Absolute rate constants for the addition of atomic hydrogen to monosubstituted and trisubstituted olefins

Article abstract of DOI:10.1021/jp960189u

A mechanism is established for the formation of the products resulting from the solution phase regioselective addition of atomic hydrogen to 1-methylcyclohexene. From this data and new data for the reactions of 1-octene, the absolute rates and activation parameters for the addition of hydrogen atom to an olefin can be extracted. A method was established to determine the absolute rate of addition of a hydrogen atom to a terminal olefin in the solution phase. The addition rate constants, k_a (25°C), to 1-octene [k_a = (4.2 ± 3.6) × 10⁹ M^-1 s^-1] and 1-methylcyclohexene [k_a = (4.6 ± 0.8) × 10⁶ M^-1 s^-1, -78°C] are found to be in reasonable agreement with the published values for the vapor phase rate of addition to ethylene. The large rate constants are supported by the observation that the activation parameters (E_a = 5.3 ± 2.9 kcal/mol and log A = 14 ± 3.5 M^-1 s^-1 for 1-octene) are consistent with the values expected for this fast reaction.

Full text of DOI:10.1021/jp960189u

J. Phys. Chem. 1996, 100, 11319-11324
11319
Absolute Rate Constants for the Addition of Atomic Hydrogen to Monosubstituted and
Trisubstituted Olefins
Dennis D. Tanner,* Liying Zhang,1 and Pramod Kandanarachchi2
Department of Chemistry, UniVersity of Alberta, Edmonton, Alberta, Canada T6G 2G2
ReceiVed: January 22, 1996X
A mechanism is established for the formation of the products resulting from the solution phase regioselective
addition of atomic hydrogen to 1-methylcyclohexene. From this data and new data for the reactions of 1-octene,
the absolute rates and activation parameters for the addition of hydrogen atom to an olefin can be extracted.
A method was established to determine the absolute rate of addition of a hydrogen atom to a terminal olefin
in the solution phase. The addition rate constants, ka (25 °C), to 1-octene [ka ) (4.2 ( 3.6) × 109 M-1 s-1]
and 1-methylcyclohexene [ka ) (4.6 ( 0.8) × 106 M-1 s-1, -78 °C] are found to be in reasonable agreement
with the published values for the vapor phase rate of addition to ethylene. The large rate constants are
supported by the observation that the activation parameters (Ea ) 5.3 ( 2.9 kcal/mol and log A ) 14 ( 3.5
M-1 s-1 for 1-octene) are consistent with the values expected for this fast reaction.
Introduction
atomic hydrogen have been determined from the relative rates
of reaction with a large variety of structurally different substrates
Recently we published the results of a study of the mechanism
of formation of the products resulting from the addition of
atomic hydrogen to 1-octene.3 One of the products, octane, was
formed from the reaction of hydrogen with the 1-methylheptyl
radical:
compared to the known absolute rate of the solution phase
reaction of hydrogen with ethylene.5
The rate of formation of the total products resulting from
the initial addition of a hydrogen atom to an olefin in a given
time, (∆P/∆t)total, is proportional to the concentration of the
reactants. In the case of a nonhomogeneous reaction, only the
first molecules that react obey these kinetics. Under nonho-
mogeneous conditions reaction occurs only upon the encounter
of the reactants in the volume of the reaction zone, i.e., the true
concentration is achieved only when the initial concentration
of olefin, [olefin]0, approaches zero. Under these conditions,
an estimate of ka can be obtained:
The capping reaction, eq 1, was only ambiguously defined since
the concentration of atomic hydrogen in this bimolecular
reaction was estimated to be too low to account for the formation
of the products, and the activation energy for the reaction of a
secondary radical with molecular hydrogen defined a rate that
was also prohibitively slow.3
(∆P/∆t)total ) ka[H•][olefin]0
(3)
The concentration of atomic hydrogen was estimated by using
the values of the absolute rate constant, ka (109 M-1 s-1),
reported for the addition of hydrogen to an olefin in both the
vapor phase4 and in aqueous solution.5 Since the velocity of
the reaction of neat 1-octene [7.05 M, -78 °C, with atomic
hydrogen, (∆P/∆t)total ) 3.36 × 10-4 M s-1] was determined,
the concentration of atomic hydrogen could be estimated by
using the reported rate constant for the addition of atomic
hydrogen to an olefin.4,5
Since the products of combination (C) and disproportionation
(D) are diffusion controlled, the concentration of radicals formed
from addition can be calculated from those products if one
determines the time dependence of the products formed from
their reactions:
(∆P/∆t)C,D ) k [R•]2
(4)
•
R
•
The diffusion rate constant, kR , can be estimated using the
Stokes-Einstein equation by assuming slip conditions and
applying a spin statistic correction:6
(∆P/∆t)total ) ka[1-octene][H•]
(2)
(8RT)103
[H•] ) (∆P/∆t)/(ka[1-octene]) ) 4.8 × 10-14
M
kR
)
3
(5)
•
3η
The concentration of R• can then be calculated from eq 4.
