DISPROPORTIONATION/COMBINATION RATE CONSTANT RATIOS FOR HALOCARBON RADICALS
557
and with CF3CH2CHCH3 are the same, the combina-
tion rates must also be the same within our experimen-
tal error. This demonstrates that substituents’ electronic
natures are unimportant in combination reactions, be-
cause CH3 and CF3 are very different in electronic na-
ture but similar in steric size [5]. This agrees with the
MO calculations [12], which predicts that combina-
tion rates are not influenced by a substituent’s elec-
tronic nature. The radical center is probably becom-
ing more crowded as X changes along the series (X =
H, Cl, CF3, or CH3), which might reduce the combi-
nation rate causing the kd/kc to increase. Rabinovitch
and coworkers [21] have noted that steric hinderance
may also influence the relative availability of the H
atoms in disproportionation reactions; however, this
would decrease the kd/kc, if the kc remained constant,
as the X changes along the series X = H, Cl, CF3, and
CH3.
position of CF3CH2CHX radicals and the relative rate
constant ratio increased by about a factor of 3 as X
changes from H to Cl, CF3, and CH3. One of the
more surprising results is that the disproportionation
rate constant nearly equals the combination rate con-
stant when two CF3CH2CHCH3 radicals collide; the
0
0
kd /kc = 0.74 0.05 and this may reflect the steric
influence of a bulky radical center on the combination
rate constant.
BIBLIOGRAPHY
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All the disproportionation/combination rate con-
stants that we have previously measured [1–4] for flu-
orocarbon radicals have ranged between 0.022 and
0.125. When a CF3 removes an H from the ␣-carbon
of the CF3CH2CHCH3 radical the kd/kc is within this
range, but when the H is from the CH3 position or when
the CF3CH2CHCH3 radicals self-disproportionate the
values are much higher, 0.24–0.47. Pritchard and
coworkers [17] also found that hydrocarbon acceptor
radicals have much larger kd/kc ratios than perfluoro-
carbon radicals. As mentioned previously, Pritchard
and coworkers [16–20] found that methyl, ethyl, and
n-propyl radicals, accepting an H from CHF2, all had
kd/kc near 0.3, whereas the kd/kc values were between
0.06 and 0.09 when the acceptor radical was a n-C3F7
or a CF3 radical.
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CONCLUSIONS
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Disproportionation/combination rate constant ratios
have been measured as 0.07 0.02 for the reactions of
CF3 with CF3CH2CHCH3 radicals when the H is re-
moved from the CH2 position and as 0.24 0.03 when
the H is removed from the CH3 position. For the self-
reaction of CF3CH2CHCH3 radicals, kd/kc is 0.27
0.03 when the H is removed from the CH2 position
and 0.47
0.04 when the H is removed from the
CH3 position. Comparison of a CF3 substituent at the
CH2 position versus an H substituent at the CH3 po-
sition, when the acceptor radical is CF3, reveals that a
CF3 substituent hinders disproportionation, with only
30% of the total disproportionation occurring at the
carbon with the CH2. The results agree with predic-
tions based on the MYFF model. Also, relative dis-
proportionation/combination rate constant ratios were
compared for CF3 removing an H from the CH2