Process for the autoxidation of cyclohexylbenzene to form cyclohexylbenzene hydroperoxide

Cyclohexylbenzene and dicyclohexylbenzenes are converted to the corresponding hydroperoxides in the presence of t-butyl, cumene, or p-diisopropylbenzene hydroperoxides and a free radical initiator. The use of the combination of hydroperoxide and free radical initiator enables the reaction to be carried out at lower temperatures (80.degree.-105.degree. C.) than can be employed with hydroperoxides or free radical initiators alone and gives high (90%) selectivity and good conversion (20% or higher).

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an improved process for the preparation of 
cycloalkyl aromatic hydroperoxides and is more particularly concerned with 
an improved process for the preparation of cyclohexylbenzene hydroperoxide 
and dicyclohexylbenzene dihydroperoxides. 
2. Description of the Prior Art 
It is known that cyclohexylbenzene can be converted to cyclohexylbenzene 
hydroperoxide by the autoxidation of cyclohexylbenzene in the absence of 
any catalyst; see U.S. Pat. No. 3,846,499. The latter process requires a 
first reaction temperature of 130.degree. to 150.degree. C. for 1 to 3 
hours followed by a second reaction temperature of 105.degree. C. to 
125.degree. C. for 2 to 4 hours. The selectivity is relatively low (of the 
order of 80 percent) because cyclohexylbenzene hydroperoxide can either 
decompose to 1-phenylcyclohexanol or rearrange to benzoylpentane 
derivatives. 
It is also known that the autoxidation of cyclohexylbenzene can be carried 
out in the presence of a number of catalysts including t-butyl 
hydroperoxide; see Furukawa, Nenryo Kyokai-shi 40 (9), 711, 1961. The 
reaction was carried out at 110.degree. C. and the conversion was low 
(10.3%) with no details given of selectivity to the desired hydroperoxide. 
Because of the nature of the autoxidation reaction, with formation of 
hydroperoxides which are potentially dangerous to handle in large scale 
operations, it is desirable to keep the reaction temperature as low as 
possible but at the same time to obtain satisfactory yields of 
cyclohexylbenzene hydroperoxide coupled with high selectivity of the 
latter in the overall reaction product. 
We have now found that these objectives can be achieved by use of a free 
radical initiator in cooperation with certain hydroperoxides in the 
autoxidation of cyclohexylbenzene and dicyclohexylbenzenes as will be 
described in more detail hereinafter. 
SUMMARY OF THE INVENTION 
This invention comprises a process for the autoxidation of mono- and 
dicyclohexylbenzenes in the presence of a hydroperoxide selected from the 
class consisting of t-butyl, cumene and p-diisopropylbenzene 
hydroperoxides to yield the corresponding hydroperoxides wherein the 
improvement comprises carrying out the reaction in the presence of a 
catalytic amount of a free radical initiator and at a temperature in the 
range of about 80.degree. C. to about 105.degree. C. 
The cyclohexylbenzene hydroperoxide which is produced in accordance with 
the invention is useful as an intermediate in the formation of phenol and 
cyclohexanone. The latter rearrangement can be achieved by processes 
well-known in the art; see, for example, S. Furukawa, supra, M. I. 
Farberov et al., Zh. Org. Khim., 10 (1) 50 (1974), and M. I. Farberov et 
al., U.S.S.R., 422, 181 (1974): C.A. 81 169291d (1974). The 
dicyclohexylbenzene dihydroperoxides which are also produced in accordance 
with the invention are useful as intermediates in the formation of 
cyclohexanone and the corresponding dihydroxybenzene. Thus, 
p-dicyclohexylbenzene dihydroperoxide, when submitted to rearrangement 
using conventional procedures such as that set forth above, yields a 
mixture of cyclohexanone and hydroquinone. m-Dicyclohexylbenzene 
dihydroperoxide under the same conditions of rearrangement yields a 
mixture of cyclohexanone and resorcinol. 