At these concentrations, a bimolecular reaction, kc, between an
alkyl radical (∼10-7 M)6 and an atom of hydrogen reacting with
a diffusion-controlled rate constant, ∼109 M-1 s-1, would have
achieved a rate of conversion of only 4.8 × 10-12 M s-1. The
preceding kinetic analysis will be correct only if the reaction
describes a homogeneous process between freely diffusing
species; however, with very fast reactions between a vapor and
a solution, reaction can take place at the surface of the solution
and will deplete the concentration of the initial substrate (i.e.,
olefin) in that region and increase the concentration of the
radicals formed.
The products from the reaction of hydrogen atoms with the
alkyl radicals, the capping reaction (eq 1), can be determined
from the products resulting from 1,2-addition:
(∆P/∆t)RH ) K [R•][H•]
(6)
•
H
2
Since this radical-radical coupling will also be diffusion
controlled, an estimation of the hydrogen atom concentration
can be made. As the hydrogen atom concentration has now
been determined, the addition of hydrogen to a primary olefin
can likewise be determined, eq 3, if the time dependent
concentrations of the reactants and the products formed in the
reaction zone, (∆n/V/∆t)RZ are known. Since the volume of
the reaction zone is only a fraction, f, of the bulk volume, the
kinetic expressions listed, eqs 2, 4, and 6, are valid only at
The establishment of a rate constant for the addition of a
hydrogen atom to an olefin is an important undertaking since
most of the solution phase rate constants for the reactions of
X Abstract published in AdVance ACS Abstracts, June 1, 1996.
S0022-3654(96)00189-X CCC: $12.00 © 1996 American Chemical Society

11320 J. Phys. Chem., Vol. 100, No. 27, 1996
Tanner et al.
f
f
c
c
c
c
d
c
d
c
e
e
d
c
d
c

Addition of Atomic Hydrogen to Olefins
J. Phys. Chem., Vol. 100, No. 27, 1996 11321
TABLE 3: Absolute Rate Constants (f1/2ka) for the Addition of Hydrogen Atoms to 1-Octene at -78 °Ca
104(∆P/∆t) (mol L-1 s-1
)
(∆P/∆t)C,D
107[R•]c
(∆P/∆t)RH
107[H•]d
f1/2kae (×103 M-1 s-1
)
bulk
total
b
reaction
2
1-6
7
8
9
3.36
1.58
2.53
0.61
0.21
0.16
0.09
0.57
0.81
1.16
0.74
0.18
0.71
0.072
0.44
2.61
1.26
1.99
0.52
0.18
0.11
0.07
0.49
0.70
1.04
0.69
0.14
0.65
0.55
0.39
5.90 ((0.7)
4.54
0.75
0.32
0.53
0.08
0.03
0.05
0.02
0.08
0.11
0.12
0.05
0.04
0.06
0.02
2.33
1.80 ((0.5)
1.13
0.24 ((0.05)
0.41
0.99
2.2
5.71
1.52
2.93
0.48
10
11
12
13
14
15
16
17
18
19
20
1.71
0.30
6.3
1.26
0.54
28.1
70.2
1.34
1.54
2.5
1.10
0.25
2.59
0.39
3.08
0.48
3.76
0.42
3.06
0.23
2.9
4.3
34.9
5.7
65.2
1.32
0.56
2.98
0.27
0.794
2.33
0.43
0.23
a Calculated by using data in Table 2. b Experimentally measured as the change in the concentration of 1-octene with time. c Calculated by using
eq 4. d Calculated by using eq 6. e Calculated by using eq 7.
