Photolysis of solutions of 3-terf-butylperoxy-3-methyl-1-butyne

Article abstract of DOI:10.1023/A:1012303806631

Photolysis of solutions of 3-tert-butyl-3-methyl-1-butyne in CD₃OD and C₆D₁₂ was studied by means of ¹H NMR spectroscopy, chemical nuclear polarization of the reaction products in the range 213-333 K, and kinetic measurements. It is shown that 3-tert-butyl-3-methyl-1-butyne decomposes primarily from a singlet electronic state. A scheme is proposed of the most probable reactions involving the radicals formed and solvent molecules. It is found that secondary processes play an important role in the initiation of the chemical nuclear polarization and the photolysis mechanism.

Full text of DOI:10.1023/A:1012303806631

Russian Journal of General Chemistry, Vol. 71, No. 4, 2001, pp. 627 632. Translated from Zhurnal Obshchei Khimii, Vol. 71, No. 4, 2001,
pp. 672 677.
Original Russian Text Copyright
2001 by Shutova, Skakovskii, Agabekov, Murashko, Dikusar.
Photolysis of Solutions of 3-tert-Butylperoxy-3-methyl-1-butyne
T. G. Shutova, E. D. Skakovskii, V. E. Agabekov, V. L. Murashko, and E. A. Dikusar
Institute of New Material Chemistry, Academy of Sciences of Belarus, Minsk, Belarus
Institute of Physical Organic Chemistry, Academy of Sciences of Belarus, Minsk, Belarus
Received September 2, 1999
Abstract Photolysis of solutions of 3-tert-butyl-3-methyl-1-butyne in CD3OD and C6D12 was studied by
1
means of H NMR spectroscopy, chemical nuclear polarization of the reaction products in the range 213
333 K, and kinetic measurements. It is shown that 3-tert-butyl-3-methyl-1-butyne decomposes primarily from
a singlet electronic state. A scheme is proposed of the most probable reactions involving the radicals formed
and solvent molecules. It is found that secondary processes play an important role in the initiation of the
chemical nuclear polarization and the photolysis mechanism.
Over the last years there have been extensive re-
search into use of derivatives of acetylenic peroxides
for production of peroxide-containing copolymers,
grafted polymers, and surfactants, for immobilization
of radical polymerization initiators on solid supports,
and creation of new types of adhesives for fixing
grafted copolymers on phase interfaces [1, 2].
almost linear (Fig. 2), i.e. the consumption of per-
oxide I is described by a first-order equation; some
deviations occur only at profound decomposition of
the starting peroxide. The table lists the rate constants
(k1) of the decomposition of peroxide I in CD3OD on
irradiation in various conditions.
To gain insight into the mechanisms of reactions
At the same time, despite the great practical
that occur in the photolysis of I, we studied effects of
promise of acetylenic peroxides, scarce data have been chemical nuclear polarization on protons of the reac-
available on kinetics and mechanisms of their ther-
molysis and photolysis [3].
tion products. Alkyl peroxides are not suitable objects
for observation of chemical nuclear polarization, since
the alkoxyl radicals formed have low hyperfine coupl-
ing constants and inconsiderable electron relaxation
times, which hinders appearance and retention of non-
equilibrium population of Zeeman levels [4]. But here,
too, chemical nuclear polarization can be induced by
varying reaction conditions (initiation mode, tempera-
ture, solvent, etc.) [5].
The aim of the present work was to study the ki-
netics and mechanism of and chemical nuclear polari-
zation effects in photolysis of 3-tert-butyl-3-methyl-1-
butyne (I) in various solvents.
