with a chemical structure that is similar to that of an existing drug. Designer drugs are produced either to
increase potency or to avoid detection. An example is ecstasy (MDMA),
which is a variant of MDA.
desorption The removal of adsorbed atoms, molecules, or ions
from a surface.
destructive distillation The
process of heating complex organic
substances in the absence of air so
that they break down into a mixture
of volatile products, which are condensed and collected. At one time
the destructive distillation of coal (to
give coke, coal tar, and coal gas) was
the principal source of industrial organic chemicals.
desulphuration The removal of
sulphur from a compound. Desulphuration has been implicated in the
toxicity of a number of sulphurcontaining organic compounds: it is
postulated that the atomic sulphur
released into a cell by desulphuration, which is highly electrophilic,
can bind to proteins and thereby
alter their function.
detailed balance The cancellation
of the effect of one process by another process that operates at the
same time with the opposite effect.
An example of detailed balance is
provided by a chemical reaction between two molecular species A and
B, which results in the formation of
the molecular species C and D. Detailed balance for this chemical reaction occurs if the rate at which the
reaction A + B → C + D occurs is
equal to the rate at which the reaction C + D → A + B occurs. The equilibrium state in thermodynamics is
characterized by detailed balance.
detergent A substance added to
water to improve its cleaning properties. Although water is a powerful
solvent 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 original 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 hydrocarbon 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 developed from petrochemicals. The
commonest, used in washing powders, is sodium dodecylbenzenesulphonate, which contains
CH 3 (CH 2 ) 11 C 6 H 4 SO 2 O
– ions. This, like
soap, is an example of an anionic detergent, i.e. one in which the active
part is a negative ion. Cationic detergents have 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 nonionic 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 stabilizers for foam.
detonating gas See electrolytic
gas.
deuterated compound A compound in which some or all of the
hydrogen–1 atoms have been replaced by deuterium atoms.
deuterium (heavy hydrogen) Symbol D. The isotope of hydrogen that
has a mass number 2 (r.a.m. 2.0144).
Its nucleus contains one proton and
desorption
170
d
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increase potency or to avoid detection. An example is ecstasy (MDMA),
which is a variant of MDA.
desorption The removal of adsorbed atoms, molecules, or ions
from a surface.
destructive distillation The
process of heating complex organic
substances in the absence of air so
that they break down into a mixture
of volatile products, which are condensed and collected. At one time
the destructive distillation of coal (to
give coke, coal tar, and coal gas) was
the principal source of industrial organic chemicals.
desulphuration The removal of
sulphur from a compound. Desulphuration has been implicated in the
toxicity of a number of sulphurcontaining organic compounds: it is
postulated that the atomic sulphur
released into a cell by desulphuration, which is highly electrophilic,
can bind to proteins and thereby
alter their function.
detailed balance The cancellation
of the effect of one process by another process that operates at the
same time with the opposite effect.
An example of detailed balance is
provided by a chemical reaction between two molecular species A and
B, which results in the formation of
the molecular species C and D. Detailed balance for this chemical reaction occurs if the rate at which the
reaction A + B → C + D occurs is
equal to the rate at which the reaction C + D → A + B occurs. The equilibrium state in thermodynamics is
characterized by detailed balance.
detergent A substance added to
water to improve its cleaning properties. Although water is a powerful
solvent 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 original 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 hydrocarbon 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 developed from petrochemicals. The
commonest, used in washing powders, is sodium dodecylbenzenesulphonate, which contains
CH 3 (CH 2 ) 11 C 6 H 4 SO 2 O
– ions. This, like
soap, is an example of an anionic detergent, i.e. one in which the active
part is a negative ion. Cationic detergents have 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 nonionic 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 stabilizers for foam.
detonating gas See electrolytic
gas.
deuterated compound A compound in which some or all of the
hydrogen–1 atoms have been replaced by deuterium atoms.
deuterium (heavy hydrogen) Symbol D. The isotope of hydrogen that
has a mass number 2 (r.a.m. 2.0144).
Its nucleus contains one proton and
desorption
170
d
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