TABLE 4: Absolute Rate Constants (f1/2ka) for the Addition of Hydrogen Atoms to 1-Octene at -91 °Ca
104(∆P/∆t) (mol L-1 s-1
)
(∆P/∆t)C,D
107[R•]c
(∆P/∆t)RH
107[H•]d
f1/2kae (×103 M-1 s-1
)
bulk
total
b
reaction
2
25-28
29
2.59
2.59
0.38
0.51
0.06
1.95
1.57
0.31
0.42
0.05
5.40 ((0.8)
5.10
0.63
1.02
0.07
0.09
0.01
3.1 ((0.9)
3.71
0.10 ((0.01)
0.12
30
2.51
0.54
1.23
31
2.43
0.72
5.90
32
0.99
0.24
45.7
a Calculated by using data from Table 3. b Experimentally measured as the change in the concentration of 1-octene with time. c Calculated by
using eq 4. d Calculated by using eq 6. e Calculated by using eq 7.
extremely low concentrations where
∆n/V RZ
∆t
∆n/V bulk
∆t
=
(
)
(
)
An expression (eq 7) that defines the rate constant for addition,
ka, can be derived by combining eqs 2-6:
1/2
(∆P/∆t)total(kdiff(∆P/∆t)C,D
)
ka ) f-1/2
(7)
[
]
[olefin](∆P/∆t)RH
2
Results and Discussion
Hydrogen Atom Addition to 1-Octene. To determine an
accurate value for the absolute rate constant for the solution
phase reactions of a hydrogen atom with an olefin, the data
reported previously for the hydrogen atom addition to 1-octene3
(-78 °C) were expanded to include varying concentrations of
olefin at several temperatures (see Tables 1 and 2).
Absolute Rate of Addition of H• to 1-Octene. The
experimental data (Tables 1 and 2) were used to calculate the
concentrations of the reactants ([olefin], [R•], [H•]) and the rate
constants for each reaction at each concentration of olefin. The
apparent rate constants (f1/2ka) of the substrates in their reaction
zone are listed in Tables 3 and 4. By using the data in Tables
3 and 4, a plot of the calculated values of f1/2ka Vs their olefin
concentration was constructed (Figure 1).
The data were fit to an equation of the form log f1/2ka )
a[olefin]b + c. The function is monotonic and steeply dependent
upon concentration. An approximation of the rate constant
[upper limits ) (1.4 ( 0.4) × 108 M-1 s-1, -78 °C], f1/2ka,
where f equals unity, was obtained from the intercept of a plot
of log f1/2ka (the apparent rate constant) Vs [1-octene] (see Figure
1). The value of f can be calculated for the reactions of neat
octene using f1/2ka at -78 °C [f ) (3.0 ( 1.5) × 10-12]. The
reaction volume (2.8 × 10-11 mL) for neat 1-octene (-78 °C)
can be estimated as a fraction of the total volume of the solution
(3.0 × 10-12 × 9.0 mL). Since the surface area of the cell
(54.1 cm2) contains approximately 2.8 × 1015 molecules of
1-octene7 and the reaction zone contains only 1.1 × 1011
molecules (f multiplied by the total number of molecules in the
Figure 1. Variation in observed rate constant log(f1/2ka) Vs [1-octene]0
at -78 °C, O (a, -5.02; b, 0.07; c, 8.14), and -91 °C, 4 (a, -4.79;
b, 0.09; c, 7.69). The extrapolated rate constants, log ka, for -78 (b)
and -91 °C (2) are also shown.