The process of the invention therefore constitutes a significant 
improvement in a key step in the overall conversion of benzene to phenol 
and related hydroxybenzenes. The overall process comprises the 
hydrodimerization of benzene by known procedures to form cyclohexylbenzene 
as the major product and dicyclohexylbenzenes as a minor product of the 
same reaction. The cyclohexylbenzene and dicyclohexylbenzenes, after 
separation if desired, are then submitted to the process of the present 
invention and the hydroperoxides are rearranged as set forth above to 
produce phenol and or hydroquinone and or resorcinol. 
DETAILED DESCRIPTION OF THE INVENTION 
The process of the invention is carried out conveniently by admixing the 
cyclohexylbenzene or dicyclohexylbenzene with the appropriate 
hydroperoxide. (i.e. one of the three named above) and maintaining the 
mixture at a temperature in the range of about 80.degree. C. to about 
105.degree. C., in the presence of oxygen or a gaseous mixture rich in 
oxygen such as air. The free radical initiator is added to the mixture so 
obtained and the reaction temperature is maintained in the above range 
until the end point of the reaction is reached. The end point is the point 
at which the initially fast rate of formation of hydroperoxide begins to 
subside. The end point is readily determined by routine analytical 
procedures, such as high pressure liquid chromatography (HPLC), infrared 
or nuclear magnetic resonance spectroscopy, carried out on an aliquot. 
The resulting product is then worked up using conventional isolation 
procedures. Illustratively, in the case where t-butyl or cumene 
hydroperoxide has been used as catalyst, the mixture is subjected to 
distillation, advantageously under reduced pressure, to recover the 
hydroperoxide catalyst and the unreacted cyclohexylbenzene. In the case 
where diisopropylbenzene hydroperoxide has been used as catalyst the 
latter normally crystallizes upon cooling the reaction mixture and can be 
removed by filtration before distilling the filtrate to recover unreacted 
cyclohexylbenzene. The residual cyclohexylbenzene hydroperoxide or 
dicyclohexylbenzene dihydroperoxide can, if desired, be purified by 
conventional procedures such as column chromatography or fractional 
recrystallization. In general, such purification is unnecessary and the 
crude product can be submitted, without purification, to rearrangement to 
produce the desired hydroxybenzene and cyclohexanone in the manner 
discussed above. 
The amount of tertiary-butyl, cumene, or diisopropylbenzene hydroperoxide 
employed in the above reaction is generally not more than about 6 weight 
percent of cyclohexylbenzene or dicyclohexylbenzene, although higher 
proportions can be used if desired. Preferably, the amount of 
tertiary-butyl, cumene, or diisopropylbenzene hydroperoxide employed in 
the reaction is within the range of about 2 percent to about 5 percent by 
weight based on cyclohexylbenzene or dicyclohexylbenzene. The actual 
amount employed varies with the reaction temperature; the higher the 
temperature, the lower the amount of catalyst used within the limits set 
forth above. The amount of tertiary-butyl, cumene, or diisopropylbenzene 
hydroperoxide actually consumed in the reaction is very small and is 
within the range of about 0.4 percent to about 0.1 percent by weight based 
on cyclohexylbenzene or dicyclohexylbenzene employed as starting material. 
Some 90 percent by weight or even higher amounts of catalyst are generally 
recovered. 
The amount of free radical initiator employed in the process of the 
invention is catalytic and advantageously is within the range of about 0.1 
percent to about 5 percent by weight based on cyclohexylbenzene or 
dicyclohexylbenzene employed as starting material. Preferably, the amount 
of free radical initiator employed in the reaction is within the range of 
about 0.5 percent to about 1.0 percent by weight based on 
cyclohexylbenzene or dicyclohexylbenzene. 
