Page 103 - Mechanism and Theory in Organic Chemistry

The principle of microscopic reversibility, required by the laws of thermo-
dynamics, specifies that there must be a reverse for every microscopic process. It is
therefore strictly speaking incorrect to omit reverse steps, and doing so is justified
only when the omitted reverse step is occurring so slowly as to have no observable
effect on the reaction during the time it will be under observation.

The bimolecular reaction We move next to a slightly more complex
case, a single step with more than one reactant (Equation 2.31). The rate equa-
tion is 2.32; if the rcverse reaction may be safely omitted, these equations sim-

-- d[A1 - kl [A] [B] - k - [C] [Dl (2.32)
-
dt
A
plify to 2.33 and 2.34. T h e : Interaction __- with B is
of
-
required. The predicted lunetic bchavinr (Equation 2.34~js~eco~der overall,

first-order in.JAJ and.&.st-prrder. in .[B],_and the_._obxpved - second-order -. .- rate
constant, kobn, is equal to the microscopic constant k,.
_ZI___/__..
The pseudo first-order reaction It may be possible in a reaction in
ach two substances take part to arrange that one concentrat& is effectively
-\ -. ---
--
constan~&Fingsfie kinetic experiment. The most obvious example is when one ,
reactant can be buffered, asii?">ri"-acid-catalyzed process, but it can=-
accomplished simplv bv having reagent B present in large excess over -- A, -. so~hat -- _-
the proportional change inawry small while the pro~ortional change in&]
is large. Then the constant concentration becomes effectively part of the-rfat_e
-
c%tant and the rateequation 2.34 reduces to

kobs = kl[B] [B] constant (2.36)
The predicted kinetic behavior under these circumstances is therefore first-order,
with a first-order k,,, related to the microscopic second-order constant k, by
Equation 2.36. Such reactions are said to follow pseudojrst-order kinetics.

Multistep Reactions
Kinetic treatment is more difficult for mechanisms with more than one element-
ary step. Here we shall restrict the discussion to two commonly encountered
special cases. Let us look first at a simple two-step process (Equations 2.37 and
2.38) in which we are justified by the chemistry in ignoring reverse reactions.