LCAO(linear combination of atomic orbitals) A
form of expression in showing molecular orbitals.
Written as linear combinations of atomic orbital wave
functions.
lead discovery The process of identifying active new
chemical entities that, by subsequent modification, may
be transformed into a clinically useful DRUG.
lead generation The term applied to strategies devel-
oped to identify compounds possessing a desired but
nonoptimized biological activity.
lead optimization The synthetic modification of a
biologically active compound to fulfill all stereoelec-
tronic, physicochemical, pharmacokinetic, and toxico-
logic requirements for clinical usefulness.
lead storage battery Secondary voltaic cell used in
most automobiles, where the anode is lead and the
cathode is lead coated with lead dioxide.The electrolyte
is sulfuric acid.
least nuclear motion, principle of The hypothesis
that, for given reactants, the reactions involving the
smallest change in nuclear positions will have the low-
est ENERGY OF ACTIVATION. (It is also often simply
referred to as the principle of least motion.)
leaving group An atom or GROUP (charged or
uncharged) that becomes detached from an atom in
what is considered to be the residual or main part of
the SUBSTRATEin a specified reaction. For example, in
the heterolytic SOLVOLYSISof benzyl bromide in acetic
acid
PhCH 2 Br + AcOH →PhCH 2 OAc + HBrthe leaving group is Br–; in the reaction
MeS–+ PhCH 2 N+Me 3 →MeSCH 2 Ph + NMe 3the leaving group is NMe 3 ; in the ELECTROPHILICnitra-
tion of benzene, it is H+. The term has meaning only in
relation to a specified reaction. The leaving group is
not, in general, the same as the SUBSTITUENTgroup pres-
ent in the substrate (e.g., bromo and trimethylammonio
in the substrates of the first two examples above).
A slightly different usage of the term prevails in the
(nonmechanistic) naming of TRANSFORMATIONs, where
the actual substituent group present in the substrate
(and also in the product) is referred to as the leaving
group.
See alsoELECTROFUGE; ENTERING GROUP; NUCLE-
OFUGE.Le Châtelier’s principle In 1888 French chemist
Henri-Louis Le Châtelier stated that a system at equi-
librium, or striving to attain equilibrium, responds in
such a way as to counteract any stress placed upon it,
and a new equilibrium position will be reached.Leclanché cell A common name for a type of dry cell
and one of the earliest practical nonrechargeable bat-
teries created by French scientist Georges-Lionel
Leclanché (1839–82). It consists of a zinc ANODE(neg-
ative electrode) and a manganese dioxide CATHODE
(positive electrode) with ammonium chloride solution
as electrolyte. Became the first DRY CELL.162 LCAO
Leclanché cell. A common name for a type of dry cell and one of
the earliest practical nonrechargeable batteries created by
French scientist Georges-Lionel Leclanché (1839–82)