Note that the steady-state approximation does not
imply that [X] is even approximately constant, only
that its absolute rate of change is very much smaller
than that of [A] and [D]. Since according to the reac-
tion scheme d[D]/dt = k 2 [X][C], the assumption that
[X] is constant would lead—for the case in which C is
in large excess—to the absurd conclusion that forma-
tion of the product D will continue at a constant rate
even after the reactant A has been consumed. (2) In a
stirred-flow reactor, a steady state implies a regime so
that all concentrations are independent of time.
stellacyanin An ELECTRON-TRANSFER PROTEIN, con-
taining a TYPE 1 COPPERsite, isolated from exudates of
the Japanese lacquer tree.
stepwise reaction A CHEMICAL REACTIONwith at
least one reaction INTERMEDIATEand involving at least
two consecutive ELEMENTARY REACTIONS.
See alsoCOMPOSITE REACTION.
stereochemical Refers to the three-dimensional view
of a molecule either as such or in a projection.
stereoelectronic Pertaining to the dependence of the
properties (especially the energy) of a MOLECULAR
ENTITYin a particular electronic state (or of a TRANSI-
TION STATE) on relative nuclear geometry. The elec-
tronic GROUND STATE is usually considered, but the
term can apply to excited states as well. Stereoelec-
tronic effects are ascribed to the different alignment of
electronic orbitals in different arrangements of nuclear
geometry.
stereoelectronic control Control of the nature of
the products of a CHEMICAL REACTION(or of its rate)
by STEREOELECTRONIC factors. The term is usually
applied in the framework of an orbital approximation.
The variations of MOLECULAR ORBITALenergies with
relative nuclear geometry (along a REACTION COORDI-
NATE) are then seen as consequences of variations in
basis-orbital overlaps.stereoisomeric Isomerism due to the difference in
the spatial arrangement of atoms without any differ-
ence in connectivity or bond multiplicity between the
isomers.stereoisomers Compounds with the same chemical
formula but having different three-dimensional struc-
tures, differing only in the way that atoms are oriented
in space.stereoselectivity, stereoselective Stereoselectivity is
the preferential formation in a CHEMICAL REACTIONof
one stereoisomer over another. When the stereoisomers
are ENANTIOMERs, the phenomenon is called enantiose-
lectivity and is quantitatively expressed by the enan-
tiomer excess; when they are DIASTEREOISOMERs, it is
called diastereoselectivity and is quantitatively expressed
by the diastereomer excess. Reactions are termed (100
percent) stereoselective if the discrimination is com-
plete or partially (x percent) stereoselective if one
product predominates. The discrimination may also
be referred to semiquantitatively as high or low stere-
oselectivity.stereospecificity, stereospecific (1) A reaction is
termed stereospecific if starting materials differing
only in their CONFIGURATION are converted into
STEREOISOMERICproducts. According to this defini-
tion, a stereospecific process is necessarily STEREOSE-
LECTIVE, but not all stereoselective processes are
stereospecific. Stereospecificity may be total (100 per-
cent) or partial. The term is also applied to situations
where a reaction can be performed with only one
stereoisomer. For example, the exclusive formation of
trans-1,2-dibromocyclohexane upon bromination of
cyclohexene is a stereospecific process, although the
analogous reaction with (E)-cyclohexene has not been
performed.
(2) The term has also been applied to describe a
reaction of very high stereoselectivity, but this usage is
unnecessary and is discouraged.d[D]/dt = – d[A]/dt =[A][C]
[C]kk
kk12- 12 +
256 stellacyanin