Absolute rate constants and Arrhenius parameters for the addition of the methyl radical to unsaturated compounds: The methyl affinities revisited

Article abstract of DOI:10.1021/ja973128y

Absolute rate constants and their Arrhenius parameters are reported for the addition of the methyl radical to 21 monosubstituted and 1,1- disubstituted alkenes and to 6 benzenes in 1,1,2-trifluoro-1,2,2- trichloroethane solution. They are used to convert relative reaction rates known as methyl affinities from the work of M. Szwarc and others for about 250 additional unsaturated compounds to the absolute scale. An analysis shows that the addition rates depend on the reaction enthalpy but also indicates a moderate nucleophilic polar effect for the liquid-phase reactions. It is pointed out that this polar effect may be smaller in the gas phase.

Full text of DOI:10.1021/ja973128y

J. Am. Chem. Soc. 1997, 119, 12869-12878
12869
Absolute Rate Constants and Arrhenius Parameters for the
Addition of the Methyl Radical to Unsaturated Compounds:
The Methyl Affinities Revisited
Torsten Zytowski and Hanns Fischer*
Contribution from the Physikalisch-Chemisches Institut, UniVersit a¨ t Z u¨ rich,
Winterthurerstrasse 190, CH-8057 Z u¨ rich, Switzerland
X
ReceiVed September 5, 1997
Abstract: Absolute rate constants and their Arrhenius parameters are reported for the addition of the methyl radical
to 21 monosubstituted and 1,1-disubstituted alkenes and to 6 benzenes in 1,1,2-trifluoro-1,2,2-trichloroethane solution.
They are used to convert relative reaction rates known as methyl affinities from the work of M. Szwarc and others
for about 250 additional unsaturated compounds to the absolute scale. An analysis shows that the addition rates
depend on the reaction enthalpy but also indicates a moderate nucleophilic polar effect for the liquid-phase reactions.
It is pointed out that this polar effect may be smaller in the gas phase.
4
5
6
Introduction
of the ethyl, the n-propyl, the cyclopropyl, and the tri-
1
a,d,7-11
1
fluoromethyl
Though some reviews are available
only been quoted occasionally in the more recent literature, and
radicals.
About four decades ago, M. Szwarc and co-workers mea-
sured an impressive set of about 200 relative rate constants for
the addition of the methyl radical to various unsaturated organic
compounds. These are ratios of the addition rate constant to
the rate constant for hydrogen abstraction from 2,2,4-trimeth-
ylpentane (isooctane), and were determined mostly at 50-85
12-14
Szwarc’s results have
1
5
some criticism of the methodology has arisen. However, in
previous work we found an excellent correlation of the methyl
affinities with absolute rate constants for the addition of methyl
to 20 alkenes at room temperature as obtained by time-resolved
ESR spectroscopy. This reassured the quality of the earlier
relative data.
Continuing our work on absolute rate constants for the
addition of carbon centered radicals to unsaturated compounds
and the factors controlling them
rate data and Arrhenius parameters for the reaction of methyl
with alkenes and other unsaturated compounds. These allow
us to convert the methyl affinities to absolute rate constants
and their Arrhenius parameters, and lead to an unparalleled large
set of absolute addition rate data now available for further
analysis and discussion. On the basis of the new experimental
°
C. From the so-called methyl affinities Szwarc deduced a
1
6
variety of factors which influence the reaction barrier, such as
the exothermicity of the reaction, the energy location of the
first excited triplet state of the reactants, and the steric blocking
by bulky substituents at the attacked carbon atom. He even
correctly concluded the now accepted structure of the transition
state. Polar substituent effects were found to be small and in
the direction of a slight radical nucleophilicity. The methods
were simple and allowed the detection of abstraction besides
addition reactions. Hence, they were also applied by others to
1
6,17
we now present additional
obtain additional methyl affinities2,3 and extended to additions
X
Abstract published in AdVance ACS Abstracts, December 15, 1997.
(1) (a) Levy, M.; Szwarc, M. J. Chem. Phys. 1954, 22, 1621. (b) Leavitt,
(3) (a) Sass, V. P.; Serov, S. I.; Sokolov, S. V. Zh. Org. Khim. (Engl.
Ed.) 1977, 13, 2140. (b) Nadervel, T. A.; Sass, V. P.; Serov, S. I.; Sokolov,
S. V. Zh. Org. Khim. (Engl. Ed.) 1977, 13, 2136. (c) Nadervel, T. A.; Sass,
V. P.; Parshina, L. G.; Fedorowa, G. B.; Dolgopolskii, I. M.; Sokolov, S.
V. Zh. Org. Khim. (Engl. Ed.) 1979, 15, 977. (d) Sass, V. P.; Bresler, L.
S.; Sokolov, S. V. Zh. Vses. Khim. Obshch. 1972, 17, 459. (e) Sass, V. P.;
Nadervel, T. A.; Rondarev, L. S.; Bresler, L. S.; Sokolov, S. V. Zh. Org.
Khim. 1973, 9, 225.
(4) Smid, J.; Szwarc, M. J. Am. Chem. Soc. 1956, 78, 3322.
(5) Smid, J.; Szwarc, M. J. Am. Chem. Soc. 1957, 79, 1534.
(6) Stefani, A. P.; Chuang, L. Y.; Todd, H. E. J. Am. Chem. Soc. 1970,
92, 4168.
(7) Stefani, A. P.; Herk, L.; Szwarc, M. J. Am. Chem. Soc. 1961, 83,
4732.
(8) Stefani, A. P.; Szwarc, M. J. Am. Chem. Soc. 1962, 84, 3661.
(9) Whittemore, I. M.; Stefani, A. P.; Szwarc, M. J. Am. Chem. Soc.
1962, 84, 3799.
F.; Levy, M.; Szwarc, M.; Stannett, V. J. Am. Chem. Soc. 1955, 77, 5493.
c) Levy, M.; Newman, M. S.; Szwarc, M. J. Am. Chem. Soc. 1955, 77,
225. (d) Rembaum, A.; Szwarc, M. J. Am. Chem. Soc. 1955, 77, 4468.
e) Levy, M.; Szwarc, M. J. Am. Chem. Soc. 1955, 77, 1949. (f) Szwarc,
M. J. Polym. Sci. 1955, 16, 367. (g) Szwarc, M. J. Chem. Phys. 1955, 23,
04. (h) Szwarc, M.; Leavitt, F. J. Am. Chem. Soc. 1956, 78, 3590. (i)
Buckley, R. P.; Leavitt, F.; Szwarc, M. J. Am. Chem. Soc. 1956, 78, 5557.
j) Buckley, R. P.; Szwarc, M. J. Am. Chem. Soc. 1956, 78, 5696. (k) Bader,
A. R.; Buckley, R. P.; Leavitt, F.; Szwarc, M. J. Am. Chem. Soc. 1957, 79,
621. (l) Buckley, R. P.; Szwarc, M. Proc. R. Soc. 1957, A240, 396. (m)
Buckley, R. P.; Rembaum, A.; Szwarc, M. J. Polym. Sci. 1957, 24, 135.
n) Gazith, M.; Szwarc, M. J. Am. Chem. Soc. 1957, 79, 3339. (o) Heilman,
(
4
(
2
(
5
(
W. J.; Rembaum, A.; Szwarc, M. J. Chem. Soc. 1957, 1127. (p) Rajbenbach,
A.; Szwarc, M. Proc. R. Soc. (London) 1957, A251, 394. (q) Buckley, R.
P.; Rembaum, A.; Szwarc, M. J. Chem. Soc. 1958, 3442. (r) Leavitt, F.;
Stannett, V.; Szwarc, M. J. Polym. Sci. 1958, 31, 193. (s) Rajbenbach, A.;
Szwarc, M. Proc. Chem. Soc. 1958, 347. (t) Binks, J. H.; Szwarc, M. J.
Chem. Phys. 1959, 30, 1494. (u) Carrock, F.; Szwarc, M. J. Am. Chem.
Soc. 1959, 81, 4138. (v) Gresser, J.; Binks, J. H.; Szwarc, M. J. Am. Chem.
Soc. 1959, 81, 5004. (w) Binks, J. H.; Gresser, J.; Szwarc, M. J. Chem.
Soc. 1960, 3944. (x) Steel, C.; Szwarc, M. J. Chem. Phys. 1960, 33, 1677.
(10) Komazawa, H.; Stefani, A. P.; Szwarc, M. J. Am. Chem. Soc. 1963,
85, 2043.
