Liquid-phase oxidation of alkyl-substituted cyclohexylbenzenes

Article abstract of DOI:10.1134/S1070428008040143

The reactivity of alkyl-substituted cyclohexylbenzenes in liquid-phase oxidation was estimated by the k₂/√k₆ value which considerably decreased as the number of methyl groups in the substrate molecule increased. The observed difference in the reactivity of the title compounds was attributed to the degree of coplanarity of intermediate radical species.

Full text of DOI:10.1134/S1070428008040143

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2008, Vol. 44, No. 4, pp. 553–556. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © G.N. Koshel’, E.A. Kurganova, E.V. Smirnova, S.G. Koshel’, V.V. Plakhtinskii, M.S. Belysheva, 2008, published in Zhurnal
Organicheskoi Khimii, 2008, Vol. 44, No. 4, pp. 558–561.
Liquid-Phase Oxidation of Alkyl-Substituted Cyclohexylbenzenes
G. N. Koshel’, E. A. Kurganova, E. V. Smirnova, S. G. Koshel’,
V. V. Plakhtinskii, and M. S. Belysheva
Yaroslavl State Technical University, Moskovskii pr. 88, Yaroslavl, 150023 Russia
e-mail: koshelgn@ystu.ru
Received January 18, 2007
Abstract—The reactivity of alkyl-substituted cyclohexylbenzenes in liquid-phase oxidation was estimated by
the k2/√k6 value which considerably decreased as the number of methyl groups in the substrate molecule
increased. The observed difference in the reactivity of the title compounds was attributed to the degree of
coplanarity of intermediate radical species.
DOI: 10.1134/S1070428008040143
Hydroperoxide oxidation of cycloalkylbenzenes
Ia–Ih (Scheme 1) follows an analogous pattern to
the oxidation of cumene (Va) and cymene (Vb)
(Scheme 2) [1]; these reaction underlie procedures for
the joint synthesis of alkylphenols IIIa–IIIe and cyclo-
alkanones IVa–IVd. The reactivity of cycloalkylben-
zenes Ia–Ih in the oxidation process attracts certain
interest from both theoretical and practical viewpoints.
zenes Ic–If (that are potential starting materials for the
joint preparation of alkylphenols and cyclohexanone)
in liquid-phase oxidation was not studied.
The rates of oxidation of hydrocarbons Ib, Ic, Ie,
and If (Fig. 1, plot 1) and accumulation of the corre-
sponding hydroperoxide (Fig. 1, plot 2) were different
despite similar conditions and the same reaction direc-
tion (in all cases, the oxidation involved the tertiary
C–H bond). Increase in the number of methyl groups
in the benzene ring of compounds Ib, Ic, Ie, and If was
accompanied by decrease in the reaction rate. How-
ever, the rate of oxidation of cumene (Va) and cymene
(Vb) that are structural analogs of cyclohexylbenzene
(Ib) and p-cyclohexyltoluene (Ic) was considerably
higher. p-Cyclohexylcumene (Id) was characterized by
the highest rates of oxidation and accumulation of the
corresponding hydroperoxide.
We previously found [2] that the rate of oxidation
and the conversion of cycloalkylbenzenes Ia, Ib, Ig,
and Ih containing 5 to 12 carbon atoms in the cyclo-
alkyl group strongly depend on the size of the latter.
The observed relation was interpreted in terms of spe-
cific structure of substrates Ia, Ib, Ig, and Ih, which is
contributed by the following factors: (1) degree of
σ,π-conjugation between the tertiary C–H bond in the
carbocycle and π-electron system of the benzene ring;
(2) coplanarity of intermediate radical species; and
(3) steric hindrances to attack by oxygen (radical) on
the C–H bond at the tertiary carbon atom. On the other
hand, the reactivity of alkyl-substituted cyclohexylben-
The reactivity of compounds Ib–If in liquid-phase
oxidation was estimated by the ratio of the rate con-
stants for chain propagation and chain termination
k2/√k6 (so-called oxidizability parameter). To exclude
Scheme 1.
R2
R2
R2
( )n
OOH
R3
OH
R3
[H+]
( )n
O2
O
+
( )n
R1
R3
R1
R1
R4
Ia–Ih
R4
R4
IIa–IIh
IIIa–IIIe
IVa–IVd
I, II, R1 = R2 = R3 = R4 = H, n = 1 (a), 2 (b), 4 (g), 8 (h); n = 2, R2 = R3 = R4 = H, R1 = Me (c), Me2CH (d), R1 = R3 = H, R2 = R4 =
Me (e), R1 = R2 = R3 = Me, R4 = H (f); III, R1 = R2 = R3 = R4 = H (a), R2 = R3 = R4 = H, R1 = Me (b), Me2CH (c), R1 = R3 = H,
R2 = R4 = Me (d), R1 = R2 = R3 = Me, R4 = H (e); IV, n = 1 (a), 2 (b), 4 (c), 8 (d).
553

