steric-approach control Control of STEREOSELEC-
TIVITYof a reaction by steric hindrance toward attack
of the reagent, which is directed to the less hindered
face of the molecule. Partial bond making is strong
enough at the TRANSITION STATEfor steric control to
take place. This suggests that the transition state
should not be close to products.
See alsoPRODUCT-DEVELOPMENT CONTROL.
steric effect The effect on a chemical or physical
property (structure, rate, or equilibrium constant) upon
introduction of substituents having different steric
requirements. The steric effect in a reaction is ascribed
to the difference in steric energy between, on the one
hand, reactants and, on the other hand, a TRANSITION
STATE(or products). A steric effect on a rate process
may result in a rate increase (steric acceleration) or a
decrease (steric retardation). (The adjective stericis not
to be confused with stereochemical.)
Steric effects arise from contributions ascribed to
STRAINas the sum of (1) nonbonded repulsions, (2)
bond angle strain, and (3) bond stretches or compres-
sions.
For the purpose of CORRELATION ANALYSISor LIN-
EAR FREE-ENERGY RELATIONs, various scales of steric
parameters have been proposed, notably A VALUES,
Taft’s Es, and Charton’s υscales.
In a reactant molecule RY and an appropriate ref-
erence molecule RoY, the “primary steric effect” of R is
the direct result of differences in compressions, which
occur because R differs from R°in the vicinity of the
reaction center Y. A “secondary steric effect” involves
the differential moderation of electron delocalization
by nonbonded compressions.
Some authors make a distinction between “steric”
effects attributed to van der Waals repulsions alone,
and “strain” effects, attributed to deviations of bond
angles from “ideal” values.
See also STABILITY CONSTANT; TAFT EQUATION;
VAN DERWAALS FORCES.
steric hindrance The original term for a STERIC
EFFECTarising from crowding of substituents.
steric isotope effect SeeISOTOPE EFFECT, STERIC.
steroid hormone(steroid drug) A large family of
structurally similar chemicals. Various steroids have
sex-determining, anti-inflammatory, and growth-regu-
latory roles. Examples include corticosteroid and glu-
cocorticoid.stimulus The cause for making a nerve cell respond;
can be in the form of light, sounds, taste, touch, smell,
temperature, etc.
See alsoSENSORY RECEPTOR.stochastic Mathematical probability; a stochastic
model takes into account variations in outcome that
occur because of chance alone.stock number SeeOXIDATION NUMBER.stoichiometric number SeeRATE OF REACTION.stoichiometry The calculation of the quantities of
reactants and products (among elements and com-
pounds) involved in a chemical reaction.stopped flow A technique for following the kinetics
of reactions in solution (usually in the millisecond time
range) in which two reactant solutions are rapidly
mixed by being forced through a mixing chamber. The
flow of the mixed solution along a uniform tube is then
suddenly arrested. At a fixed position along the tube,
the solution is monitored (as a function of time follow-
ing the stoppage of the flow) by some method with a
rapid response (e.g., photometry).
See alsoMIXING CONTROL.strain(strain energy) Strain is present in a MOLECU-
LAR ENTITYor TRANSITION STRUCTUREif the energy is
enhanced because of unfavorable bond lengths, bond
angles, or dihedral angles (torsional strain) relative to a
standard.
It is quantitatively defined as the standard enthalpy
of a structure relative to a strainless structure (real orstrain 257