(11) Serov, S. I.; Zhuravlev, M. V.; Sass, V. P.; Sokolov, S. V. Zh. Org.
Khim. 1981, 17, 53.
(12) Szwarc, M. J. Phys. Chem. 1957, 61, 40.
(
y) Feld, M.; Szwarc, M. J. Am. Chem. Soc. 1960, 82, 3791. (z) Gresser,
(13) Szwarc, M.; Binks, J. H. In Theoretical Organic Chemistry; Kekule
Symposium; Butterworth: London, 1959; p 262.
(14) Szwarc, M. In The Transition State; Spec. Publ. No. 16, Chemical
Society: London, 1962, p 91.
J.; Rajbenbach, A.; Szwarc, M. J. Am. Chem. Soc. 1960, 82, 5820. (aa)
Gresser, J.; Rajbenbach, A.; Szwarc, M. J. Am. Chem. Soc. 1961, 83, 3005.
(ab) Matsuoka, M.; Szwarc, M. J. Am. Chem. Soc. 1961, 83, 1260. (ac)
Herk, L.; Stefani, A.; Szwarc, M. J. Am. Chem. Soc. 1961, 83, 3008. (ad)
Feld, M.; Stefani, A. P.; Szwarc, M. J. Am. Chem. Soc. 1962, 84, 4451.
(15) Leffler, J. E. An Introduction to Free Radicals; J. Wiley: New York,
1993, p 163.
(16) Zytowski, T.; Fischer, H. J. Am. Chem. Soc. 1996, 118, 437.
(2) Stefani, A. P.; Todd, H. E. J. Am. Chem. Soc. 1970, 93, 2982.
S0002-7863(97)03128-4 CCC: $14.00 © 1997 American Chemical Society

12870 J. Am. Chem. Soc., Vol. 119, No. 52, 1997
Zytowski and Fischer
data and the converted methyl affinities we further discuss the
factors governing the barriers for the addition of methyl with
special emphasis on alkenes and on the role of the polar effect
18
which has recently severely been questioned.
Experimental Procedures and Analysis
The general experimental arrangement and the procedures for kinetic
ESR spectroscopy have been described previously.16,17 In principle,
deoxygenated solutions of a photochemical radical precursor in an inert
solvent flow slowly through a thermostated flat cell in the ESR cavity
and are irradiated by UV light. During continuous photolysis and in
the absence of reactive substrates one observes the radical whose
kinetics is to be studied. In the presence of reactants its ESR spectrum
is replaced by those of secondary radicals, and their structures reveal
the type of reaction. Intermission of the photolysis by a rotating sector
and coherent sampling of the time-dependent signal of the primary
radical leads to kinetic traces which are then analyzed according to the
appropriate rate laws. In the absence of substrate and in inert solvents
one observes a pure second-order radical self-termination, possibly
slightly disturbed by a pseudo-first-order reaction with the solvent, the
precursor, or impurities. In the presence of reactants the pseudo-first-
order contribution increases due to the reaction to be examined. In
the off-period of photolysis the radical decay profile obeys the
following:
Figure 1. ESR spectrum of the methyl radical (inset) and kinetic traces
in 1,1,2-trifluoro-1,2,2-trichloroethane.
Since both the cumyloxy and the methyl radical are highly reactive,
various precautions had to be taken to avoid side reactions. Thus, the
peroxide (Aldrich) was recrystallized at least 4 times from methanol,
and 1,1,2-trichloro-1,2,2-trifluoroethane of the highest available purity
(SDS, Peypin, >99.5%) was chosen as a fairly inert solvent. It was
found that the product acetophenone photosensitizes the peroxide
decomposition,1 and, hence, it was added deliberately. Optimum
concentrations were 0.022 M peroxide and 0.020 M acetophenone.
GLC analysis of the photolyzate in the absence of further substrates
yielded ethane and acetophenone as major products (>90%), as well
as methane, methyl chloride, benzene, chlorobenzene, toluene, possibly
acetone, and further unidentified compounds in minor amounts (total
<10%). Obviously, reactions 2 and 3 are dominant, and the minor
products point to reactions of methyl with the solvent, the peroxide,
and acetophenone, and to a minor cleavage pathway of cumyloxy
leading to acetone and phenyl radicals.20
9b
R(t) ) R(0) exp(-t/τ )/(1 + t/τ )
(1)
1
2
-
1
) τ10-1 + k[S] reflects the side
The pseudo-first-order rate τ
reaction (τ10) and the reaction with the substrate (k), and the second-
1
-
1
order lifetime τ
2
t
) 2k R(0) is due to the termination reactions.
Equation 1 is exact if all concurring termination reactions have the
same rate constants and/or if the pseudo-first-order processes contribute
little to the overall decay. Though the first condition is approximately
Figure 1 shows the familiar ESR spectrum of the methyl radical
(inset, 297 K, g ) 2.0025, aH ) -22.83 G) and its concentration vs
time profiles in the absence of substrates at 303 and 258 K. For T >
273 K the decay is well described by eq 1, but for lower temperatures
there are deviations because then reaction 3 is noninstantaneous on
the experimental time scale. Simulations of decay profiles showed that
this delayed formation of methyl can simply be taken into account by
a shift of zero time t′ in eq 1
fulfilled for all small alkyl radicals, the latter (τ
here by using low enough substrate concentrations.
1
2
. τ ) was also obeyed
In the course of this work the experimental arrangement was
improved considerably by installation of an ESR spectrometer with a
higher time resolution (Bruker EMX, 10 µs minimum response time),
by increasing the stability of the sector frequency to 0.05% and by
using a new home-built PC-based control, data aquisition, and analysis
system with a common precise clock.
R(t) ) R(0) exp(-(t - t′)/τ )/(1 + (t - t′)/τ )
(4)
-1, i.e., the rate of the first-order cumyloxy fragmentation
3
Methyl radicals were produced by photolysis of dicumyl peroxide,
i.e., the reactions
1
2
1
9
where t′ ) k
hV
of reaction 3. Equation 4 was found generally useful for the study of
radical rearrangements and other reactions as will be discussed
elsewhere, and fits to this formula are superimposed on the experimental
data in Figure 1. Analyses at various temperatures lead to the Arrhenius
C H (CH ) COOC(CH ) C H
9
8 2C H (CH ) CO˙
(2)
(3)
6
5
3 2
3 2
6
5
6
5
3 2
C H (CH ) C O˙ f C˙ H + CH COC H
5
6
5
3 2
3
3
6
parameters for the fragmentation of the cumyloxy radical (E
a
given in
kJ/mol and errors in units of the last quoted digit)
(
17) (a) Fischer, H. In Free Radicals in Biology and EnVironment;
Minisci, F., Ed.; Kluwer Acad. Pub.: Dordrecht, 1997. (b) M u¨ nger, K.;
Fischer, H. Int. J. Chem. Kinet. 1985, 17, 809. (c) Fischer, H. In Substituent
Effects in Radical Chemistry; Eds. Viehe, H. G. et al.; Reidel: Dordrecht,
log(k /s- ) ) 13.3(3) - 45.2(10)/2.303RT
1
(5)
3
1
986. (d) Beranek. I.; Fischer. H. In Free Radicals in Synthesis and Biology;
the self-termination of methyl in 1,1,2-trichloro-1,2,2-trifluoroethane
Minisci, F., Ed.; Kluwer Acad. Pub.; Dordrecht, 1988. (e) Heberger, K.;
Walbiner, M.; Fischer, H. Angew. Chem., Int. Ed. Engl. 1992, 31, 635. (f)
Heberger, K.; Fischer, H. Int. J. Chem. Kinet. 1993, 25, 249. (g) Heberger,
K.; Fischer, H. Int. J. Chem. Kinet. 1993, 25, 913. (h) Salikhov, A.; Fischer,
H. Appl. Magn. Reson. 1993, 5, 445. (i) Wu, J. Q.; Fischer, H. Int. J. Chem.
Kinet. 1995, 27, 167. (j) Wu, J. Q.; Beranek, I.; Fischer, H. HelV. Chim.
Acta 1995, 78, 194. (k) Walbiner, M.; Wu, J. Q.; Fischer, H. HelV. Chim.
Acta 1995, 78, 910. (l) Rubin, H.; Fischer, H. HelV. Chim. Acta 1996, 79,
log(2k /M- s-1) ) 12.8(1) - 14.7(4)/2.303RT
1
(6)
t
and the rate of the pseudo-first-order side reaction
log(τ10- /s-1) ) 4.2(3) - 12.4(11)/2.303RT
1
(7)
1
670. (m) Batchelor, S. N.; Fischer, H. J. Phys. Chem. 1996, 100, 9794.