cell), the reaction is limited to only a fraction of the molecules
on the surface of the cell. By using the same method, the value
of ka [(4.9 ( 1.1) × 107 M-1 s-1] was obtained at another
temperature, -91 °C (Figure 1). A plot of log ka Vs 1/T gave
Arrhenius parameters (Ea ) 5.3 ( 2.9 kcal/mol, log A ) 14 (
3.5 M s-1) that allowed the calculation of ka ) (4.2 ( 3.6) ×
109 M-1 s-1 at 25 °C. The calculated values of the rate
constants and activation parameters are in excellent agreement
with the previously reported rate constants, which ranged over
ka ) (0.88-8.19) × 108 M-1 s-1 (25 °C)4,8,9 for the addition of
atomic hydrogen to ethylene. The previously reported activation
energies ranged from 0.5 to 3.3 kcal/mol,4 while a value of Ea
) 3.18 kcal/mol was calculated by Baldwin et al.9 on the basis
of a combination of the results of Yang10 and Jennings and
Cvetanovic.8b The observed decrease in both concentrations
of the 1,2-addition product (Tables 1 and 2) and the [H•]calc
(Tables 3 and 4) as the concentration of 1-octene is decreased
is compatible with the proposed reaction model since as the
[olefin] decreases, the reaction volume increases and (∆n/V)RZ
approaches (∆n/V)bulk and limits the formation of the 1,2-
addition product. Rapid stirring of the reaction mixture, entries

11322 J. Phys. Chem., Vol. 100, No. 27, 1996
Tanner et al.
TABLE 5: Product Distribution of the Addition of Hydrogen Atoms to 1-Methylcyclohexene at -78 °C
b
b
b
c
b
reaction reaction % [1-methylcyclohexene]a time (min)
othersd
1-2
9.8-15.9
5.4
9.24
10-20
4
15.4
15.2
19.1
18.3
20.6
20.5
20.8
9.0
8.6
10.8
10.4
11.7
11.6
11.8
19.2
7.8
3.2
3.8
3.8
4.3
4.0
39.1
44.8
47.1
46.7
46.7
47.0
47.2
14.6
1.4
1.6
1.7
1.7
1.7
1.7
1.7
1.2
1.0
0.9
1.2
1.1
3
4
5
6
7
8
1.14
21.0
17.2
17.9
14.4
144.8
14.5
8.8
0.23
0.12
0.044
0.011
0.006
2
6.2
1
5.3
0.75
0.75
1.5
8.6
18.7
a Reactions 1 and 2 were carried out with neat 1-methylcyclohexene; reactions 3-8 were acetone solutions. The concentrations were corrected
for -78 °C. b Products from disproportionation reactions. Calculated by using the product distributions obtained from the experiments carried out
with deuterium (see Table 6). kd/kc (-78 °C) was taken as 27.8 and FMDO/FMDA was taken as 1. c Product from the 1,2-addition of hydrogen to
1-methylcyclohexene. d Four unidentified dimers formed in a ratio of 1/0.6/0.6/1 (<1.2%).
2.05, 2-deuterium of methylenecyclohexane (I5); and 5.33,
2-deuterium of 1-methylcyclohexene, (I6).
The ratio of dideuterated (C) and monodeuterated methyl-
cyclohexane (D) was obtained from an examination of their GC/
MS spectra. From the concentrations of the products (GC
2
analysis) and the H NMR intensity ratios, the concentrations
of the deuterated products listed in Table 6 are calculated by
using the following relationship: [E] ) γI5, [A] ) γI4, [B] )
γI6, and [C] + [D] ) γI2. The proportionality constant γ )
2
[E]/I5 was used to calibrate the H NMR concentrations. The
consistency of the analysis can be checked since the concentra-
tions of monodeuterated alkenes and the monodeuterated alkane
will be formed from disproportionation in a 1/1 ratio ∑([A] +
[B] + [E]/[D]) ) 1. The disproportionation products are
formed, within experimental error, in the predicted ratio (see
Table 6).