KOSHEL’ et al.
554
Scheme 2.
Me
Me
OH
O
[H+]
OOH
Me
O2
Me
+
Me
Me
R
R
R
Va, Vb
VIa, VIb
VIIa, VIIb
VIII
R = H (a), Me (b).
possible errors, the kinetics of AIBN-initiated oxida-
tion of hydrocarbons Ib–If, Va, and Vb were studied
using the same setup. The kinetic parameters (k2/√k6
ratios, E2 – 1/2E6 values, and logarithms of the preex-
ponential factors logA in the Arrhenius equation) for
the examined transformations of cyclohexylbenzenes
Ib–If are collected in Table 1. It is seen from Fig. 2
that the character of variation of the rate of initiated
oxidation (Wio) and k2/√k6 is analogous to the character
of variation of the corresponding parameters in the
“gross” oxidation (Fig. 1).
The observed differences in the reactivity in the
series of cyclohexylbenzenes Ib–If, as well as between
hydrocarbons Ib–If and compounds Va and Vb, may
be attributed to different structures of the substrates
and radical species derived therefrom. It is known that
the formation of radical from cumene molecule is
accompanied by transformation of the tertiary carbon
atom from sp3- to sp2-hybrid state, and the bond angles
at that atom change to 120°. As a result, the α- and
β-carbon atoms and the benzene ring appear in one
plane, and the radical thus formed is stabilized by
conjugation between the unpaired p electron and π
electrons in the benzene ring [3, 4].
0.20
0.15
0.10
0.05
0.00
0.20
0.15
0.10
0.05
0
2
1
The three C–C bonds at the α-carbon atom in mole-
cule Va can readily be arranged in the benzene ring
plane, so that isopropyl radical is a coplanar system.
Unlike cumene (Va), carbon atoms in the cyclohexyl
fragment of compound Ib are located in different
planes, and no coplanar system is formed in the radi-
cals derived therefrom (only a part of carbon atoms,
namely Cα, Cβ, and Carom, can be arranged in one plane;
flattening of the cyclohexane ring should be accom-
panied by appearance of considerable steric strains).
Presumably, just that factor is responsible for the lower
reactivity of radicals derived from cyclohexylbenzene
(Ib) and hence for the lower reactivity of Ib as com-
pared to cumene (Va). Reactions of phenylcyclohexyl
radicals with hydrocarbon molecules having nonplanar
structure involve steric hindrances, i.e., mutual orienta-
tion of the reactants must be more rigorous than in
reactions with planar radicals. The effect of coplanarity
in liquid-phase oxidation of alkylaromatic hydro-
carbons (such as isopropylxylenes and isopropylnaph-
thalenes) was demonstrated in [5].
0
1
2
3
4
m
Fig. 1. Plots of (1) the rate of oxidation of cyclohexylben-
zenes Ib, Ic, Ie, and If and (2) accumulation of the corre-
sponding hydroperoxides ([HP]) at high substrate conver-
sions versus the number of methyl groups in their molecules
(m); temperature 120°C, reaction time 2 h.
1.2
1.0
0.8
0.6
0.4
0.2
0.2
1.2
1.0
0.8
0.6
0.4
0.2
0
2
1
The presence of alkyl groups in the benzene ring of
compounds Ic–If is likely to reinforce noncoplanarity
of their molecules and radicals, thus reducing their
reactivity as compared to cyclohexylbenzene (Ib) and
cumene (Va). It was found that the calculated (AM1)
ionization potentials (I) of the initial molecular sys-
0
1
2
3
4
m
Fig. 2. Plots of (1) k2/√k6 and (2) rates of AIBN-initiated
oxidation of cyclohexylbenzenes Ib, Ic, Ie, and If versus the
number of methyl groups in their molecules (m); temperature
70°C, AIBN concentration 0.063 M, reaction time 10 min.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 4 2008