(
1
n) Martschke, R.; Farley, R. D.; Fischer, H. HelV. Chim. Acta 1997, 80,
At 303 K the rate constant for the fragmentation in reaction 3 is k
3
363. (o) Salikhov, A.; Fischer, H. Theor. Chem. Acc. 1997, 96, 114.
18) (a) Wong, M. W.; Pross, A.; Radom, L. J. Am. Chem. Soc. 1993,
-1
-1
)
300 000 s , and agrees very well with k
3
) 340 000 s reported
solvent. Also, the Arrhenius parameters are
/s- ) ) 12.4(6) - 36.0(19)/2.303RT for chlorobenzene
solvent.21 The self-termination of methyl is at the diffusion controlled
(
19a
by Avila et al. for CCl
4
1
3
1
1
8
15, 11050. (b) Wong, M. W.; Pross, A.; Radom, L. Isr. J. Chem. 1994,
3, 415. (c) Wong, M. W.; Pross, A.; Radom, L. J. Am. Chem. Soc. 1994,
16, 6284. (d) Wong, M. W.; Pross, A.; Radom, L. J. Am. Chem. Soc.
994, 116, 11938. (e) Wong, M. W.; Radom, L. J. Phys. Chem. 1995, 99,
582.
1
close to log(k
3
10
-1 -1
limit (2k
t
(298) ) 1.7 × 10
M
s ), and eq 7 was used later on to
(
19) (a) Avila, D. V.; Brown, C. E.; Ingold, K. U.; Lusztyk, J. J. Am.
(20) Norrish, R. G. W.; Searby, M. H. Proc. R. Soc. 1956, A 237, 464.
(21) Baign e´ e, A.; Howard, J. A.; Scaiano, J. C.; Stewart, L. C. J. Am.
Chem. Soc. 1983, 105, 6120.
Chem. Soc. 1993, 115, 446. (b) Walling, C.; Gibian, M. J. J. Am. Chem.
Soc. 1965, 87, 3413.

Addition of Me Radical to Unsaturated Compounds
J. Am. Chem. Soc., Vol. 119, No. 52, 1997 12871
Table 1. Absolute Rate Constants and Arrhenius Parameters for the Addition of Methyl Radicals to Unsaturated Compounds
substrate
propene
ethene
na
T, K
297
297
297
297
297
297
273-313
297
297
273-313
297
273-313
263-313
253-313
297
273-313
297
log A, M-1 s-1
E
a
, kJ/mol
k
297, /M-1 s-1
4300
12
16
14
11
19
30
68
26
12
160
31
39
90
78
33
86
17
16
70
106
35
c
7000
7600
8500
12000
12000
14000
14000
20000
23000
1
2
2
-butene
-methylpropene
-methoxypropene
isopropenyl acetate
vinyl acetate
ethyl vinyl ether
vinyl chloride
trimethylvinylsilane
isopropenyl chloride
styrene
8.4 (+2/-4)b
8.1 (+1/-2)
23.9 (15)
20.8 (7)
35000
8.9 (+2/-3)
8.8 (+1/-2)
8.9 (+2/-3)
19.9 (11)
18.9 (8)
19.1 (13)
260000
310000
320000
340000
490000
610000
740000
760000
760000
780000
R-methylstyrene
1
,1-dichloroethene
methyl acrylate
methyl methacrylate
acrylonitrile
acrolein
isoprene
8.9 (+1/-2)
18.2 (10)
297
273-313
283-313
263-313
8.6 (+2/-4)
8.9 (+2/-4)
7.9 (+1/-2)
17.1 (15)
16.9 (14)
11.3 (8)
methacrylonitrile
1
,1-diphenylethene
benzene
39
50
34
5
7
6
283-333
273-313
296-313
296-313
296-313
296-313
8.2 (+1/-2)
8.9 (3)
10.2 (4)
10.3 (7)
9.7 (5)
37.2 (8)
46
54
170
500
860
4000
fluorobenzene
chlorobenzene
benzonitrile
40.9 (22)
45.3 (47)
43.2 (113)
42.8 (55)
35.8 (42)
1
1
,4-dichlorobenzene
,3,5-trichlorobenzene
9.9 (5)
a
Number of kinetic runs. b Errors in units of the last digit given. c 3500 M-1 s-1 per CH
group.
2
Figure 3. Temperature dependence of the addition rate constant of
the methyl radical to R-methylstyrene and 1,1-diphenylethene and fits
to the Arrhenius equation. The squared symbols are data derived from
methyl affinities.
Figure 2. Pseudo-first-order plots for the reaction of the methyl radical
with R-methylstyrene and trimethylvinylsilane.
supply the correction factor t10 in the analyses of the reactions of methyl
with unsaturated substrates.
1
addition only, as established earlier in Szwarc’s work and in
For several alkene substrates used in relatively high concentrations
ESR spectra of the secondary radicals were taken during continuous
photolysis. As expected, they are adducts to the unsubstituted alkene
carbon atom. Their ESR data are given in the Supporting Information,
part confirmed here by the direct observation of the adduct
radicals. The statistical errors of the rate constants are below
2
0%. However, for ethyl vinyl ether and 2-methoxypropene
1
hydrogen abstraction may compete, and the rate constants are
therefore tentative. Small deviations from our previous data16
are due to the now increased number of experiments.
2
2
and agree with earlier literature.
Figure 2 displays pseudo-first-order plots for the reactions of methyl
with R-methylstyrene and trimethylvinylsilane. The intercept of the
ordinate reflects the side reactions, and the rate constants for the reaction
with the alkenes follow from the slopes of the straight lines. Measure-
ments at various temperatures in the range 263 K < T < 313 K gave
the Arrhenius parameters of the reaction rate constants, and two fits of
the Arrhenius equation to the data are shown in Figure 3.
To convert Szwarc’s methyl affinities to absolute values the
Arrhenius parameters for the hydrogen abstraction reaction from
2,2,4-trimethylpentane are required. Unfortunately, the kinetics
of this reaction could be directly studied only with low precision
and at room temperature, since the rate constant is rather low,
and since secondary reactions of the cumyloxy radical and of
excited acetophenone with the alkane strongly interfered. The
Results
-1 -1
result is kTMP(297) ) (190 + 50/-100) M s . More reliable
rate constants were obtained indirectly in the following way:
Szwarc et al. measured the Arrhenius parameters for the methyl
affinities of ethene, propene, 2-methylpropene, styrene, R-meth-
ylstyrene, and isoprene (for references, see Table 2). Combi-
nation with our addition rate constants measured at 297 K
Table 1 gives absolute rate constants at room temperature
and Arrhenius parameters for the reaction of the methyl radical
with 20 mono- and 1,1-disubstituted alkenes, one diene, and 6
substituted benzenes. All reactions are believed to be by
(22) Fischer, H.; Giacometti, G. J. Polym. Sci. C 1967, 16, 2763.