The first step in the addition of a H atom to 1-methylcyclo-
hexene no doubt is addition to the secondary position of the
double bond to give a tertiary radical:
Figure 2. Plot of log(f1/2ka) Vs [1-methylcyclohexene]0 (a, -2.76; b,
0.19; c, 6.64). The extrapolated rate constant, log ka, at -78 °C (O) is
shown.
13-20 of Table 1, represents experiments in which the
“apparent volume” is increased. Presumably if stirring is rapid
enough and competitive with diffusion, f1/2ka will equal ka.
Unfortunately, although f1/2ka is increased with stirring, only a
11-fold increase in f1/2ka could be achieved by stirring (0-4000
rpm).
Regioselective Addition of Atomic Hydrogen to 1-Meth-
ylcyclohexene. When atomic hydrogen (0.70-4.44 mmol) is
passed over neat 1-methylcyclohexene or acetone solutions of
1-methylcyclohexene at two temperatures (-78 and -91 °C),
the products of the reaction are methylcyclohexane, methyl-
enecyclohexane, two isomers of 1,1′-dimethylbicyclohexyl, and
four other minor dimeric products (see Table 5).
When the tertiary radical encounters another deuterium atom,
1,2-dideuterio-1-methylcyclohexane is formed:
In the original report3 on the hydrogenation of 1-octene, the
capping reaction could not be definitely attributed to the reaction
of a second hydrogen atom, as depicted in eq 9. However, it is
now clear that the mode of formation of the 1,2-dideuterated
alkanes follows this sequence of reactions.
Some insight into the mechanism of the formation of these
products is obtained by carrying out the reduction with
deuterium atoms (-78 and -91 °C). The distribution of the
products from these reactions is listed in Table 6. By comparing
1
2
the mass and H and H NMR spectra of the protiated and
deuterated products, it was obvious that in each of the products,
whether monomer or dimer, at least one deuterium is attached
to a secondary position attributable to the original olefin (see
Table 6). The mass spectra (GC/MS) of the deuterated products
showed 1-methylcyclohexene (d1, A and B), methylcyclohexane
(d2, C; d1, D), methylenecyclohexane (d1, E), 1,1′-dimethylbi-
cyclohexyl (d2, F), and four other minor dimeric products (see
Table 5). If the assumption is correct that the initial addition
to the secondary position of 1-methylcyclohexene is regiospe-
cific, then the deuterated products are those listed in Table 6.
The 2H NMR (see Figure 2) showed absorption with intensity,
In: δ 0.83, axial 2-deuterium of methylcyclohexane (I1); 1.25,
tertiary deuterium of methylcyclohexane and 1,2-dideuteriocy-
clohexane (I2); 1.55-1.60, equatorial 2-deuterium of methyl-
cyclohexane (I3); 1.82, 6-deuterium of 1-methylcyclohexene (I4);
The major saturated or unsaturated hydrocarbons, however,
contain only one deuterium atom. The incorporation of protium
into methylcyclohexane can be attributed to disproportionation
(eq 10) or allylic abstraction (eq 11).
