across the inner membrane that provides the energy
needed for ATPsynthesis. The function of this chain is
to permit the controlled release of free energy to drive
the synthesis of ATP.
See alsoPROTON PUMP.
electrophile(electrophilic reagent) An electrophile
(or electrophilic reagent) is a reagent that forms a BOND
to its reaction partner (the NUCLEOPHILE) by accepting
both bonding electrons from that reaction partner.
An electrophilic SUBSTITUTION REACTIONis a het-
erolytic reaction in which the reagent supplying the
entering group acts as an electrophile. For example
C 6 H 6 + NO 2 +(electrophile) C 6 H 5 NO 2 + H+
(ELECTROFUGE)
Electrophilic reagents are LEWIS ACIDs. Electrophilic
catalysis is catalysis by Lewis acids.
The term electrophilicis also used to designate the
apparent polar character of certain RADICALs, as
inferred from their higher relative reactivities with reac-
tion sites of higher electron density.
See alsoELECTROPHILICITY; HETEROLYSIS.
electrophilicity (1) The property of being elec-
trophilic. (SeeELECTROPHILE.)
(2) The relative reactivity of an electrophilic
reagent. (It is also sometimes referred to as “elec-
trophilic power.”) Qualitatively, the concept is related
to LEWIS ACIDITY. However, whereas Lewis acidity is
measured by relative equilibrium constants, ELEC-
TROPHILICITYis measured by relative RATE CONSTANTs
for reactions of different electrophilic reagents toward
a common SUBSTRATE (usually involving attacking a
carbon atom).
See alsoNUCLEOPHILICITY.
electrophoresis Migrating charged particles (e.g.,
colloidal particles that acquire a charge through
adsorption of ions, charged macromolecules such as
proteins) in an electric field by virtue of their charge.
A method of separating charged molecules through
their different rates of migration under an electric field.
Usually a medium that minimizes diffusion is used, e.g.,
gel, paper, or capillary electrophoresis.
element A substance that consists of atoms that have
the same number of protons in their nuclei. Elements
are defined by the number of protons they possess.elementary process Each step of a reaction mecha-
nism.elementary reaction A reaction for which no reac-
tion intermediates have been detected or need to be
postulated in order to describe the chemical reaction on
a molecular scale. An elementary reaction is assumed to
occur in a single step and to pass through a single
TRANSITION STATE.
See alsoSTEPWISE REACTION.element effect The ratio of the rate constants of two
reactions that differ only in the identity of the element
of the atom in the LEAVING GROUP, e.g., kBr/kCl. As for
ISOTOPE EFFECTs, a ratio of unity is regarded as a “null
effect.”elimination (1) The process achieving the reduction
of the concentration of a XENOBIOTIC compound
including its METABOLISM.
(2) The reverse of an ADDITION REACTION or
TRANSFORMATION. In an elimination, two groups
(called eliminands) are lost most often from two differ-
ent centers (1/2/elimination or 1/3/elimination, etc.)
with concomitant formation of an unsaturation in the
molecule (double bond, triple bond) or formation of a
new ring.
If the groups are lost from a single center (α-elimi-
nation, 1/1/elimination), the resulting product is a car-
bene or a carbene analog.
See alsoALPHA(α) ELIMINATION.empirical formula The simplest whole-number ratio
of atoms in a compound.EMR(electron magnetic resonance) See ELECTRON
PARAMAGNETIC RESONANCE SPECTROSCOPY.EMR 95