12872 J. Am. Chem. Soc., Vol. 119, No. 52, 1997
Zytowski and Fischer
Table 2. Representative Methyl Affinities Converted to Absolute Rate Constants and Arrhenius Parameters
338, M- s-1
1
log A, M-1 s-1
E
, kJ/mol
ref
compound
k/kTMP (338 K)
k
a
Alkenes and Isolated Double Bonds
4
9.3a
ethene
propene
38.4
3.3 × 10
31.4
32.3
22.5
1l,j,m,y,ad
1l,p,x,y,ad
1l,m,p,x
1l,p
1l,p
1l,p
1r
1b,f,r
1ac
b
4
a
22.0
1.9 × 10
9.3
7.8
b
4
a
1
1
1
1
-butene
27
2.3 × 10
b
4
-pentene
-heptene
-decene
25
2.1 × 10
b
4
26
2.2 × 10
b
4
22
1.9 × 10
ethyl vinyl ether
vinyl acetate
methyl acrylate
methyl vinyl ketone
acrylonitrile
8
34
6900
4
7.4a
9.0a
2.9 × 10
18.9
5
1030
1900
1730
790
8.8 × 10
6
1.6 × 10
1ac
6
1.5 × 10
18.1
21.6
1f,ac
1b,f,r,p,y
1u
1u
1u
1u
1u
1u
1u
1u
2
2
2,3a
1l,m,p,y
1r,y
1f,r,1ac
1ac
5
a
styrene
6.8 × 10
9.2
b
4
2
2
3
4
2
1
1
9
,4,6-trimethylstyrene
-chlorostyrene
-chlorostyrene
-chlorostyrene
,5-dichlorostyrene
-vinylnaphthalene
-vinylanthracene
-vinylanthracene
104
8.9 × 10
5
1000
1040
1020
1210
810
8.6 × 10
5
8.9 × 10
5
8.8 × 10
6
1.0 × 10
5
7.0 × 10
6
1350
440
1.2 × 10
5
3.8 × 10
d
5
vinyl bromide
vinyl chloride
vinyl fluoride
292
2.5 × 10
d
5
219
1.9 × 10
d
4
20.3
36
1.7 × 10
b
4
8.9a
2
-methylpropene
3.1 × 10
28.1
18.7
16.3
5
a
R-methylstyrene
methyl methacrylate
methacrylonitrile
930
1450
2120
8.0 × 10
8.8
8.6
6
a
1.2 × 10
6
1.8 × 10
d
5
1
1
1
,1-dichloroethene
,1-difluoroethene
,1-diphenylethene
1020
8.8 × 10
2
d
4
24.9
2.1 × 10
2,3a
1b,f,k
1aa
1aa
1aa
6
7.7a
1600
5.8
1.4 × 10
10.5
cyclopentene
cyclohexene
cycloheptene
cyclooctene
5000
770
0.9
4.2
3600
5800
5900
2900
6.7
1aa
b
(
(
(
(
(
(
E)-2-butene
Z)-2-butene
E)-â-methylstyrene
Z)-â-methylstyrene
E)-stilbene
6.9
7.7
7.4
25.3
25.2
1d,i,l,m,p
1d,i,l,m,p
1p,u
1u
1b,e,f,k
1k
1k,r
1k,r
1b,f,k,r
1k,r
1p
1u
1t
1ac
1ac
1b,f
2,3a
1u
1b
1j
b
3.4
b
4
90
7.7 × 10
b
4
40
3.4 × 10
4
105
29.0
2000
1930
333
1930
5.6
66
15.4
12
9.0 × 10
8.3
8.2
6.9
8.5
7.7
9.9
21.4
24.2
4.1
14.2
14.4
23.6
4
Z)-stilbene
2.5 × 10
6
diethyl fumarate
fumarodinitrile
diethyl maleate
maleodinitrile
1.7 × 10
6
1.7 × 10
5
2.9 × 10
6
1.7 × 10
2
-methyl-2-butene
4800
4
R,â-dimethylstyrene
â,â-dimethylstyrene
â,â-dimethyl methylacrylate
â,â-dimethylacrylonitrile
triphenylethene
5.7 × 10
4
1.3 × 10
4
1.0 × 10
4
23.5
46
2.0 × 10
4
4.0 × 10
7.4
8.8
18.1
21.2
d
4
trifluoroethene
R,â,â-trimethylstyrene
tetraphenylethene
tetrachloroethene
tetrafluoroethene
hexafluoropropene
perfluoro-1-octene
5b7.5
4.9 × 10
4
20
1.7 × 10
<10
<8600
<2600
<0.3
d
5
387
3.3 × 10
1j,2,3a
3d
3b,e
1p
4
60
39
5.2× 10
4
3.4 × 10
b
4
1
1
,5-hexadiene
,4-cyclohexadiene
68
5.8 × 10
4
<20
<1.7 × 10
1s,aa
Conjugated Multiple Bonds
6
1
,3-butadiene
2015
2090
7540
2230
840
1200
1000
4750
1015
500
2290
180
270
665
156
1.7 × 10
9.6
9.0
21.8
17.6
1p,r,y,ad
1r,p,y
1p
1r,p
1p
1p
1p
1z
1z
1p
1p
6
a
isoprene
chloroprene
1.8 × 10
6
6.5 × 10
6
2
,3-dimethyl-1,3-butadiene
1.9 × 10
5
(E)-1,3-pentadiene
Z)-1,3-pentadiene
7.2 × 10
6
(
1.1 × 10
5
4
1
1
1
1
2
-methyl-1,3-pentadiene
,2-dimethylenecyclobutane
,2-dimethylenecyclohexane
-methoxy-1,3-butadiene
-phenyl-1,3-butadiene
,4-hexadiene
8.6 × 10
6
4.1 × 10
5
8.7 × 10
5
4.3 × 10
6
2.0 × 10
5
1.5 × 10
8.3
20.2
1p
5
cyclopentadiene
,3-cyclohexadiene
2.3 × 10
1aa
1aa
1aa
1aa
5
1
5.7 × 10
5
cycloheptatriene
cyclooctatetraene
1.3 × 10
4
81
7.0 × 10

Addition of Me Radical to Unsaturated Compounds
J. Am. Chem. Soc., Vol. 119, No. 52, 1997 12873
Table 2 (Continued)
338, M- s-1
1
log A, M-1 s-1
E
, kJ/mol
ref
compound
k/kTMP (338 K)
k
a
Alkynes
930 02 .5 × 10
1.2 × 10
1.5 × 10
1.7 × 10
1200
4
ethyne
propyne
29.4
10.9
9.7
8.8
34.3
31.1
1n
1n
1n
1n
1n
1n
1n
b
4
4
5
1
1
-pentyne
-hexyne
14
17.5
b
phenylethyne
-butyne
195
1.4
12.0
9.2
25.9
42.1
2
4
diphenylethyne
1.0 × 10
Cumulated Multiple Bonds
10.5
4
allene
17.6
14.8
1.5 × 10
7.6
8.3
22.2
26.8
1p
1p
1p
1p
1p
4
1,2-butadiene
1,2-pentadiene
2,3-pentadiene
1.3 × 10
b
4
19
1.7 × 10
b
4
14
1.2 × 10
e
4
tetraphenylallene
vinylacetylene
52
4.5 × 10
6
2260
1.9 × 10
1ac
Benzenes
250
benzene
toluene
anisol
ethyl benzoate
acetophenone
benzonitrile
bromobenzene
chlorobenzene
fluorobenzene
0.29
0.48
9.5a
9.2a
46.0
36.9
1a,e,f
1i
b
410
163
1300
600
3000
900
1050
550
3100
2900
c
0.19
1o
1o
1o
1o
1o
1o
1o
1o
1o
c
1.5
c
0.70
c
3.6
c
1.05
1.23
c
8.8a
37.1
47.3
c
10.1a
0.64
c
1
1
,3-dichlorobenzene
,4-dichlorobenzene
3.6
3.4
c
7.5a
25.8
27.7
Condensed Aromatic Compounds
naphthalene
9.4
8.1
13.0
8.8
8100
7000
760 01 .1 × 10
8.2
1a,e,v
1v
1v
1v
1v
1v
1v
1v
1v
1v
1v
1v
1v
1v
1v
1v
1v
1v
1v
1a,e,t,w
1t
1t
1t
1a,e,g,t
1t
1t
1t
1f
1h
1c
1a,e
1f
1h
1h
1h
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
-methylnaphthalene
-methylnaphthalene
-methoxynaphthalene
-methoxynaphthalene
-methyl naphthoate
-methyl naphthoate
-acetonaphthone
-acetonaphthone
-naphthonitrile
-naphthonitrile
-bromonaphthalene
-bromonaphthalene
-chloronaphthalene
-chloronaphthalene
-fluoronaphthalene
-fluoronaphthalene
,4-dichloronaphthalene
4
7.5
6400
4
19.4
40.3
20
44
31
46
8
21.2
13.2
21.6
10.5
13.2
20.6
4.7
1.7 × 10
4
3.5 × 10
4
1.7 × 10
4
3.8 × 10
4
2.6 × 10
4
3.9 × 10
7000
4
1.8 × 10
4
1.1 × 10
4
1.9 × 10
9000
4
1.1 × 10
4
1.8 × 10
acenaphthene
anthracene
4000
5
385
3.3 × 10
9.4
9.3
25.0
34.8
5
1-methylanthracene
2-methylanthracene
9-methylanthracene
355
355
190
10.2
13
12
3.1 × 10
5
3.1 × 10
5
1.6 × 10
phenanthrene
8800
4
2
3
9
-methylphenanthrene
-methylphenanthrene
,10-dimethylphenanthrene
1.1 × 10
4
1.1 × 10
6.0
5200
6
naphthacene (358 K)
benzo[a]anthracene (358 K)
benzo[c]phenanthrene (358 K)
pyrene (358 K)
3700
206
25.5
49
6.2 × 10
e
5
3.4 × 10
e
4
4.2 × 10
4
8.1 × 10
4
chrysene (358 K)
benzo[a]pyrene (358 K)
dibenz[a,h]anthracene (358K)
hexahelicene (358 K)
23
268
3.8 × 10
e
5
4.5 × 10
e
5
148
114
2.5 × 10
e
5
1.9 × 10
Quinones
1
2
2
2
2
2
2
2
,4-benzoquinone
6050e
4150
3180
5.2 × 10
6
1d,f
1d,f
1f
1d,f
1q
1q
1q
6
-methylbenzoquinone
-methoxybenzoquinone
-chlorobenzoquinone
,3-dimethylbenzoquinone
,6-dimethoxybenzoquinone
,3-dichlorobenzoquinone
,6-dichlorobenzoquinone
3.6 × 10
6
2.7 × 10
4
6
1 × 10
8.9 × 10
e
6
3090
2.7 × 10
e
5
1020
4000
8.8 × 10
e
6
3.4 × 10
4
e
7
4
5
6
duroquinone
chloranil
3 19 .6 × 10
120
1370
1.3 × 10
3.4 × 10
1.0 × 10
1.2 × 10
1f
1