The free radical initiator employed in the process of the invention can be 
any of the azo type free radical initiators described in Encyclopedia of 
Polymer Science and Technology, Volume 2, page 278 et seq., 1965 or any of 
the peroxide or hydroperoxide type free radical initiators described in 
the same publication at Volume 9, page 814 et seq., 1968. Illustrative of 
azo type free radical initiators are 2,2'-azobis(aliphatic nitriles) such 
as 2,2'-azobisisopropionitrile, 2,2'-azobisisobutyronitrile, 
2,2'-azobishexanonitrile, and the like, and bisazoalkanes such as 
1,1'-azobisbutane, 1,1'-azobishexane, 1,1'-azobisoctane, and the like. 
Illustrative of peroxide type free radical initiators are alkyl-aromatic 
peroxides such as dicumyl peroxide, and the like; dialkyl peroxides such 
as di-t-butyl peroxide, diisobutyl peroxide, diisopropyl peroxide, 
diisohexyl peroxide, and the like; diperoxy ketals such as 
2,2-bis(t-butylperoxy)butane, n-butyl 4,4-bis(t-butylperoxy) valerate, and 
the like; diacyl peroxides such as dibenzoyl peroxide, diacetyl peroxide, 
dilauroyl peroxide, dipropionyl peroxide, and the like; peroxy esters such 
as t-butyl peroxypivalate, t-butyl peroxyacetate, t-butyl peroxybenzoate, 
and the like; dialkyl peroxydicarbonates such as di-isobutyl 
peroxydicarbonate, dihexyl peroxydicarbonate, and the like; ketone 
peroxides such as methyl ethyl ketone peroxide, cyclohexanone peroxide, 
and the like. 
A preferred group of free radical initiators for use in the process of the 
invention is azobisisobutyronitrile, t-butyl perbenzoate, and dicumyl 
peroxide. 
As set forth above, the use of free radical initiators in accordance with 
the process of the invention enables the autoxidation of cyclohexylbenzene 
and or dicyclohexylbenzenes to be carried out at significantly lower 
temperatures than hitherto employed and to obtain good conversion of the 
starting materials to the corresponding hydroperoxides with a high degree 
of selectivity. As will be seen from the data set forth in the Examples 
below, the use of reaction temperatures higher than about 105.degree. C. 
results in deterioration in the degree of selectivity, particularly when 
t-butyl hydroperoxide is employed as a catalyst. It will also be seen that 
use of the t-butyl, cumene or diisopropylbenzene hydroperoxides without 
the free radical initiator, or the use of the latter without the 
hydroperoxides, gives markedly inferior results than does the use of the 
combination of hydroperoxide and initiator. The use of this combination 
permits much faster reaction times in addition to improved yields and high 
selectivity. Further, the reaction temperatures which are employed in the 
process of the invention, unlike the higher reaction temperatures employed 
hitherto, do not result in decomposition of the hydroperoxides employed in 
the catalyst combination. The ability to recover the hydroperoxide 
catalysts for reuse in subsequent oxidations makes possible a significant 
reduction in raw material costs of the overall process. 
The process of the invention can also be accomplished by forming the cumene 
hydroperoxide or the diisopropylbenzene dihydroperoxide in situ in the 
reaction mixture rather than employing the preferred hydroperoxides. This 
can be achieved by carrying out the process of the invention as described 
hereinabove but adding cumene or diisopropylbenzene to the initial 
reaction mixture in place of the corresponding hydroperoxides. The amounts 
of cumene or diisopropylbenzene employed in this embodiment of the process 
of the invention are within the same range, on a percentage weight basis, 
as that quoted above for the corresponding hydroperoxides. 
The following examples describe the manner and process of making and using 
the invention and set forth the best mode contemplated by the inventors of 
carrying out the invention but are not to be construed as limiting.

