reaction step An ELEMENTARY REACTION, constitut-
ing one of the stages of a STEPWISE REACTIONin which
a reaction INTERMEDIATE(or, for the first step, the
reactants) is converted into the next reaction interme-
diate (or, for the last step, the products) in the
sequence of intermediates between reactants and
products.
reactive, reactivity As applied to a CHEMICAL
SPECIES, the term expresses a kinetic property. A species
is said to be more reactive or to have a higher reactivity
in some given context than some other (reference)
species if it has a larger rate constant for a specified
ELEMENTARY REACTION. The term has meaning only by
reference to some explicitly stated or implicitly
assumed set of conditions. It is not to be used for reac-
tions or reaction patterns of compounds in general.
The term is also more loosely used as a phenomenolog-
ical description not restricted to elementary reactions.
When applied in this sense, the property under consid-
eration may reflect not only rate but also equilibrium
constants.
See alsoSTABLE; UNREACTIVE; UNSTABLE.
reactivity index Any numerical index derived from
quantum mechanical model calculations that permits
the prediction of relative reactivities of different molec-
ular sites. Many indices are in use, based on a variety
of theories and relating to various types of reaction.
The more successful applications have been to the SUB-
STITUTION REACTIONs of CONJUGATED SYSTEMs, where
relative reactivities are determined largely by changes
of pi-electron energy.
reactivity-selectivity principle (RSP) This idea can
be expressed loosely as: the more REACTIVEa reagent
is, the less selective it is.
Consider two substrates S^1 and S^2 undergoing the
same type of reaction with two reagents R^1 and R^2 , S^2
being more reactive than S^1 , and R^2 more reactive
than R^1 in the given type of reaction. The relative
reactivities (in log units; seeSELECTIVITY) for the four
possible reactions can notionally be represented as
follows:
With the positions of (S^1 + R^1 ), (S^2 + R^1 ), and (S^1 +
R^2 ) fixed, there are three types of positions for (S^2 +
R^2 ):
In position (i) the selectivity of R^2 for the two sub-
strates, measured by a,is the same as the selectivity of
R^1 for the two substrates, also a.
In position (ii) the selectivity of R^2 for the two sub-
strates, measured by b,is less than the selectivity of R^1
for the two substrates, i.e., b< a.It is this situation that
is in accord with the RSP.
In position (iii) the selectivity of R^2 for the two
substrates, measured by c,is greater than the selectivity
of R^1 for the two substrates, i.e., c> a.This situation
can be described as anti-RSP.
There are many examples in which the RSP is fol-
lowed, but there are also many examples correspond-
ing to situations (i) and (iii). The RSP is in accord
with intuitive feeling and certainly holds in the limit-
ing case when reactivity is controlled by DIFFUSION.
However, the validity of the RSP is a matter of great
controversy, and diverse opinions have been
expressed, from declaring the reactivity-selectivity
principle as a universal law up to virtually useless in
practice as a general rule.real gas Gases under physical conditions that give
nonideal behavior. The repulsion between molecules at
very close distances is due to the repulsion between the
nuclei of the two molecules. These forces give rise to
relationships between the pressure, temperature, vol-
ume, and quantity of a substance that do not exactly
obey the ideal gas law and therefore give nonideal
behavior.real gas 233