,2-naphthoquinone

1
2874 J. Am. Chem. Soc., Vol. 119, No. 52, 1997
Table 2 (Continued)
compound
Zytowski and Fischer
1
log A, M-1 s-1
k/kTMP (338 K)
k
338, M- s-1
E
a
, kJ/mol
ref
1q
Quinones
2.8 × 10
1.2 × 10
5.6 × 10
3.1 × 10
2.4 × 10
6
6
6
4
5
1
2
2
2
,4-naphthoquinone
3240
1350
6500
36
280
-methyl-1,4-phthoquinone
-chloro-1,4-naphthoquinone
-tert-butylanthraquinone
1d,f
1q
1d,f
1d,f
e
phenanthraquinone
Heterocycles
2000
pyridine (358 K)
acridine
quinoline
1.2
187
13
7.0
9.8
7.9
25.6
29.4
24.7
1a,e
5
4
4
4
1.6 × 10
1.2 × 10
2.3 × 10
9.2 × 10
1e,f,w
1a,e,w
1e,f,w
1w
isoquinoline (358 K)
phenazine
14
107
a
From converted methyl affinities and rate constants measured in this work. Abstraction also observed. c Original data relative to the reactivity
b
d
of benzene, converted via kBen/kTMP ) 251/858 at 65 °C. Original data relative to the reactivity of ethene, converted via kEth/kTMP ) 38.4 at 65
1
ad
e
Original data relative to the reactivity of toluene, converted via kTol/kTMP ) 3.1i
°
C.
Table 2 gives a representative selection of methyl affinities
converted to absolute rate constants and their temperature
dependence. Our new experimental data at room temperature
were included in the redetermination of the Arrhenius parameters
which, therefore, differ slightly from those given in Table 1.
Data for 95 additional compounds are available as Supporting
Information.
For comparison only a few absolute rate constants for the
addition of methyl radicals in liquid solutions are available. By
23
pulse radiolysis and for aqueous solution Thomas reported
3
3
4
4
k(297) ) 4.9 × 10 , 5.3 × 10 , 3.0 × 10 , 3.9 × 10 , and 1.2
6
-1 -1
×
10 M for ethene, propene, 1-butene, 2-methylpropene
s
and 1,3-butadiene, respectively. Most of these data are slightly
higher than ours and the converted methyl affinities (Tables 1
and 2), but the overall agreement is fair. The same holds for
Figure 4. Temperature dependence of the reaction rate constant of
methyl with 2,2,4-trimethylpentane and fit to the Arrhenius equation.
2
4
5
6
5
-1
Gilbert’s k(297) ) 5.0 × 10 , 5.8 × 10 , and 1.0 × 10 M
-1
s
for methacrylic acid, methyl methacrylic acid, and crotonic
1
(
3
Table 1) gives kTMP(297) ) (162 ( 25) M- s-1. For 323,
acid in water, which are slightly higher but similar to our values
6
-1 -1
38, and 358 K, Szwarc’s methyl affinities for the addition to
for the esters, and for k(297) ) 1.2 × 10
M
s
for
2
5
styrene, R-methylstyrene, and isoprene and the Arrhenius
parameters from this work were used for the conversion. The
same procedure was applied for 338 and 358 K for 1,1-
diphenylethene and methyl methacrylate, and finally for 338 K
for 1,1-dichloroethene and methacrylonitrile. The resulting
temperature dependence of kTMP is shown in Figure 4 and is
given by
benzoquinone, again in aqueous medium. Since there are no
gross differences between the rate constants in the low polar
1,1,2-trichloro-1,2,2-trifluoroethane (ꢀ ) 2.42(2)) and in aqueous
solution, the addition behavior of the methyl radical is not
strongly solvent dependent, though the rate constants seem to
increase slightly with increasing solvent polarity. This also
ensures the validity of the conversion procedure for the methyl
affinities which were measured in the unpolar solvent 2,2,4-
log(kTMP/M- s ) ) 8.13(7) - 33.6(5)/2.303RT (8)
1
-1
1
trimethylpentane and converted with rate data obtained here
in 1,1,2-trifluoro-2,1,1-trichloroethane.
A comparison of the solution with gas-phase data (see
Supporting Information) shows that for all compounds except
benzene the latter are lower by more than 1 order of magnitude.
For ethene and ethyne the gas-phase data have been reevaluated
with Ea in kJ/mol.
Moreover, the Arrhenius parameters of k/kTMP for styrene,
R-methylstyrene, and isoprene are known.1
b,f,r,p,y
Combination
with our data of Table 1 yields
2
6,27
several times
and are believed to be accurate to a factor of
log(kTMP/M-1 s ) ) 8.0(2) - 32.5(12)/2.303RT (9)
-1
2, so that their differences to the solution rates are particularly
noteworthy. Averaging of the frequency factor differences for
-
1
the individual compounds in the two phases gives log(Al/M
By using all available conversions and proper error weighting
the average becomes
-
1
-1 -1
s ) - log(Ag/M s ) ) +0.41 ( 0.69, i.e., an insignificant
(
23) Thomas, J. K. J. Phys. Chem. 1967, 46, 1694.
log (kTMP/M-1 s-1) ) 8.08(30) - 33.30(200)/2.303RT (10)
(24) Gilbert, B. C.; Smith, J. R. L.; Milne, E. C.; Whitwood, A. C. J.
Chem. Soc., Perkin Trans. 2 1993, 2025.
(
25) Veltwisch, D.; Asmus, K. D. J. Chem. Soc., Perkin Trans. 2 1982,
147.
26) Baulch, D. L.; Cobos, C. J.; Cox, R. A.; Esser, C.; Frank, P.; Just,
T.; Kerr, J. A.; Pilling, M. J.; Troe, J.; Walker, R. W.; Warnatz, J. J. Phys.
Chem Ref. Data 1992, 21, 411; 1994, 23, 847. Baulch, D. L.; Cobos, C. J.;
Cox, R. A.; Frank, P.; Hayman, G.; Just, T.; Kerr, J. A.; Murrells, T.; Pilling,
M. J.; Troe, J.; Walker, R. W.; Warnatz Combust. Flame 1994, 98, 59.
1
and this relation was used to obtain the final absolute data from
the methyl affinities. To check the procedure we included the
rate constants derived from methyl affinities at higher temper-
atures in the plots of the temperature dependence observed here.
As demonstrated in Figure 3, they agree very well with the
Arrhenius parameters derived from our experimental data which
are measured at lower temperatures.
(
(27) Kerr, J. A.; Moss, S. J. Handbook of Bimolecular and Termolecular
Gas Reactions; CRC Press: Boca Raton, 1984, Vol. 2, and references given
therein.

Addition of Me Radical to Unsaturated Compounds
J. Am. Chem. Soc., Vol. 119, No. 52, 1997 12875
value. If benzene is omitted, the average difference of the
experimental activation energies is Eag - Eal ) (3.0 ( 4.7) kJ/
mol, and from the ratio of rate constants one obtains for equal
frequency factors in the two phases Eag - Eal ) (6.5 ( 2.8)
kJ/mol. This difference may be significant and may reflect a
true solvation effect on the solution data. It would be in keeping
with the slight solvent polarity dependence noticed above. On
the other hand, the gas-phase data have all been determined
relative to other rate constants and date back many years.