EXAMPLE 1 
A series of experiments were carried out using varying amounts of the 
various free radical initiators listed in Table I in the autoxidation of 
cyclohexylbenzene in the presence of catalytic amounts of tertiary-butyl 
hydroperoxide (the amount used in each run is shown in Table I). The 
following standard procedure was used in all runs: 
The cyclohexylbenzene (16.01 g.:0.1 mole) was charged to a three-necked 
round bottomed flask fitted with stirrer, thermometer, gas inlet and 
reflux condenser. The flask and contents were heated to the desired 
temperature (see Table I) and a stream of dry oxygen at a flow rate of 
circa 5 ml/min. was introduced. Thereafter, the required amounts (see 
Table I) of tertiary-butyl hydroperoxide and the free radical initiator 
were introduced with vigorous stirring. The temperature of the reaction 
mixture was maintained at the above temperature with stirring and the 
progress of the reaction was monitored by HPLC analysis of aliquots of the 
mixture. When the rate of formation of the desired cyclohexylbenzene 
1-hydroperoxide slowed significantly, the reaction mixture was cooled to 
room temperature. The tertiary-butyl hydroperoxide and unreacted 
cyclohexylbenzene were removed by distillation under reduced pressure. An 
aliquot of the residue was analyzed by high pressure liquid chromatography 
to determine the proportion of desired cyclohexylbenzene hydroperoxide and 
principal by-products. The residue was then diluted with hexane (5 ml.) 
and allowed to stand at 0.degree. C. whereupon the cyclohexybenzene 
1-hydroperoxide separated as a crystalline solid having a melting point of 
61.degree. C. 
The data given in Table I below shows the identity and amount of each free 
radical initiator, the amount of tertiary-butyl hydroperoxide (TBHP), the 
reaction temperature and time of reaction, the % conversion (determined by 
amount of cyclohexylbenzene recovered) and the distribution of products in 
the crude residue from the reaction (determined by quantitative analysis 
using HPLC). The cyclohexylbenzene 1-hydroperoxide is shown as "P" and the 
two principal impurities as "P-1" and "P-2". The by-product "P-1" is a 
mixture of cyclohexylbenzene 2-hydroperoxide and 1-phenylcyclohexanol and 
the by-product "P-2" is a mixture of 5-benzoylpentyl hydroperoxide and 
5-benzoylpentanol. The first run shown in Table I was a control experiment 
carried out in the absence of free radical initiator to illustrate the 
lower level of conversion achieved under identical conditions. 
The results of Runs 4 and 6, which were carried out at temperatures above 
105.degree. C., as compared with Run 5, carried out at reaction 
temperature within the presently claimed range of this invention, 
dramatically illustrate the drop in degree of selectivity which results 
from the use of the higher temperatures. 
A second series of experiments was carried out, using exactly the procedure 
described above but using double the amount of phenylcyclohexane employed 
(i.e. 32 g. in place of 16.01 g.), to show the effect of using free 
radical initiators alone, i.e. in the absence of t-butyl, cumene or 
diisopropylbenzene hydroperoxides. The results are shown in Table II and 
it is clearly apparent that the use of the free radical initiators alone 
gives much lower conversion and a markedly lower degree of selectivity 
(the yields in Runs 9 and 10 were so low that no analysis of the oxidation 
product, to determine selectivity, was possible). 
TABLE I 
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Product Dis- 
Reaction 
Reaction 
% tribution % w/w 
Run 
Initiator (wt. %) 
TBHP (wt. %) 
temp. (.degree. C.) 
Time (hr.) 
