,
I
ROBERT A. SANFORD, STEPHEN M. KOVACH, and BERNARD S. FRIEDMAN Sinclair Research Laboratories, Inc., Harvey, 111.
Intact Alkylation of Benzene Some Unusual Differences in Catalyst Systems Coverage of the subzero reaction temperature range makes these data a valuable contribution to the alkylation reaction literature
Several catalysts effect octylation of toluene in good yields. In general, intact octylation of benzene is much more difficult. No catalyst tried afforded pure 1,1,3,3-tetramethylbutylbenzene, and only the hydrogen fluoride-boron fluoride and aluminum chloride-carbon tetrachloride octylbenzene products were free of di-tert-butylbenzene and polymer contaminants. With hydrogen fluoride as catalyst and a t temperatures between -20' and -40" C., reactivity decreased in the following order: toluene, o-xylene, benzene, tert-butylbenzene, and tert-octylbenzene.
INTACT
alkylations of aromatic compounds such as biphenyl ( 8 ) ,naphthalene (7, 5), thiophenes (6, 32), and phenols (2, 27, 30, 31) with diisobutene (DIB) have been reported. However, with but few exceptions, all those (7, 75, 77, 22, 25, 29) who studied the reaction of benzene and its homologs with diisobutene reported that depolyalkylation (75) occurs, resulting in production of tert-bu tyl derivatives. I n only three instances was intact alkylation accomplished (70, 27, 28). Later, it was reported (79) that by proper choice of catalyst and conditions, either depolyalkylation or intact alkylation could be made to predominate, but no data were given regarding intact alkylation with diisobutene. T h e work described here is a part of a study of the intact alkylation of benzene and some of its homologs with isobutene polymers (26). Several catalysts produced good yields in the octylation of toluene without olefin cleavage or polymerization. However, the intact octylation of benzene was much more difficult.
Experimental All syntheses, with the exception of
those employing hydrogen fluoride were performed in glass equipment. A fournecked, fluted flask was fitted with a stirrer, dropping funnel, thermometer, and reflux condenser holding a calcium chloride drying tube. When necessary, hydrogen chloride was dispersed into reaction mixtures through a fritted-disk sparger. A three-necked, stainless steel flask fitted with a stainless steel stirrer and copper thermowell was employed for reactions using hydrogen fluoride. I n the low temperature alkylation of benzene with catalysts other than hydrogen fluoride, n-pentane or n-heptane was employed to maintain the aromatic in solution. I n general, the catalyst was first added to 4 moles of aromatic hydrocarbon. Ordinary precautions were taken to minimize exposure of the reagents to atmospheric moisture and oxygen. T o this stirred mixture, the olefin (1 mole) diluted with aromatic (1 mole) was added slowly. \Vhen aluminum chloride-hydrogen chloride was used as a catalyst, a weighed amount of hydrogen chloride was passed into a suspension of aluminum chloride in the aromatic hydrocarbon. I n some
instances the gas was continuously introduced. Other catalyst complexes such as aluminum chloride-nitromethane were prepared separately and then added to the aromatic hydrocarbon. Concentrated solutions of sulfuric acid were readily used at sub-zero temperatures if the aromatic and acid were first mixed a t temperatures above - 10' C. Sufficient sulfonation occurred to lower the freezing point of the concentrated sulfuric acid to about -55' C. T h e same effect was achieved simply by adding a small amount of p-toluenesulfonic acid to the sulfuric acid before cooling. The alkylation products were quenched in ice water, washed with water and sodium bicarbonate solution, and dried over anhydrous potassium carbonate. The products were distilled through a n 18 inch X 25 mm. column (Heligrid packing) with reflux ratio of 50 to 1. A small amount of solid potassium carbonate was added to the pot to prevent acidic impurities from causing catalytic rearrangement of alkyl groups. All yields are based on weight percentage of diisobutene converted to a given product. VOL. 51, NO. 12
DECEMBER 1959
1455
Identification of Products
Analysis for C13H19NO
T h e configuration as well as the number and position of the side chains was determined by infrared absorption. T h e amount of the 1,1,3,3-tetramethylbutyl side-chain was estimated on the basis of absorption in the 7- to 8.5-micron region characteristic for neopentyl groups, Contamination of the octylated fractions with di-tert-butyl derivatives was determined by absorption a t 11-30 microns for m - and a t 12.06 microns for p-ditert-butylbenzenes and a t 11.6 to 11.7 microns for di-tert-butyltoluene. tert-Octylbenzene. Nitration (73,7 4 , reduction, and acetylation (76) of this compound by various means failed to give satisfactory derivatives. Comparison of its physical properties and infrared spectrum with a n authentic sample kindly furnished by Herman Pines (24) identified it as 2,2,4-trimethyl-4-phenylpentane. p-tert-Octyltoluene. T h e infrared spectrum of this compound supports assignment of the structure: 2,2,4-trimethyl-4-ptolylpentane. Its acetamino (76) derivative was a colorless, fluffy solid, melting a t 149' to 150' C. (heptane-benzene) . T h e following groups were present by infrared examination: mono-substituted amide, aromatic ring (1,2,4-trisubstituted), tert-butyl, isolated methyl. T h e octyl group was absent indicating cleavage had occurred during nitration or reduction. T h e analytical data indicated that the solid derivative is 2-ace t a m i n o -4 -tert- b u t y 1t o l u e n e (CisHi9NO).
Calcd. C H
76.0 9.35
Mol. wt.
205
Analysis for CMH~INO Calcd. Found
C H
76.8 9.67
Mol. wt.
