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 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 preparation of some new β,β-dimethylstyrenes
and α-methylstilbenes is described and their p.m.r. spectra are reported.
The long-range shielding effects in β,β-dimethylstyrenes and other
styrene derivatives are discussed. The differences between the cisoid and
transoid allylic coupling constants in β,β-dimethylstyrenes are much
smaller than the corresponding differences in simple propenes. No appreciable
(J ≥ 0.3 c/s) coupling between the non-equivalent methyl protons in β,β-dimethylstyrenes
has been observed. Long-range coupling in the compounds investigated appears to
be independent of ring substitution. Some general features of allylic coupling
are discussed.
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