Conversion 
P P-1 
P-2 
__________________________________________________________________________ 
1 0 (0) 5 100 .+-. 3 
8 11.4 90.5 
4.7 
4.8 
2 dibenzoyl peroxide (1.0) 
5 98 .+-. 2 
7.5 15.1 86.1 
9.1 
4.8 
3 dibenzoyl peroxide (2.0) 
5 98 .+-. 2 
5 17.8 91.5 
5.8 
2.7 
4 2,2'-azobisisobutyronitrile (2) 
4.0 115 .+-. 2 
5.5 24.5 87.7 
7.3 
5.1 
5 2,2'-azobisisobutyronitrile (2) 
5.0 100 .+-. 2 
7.0 20.0 91.5 
5.8 
4.7 
6 2,2'-azobisisobutyronitrile (2) 
2.0 115 .+-. 2 
7.0 23.1 87.0 
8.0 
5.0 
7 t-butyl perbenzoate (2) 
4.0 100 .+-. 3 
6.0 17.5 91.7 
6.4 
1.9 
8 cumene peroxide (2) 
4.0 100 .+-. 2 
8.0 15.0 92.0 
4.3 
3.7 
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TABLE II 
__________________________________________________________________________ 
Product Dis- 
Run Reaction 
% tribution % w/w 
No. 
Initiator (wt. %) 
Reaction temp. (.degree. C.) 
time (hr.) 
Conversion 
P P-1 
P-2 
__________________________________________________________________________ 
9 2,2'-azobisisobutyronitrile (2) 
100 .+-. 3.degree. C. 
7.0 4.4 -- -- -- 
10 dicumyl peroxide (2) 
100 .+-. 3.degree. C. 
6.0 1.6 -- -- -- 
11 t-butyl perbenzoate (2) 
100 .+-. 3.degree. C. 
7.5 9.2 85.7 
4.8 
9.6 
12 benzoyl peroxide (2) 
100 .+-. 3.degree. C. 
7.5 10.8 83.0 
6.2 
10.8 
__________________________________________________________________________ 
EXAMPLE 2 
A mixture of 32.1 g. (0.2 mole) of cyclohexylbenzene and 1.6 g. (5 percent 
by weight) of tertiary-butyl hydroperoxide was heated to 80.degree. C. and 
maintained thereat while oxygen was bubbled into the mixture at a rate of 
circa 5 ml. per minute and 0.16 g. (0.5 percent by weight) of 
azobis-isobutyronitrile was added. The progress of the reaction was 
monitored hourly using high pressure liquid chromatography. Two further 
additions of azobis-isobutyronitrile, each of 0.16 g., were made at the 
end of 60 minutes and 180 minutes from the time of the first addition. 
After 7 hours of reaction, with the temperature maintained at 80.degree. 
C. throughout, it was found by high pressure liquid chromatography that 16 
percent of the cyclohexylbenzene had been oxidized with a selectivity to 
cyclohexylbenzene hydroperoxide of 91.8 percent. The amounts of impurities 
P-1 and P-2 (see Example 1) were found to be 4.8 and 3.6 percent, 
respectively. 
EXAMPLE 3 
A mixture of 32.1 g. (0.2 mole) of cyclohexylbenzene, and 1.3 g. (4 percent 
by weight) of tertiary-butyl hydroperoxide was heated to 
100.degree..+-.3.degree. C. and maintained thereat while oxygen was 
bubbled into the mixture at a rate of circa 5 ml. per minute and 0.65 g. 
(2 percent by weight) of tertiary-butyl perbenzoate and 0.65 g. (2 percent 
by weight) of sodium adipate (present to neutralize the acid generated by 
the perbenzoate) were added. The mixture was maintained for 7 hours at the 
above temperature. At the end of this time it was found by high pressure 
liquid chromatography that 17.5 percent of the cyclohexylbenzene had been 
oxidized with a selectivity to cyclohexylbenzene hydroperoxide of 91.7 
percent. 
EXAMPLE 4 
The procedure described in Example 3 was repeated with the sole exception 
that the tertiary-butyl perbenzoate employed as initiator was replaced by 
0.64 g. (2 percent by weight) of dicumyl peroxide and the sodium adipate 
was omitted. After 7.5 hours of reaction at 105.degree..+-.2.degree. C. it 
was found that 19.1 percent of the cyclohexylbenzene had been oxidized 
with a selectivity to cyclohexylbenzene hydroperoxide of 92.3 percent. 