LIQUID-PHASE OXIDATION OF ALKYL-SUBSTITUTED CYCLOHEXYLBENZENES
555
Table 1. Ratios of rate constants for chain propagation and chain termination and activation parameters of liquid-phase
oxidation of cycloalkylbenzenes Ib–If and alkylbenzenes Va and Vb
Temperature, °C
Compound
no.
E2 – 1/2E6,
kJ/mol
60
65
70
75
logA
ΔS≠, J mol–1 K–1 ΔG≠, kJ/mol
k2/√k6 ×103, l1/2 mol–1/2 s–1/2
Ib
Ic
0.58
0.36
4.51
0.35
0.34
3.79
2.39
0.75
0.53
5.28
0.43
0.44
4.50
3.20
1.03
0.71
6.10
0.62
0.54
5.07
3.85
1.20
0.97
7.01
0.77
0.66
5.71
4.66
47.70
63.05
28.16
52.62
41.13
35.36
42.01
4.25
6.44
2.07
4.78
2.99
3.10
3.82
436.42
486.51
363.34
454.94
425.87
385.98
408.08
0–87.00
–104.22
0–98.31
–105.51
–107.70
0–99.88
–101.10
Id
Ie
If
Va
Vb
tems (Table 2) are related to log(k2/√k6) values through
Eq. (1):
The rate of oxidation of compound Id considerably
exceeds those found for the other examined hydro-
carbons. The k2/√k6 value for Id is approximately
equal to the sum of the k2/√k6 values for Va and Ib.
This means that hydrocarbon Id can be oxidized at the
C–H bonds in both isopropyl and cyclohexyl frag-
ments simultaneously.
log(k2/√k6) = (–4.26±0.54) + (0.565±0.061)I;
r = 0.989, s = 0.0223 (7.9%).
(1)
The greater the ionization potential I, the higher the
reactivity. An analogous relation was found in [6] for
the oxidation of phenyl- and vinylphenylmethanes.
EXPERIMENTAL
We performed AM1 semiempirical calculations of
the localization energies ΔΔH for the stage of forma-
tion of radicals from cyclohexylbenzenes Ib, Ic, Ie,
and If; the ΔΔH values were calculated as the differ-
ence between the energies of formation ΔHf of the
radicals and parent molecular systems (Table 2). The
reactivity parameters log(k2/√k6) of hydrocarbons Ib,
Ic, Ie, and If showed a good correlation with the cal-
culated ΔΔH values [Eq. (2)] and Gibbs energies ΔG≠
(Table 1; Eq. (3)].
Cyclohexylbenzene (Ib), p-cyclohexyltoluene (Ic),
p-cyclohexylcumene (Id), 2-cyclohexyl-1,4-dimethyl-
benzene (Ie), and cyclohexylmesitylene (If) were syn-
thesized by alkylation of toluene, isopropylbenzene,
p-xylene, and mesitylene, respectively, with cyclo-
hexanol in the presence of sulfuric acid [7]. The purity
of hydrocarbons Ib–If was checked by GLC. The ox-
idation of compounds Ib–If was performed on a kinet-
ic setup ensuring variation of the oxygen absorption
rate from 7×10–6 to 4×10–5 mol l–1 s–1 with a conver-
sion of 0.05 to 0.5% [8]. The experimental error did
not exceed 5%. The reactions were carried out at 65 to
80°C to avoid degenerate chain branching (initiation
log(k2/√k6) = (1.64±0.16) + (0.0179±0.0024)ΔΔH≠; (2)
r = 0.982, s = 0.0276 (9.8%);
log(k2/√k6) = (1.11±0.16) + (0.0126±0.0015)ΔG≠; (3)
r = 0.985, s = 0.0258 (9.2%).
Table 2. Calculated (AM1) ionization potentials I, localiza-
tion energies ΔΔH, and energies of formation of hydro-
carbons Ib, Ic, Ie, and If [ΔHf(RH)] and radicals derived
therefrom [ΔHf(R·)]
Thus the linear free energy relationship principle
is obeyed. The examined reaction series is isokinetic:
a linear relation exists between the enthalpy of activa-
tion ΔH≠ and entropy of activation ΔS≠. Our results
provide an additional support to the fact that increase
in the number of methyl groups in the benzene ring in
the series of hydrocarbons Ib, Ic, Ie, and If is accom-
panied by increase if the entropy contribution, which
affects the rates of the chain propagation and chain
termination steps.
Compound
no.
ΔHf(RH), ΔHf(R·),
ΔΔH,
kJ/mol
I, eV
kJ/mol
–37.221
–62.281
–61.628
–90.926
kJ/mol
–38.365
–05.469
–02.789
–31.116
Ib
Ic
Ie
If
9.318
9.055
8.941
8.821
75.586
67.750
64.427
59.810
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 4 2008