Moreover, the most recent work on ethene, ethyne, and
or polar substituent effects.17,30,31 If the enthalpy effect
dominates, the activation energies decrease and the rate constants
increase with increasing reaction exothermicity. This behavior
has been found clearly expressed for two cyano- and a carboxy-
1
7g,j
substituted alkyl radical
species. Polar effects are due to a partial charge transfer in
the transition state. Within the frame of the state correlation
and for the benzyl and the cumyl
1
7k
3
2
diagram for radical additions they reflect contributions of the
+
-
-
+
configurations R A and R A to the wave function where R
denotes the radical and A the alkene. These are of particular
importance for the transition state geometry since there the
energies of the charge transfer states are considerably lowered
by the Coulomb attraction. For several radicals with low
ionization energies, such as hydroxymethyl, 2-hydroxypropyl,
2
8
benzene gives errors which still allow order of magnitude
variations.
Discussion
+
-
and tert-butyl, the contribution of the state R A was found
In the following we will mainly discuss the factors controlling
the rates for the addition of methyl to the mono- and 1,1-
substituted alkenes. Their frequency factors are in the range
dominant. The barriers for their additions decreased with
17
increasing electron affinity of the alkene, and there was no
clear correlation with the reaction enthalpy. These radicals react
with alkenes carrying strongly electron withdrawing substituents
even faster than with phenyl-substituted compounds such as
styrene for which the exothermicity is higher, and this can be
taken as a clear criterion for a polar effect. On the other hand,
radicals with high electron affinities, such as perfluoroalkyls
and dicyanomethyl, are electrophilic, and their reaction barriers
-
1
-1
7
.4 < log(A/M s ) <9.2 (Table 1) and show no specific
variation with the substituents. Therefore, we consider the
spread insignificant and likely to be caused by error compensa-
tion of the Arrhenius parameters. The average of all data is
-
1 -1
log(A/M s ) ) 8.6 ( 0.5. This is very close to the average
frequency factors for the addition of other primary radicals to
-
1
-1
the same alkenes as log(A/M s ) ) 8.7 ( 0.3 for cyano-
33
increase with decreasing alkene ionization energy.
-
1
-1
methyl, log(A/M s ) ) 8.4 ( 0.1 for tert-butoxycarbonyl-
Now, the methyl radical has a rather high ionization energy
-
1
-1
methyl, log(A/M s ) ) 8.1 ( 0.1 for hydroxymethyl, and
34
of 9.84 eV and a low electron affinity of 0.08 eV. Hence,
-
1
-1
17a
log(A/M s ) ) 8.6 ( 1.3 for benzyl. From symmetry
the polar effects should not be large. The bond dissociation
-
1
-1
considerations methyl should have a ∆ log(A/M s ) ) ∆
log 3 ) 0.477 higher frequency factor than the other species,
but this is within the error limits. Hence, the data suggest a
common value of log(A/M s ) ) 8.5 ( 0.5 for additions of
primary alkyl radicals to the CH2 group of mono- and 1,1-
disubstituted alkenes both in solution and in the gas phase. If
correct, this suggests a minimum error of the activation energies
of about 3 kJ/mol.
35
energy of methane exceeds that of other CH bonds, and makes
the addition of methyl more exothermic than that of other alkyl
radicals.
-
1 -1
For the methyl radical strong effects of the reaction enthalpy
1
have already been pointed out by Szwarc in terms of the adduct
radical stabilization, of the radical localization energies, and of
strain in the attacked bond, and there are many clear-cut
examples in Table 2 (cf. ethene vs 1,3-butadiene, benzene vs
The common frequency factor means that for all cases studied
the structures of the transition states are very similar, and this
naphthalene, etc.). Szwarc also noticed a weak nucleo-
1o,r,u,v,ac
philicity
(Table 2, cf. chlorostyrenes vs styrene, chlo-
agrees with the results of several quantum chemical calcula-
robenzene and benzonitrile vs benzene, etc.), and remarked that
tions.1
8,29
Hence, the variation of the addition rate constants
the methyl radical is more polar in its addition behavior than
with the alkene substituents is caused mainly by their influence
on the activation energy, as has been stated previously by
Tedder.30 At 297 K, the rate constants vary with substitution
30,31,36
phenyl. Later reviews
supported and, in part, overem-
phasized this point. To the contrary, recent high level ab initio
studies on the addition of methyl to a variety of alkenes did
-
1 -1
from 4 300 to 780 000 M s , i.e., by a factor of 200, and the
activation energies range from 10.5 to 31.4 kJ/mol (tables). In
comparison to several other alkyl radicals17 these ranges are
small. In particular, the methyl radical is much less selective
18,29c
not reveal a significant charge transfer in the transition state.
Besides that, very good correlations of the calculated reaction
barriers with the calculated reaction enthalpies were found, and
18a
hence, Radom et al. concluded that polar contributions to the
1
7i
and for some alkenes also less reactive than the hydroxymethyl,
the 2-hydroxy-2-propyl,
reactivity of methyl toward alkenes are generally insignifi-
cant.
17g,n
17b
and the tert-butyl radical, and
its reaction behavior resembles that of the cyanomethyl and the
tert-butoxycarbonylmethyl species,17j though the latter radicals
(31) Giese, B. Angew. Chem., Int. Ed. Engl. 1983, 22, 573.
(32) (a) Shaik, S. S.; Hiberty, P. C.; Lefour, J.-M.; Ohanessian, G. J.
are generally more reactive by up to a factor of 10.1
7a
Am. Chem. Soc. 1987, 109, 363. (b) Shaik, S. S.; Hiberty, P. C.; Ohanessian,
G.; Lefour, J.-M. J. Phys. Chem. 1988, 92, 5086. (c) Maitre, P.; Hiberty,
P. C.; Ohanessian, G.; Shaik, S. S. J. Phys. Chem. 1990, 94, 4089. (d)
Shaik, S. S.; Canadell, E. J. Am. Chem. Soc. 1990, 112, 1446. (e) Shaik, S.
S. Pure Appl. Chem.1991, 63, 195. (f) Pross, A.; Yamataka, H.; Nagase, S.
J. Phys. Org. Chem. 1991, 4, 135.
For mono- and 1,1-disubstituted alkenes steric substituent
effects on the addition rates should be small. Therefore, the
reaction barriers should be governed mainly by enthalpic and/
(
28) Holt, P. M.; Kerr, J. A. Int. J. Chem. Kinet. 1977, 9, 185.
(33) (a) Avila, D. V.; Ingold, K. U.; Lusztyk, J.; Dolbier, W. R.; Jr.;
Pan, H. Q. J. Am. Chem. Soc. 1993, 115, 1577. Avila, D. V.; Ingold, K.
U.; Lusztyk, J.; Dolbier, W. R. Jr.; Pan, H. Q.; Muir, M. J. Am. Chem. Soc.
1994, 116, 99. Avila, D. V.; Ingold, K. U.; Lusztyk, J.; Dolbier, W. R., Jr.;
Pan, H. Q. J. Org. Chem. 1996, 61, 2027. (b) Riemenschneider, K.; Bartels,
H. M.; Dornow, R.; Drechsel-Grau, E.; Eichel, W.; Luthe, H.; Michaelis,
Y. M.; Boldt, P. J. Org. Chem. 1987, 52, 205.
(29) (a) Gonzalez, C.; Sosa, C.; Schlegel, H. B. J. Phys. Chem. 1989,
9
3, 2435. (b) Arnaud, R.; Subra, D.; Barone, V.; Lelj, F.; Olivella, S.; Sole,
A.; Russo J. Chem. Soc., Perkin Trans. 2 1986, 1517. (c) Fueno, T.;
Kamachi, M. Macromolecules 1988, 21, 908. (d) Houk, K. N.; Paddon-
Row: M. N.; Spellmeyer, D. C.; Rondan, N. G.; Nagase, S. J. Org. Chem.
1
986, 51, 2874. (e) Arnaud, R. New J. Chem. 1989, 13, 545. (f) Zipse, H.;
He, J.; Houk, K. N.; Giese, B. J. Am. Chem. Soc. 1991, 113, 4324. (g)
Arnaud, R.; Postlethwaite, H.; Barone, V. J. Phys. Chem. 1994, 98, 5913.
(34) (a) Blush, J. A.; Chen, P.; Wiedmann, R. T.; White, M. G. J. Chem.