1
The H NMR spectrum of a solution of peroxide I
in methanol-d4 at 293 K shows the following signals,
, ppm: 1.43 s [(CH3)3C], 1.64 s (CH3)2, and 2.91 s
( CH). As the internal reference we used a signal of
CD2HOD (3.50 ppm) which is present as admixture
in methanol-d4. In the course of irradiation, the fol-
lowing signals appeared, , ppm: 1.41 s [CH3)3; t-
BuOH (t-BuOD)], 1.65 s [(CH3)2]; ethynyldimethyl-
carbinol (II)], 2.90 s ( CH), and 2.35 s (CH3;
acetone). At deeper conversion of peroxide I, weak
signals of methane, ethane, CH3CD2OD, and certain
unidentified products also appeared.
1
The H NMR spectra of methanol-d4 solutions of
peroxide I, measured in the course of the photolysis at
233 K, show, along with the peroxide and solvent
signals, an emission signal of acetone protons
(Fig. 3a). Raising the temperature to 273 K (Fig. 3b)
increases the intensity of the acetone signal and gives
rise to emission signals of ethane (1.04 ppm, s) and
methane (0.38 ppm, s), as well as positively enhanced
signals of ethanol methyl protons (CH3CD2OD)
(1.35 ppm, septet). In addition, emission signals of
unidentified compounds are observed: a broadened
singlet at 3.66 ppm, a multiplet at 2.31 ppm, and two
singlets at 1.79 and 1.72 ppm. The spectral pattern for
the photolysis at 313 K (Fig. 3c) is slightly changed.
Instead of the emission signal of acetone, we observe
its absorption signal with equilibrium polarization,
while the emission singlet of methane is replaced by
Figure 1 shows the kinetic curves of consumption
of compound I and accumulation of principal decom-
position products, viz. t-BuOD, alcohol II, and
acetone, in the photolysis of compound I in CD3OD.
The concentrations are given per 1 mol of peroxide I.
The semilog anamorphoses of the kinetic curves of
consumption of peroxide I in deuteromethanol are
1070-3632/01/7104-0627$25.00 2001 MAIK Nauka/Interperiodica

628
SHUTOVA et al.
C/C0
C/C0
(a)
1.0
2
1.5
4
0.8
0.6
0.4
1
1.0
0.5
0
3
2
1
3
0.2
0
4
60
120
180
240
0
30
60
90
120
t, min
t, min
Fig. 2. Semilog anamorphoses of the kinetic curves of
peroxide for the photolysis in CD OD in
(1) pyrex and (2) quartz inserts and in C D (quartz) at
I
C/C0
3
(b)
6
12
1.0
(3) 293 and (4) 333 K.
2
3
character of the temperature dependence of the non-
equilibrium polarization of acetone and its occurrence
even at a low temperature (213 K), when no other
polarized protons are observed, suggests that the
polarization has arisen by an optical nuclear polariza-
tion in the starting compound I.
0.8
0.6
0.4
0.2
For optical nuclear polarization to arise in a photo-
chemically excited compound I, the latter should have
a bonding triplet term. Scaiano and Wubbels [7] in
their study on photochemically initiated dissociation
of tert-butyl peroxide came to a conclusion that all
lower excited singlet and triplet terms are repulsive
in nature. Malkin and Shepelin [8] showed that the
dissociation of the O O bond in aromatic diacyl per-
oxides, induced by direct photoexcitation, occurs both
from the S1( , *) state and from a triplet eximer
formed by the peroxide in the T1( , *) and S0 states.
Thus, unlike tert-butyl peroxide, peroxide I, due to
1
4
0
60
120
180
t, min
Fig. 1. Kinetic curves of (1) consumption of peroxide I
and accumulation of (2) compound II, (3) t-BuOD, and
(4) acetone on the photolysis of compound I in CD OD
3
in (a) Pyrex and (b) quartz inserts. [I] 0.14 M. (Lines)
Simulation and (points) experiment.
Rate constants for photolysis of peroxide I in various
solvents; [I]0 0.14 M
an emission triplet of its monodeuterated analog at
0.37 ppm. Moreover, all the other signals have in-
creased intensity.
