Physical Chemistry Third Edition

610 13 Chemical Reaction Mechanisms II: Catalysis and Miscellaneous Topics

trap.^37 Molecular oxygen also reacts with free radicals, and if addition of oxygen to a

reacting system inhibits the rate of reaction it is likely that some kind of a free radical

intermediate is present in the reaction mechanism.

Another technique that can be used to detect the presence of free radical intermediates

is based on the fact that almost any free radical catalyzes the conversion ofortho-H 2

topara-H 2 and vice versa. Inortho-H 2 the nuclear spins are parallel, and inpara-

H 2 the nuclear spins are antiparallel. At room temperature equilibrium H 2 consists

of 75%ortho-H 2 and 25%para-H 2 , whereas at low temperatures the equilibrium

mixture is nearly 100%para-H 2 .Ifpara-H 2 prepared at low temperature is brought in

contact with free radicals, the rate of conversion to the equilibrium mixture is a measure

of the amount of free radicals present. However, these techniques for the detection

of free radical intermediates do not distinguish one free radical intermediate from

another.

Another technique is the use ofisotopicsubstitution. For example, the decomposition

of acetaldehyde

CH 3 CHO−→CH 4 +CO (13.6-1)

is thought to proceed by the mechanism

(1) CH 3 CHO−→CH 3 +CHO (13.6-2a)

(2) CH 3 +CH 3 CHO−→CH 4 +CH 3 CO (13.6-2b)

(3) CH 3 CO−→CH 3 +CO (13.6-2c)

A mixture of CH 3 CHO and CD 3 CDO was reacted, where D stands for deuterium

(^2 H).^38 The product mixture was found to contain the statistically expected mixture

of randomly isotopically substituted methanes, which increased the plausibility of this

mechanism, since a mechanism without free radicals such as CH 3 would not have

mixed the isotopes randomly.

Molecular Beam Reactions

In this technique the reaction is carried out by forming beams of reactants in an other-

wise evacuated chamber. Figure 13.20 shows schematically an apparatus for generating

a molecular beam. The material is vaporized in an oven and the molecules exit from a

small aperture into an evacuated chamber. The molecules pass through a second bar-

rier with a small aperture into a chamber that is evacuated to even lower pressure. This

chamber can contain a velocity selector, which passes only those molecules in a nar-

row range of speeds. Figure 13.21 shows schematically one type of velocity selector,

called atime-of-flight (TOF) selector. The rotating disks have slots and molecules can

pass through the apparatus only if they reach each disk when one of its slots is in the

beam position. Varying the speed of rotation of the disks allows different speeds to be

selected. The molecules then pass into a third chamber in which the beam molecules

can react with a second substance.

State
selector
Source
compartment

Scattering

compartment

To pumps

Detector

compartment

Figure 13.20 An Apparatus for Gen-
erating a Molecular Beam (Sche-
matic).

Molecular beam

Rotating

shaft

Slotted disks to pass

molecules within a certain

range of speeds (depending

on rotational speed)

Figure 13.21 A Velocity-Select-

ing Apparatus (Schematic). (^37) S. W. Benson,op. cit., p. 101 (note 6) gives a table of free radicals and metals that had been studied as
of the 1950s.
(^38) S. W. Benson,op. cit., p. 108 (note 6).