Organic Chemistry

1010 CHAPTER 24 Catalysis

24.6 Intramolecular Reactions

The rate of a chemical reaction is determined by the number of molecular collisions

with sufficient energy andwith the proper orientation in a given period of time

(Section 3.7):

Because a catalyst decreases the energy barrier of a reaction, it increases the reaction

rate by increasing the number of collisions that occur with sufficient energy to over-

come the barrier.

The rate of a reaction can also be increased by increasing the frequency of the col-

lisions andthe number of collisions that take place with the proper orientation. We

have seen that an intramolecular reaction that results in formation of a five- or a six-

membered ring occurs more readily than the analogous intermolecular reaction. This

is because an intramolecular reaction has the advantage that the reacting groups are

tied together in the same molecule, giving them a better chance of finding each other

than if they were in two different molecules in a solution of the same concentration

(Section 11.11). As a result, the frequency of the collisions increases.

If, in addition to being in the same molecule, the reacting groups are juxtaposed in

a way that increases the probability that they will collide with each other in the proper

orientation, the rate of the reaction is further increased. The relative rates shown in

Table 24.2 demonstrate the enormous increase that occurs in the rate of a reaction

when the reacting groups are properly juxtaposed.

Rate constants for a series of reactions are generally compared in terms of relative

rates because relative rates allow one to see immediately how much faster one reaction

is than another. Relative ratesare obtained by dividing the rate constant for each

of the reactions by the rate constant of the slowest reaction in the series. Because an

intramolecular reaction is a first-order reaction (it has units of ) and an inter-

molecular reaction is a second-order reaction (it has units of the relative

rates in Table 24.2 have units of molarity (M) (Section 3.7).

The relative rates shown in Table 24.2 are also called effective molarities. Effective

molarityis the concentration of the reactant that would be required in an intermolecu-

larreaction for it to have the same rate as the intramolecularreaction. In other words,

the effective molarity is the advantage given to a reaction by having the reacting

groups in the same molecule. In some cases, juxtaposing the reacting groups provides

such an enormous increase in rate that the effective molarity is greater than the con-

centration of the reactant in its solid state!

The first reaction shown in Table 24.2 (A) is an intermolecular reaction between an

ester and a carboxylate ion. The second reaction (B) has the same reacting groups in a

single molecule. The rate of the intramolecular reaction is 1000 times faster than the

rate of the intermolecular reaction.

The reactant in B has four bonds that are free to rotate, whereas the reactant

in D has only three such bonds. Conformers in which the large groups are rotated away

from each other are more stable. However, when these groups are pointed away from

each other, they are in an unfavorable conformation for reaction. Because the reactant

in D has fewer bonds that are free to rotate, the groups are less apt to be in a confor-

mation that is unfavorable for a reaction. Therefore, reaction D is faster than reaction

B. The relative rate constants for the reactions shown in Table 24.2 are quantitatively

C¬C

relative rate=

first-order rate constant

second-order rate constant

=

time-^1

time-^1 M-^1

=M

time-^1 M-^1 ),

time-^1

rate of reaction=

number of collisions

unit of time

* fraction

with

sufficient energy

* fraction

with

proper orientation