Phys. 1993, 98, 3557. (b) Ellison, G. B.; Engelking, P. C.; Lineberger, W.
C. J. Am. Chem. Soc. 1978, 100, 2556.
(35) Berkowitz, J.; Ellison, G. B.; Gutman, D. J. Phys. Chem. 1994, 98,
2744.
(
h) Arnaud, R.; Barone, V.; Olivella, S.; Sole, A. Chem. Phys. Lett. 1985,
18, 573.
30) (a) Tedder, J. M. Angew. Chem., Int. Ed. Engl. 1982, 21, 401. (b)
Tedder, J. M.; Walton, J. C. AdV. Free Radical Chem. 1980, 6, 155.
1
(
(36) Minisci, F.; Vismara, E.; Fontana, F. Heterocycles 1989, 28, 489.

12876 J. Am. Chem. Soc., Vol. 119, No. 52, 1997
Zytowski and Fischer
Table 3. Energy Quantities for the Addition of Methyl Radicals to Alkenes (Errors Are Given in Units of the Last Digit Quoted; Numbers in
Italic Letters Are from (QCISD(T)/6-311G**+ZPVE) Calculations18a,b
)
1
0(R-H), kJ/molc
0(A), kJ/molc
52(1)
X, Y
H, H
k
297, M- s-1
E
a
, kJ/mol
31.4
5.2
28.3
5.7
32.3
3.3
h
f
h
f
BDE(R-H), kJ/mol
r
h , kJ/mol
3500a
-105(1)
-286(2)
-127(1)
423(2)g
-99(4)
-92.3
3
4200b
4300
b
-139(2)
410(10)h
413(2)g
-102(14)
-93.0
H, F
3
H, Me
20(1)
-99(4)
3
-92.4
H, Et
6700
8500
12000
12000
14000
22.5
28.1
25.7
25.7
25.3
-147(1)
-154(1)
-272(10)
-501(5)
-272(1)
-0.4(5)
-17(1)
-146(5)
-349(5)
-141(5)
413(2)g
404(2)g
-99(4)
Me, Me
Me, OMe
Me, OAc
H, OEt
-98(4)
b
b
b
d
i
378(10)
-113(25)
-117(18)
-109(14)
j
400(8)
387(8)
i
-92.1 (OH)
H, OAc
H, Cl
14000
20000
23.9
23.9
-454(5)
-132(1)
-315(5)
406(8)j
411(8)k
-98(18)
-109(11)
-105.0
b
23(2)
2
8.5
20.8
3
9.8(SiH )
H, SiMe
3
23000
35000
-261(10)d
-123(5)
402(8)j
-101(23)
-101.1
2
b
406(10)k
Me, Cl
H, Br
H, Ph
22.5
20.2
19.9
18.9
19.1
16.9
18.2
15.3
-161(8)
-85(1)
+8(1)
-21(5)
79(2)
148(1)
113(5)
2(1)
-312(5)
-348(5)
184(5)
-99(23)
-126(8)
-148(8)
-142(14)
-125(15)
-120(25)
-114(26)
-139(15)
-127.7
b
b
n
90000
403(5)
j
260000
310000
320000
340000
490000
610000
357(6)
j
Me, Ph
Cl, Cl
-17(1)
353(8)
d
j
-151(8)
393(6)
b
b
b
l
H, CO
2
Me
Me
-452(5)
385(15)
379(15)
d
l
Me, CO
H, CN
2
-476(6)
+34(1)e
-208(2)
+5(5)
376(9)m
372(11)o
362(8)m
2
0.4
15.0
4.1
H, CHO
740000
-77(5)
-124(18)
-119.6
-128(18)
-181(22)
2
Me, CN
Ph, Ph
760000
780000
16.9
11.3
130(5)
246(4)
f
j
+91(12)
339(6)
a
group. b Calculated from k338 (Table 2) or k297 (for E
c
Per CH see Table 1) and log A ) 8.6. Data without comments are from the standard
2
a
3
7
d
0
0
reference. When no errors were available the error was set to 5 kJ/mol. From h
f
(Me
) ) (-501 ( 6) kJ/mol, h
increment of (-20 ( 5) kJ/mol for replacement of one Me- by an Et-group, as derived from related pairs found in ref.
2
CHOMe) ) (-252.0 ( 1.0) kJ/mol, h
f
2 3
(MeCH SiMe ) )
0
0
0
-
241.0 kJ/mol, h
f
(MeCHCl
2
) ) (-131 ( 3) kJ/mol, h
f
(MeCHF
2
f
(Me
2
CHCO
H CH
g
2
Me) ) (-456 ( 1) kJ/mol and the
37
e
Rakus, K.; Verevkin,
f
0
S. P.; Beckhaus, H.-D.; R u¨ chardt, C. Chem. Ber. 1994, 127, 2225. From h
) kJ/mol for replacement of an H-atom by an Et-group, as derived from CH
f
) (140 (3) kJ/mol for C
6
5
2
C
6
H
5
and the increment of (-49 (
Recommended values for BDE(Me C-H),
BDE (MeCHOEt-H) incremented for Me COEt-H from ∆BDE (HOCHCH
SiCH CCl
SiCHEt-H incremented by ∆BDE (AlkCHMe-H/AlkCH
37
1
2
Me
2
and EtCHMe
2
.
3
35
h
38
i
39
BDE(H-CHMe
2
), and BDE(H-CH
-H) ) -9 kJ/mol.
2
Me).
BDE(CH
3
CHF-H).
BDE(H-CH OCOPh), BDE(Me
2
3
-
4
0
j
H/HOCMe
2
2
3
2
-H), BDE(CHCl
2
2
-H), BDE(PhCMeH-H) and BDE(PhCMe
2
-
-
4
1
H), for H, OAc, and Me
H/AlkCH -H) ) (-19 ( 3) kJ/mol.
3
2
-H) ) (-13 ( 3) kJ/mol, for Me, OAc by ∆BDE(AlkCMe
2
3
5
k
42a
2
BDE(CH
3
CHCl-H) and ∆BDE(CH
3
CHCl-H/CH
2
Cl-H) ) (-5 kJ/mol). BDE ) (401 ( 4) is reported
l
43
in ref 42b. BDE(H-CH
2
CO
2
CH
3
)
incremented by ∆BDE(AlkCHMe-H/AlkCH
2
-H) )(-13 ( 3) kJ/mol and ∆BDE(AlkCMe
2
-H/AlkCH -H)
2
3
5
m
44
n
45
o
46
)
∆
-(19 ( 3) kJ/mol.
BDE(Me
-H) ) (-13 ( 3) kJ/mol.
2
CCN-H) and BDE(MeCHCN-H).
BDE(MeCHBr-H).
BDE(OHCCH
2
-H) incremented by
3
5
BDE(AlkCHMe-H/AlkCH
2
0
37
Here, we again consider the correlations between the experi-
BDE. For methane hf ) -74.5(5) kJ/mol and BDE ) 439(1)
35
mental activation energies and log(k) and the reaction enthalpies,
the alkene ionization energies, and electron affinities for the
addition of methyl to mono- and 1,1-disubstituted alkenes based
on the now available larger data set. 1,1-Difluoroethene is not
included in the analysis because there is addition to the CF2
kJ/mol were used. Table 3 gives the energy quantities needed
and the reaction enthalpies including all errors, with footnotes
referring to the details of the determination. Small differences
to enthalpies given earlier are due to the inclusion of more
precise recent data. Table 3 also gives the enthalpies and the
1
7i
2
7,31
18a
carbon, also.
The alkene ionization energies and electron affinities were
reaction barriers calculated by Radom et al. with a refined
quantum chemical method. Considering the appreciable errors,
the experimental and theoretical reaction enthalpies agree fairly
well though the calculated exothermicities appear on average
somewhat lower.
1
7
taken from ref 37 or the literature quoted in our earlier work.
