larger than molecular scale. However, they do not set-
tle out.
colloidal bismuth subcitrate(CBS) SeeDE-NOL.
combination reaction A reaction in which two sub-
stances, either elements or compounds, combine with
each other to form one substance or compound.
combinatorial library A set of compounds prepared
by COMBINATORIAL SYNTHESIS.
combinatorial synthesis A process for preparing
large sets of organic compounds by combining sets of
building blocks in all possible combinations.
combustible Refers to any material that will burn.
combustion An exothermic oxidation-reduction reac-
tion between molecular oxygen and a nonmetallic
substance.
CoMFA See COMPARATIVE MOLECULAR FIELD
ANALYSIS.
common-ion effect(on rates) A reduction in the
RATE OF REACTIONof a SUBSTRATERX in solution (by a
path that involves a PRE-EQUILIBRIUMwith formation
of R+[or R–] ions as reaction intermediates) caused by
the addition to the reaction mixture of an electrolyte
solute containing the “common ion” X–(or X+). For
example, the rate of solvolysis of diphenylmethyl chlo-
ride in acetone-water is reduced by the addition of salts
of the common ion Cl–, which causes a decrease in the
quasi-equilibrium concentration of the diphenylmethyl
cation in the scheme
Ph 2 CHCl Ph 2 CH++ Cl–(free ions, not ion pairs)Ph 2 CH++ OH 2 →^2 Ph 2 CHOH + H+(solvated)
This phenomenon is a direct consequence of the MASS-
LAW EFFECT on ionization equilibria in electrolytic
solution.
More generally, the common-ion effect is the influ-
ence of the “common ion” on the reactivity due to the
shift of the dissociation equilibrium. It may also lead to
an enhancement of the rates of reaction.comparative molecular field analysis(CoMFA) A
THREE-DIMENSIONAL QUANTITATIVE STRUCTURE-ACTIV-
ITY RELATIONSHIP(3D-QSAR) method that uses statisti-
cal correlation techniques for the analysis of (a) the
quantitative relationship between the biological activity
of a set of compounds with a specified alignment and (b)
their three-dimensional electronic and steric properties.
Other properties such as hydrophobicity and hydrogen
bonding can also be incorporated into the analysis.compensation effect In a considerable number of
cases, plots of T∆+- Svs. ∆+
- H,for a series of reactions,
e.g., for a reaction in a range of different solvents, are
straight lines of approximately unit slope. Therefore,
the terms ∆+
- H,for a series of reactions,
Hand T∆+
Sin the expression partially
compensate, and ∆+
G= ∆+
H–T∆+
Soften is a much sim-
pler function of solvent (or other) variation than ∆+
H
or T∆+
Sseparately.
See alsoISOKINETIC RELATIONSHIP.
competitive exclusion principle The condition
where one species is driven out of a community by
extinction due to interspecific competition; one species
will dominate the use of resources and have a repro-
ductive advantage, forcing the others to disappear. Also
called Gauss’s law.competitive inhibitor A substance that resembles
the substrate for an enzyme, both in shape and size,
and competes with the substrate for the substrate-bind-
ing site on the enzyme, thereby reducing the rate of
reaction by reducing the number of enzyme molecules
that are successful.complementary binding site SeeBINDING SITE.54 colloidal bismuth subcitrate