KOSHEL’ et al.
556
2. Koshel’, G.N., Glazyrina, I.I., and Farberov, M.I.,
occurred only as a result of decomposition of the initi-
ator); azobis(isobutyronitrile) was used as initiator.
The k2/√k6 values were calculated by Eq. (4) [9]:
Zh. Org. Khim., 1979, vol. 15, p. 755.
3. Nenițescu, C.D., Chimie organicǎ, Bucuresti: Technica,
1962. Translated under the title Organicheskaya khimiya,
Moscow: Inostrannaya Literatura, 1962, vol. 1, p. 56.
W = k2/√k6 [RH] √Wi.
(4)
4. Becker, H., Einführung in die Elektronentheorie orga-
nisch–chemischer Reaktionen, Berlin: Wissenschaften,
1961. Translated under the title Vvedenie v elektronnuyu
teoriyu organicheskikh reaktsii, Moscow: Mir, 1965,
p. 73.
5. Farberov, M.I., Bondarenko, A.V., and Shutov-
skaya, G.N., Dokl. Akad. Nauk SSSR, 1969, vol. 187,
p. 831.
Here, W is the rate of oxidation (mol l–1 s–1), Wi is
the rate of initiation (mol l–1 s–1), and [RH] is the sub-
strate concentration. The rates of initiation were cal-
culated according to formula (5):
Wi = ki[AIBN],
(5)
6. Russell, G.A., J. Am. Chem. Soc., 1956, vol. 78, p. 1041.
where ki is the initiation rate constant (s–1).
7. Koshel’, S.G., Lebedeva, N.V., Rudkovskii, E.K., and
Koshel’, G.N. Izv. Vyssh. Uchebn. Zaved., Ser. Khim.
Khim. Tekhnol., 1996, vol. 39, p. 172.
The oxidation of hydrocarbons Ib–If with higher
conversions was performed using a kinetic setup de-
scribed in [10]. The concentration of hydroperoxides
was determined by iodometric titration.
8. Tsepalov, V.F., Zavod. Lab., 1964, vol. 30, p. 111.
9. Emmanuel’, N.M., Zaikov, G.E., and Maizus, Z.K., Rol’
sredy v radikal’no-tsepnykh reaktsiyakh okisleniya
organicheskikh soedinenii (Effect of the Medium in
Radical Chain Oxidation of Organic Compounds),
Moscow: Nauka, 1973, p. 12.
REFERENCES
1. Kruzhalov, B.D. and Golovanenko, B.I., Sovmestnoe
poluchenie fenola i atsetona (Joint Preparation of Phenol
and Acetone), Moscow: GosKhimIzdat, 1963, p. 351.
10. Styskin, E.L., Bondarenko, A.V., and Farberov, M.I.,
Neftekhimiya, 1968, vol. 8, p. 404.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 4 2008