Since disproportionation gives equal amounts of monodeuterated
methylcyclohexane (FMDA) and monodeuterated olefin (FMDO),

Addition of Atomic Hydrogen to Olefins
J. Phys. Chem., Vol. 100, No. 27, 1996 11323
TABLE 6: Product Distribution from the Addition of Deuterium Atoms to Neat 1-Methylcyclohexene
products (mol %)
D
D
D
D
D
D
D
D
reaction temp time
(C)b
(D)a
(A)a
(B)a
(E)a
(F)
reaction
%
(°C) (min)
otherc
FMDO/FMDA
kd/kc
9
10
11
12
2.0
2.2
5.0
5.3
-78
3
3
5
5
15.3
13.8
14.8
15.1
7.9
9.9
6.2
9.5
20.2
22.0
20.8
18.2
39.4
38.0
40.7
37.9
15.3
14.2
15.6
17.0
1.5
1.4
1.4
1.3
0.5
0.7
0.6
1.0
1.0
1.0
1.1
1.1
26.2
27.1
29.0
29.2
ave 27.9 ( 1.2
13
14
3.1
2.1
-91
5
5
18.2
11.9
8.0
5.2
25.4
17.1
36.4
37.6
10.5
26.2
0.9
1.2
0.5
0.9
1.0
1.2
40.6
33.7
ave 37.2 ( 3.4
a Products from disproportionation reactions, FMDO/FMDA ) 1. b Products from the 1,2-addition of hydrogen. c Four unidentified dimers (each
with two H2) formed in a ratio of 1/0.6/0.6/1.
Figure 4. Variation in log(kd/kc) with 1/T. The dashed line represents
the extrapolation to 25 °C.
Figure 3. 2H NMR spectrum of the product mixture obtained from
the addition of deuterium atoms to 1-methylcyclohexene.
Since the disproportionation to combination ratio for the
products from the addition of protium or deuterium atoms will
be approximately the same,3 the ratio kd/kc determined from the
reactions with deuterium was used to calculate the yields of
methylcyclohexane and 1-methylcyclohexene formed from
disproportionation when the reaction of 1-methylcyclohexene
and atomic hydrogen was carried out (see Table 5, footnote b).
The yields of the unidentified dimeric addition products were
shown to be dependent upon the initial concentration of olefin
(see Table 5).
The concentration dependence of the yield of addition
products (PA) is consistent with its formation from a bimolecular
reaction and must be due to addition of the 1-methylcyclohexyl
radical to 1-methylcyclohexene (eq 13) followed by subsequent
disproportionation or addition of a hydrogen atom to the new
tertiary radical.
the monodeuterated products are formed primarily by dispro-
portionation (see Table 6). If a detectable amount of allylic
abstraction takes place, then the products from the reactions of
the allylic radicals would also be observed (eqs 12-14). Since
the products ratio, FMDA/FMDO, is approximately unity, and since
no diene products are formed [only 1-methylcyclohexane,
methylenecyclohexane (monomers), and five saturated dimeric
products are formed], it is unlikely that any appreciable allylic
abstraction takes place (see Table 5). One of the four unidenti-
fied dimers was a saturated alkane (GC/MS). The unidentified
dimeric products (<1% of the total products) are formed in a
ratio of approximately 0.3/0.2/0.2/0.3 (see Table 5).
Since the products of disproportionation can be identified,
the disproportionation to combination ratio is easily determined.
The value determined for the reaction carried out to low
conversion (2-5% reaction) is 27.9 ( 1.2 (-78 °C). When
the reaction is carried out at two temperatures, -91 and -78
°C, a plot of ln kd/kc Vs 1/T gives a slope, Ea(dis)-Ea(comb), equal
to -1.5 kcal/mol (see Figure 3). The ratio of rate constants,
kd/kc, calculated from the plot at 25 °C is 6.8 and is within
experimental error of the value (kd/kc ) 7.2) reported for the
tert-butyl radical.11
Absolute Rate of Addition of H• to 1-Methylcyclohexene
(-78 °C). The same kinetic method that was used to calculate
the absolute rate of addition of H• to 1-octene was used to treat
the data collected for 1-methylcyclohexene. The products of
the H• reaction with varying concentrations of olefin (Tables 5
and 6) yielded the kinetic data listed in Table 7. A plot of log
f1/2ka Vs [olefin]0 at -78 °C, when extrapolated to zero
concentration, yielded the absolute rate constant ka ) (4.6 (
0.8) × 106 M-1 s-1 (Figure 4). If a model reaction,4 the vapor

11324 J. Phys. Chem., Vol. 100, No. 27, 1996
Tanner et al.