4
1
Solvent
T, K k1 10 , s
k2/k3
k4/k*
It should be noted that polarized acetone protons in
the photolysis of peroxide I are observed immediately,
not with a certain delay, as with tert-butyl peroxide
[6]. Consequently, the polarization arises in primary
processes and cannot be related to chemical nuclear
polarization in radical pairs. Moreover, the extremal
CD3OD
CD3OD a
C6D12
293
293
293
333
1.90
0.75
0.84
1.42
1.15
2.00
0.33
0.39
0.16
0.04
C6D12
a
Irradiation in a Pyrex insert.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 4 2001

PHOTOLYSIS OF SOLUTIONS OF 3-tert-BUTYLPEROXY-3-METHYL-1-BUTYNE
629
the presence of a triple bond, acquires a low-lying
bonding triplet state.
(a)
However, like tert-butyl peroxide, peroxide I can
undergo a photochemically induced homolytic O O
bond cleavage to give a singlet radical pair including
tert-butoxyl and ethynyldimethylmethoxyl radicals.
The resulting radicals abstract deuterium from me-
thanol-d4 to give hydroxymethyl-d3 and, probably,
methoxyl radicals [9]. This route is, however, difficult
to trace by chemical nuclear polarization effects. It is
known [9] that, along with deuterium cleavage, tert-
butoxyl radical generates acetone and methyl radical.
CH3CCH3
O
(b)
CH3CH2OD
The chemical nuclear polarization of CH3CD2OD,
ethane, and methane can readily be explained in terms
of an uncorrelated radical pair of methyl and hydroxy-
methyl-d3 radicals. Ethane and methane are polarized
as outcome products from this radical pair. Then,
according to Kaptein’s rules [4] for polarization signs
of protons of these products, positively polarized
would be methyl protons of ethanol, and negatively,
those of methane and ethane (Fig. 3).
u
uu
u
C2H6
CH4
(c)
Note that at low temperatures (273 K) methyl
radical abstracts primarily hydrogen, while at higher
temperatures (313 K) it can also abstract deuterium
from the solvent.
CH3D
Like tert-butoxyl, ethynyldimethylmethoxyl radical
can decompose by two routes to give acetone and
ethynyl radical (route a) or ethynyl methyl ketone and
methyl radical (route b).
4
3
2
1
0
, ppm
1
To decide between the possible decomposition
routes of ethynyldimethylmethoxyl radical, we studied
the chemical nuclear polarization effects on the photo-
lysis of methanol-d4 solutions of bis(2-methyl-3-butyn-
2-yl) peroxide (III) at 253 333 K. In this temperature
range, we observed no chemical nuclear polarization
both for methane and for ethane, unlike the photolysis
of compound I and tert-butyl peroxide, but observed
polarization of double-bond protons.
Fig. 3. H NMR spectra of solutions of peroxide I in
CD OD ([I] 0.14 M), measured in the course of photo-
lysis at various temperatures, K: (a) 233, (b) 273, and
(c) 313. (u) Unidentified products.
3
(CH3)3COOC(CH3)2C CH
S
h
(CH3)3CO OC(CH3)2C CH,
(1)
k1
(CH3)3CO + OC(CH3)2C CH,
The above findings led us to suggest that route a
is preferred. Analysis of the final products of photo-
lysis of peroxide III might clarify this problem.
However, the resulting unsaturated compounds readily
enter various reactions that give a great number of
products and make analysis difficult.
k2
(CH3)3CO
CH3COCH3 + CH3 ,
(2)
(3)
k3
(CH3)3CO + CD3OD
(CH3)3COD
+ CD2OD (CD3O ),
k4
Analysis of the variations of the concentrations of
peroxide I and its photolysis products with time
(Fig. 1) with account for the above-discussed chemical
nuclear polarization effects, allow us to propose the
following mechanism of the photolysis of compound I
in deuteromethanol:
HC CC(CH3)2O + CD3OD
HC CC(CH3)2OD
II
(4)
+
CD2OD (CD3O ),
k5
HC CC(CH3)2O
CH3COCH3 + C CH,
(5)
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630
SHUTOVA et al.
