed by deuterium atoms.deuterium(heavy hydrogen) Sym-
bol D. The isotope of hydrogen that
has a mass number 2 (r.a.m. 2.0144).
Its nucleus contains one proton anddesorption 170d