219
0
/,
-50
Benzene and Toluene
Aluminum Chloride. I n the presence of aluminum chloride the optimum temperature for intact alkylation of
OCTYLTOLUENE
/
alkylating
Mole ratio: toluene-diisobutene-AlCl3 = 5 - 1 -0.08 Olefln addition, 20 minutes; additional stirring, 60 minutes
1456
OCTYLTOLUENE
20
10
0 2
TELAPERATURE, *C.
for
INDUSTRIAL AND ENGINEERING CHEMISTRY
toluene seemed to be close to -20' C. (Figure 1). At this point the formation of butyl derivatives was low, and the formation of octyl and dodecyl derivatives appeared to be near maximum. Below this temperature, polymers (mainly triisobutene and tetraisobutene) are the main product; while above it, depolyalkylation becomes important. Starting a t about -45' C. a "disproalkylation" reaction becomes manifest. In disproalkylation the diisobutene undergoes disproportionation (depolymerization and polymerization) to form Cd, (212, and C16 fragments, presumably equilibrium controlled, which then alkylate the aromatic to yield butyl, dodecyl, and hexadecyl derivatives. For benzene, no optimum temperature was observed. Only low yields of octyl derivatives were obtained. Polymer and dodecylbenzene were the main products a t -25'and -5' C. Toluene was more readily substituted with a tert-octyl group than benzene. This means that for benzene the probability of side reactions stemming from olefin depolymerization and repolymerization is greatly enhanced. When a mixture of tert-octyltoluene and toluene was stirred with aluminum chloride, tert-butyltoluene was readily formed even a t 0' C. T h e instability of the initially formed tert-octyltoluene may be one reason why many workers failed to obtain intact octylation with diisobutene using this catalyst. T h e instability is not only dependent upon the reaction temperature but also upon
A
>
Figure 1. Optimum temperature toluene was near -20' C.
76.9 10.3 220 (cryosc.)
The properties of the various tert-octylarenes are given in Table I (page 1460).
10
x
10.2 2 11 (cryosc.)
Evidence to be presented a t a later date indicates cleavage during the reduction step, a phenomenon apparently not encountered by Huston and Guile (77) in reducing the nitro derivative of tert-octylbenzene. Oxidation (27) with potassium permanganate yielded p-tert-octylbenzoic acid, meeting point 158.5' to 159.5' C. [reported to be 158' C. (lo)]. 1,2-Dimethyl-4-tert-octylbenzene. The acetamino derivative was a colorless solid, melting point 146.5' to 147.5' C. (heptane). Infrared and analytical data showed this, too, was a tert-butyl derivative (1,2-dimethy1-4-tert-butylbenzene) rather than the tert-octyl product:
2oK ethylbenzene > cumene > tert-butylbenzene > benzene (9). Of the arenes 1istedinTableI11 toluene, o-xylene. and a-methylnaphthalene appear to be the most readily octylated. Steric hindrance (4, 9 ) in the ortho positions and reduced contribution of hyperconjugation may account for the greatly reduced rate of alkylation of tertbutylbenzene and tert-octylbenzene compared to toluene. Variation of solubility in hydrogen fluoride may also contribute to the differences i n ease of alkylation observed, increased bulk in the side chain reducing the solubility. Such a possibility finds support in Klatt's ( 7 8 ) early work on aromatic hydrocarbon solubility in hydrogen fluoride a t 0' C. H e observed solubilities as follows (weight %) : benzene, 2.25% ; toluene, 1.54'G; a n d rn-xylene, 1.177,. Brown ( 3 ) interpreted these solubilities as the result of interactions of the pirather than the sigma-type.
Mole ratio: benzene-diisobutene-HF = 5-1 -5 Olefin addition, 40 minutes; additional stirring, 50 minutes
Thus the failure of tert-octylbenzene to undergo further octylation may be attributed to its limited solubility in this catalyst. However, with other catalysts tert-octylbenzene can be octylated as reaction of benzene and diisobutene in the presence of aluminum chloride afforded 32y0 dioctylbenzene a t -45'
C. Discussion Several catalysts effect octylation of toluene in good yields (Table IV). Aluminum chloride, iron(II1) chloride, sulfuric acid, aluminum chloride-nitrobenzene, and hydrogen fluoride gave the best octylation yields with a minimum of olefin cleavage or polymerization. Di-tert-butyltoluene did not form at low temperatures probably because of steric hindrance to formation of any but the
Table 111.
1,3,5-isomer and reduced rate of attack a t the 5-PO"' Jition. ' However, some of these catalysts effect skeletal changes in the 1,1,3,3tetramethylbutyl side chain or cation, resulting in the formation of isomeric octyl derivatives. M'ith aluminum chloride. aluminum chloride-nitrobenzene, hydrogen fluoride. and alkane sulfonic acids-boron fluoride, 1,1,3.3-tetramethylbutyltoluene uncontaminated with other tert-octyl isomers or with ditert-butyltoluene was obtained. In general, intact octylation of benzene is much more difficult (Table L7). No catalyst afforded pure 1:1,3,3tetramethylbutylbenzene, and only the hydrogen fluoride-boron fluoride and aluminum chloride-carbon tetrachloride octylbenzene products were free of ditert-butylbenzene and polymer contaminants.
Reactivity of the Arene Greatly Affects Product Distribution in Alkylation with Diisobutene Catalyst:
HF; Mole Ratio Ar-DIB-HF = 5-1 -5
Wt.
.Irene
Temp., O C .
Toluene o-Xylene a-Methylnaphthalene Benzene terl-Butylbenzene tert-Octylbenzene
- 40
- 20 - 40
70DIB Converted
tert-Ca.Ir
tert-CkIr
92 96