It should be noted that for the photolysis of com-
pound I in CD3OD, the relation [I]0 [I] [II] is
valid (Fig. 1). Apparently, in the experimental condi-
tions, k4[CD3OD] >> k5, and reaction (5) can be ne-
glected in calculations.
C/C0
1.0
(a)
0.8
0.6
The ratios of the rate constants of the decomposi-
tion of tert-butoxyl radical (k2) and abstraction by this
radical of D from CD3OD (k3), calculated as k2/k3 =
[acetone][CD3OD]/[t-BuOD], are listed in the table.
3
2
1
In solving the direct kinetic task, k2 was let be
0.4
0.2
equal to the rate constant of (CH3)3CO elimination in
1
CCl4 (1.2 102 s at 293 K) [9], and k3
k4. The
kinetic curves of consumption of compound I and
accumulation of principal products of the photo-
lysis in CD3OD, obtained by computer simulation
with experimental and published [9] rate constants,
fairly fit experimental data (Fig. 1).
4
0
120
240
360
t, min
The decomposition of the starting peroxide and the
accumulation of products in the photolysis of I in
C6D12 have more complicated, temperature dependent
patterns (Fig. 4.)
C/C0
(b)
1.0
0.8
Analysis of the ln[I]0/[I] time dependence for the
photolysis of compound I in C6D12 (Fig. 2) shows
that with increasing time of irradiation the reaction
slows down, which is apparently associated with
changed conditions of absorption of the UV light with
the system. The decomposition of compound I in
C6D12 is a first-order reaction only at initial stages,
when the conversion of compound I is no higher than
40 50%. The rate constants for the decomposition of
peroxide I in C6D12 are listed in the table. When the
temperature is increased from 293 to 333 K, the de-
composition rate constant increases by a factor of 1.7.
Concurrent themolysis of peroxide I in these condi-
tions is unlikely, since it is known [1, 9] that dialkyl
peroxides begin to decompose at appreciable rates at
temperatures higher than 373 K. Probably, the above
acceleration of the reaction is explained by the fact
that photoexcitation of compound I at 333 K occurs
from a higher vibrational level of the S0 state, thus
resulting in absorption of a stronger longer wave-
length irradiation. On the other hand, not excluded is
also acceleration of induced decomposition of the
peroxide under the action of radicals resulting from
photolysis.
3
0.6
0.4
0.2
1
2
4
0
60
120
180
t, min
Fig. 4. Kinetic curves of (1) consumption of peroxide I
and accumulation of (2) ethynyldimethylcarbinol, (3)
t-BuOD, and (4) acetone on the photolysis of I in C D
6
12
(quartz) at (a) 293 and (b) 313 K.
F
CH3 CD2OD
CH3CD2OD,
CH3CH3,
F
CH3 CH3
1
The H NMR spectra of a cyclohexane solution of
CH3 + CD3OD
CH3D + CD2OD (CD3O ),
CH4 + R ,
compound I shows the following signals, , ppm:
1.24 s [(CH3)3C], 1.45 s (CH3)2, and 2.11 s ( CH).
The internal reference was C6D11H ( 1.40 ppm)
which was present as an admixture in C6D12. In the
course of irradiation of the solution, the following
CH3 + RH
CD3O + RH
CD3OH + R ,
RH = compound I, t-BuOD, compound II, acetone, etc.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 4 2001

PHOTOLYSIS OF SOLUTIONS OF 3-tert-BUTYLPEROXY-3-METHYL-1-BUTYNE
631
signals appear, , ppm: 1.22 s [(CH3)3; t-BuOD],
1.46 s [(CH3)2; compound II], 2.16 s ( VH), and
2.01 s (CH3; acetone). No polarization of protons of
these compounds was observed. However, in the range
of signals characteristic of double-bond protons
(5 6 ppm), absorption and lines of reaction products
appear which are not observed in the stedy state. The
intensity of these signals is slightly higher at 333 K.