However, the reaction enthalpies are not available from experi-
ment and were determined indirectly. By considering the overall
reaction
Figures 5 and 6 show the activation energies and log(k(297)/
M- s ) plotted versus the reaction enthalpies of Table 3. Ob-
viously, the rate constants do increase and the activation energies
decrease with increasing exothermicity of the addition but the
correlations are weak. The straight lines are represented by
1 -1
CH + CH dCXY f CH + H + CH dCXY f
4
2
3
2
CH CH CXY + H f CH CH CHXY
3
2
3
2
one obtains for the reaction enthalpy
h ) h (CH CH CHXY) - h (CH dCXY) - h (CH ) +
E /kJ/mol ) 44.8(46) + 0.195(39)h /kJ/mol
a
r
2
r
f
3
2
f
2
f
4
R ) 0.560 (12)
BDE(CH CH CHXY) - BDE(CH ) (11)
3
2
4
log(k297/M- s ) ) 0.97(74) - 0.032(6)h /kJ/mol
1 -1
r
With this equation we estimated hr for 300 K from the best
available heats of formation and bond dissociation energies
2
R ) 0.569 (13)
Interestingly, the slopes of the linear correlations are similar
to those found for radicals with a more clear-cut enthalpy
(
37) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin,
R. D.; Mallard, W. G. J. Phys. Chem. Ref. Data 1988, 17, Suppl. 1.

Addition of Me Radical to Unsaturated Compounds
J. Am. Chem. Soc., Vol. 119, No. 52, 1997 12877
Figure 7. Rate constants for the addition of methyl to mono- and 1,1-
disubstituted alkenes plotted vs the alkene electron affinities. Symbols
denote the substituents, H ) ethene.
Figure 5. Rate constants for the addition of methyl to mono- and 1,1-
disubstituted alkenes plotted vs the reaction enthalpy. Symbols denote
the substituents, H ) ethene.
Figure 8. Activation energies for the addition of methyl to mono-
and 1,1-disubstituted alkenes plotted vs the alkene electron affinities.
Symbols denote the substituents, H ) ethene.
Figure 6. Activation energies for the addition of methyl to mono-
and 1,1-disubstituted alkenes plotted vs the reaction enthalpy. Symbols
denote the substituents, H ) ethene.
6
as compared to Radom’s plots is at least partly due to our
larger set of alkenes.
control, as, for log(k), -0.037 for cyanomethyl, -0.039 for tert-
butoxycarbonylmethyl, -0.039 for 2-cyano-2-propyl, and -0.045
for benzyl.1 This clearly suggests that the enthalpy effect
operates also for methyl.
Figure 5 and Tables 1 and 2 also show that the methyl radical
reacts as fast or even faster with strongly electron deficient
alkenes than with styrene and R-methyl styrene, which were
not covered by Radom. We think that this must be due to a
nucleophilic polar effect. It is weaker than for hydroxymethyl,
7a
1
8
In comparison, the calculated data of Radom et al. for 10
1
8
alkenes gave the much better correlation
2
-hydroxy-2-propyl, and tert-butyl radicals, which add to
electron deficient alkenes even faster than to 1,1-diphenylethene.
In the analysis for polar effects, correlations of log(k) or Ea
vs the alkene ionization energies gave a complete scatter. This
was expected since the methyl radical has never been found
electrophilic. On the other hand, the correlations with the
electron affinities are very reasonable and are shown in Figures
7 and 8.
2
E /kJ/mol ) 72.6 + 0.41h /kJ/mol
R ) 0.973 (14)
a
r
with a considerably larger slope than (12), However, restricting
the analysis of our data to alkenes covered in the theoretical
work (X ) H, F, Cl, Me, CN, CHO, SiH3 (here SiMe3), and
OH (here OEt)) we obtain
The straight lines are
2
E /kJ/mol ) 67.2(116) + 0.39(10)h /kJ/mol
R ) 0.702
a
r
2
Ea/kJ/mol ) 15.7(10) - 5.7(7)EA/eV
R ) 0.778 (16)
(15)
log(k297/M- s ) ) 5.78(13) + 0.97(10)EA/eV
1 -1
which is within the error limits identical with (14), i.e., there is
a good general agreement between the calculated and the
experimental barriers. Hence, the larger scatter in our Figure
2
R ) 0.854 (17)
In comparison to more clearly nucleophilic radicals17 the
slopes are smaller. Their ordering, for log(k), cf. 0.97 vs 1.53,
(
38) Martell, J. M.; Boyd, R. J.; Shi, Z. J. Phys. Chem. 1993, 97, 7208
and references therein.
39) Burkey, T. J.; Majewski, M.; Griller, D. J. Am. Chem. Soc. 1986,
08, 2218.
(
(43) Bordwell, F. G.; Zhang, X.-M.; Alnajjar, M. S. J. Am. Chem. Soc.
1992, 114, 7623.
(44) King, K. D.; Goddard, R. D. J. Am. Chem. Soc. 1971, 97, 4504, J.
Phys. Chem. 1976, 80, 546.
(45) Tschuikow-Roux, E.; Salomon, D. R.; Paddison, S. J. Phys. Chem.
1987, 91, 3037.
(46) Holmes, J. L.; Lossing, F. P.; Terlouw, J. K. J. Am. Chem. Soc.
1986, 108, 1086.
1
1
4
(40) Holmes, J. L.; Lossing, F. P.; Mayer, P. M. J. Am. Chem. Soc. 1991,
13, 3, 9723.
(
41) McMillan, D. F.; Golden, D. M. Annu. ReV. Phys. Chem. 1982, 33,
93.
42) (a) Holmes, J. L.; Lossing, F. P. J. Am. Chem. Soc. 1988, 110, 7343.
b) Tschuikow-Roux, E.; Salomon, D. R. J. Phys. Chem. 1987, 91, 699.
(
(

12878 J. Am. Chem. Soc., Vol. 119, No. 52, 1997
Zytowski and Fischer
1
2
.61, and 2.81 for methyl, hydroxymethyl, tert-butyl, and
-hydroxy-2-propyl, respectively, reflects the ordering of the
it should be kept in mind that our conclusion of an addition
behavior mainly controlled by the reaction enthalpy but noti-
cably modulated by nucleophilic polar effects is based on data
obtained for liquid solutions. On the other hand, the calculated
reaction barriers, enthalpies, and transferred charges are for
isolated reactants at 0 K, i.e., gas-phase conditions. Hence, it
may be significant that Radom’s highest level calculations give
barriers which are on the average slightly higher than our
activation energies (Table 3), and that the experimental activa-
tion energies may be larger for the gas than for the liquid phase
radical ionization energies of 9.84 vs 7.5, 6.7, and 6.5 eV,
respectively. Restricting the number of data points as above
to the alkenes covered by Radom one obtains
2
Ea/kJ/mol ) 14.7(22) - 7.1(14)EA/eV
R ) 0.801 (18)
and this is again similar to the theoretically found correlation,18a,b
namely
(
vide supra). Therefore, there may be a real difference between
2
Ea/kJ/mol ) 24.9-5.43‚EA/eV
R ) 0.825 (19)
the two phases. In the liquid-phase solvation effects may lower
the energies of the excited configurations such as the charge-
transfer configurations and increase their contribution to the
transition state. Hence, the polar effects may in fact be stronger
for the liquid phase, which would be in keeping with a slight
solvent effect, though the extensive work of Tedder et al.30 on
the addition of methyl to fluoroethenes indicates that polar
effects do exist also in gases.
It is known that the alkene electron affinities correlate with
17,18
the reaction enthalpies,
and, hence, the correlations with EA
may deceive and may be caused by the reaction enthalpy effect,
only. Nevertheless, the behavior of methyl toward the strongly
electron deficient alkenes in comparison to the styrenes forces
us to conclude that the addition to alkenes is not alone influenced
by the reaction enthalpy but also by a nucleophilic polar effect,
in keeping with all earlier notions. This is also supported by
the rate data for addition to substituted benzenes since chloro-,
bromo-, and cyano-substituted compounds react faster than the
parent benzene whereas anisol reacts slower. The same is seen
for higher condensed aromatic compounds, and we deem it very
unlikely that these substituent effects are caused by variations
in the stabilization of the reactants or the cyclohexadienyl
radicals resulting from the addition. A full discussion of these
cases is beyond the scope of this work, however, and requires
estimations of reaction enthalpies for which there is no safe
basis yet.
Acknowledgment. We gratefully acknowledge the financial
support of this work by the Swiss National Foundation for
Scientific Research and fruitful discussions with L. Radom
(Canberra). A. L. J. Beckwith (Canberra) strongly encouraged
us to to convert the methyl affinities to absolute rate constants.
Supporting Information Available: Three tables with
magnetic properties of the adduct radicals, additional methyl
affinities, and their conversion to absolute rate constants and a
comparison of gas phase and solution rate data (6 pages). See
any current masthead page for ordering and Internet access
instructions.
As mentioned above, there is no large discrepancy between
Radom’s theoretical18 and our experimental results though the
conclusions differ and subtle differences remain. In this respect,
JA973128Y