TABLE 7: Absolute Rate Constants (f1/2ka) for the Addi-
tion of Atomic Hydrogen to 1-Methylcyclohexene at -78 °Ca
identified by comparison of their GC retention times, GC/MS,
and GC/IR with those of authentic samples. 2H NMR spectra
were also obtained for products of the reactions with deuterium
(see Figure 2).
104(∆P/∆t)bulk
f1/2ka
e
total
)
b
reaction
(mol L-1 s-1
107[R•]c
107[H•]d
(×10-3 M-1 s-1
)
1
2
Instrumentation. The H, H, and 13C NMR spectra were
obtained using either a Bruker AM-400 (400 MHz), Bruker AM-
300 (300 MHz), or Bruker WH-200 (200 MHz) NMR spec-
1-2, 9-12
19.1 ((2) 4.7 ((0.4)
0.29 ((0.4)
3
4
5
6
7
8
2.47
1.62
1.14
0.52
0.21
0.11
5.55
4.61
3.85
2.60
1.65
1.22
0.47
0.15
0.15
0.10
0.07
0.05
4.77
48.3
68.1
1
115
trometer. Unless otherwise noted, the H NMR spectra are
259
referenced to TMS as an internal standard at 0.00 ppm or to
CHCl3 as an internal standard at 7.26 ppm. The 13C NMR
chemical shifts are reported in δ relative to chloroform (δ CDCl3
) 77.0).
412
a Calculated by using data from Table 5. b Experimentally measured
as the change in the concentration of 1-methylcyclohexene with time.
c Calculated by using eq 4. d Calculated by using eq 6. e Calculated by
using eq 7.
The 13C absorption spectra were examined by APT (an
attached proton test) to assign the absorptions obtained from
phase addition of H• to cyclohexene4 Vs 1-octene, is chosen as
a comparison reaction for the solution phase reaction of
1-methylcyclohexene Vs 1-octene (I, 25 °C),12 the value of the
relative rate (0.46) illustrates that the cyclic olefin reacts more
slowly than the terminal olefin. At -78 °C the relative rate of
the more highly substituted olefin, 1-methylcyclohexene (II),
1
the H NMR spectra.
GC/IR data was obtained by using an HP 5965A IRD GC/
FTIR interfaced to an HP 5890 gas chromatograph fitted with
a DB-5 (30 m × 0.25 mm) glass capillary column.
GC/MS data were obtained by using a VG-70E EI+ spec-
trometer fitted with a Varian Vista 6000 gas chromatograph
having a glass capillary column (DB-5 30 m × 0.25 mm, J. &
W. Scientific) and interfaced to a 1125 data system.
II
I
is even lower, ka /ka ) 0.03 ( 0.01.
Conclusion. The solution phase reaction rate constants of
an olefin (1-octene or 1-methylcyclohexene) with atomic
hydrogen are the same order of magnitude as the rate constants
for the diffusion of their reactants. The addition of hydrogen
takes place in a limited volume that contains the first encoun-
tered molecules of olefin. When the concentration of olefin
approaches zero, the true rate of addition, ka, can be estimated
since the reaction volume is approximately equal to the volume
of the reaction cell.
GC analyses were carried out by using a Varian Vista 6000
FID gas chromatograph fitted with a glass capillary column
(PONA, 30 m × 0.25 mm, Hewlett-Packard) and interfaced to
a Varian Vista CDS 401 chromatographic data system. The
yield of the reaction products was determined by GC analysis
using a standard calibration solution of known concentrations
of the authentic materials and an added internal standard (1-
chlorooctane or o-dichlorobenzene).3
Experimental Section
The apparatus used for the generation of atomic H• (D•) and
its reaction with 1-octene and 1-methylcyclohexene was identical
to that previously reported for the reaction of 1-octene.3
Materials. Additional materials not reported previously:3 The
purity of 1-methylcyclohexene (FLUKA, +99%) was varified
by GC prior to use. The values used to calculate the viscosities
of acetone or 1-octene were taken from the temperature
dependent literature values.13 The values of the mixture
viscosities were measured by using an Ostwald viscometer.14
The viscosity of 1-methylcyclohexene at -78 and -91 °C was
measured as a relative value using methylcyclohexane as a
standard.13
Acknowledgment. The authors thank the Natural Sciences
and Engineering Research Council of Canada and the University
of Alberta for their generous support of this work. The authors
also thank Professor M. Newcomb for his helpful discussions
concerning several points in the manuscript.