EXAMPLE 5 
For purposes of comparison the following run was carried out in the absence 
of initiator and using a higher reaction temperature than that employed in 
any of the examples set forth above. 
A mixture of 65 g. (0.4 mole) of cyclohexylbenzene and 1.3 g. (2 percent by 
weight) of tertiary-butyl hydroperoxide was kept at 120.degree. C. and 
oxygen was bubbled through the reaction mixture at circa 5 ml./minute. At 
the end of 5 hours heating at the above temperature it was found by high 
pressure liquid chromatography that 24.7 percent conversion of the 
cyclohexylbenzene had taken place but the selectivity to cyclohexylbenzene 
hydroperoxide was only 86.9 percent. The amounts of impurities P-1 and P-2 
(see Example 1) were 7.1 and 6 percent, respectively. 
EXAMPLE 6 
A mixture of 51.2 g. (0.21 mole) of p-dicyclohexylbenzene, 2 g. (3.9 
percent by weight) of tertiary-butyl hydroperoxide, 0.56 g. (1 percent by 
weight) of tertiary-butyl perbenzoate, and 15 ml. of benzene was heated at 
105.degree. to 108.degree. C. in the presence of a stream of oxygen (circa 
5 ml. per minute). The progress of the reaction was monitored by high 
pressure liquid chromatography. The reaction was discontinued after 11 
hours and the benzene was removed by distillation under reduced pressure. 
To the residual product was added 100 ml. of methanol and the mixture was 
allowed to stand at 0.degree. C. The unreacted p-dicyclohexylbenzene (35.9 
g: 70.2 percent recovery) which crystallized was isolated by filtration. 
The methanolic mother liquors were evaporated to remove methanol and the 
residue was dissolved in 100 ml. of ether and extracted with an excess of 
50 percent aqueous sodium hydroxide. The aqueous extract was neutralized 
by bubbling carbon dioxide therethrough and extracted with three portions, 
each of 20 ml., of ether. The ether extracts were combined and dried over 
anhydrous magnesium sulfate and then evaporated to dryness to give 1.75 g. 
of dicyclohexylbenzene dihydroperoxide. 
The ethereal solution remaining from the original reaction mixture after 
extraction with the sodium hydroxide solution was washed with water and 
dried over anhydrous magnesium sulfate. The dried extract was evaporated 
to dryness to obtain 8.81 g. of dicyclohexylbenzene monohydroperoxide. 
The dicyclohexylbenzene hydroperoxide obtained as described above was 
rearranged to yield hydroquinone and cyclohexanone using the following 
procedure. 
To a suspension of 1.7 g. of dihydroperoxide in 15 ml. of anhydrous benzene 
was added with stirring 6 drops of boron trifluoride etherate while the 
temperature was kept below 40.degree. C. The mixture turned light green 
and the undissolved portion of the dihydroperoxide rapidly disappeared. 
The mixture was stirred at room temperature (circa 20.degree. C.) for 1 
hour before being cooled to 0.degree. C. and filtered. The hydroquinone so 
isolated was dried; 0.51 g. (63.75 percent theoretical yield). The mother 
liquors were shown by high pressure liquid chromatography to contain 
cyclohexanone and some additional hydroquinone. 
EXAMPLE 7 
A mixture of 32.2 g. (0.2 mole) of cyclohexylbenzene, 1.6 g. (5 percent by 
weight) of cumene hydroperoxide and 0.65 g. (2 percent by weight) of 
tertiary-butyl peroxybenzoate was heated at 100.degree..+-.2.degree. C. 
and oxygen was bubbled through the mixture at circa 5 ml. per minute. 
After a total of 21.5 hours heating at the above temperature, it was found 
by high pressure liquid chromatography that 25 percent of the 
cyclohexylbenzene had been oxidized with a selectivity of 88 percent to 
cyclohexylbenzene hydroperoxide.