Thus, the photolysis of peroxide I in cyclohexane-d12
gives rise to a weak chemical polarization of protons
of products with double bonds.
EXPERIMENTAL
3-(tert-Butylperoxy)-3-methyl-1-butyne (I) and bis-
(2-methyl-3-butyn-2-yl) peroxide (III) were synthe-
sized by the procedures described in [10, 11]. The
purity of the peroxides was no less than 99%.
The photolytic and chemical nuclear polarization
studies were performed in two solvents, cyclohexane-
d12 and methanol-d4, since they both are transparent
in the near UV light but give radicals with different
physicochemical characteristics, cyclohexyl and hydr-
oxymethyl and methoxyl, respectively.
The yield of alcohol II in deuterocyclohexane is
higher than in deuteromethanol, especially at 333 K
(Fig. 4b). This is apparently explained by a much
increased contribution of polymerization in the me-
chanism of the decomposition of compound I, as
The kinetics of photolysis of compound I were
studied in methanol-d4 at 293 K and in cyclohexane-
d12 at 293 and 333 K. The concentration of peroxide
I was 0.14 M. The solutions were irradiated in quartz
and Pyrex inserts 3 mm in internal diameter and
60 mm in length, sealed in such a way that the gas-to-
liquid volume ratio was no more than 1:9. Under
these conditions all gaseous photolysis products
almost completely remained in the solution and thus
could be analyzed, both qualitatively and quantitative-
ly. Irradiation was performed with an RSh-500 mer-
cury lamp with a heat filter.
1
evidenced by the observation in the H NMR spec-
trum of deuterocyclohexane solutions of peroxide I
of signals of unidentified reaction products, probably,
of those with double bonds.
It the photolysis of compound I in C6D12, the frac-
tion of ethynyldimethylsilyl radicals invoved in deute-
rium abstraction from the solvent, leading to alcohol
II formation, decreases from 0.7 from 0.2 as the
temperature is raised from 293 to 333 K. The ratio of
the rate constant of deuterium abtraction from C6D12
(k4) to the overall rate constant (k*) of the conversion
of ethynyldimethylmethoxyl radical by reaction (5)
and oligomerization reactions, calculated as k4/k* =
[II]/([I]0 [I] [II])[C6D12], much decreases as the
photolysis temperature increases (see table). The ratio
k2/k3 (see table) was estimated in the assumption that,
like with CD3OD, the contribution of reaction (5) in
the mechanism of the decomposition of compound I is
insignificant.
1
The H NMR spectra of initial solutions and photo-
lyzates were recorded on a Tesla VS-567A spectro-
meter at 100 MHz.
The yields were determined at a relative error of
about 10%.
The chemical nuclear polarization effects were per-
formed in a modified temperature-controlled NMR
probe. The light of the DRSh-1000 mercury lamp was
focused on an outer end of a quartz light guide and
thus passed to a wall of a rotating quartz ampule [12].
Methanol-d4 solutions of compounds I and III were
illuminated at temperatures from 213 to 313 K and
from 213 to 333 K, respectively, while cyclohexane-
d12 solutions, at 293 and 333 K.
In simulating the decomposition of compound I in
C6D12, the k2 values, like with deuteromethanol, was
set equal the rate constant of (CH3)3CO elimination
Kinetic schemes of photolysis of compounds I in
various solvents were simulated using the KINETICS
(Version 2) program designed for solving the direct
kinetic task by the Gear method [13].
1
(solvent CCl4) (1.2 102 and 2.3 103 s at 293 and
333 K, respectively) [9], and k3
k4. The resulting
kinetic curves of consumption of peroxide I and
accumulation of principal products of the photolysis
in C6D12 fairly fit experimental results at low degress
of decomposition of the starting peroxide.
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