References and Notes
(1) Taken in part from the Ph.D. dissertation of Liying Zhang,
University of Alberta, Edmonton, Alberta, 1994.
(2) Postdoctoral Fellow, University of Alberta, 1994-1995.
(3) Tanner, D. D.; Zhang, L. J. Am. Chem. Soc. 1994, 116, 6683.
Identification of the Reaction Products. The products
(methylcyclohexane and methylenecyclohexane) were identified
by a comparison of their GC retention times, GC/IR spectra,
and GC/MS spectra with those of authentic samples.
1,1′-Dimethyl-1,1′-bicyclohexyl was isolated from the reac-
tion product mixture of atomic hydrogen with 1-methylcyclo-
hexene. The structure of the dimer was identified by analysis
(4) Jones, W. E.; Macknight, S. D.; Teng, L. Chem. ReV. 1973, 73,
407.
(5) Neta, P. Chem. ReV. 1972, 72, 533.
(6) (a) Calculated from the rate of combination and disproportionation
by using the diffusion rate constant. (b) Ingold, K. U. In Free Radicals;
Kochi, J. K., Ed.; Wiley-Interscience: 1973; New York, NY, Vol. 1, Chapter
2.
(7) The number of molecules on the surface of the reaction cell were
estimated from the MM2-generated surface area (193 Å2) of a rod-shaped
molecule of 1-octene using a closest packing arrangement.
1
of its GC/MS, GC/IR, 1H NMR, 13C NMR, and APT: H NMR
(300 MHz, CDCl3) δ 0.85 (s, 6H), 1.20-1.80 (m, 20H); 13C
NMR (300 MHz, CDCl3) δ 16.61 (q, 2), 30.34, 26.61, 30.34 (t,
5), 38.15 (s, 2); IR (gas phase) ν 2937, 2872, 1455, 1382, 1302
cm-1; EI+ (GC/MS, VG-70) m/z 194 (M+), 97, 96, 81, 69, 68,
67, 55, 41.
(8) (a) Cvetanovic, R. J.; Doyle, J. C. J. Chem. Phys. 1969, 50, 4705.
(b) Jenning, K. R.; Cvetanovic, R. J. J. Chem. Phys. 1961, 35, 1233.
(9) Baldwin, R. R.; Simmons, R. F.; Walker, R. W. Trans. Faraday
Soc. 1966, 62, 2486.
(10) Yang, K. J. Am. Chem. Soc. 1962, 84, 719.
(11) Tanner, D. D.; Rahimi, P. M. J. Am. Chem. Soc. 1982, 104, 225.
(12) This work.
General Procedure for the Atomic Hydrogen Reaction.
An aliquot solution was placed in the U-shaped reactor. The
reactor was cooled to the desired temperature, and a stream of
hydrogen or deuterium was passed into the system at a flow
rate of 4 mL/min. The reaction system pressure was controlled
at 3-4 torr by a make-up gas (helium). The microwave
generator was activated and the reaction allowed to proceed
for the desired time. After the reaction, the product mixture
was analyzed by GC. For each reaction, the products were
(13) Daubert, T. E.; Danner, R. P. Physical and Thermodynamic
Properties of Pure Chemicals; Hemisphere Publishing Corp.: New York,
1989; Vol. 3.
(14) Tanner, D. D.; Mahamat, H. O.; Meintzer, C. P.; Tsai, E. C.; Lu,
T. T.; Yang, D. J. Am. Chem. Soc. 1991, 113, 5397.
JP960189U