The kinetics of the
reaction between methyl radicals and anisole have been studied at temperatures
between 453 and 539�K and total pressures between 10 and 30 torr.
The concentrations of methyl radicals ranged from 2 x 10-12 to 5 x
10-11 mole and those of anisole from 10-7 to mole cm-3.
The reaction proceeds mainly by the mechanism
������������������ C6H5OCH3+CH3·
→ C6H5OCH2·+CH4���������������� (1)����������������� C6H5OCH2·+CH3·
→ C6H5OC2H5�������������������� (2)���������������� ���������C6H5OCH2·
→ C6H5CHO+H·������������������ (3)
At 487�K attack on the
aromatic ring to yield methyl anisoles is about twelve times slower than
reaction (1). The Arrhenius parameters for reactions (1) and (8) are: log10(A1 cm3
mole-1 sec-1) = 11.7 � 0.3, and E1 = 10.5 �
0.8 kcal mole-1; log10(A8
sec-1) = 12.5, and E8 = 21 kcal mole-1. The
last two values are based on the assumption that the kinetics of reaction (2)
are similar to those of the recombination of methyl radicals. The rate of
reaction (1) is about half that of the corre- sponding reaction with toluene and about five times that of
the reaction with ethane in the above temperature range.
The rate constant of a gaseous reaction
calculated from data obtained by the conventional flow method is subject to
errors arising from departures from piston flow and thermal equilibrium in the
reaction tube. An approximate theoretical analysis of the errors is given for
the first-order pyrolysis or isomerization of an organic vapour. In the case of
a reaction occurring near 1000�K in a tube 2 cm in diameter, it is shown that
for the measured value of the rate constant to be accurate within about 10% the
experimental conditions should be such that z > tc/p
> 0.5, where p cmHg is the total pressure and tc
sec the average contact time. The upper limit (z) of tc/p
increases from 3 under conditions of 50% conversion to 10 at 25% and ∞ at
0%.
The analysis is applied to measurements of
the rate of pyrolysis of toluene. Lack of thermal equilibrium could be at least
partly responsible for the observed effect of contact time on the rate constant
but does not account for the effect of pressure.
The error incurred by assuming piston flow
in an isothermal reactor when in fact viscous flow is occurring is discussed in
the Appendix.
The kinetics of abstraction of hydrogen
atoms from the methyl group of the toluene molecule by methyl radicals at
430-540�K have been determined. The methyl radicals were produced by pyrolysis
of di-t-butyl peroxide in a stirred-flow system. The kinetics ,agree
substantially with those obtained by previous authors using photolytic methods
for generating the methyl radicals.
At toluene and methyl-radical
concentrations of about 5 x 10-7 and 10-11 mole cm-3
respectively the benzyl radicals resulting from the abstraction disappear
almost entirely by combination with methyl radicals at the methylenic position.
In this respect the benzyl radical behaves differently from the iso-electronic
phenoxy radical, which previous work has shown to combine with a methyl radical
mainly at ring positions.
The investigation illustrates the
application of stirred-flow technique to the study of the kinetics of
free-radical reactions.
The uncertainty regarding temperature and
flow conditions which attaches to the conventional flow method of determining
the rate of a gaseous reaction can be substantially reduced by using a
stirred-flow reactor. The reagents, products, and carrier-gas (if any) are
mixed sufficiently vigorously for the composition of the gas in the reactor to
be virtually uniform. A reactor designed to achieve the required degree of
mixing at pressures of about 1 cmHg and reaction times of the order of 1 sec to
1 min is described.
The rate constant of the decomposition of
di-t-butyl peroxide was determined over the temperature range 430-550 �K. The
values derived on the assumption of complete mixing in the reactor were
independent of the degree of conversion and in excellent agreement with those
obtained by previous authors using the static method.
The reaction between t-butyl hydroperoxide
and titanous ion in aqueous solution produces free methyl radicals detectable
by electron spin resonance spectrometry (Dixon and Norman). However, the
presence of titanous ion in concentrations greater than 0.01M broadens the
spectrum of the methyl radical, causing it effectively to disappear at titanous
concentrations greater than 0.1M. At hydroperoxide concentrations above 0.25M
t-butyl peroxy radicals (identified by a strong single-line spectrum with
g-value 2.0136) are produced by the reaction
���������� R. + (CH3)3COOH
→ RH + (CH3)3COO.
Their concentration reaches
a maximum about 1 sec after the concentration of the methyl radicals has fallen
to an undetectable value and their half-life (≈ 5 sec) is about ten times
that of the methyl radicals.
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