THE RESEARCH LABORATORIES OF THE UNIVERSAL O I L PRODUCTS COMPANY]
[CONTRIBUTION FROM
ISOMERIZATION ACCOMPANYING ALKYLATION. 11. THE ALKYLATION OF BENZENE WITH OLEFINS, NAPHTHENES, ALCOHOLS AND ALKYL HALIDES V. N. IPATIEFF, HERMAN PINES,
AND
LOUIS SCHMERLING
Received December 19, 1939
It is well known that alkylbenzenes may be prepared by the reaction of benzene with olefins (l), naphthenes (2), alcohols (3), alkyl halides (4), ethers (5), and esters (6) in the presence of suitable catalysts such as acids and metal halides. However, because isomerization of the alkyl group in some cases does and in others does not accompany the alkylation, there is much confusion in the literature with regard to both the structure of the alkylation product and the mechanism of the reaction. In this paper, the results of a study that was undertaken for the purpose of gaining an insight into the true nature of the alkylation are given. Both important types of catalyst will be discussed. Since sulfuric acid and aluminum chloride have been most widely used, they have been chosen as representatives of the two types. A consideration of the experimental facts as summarized in Table I leads to some important conclusions concerning the mechanism of the alkylation. It is seen a t once that the results obtained with aluminum chloride are different from those with sulfuric acid. It is seen further that, contrary to the general belief, it is sulfuric acid which is the stronger isomerizing catalyst insofar as the apparent shifting of a double bond is concerned, and that in many cases aluminum chloride does not cause isomerization to accompany the alkylation. There has been considerable discussion in the literature whether the mechanism of the reaction involves an olefin or an ester as the alkylating agent. While no experimental data can be given to prove definitely which is the actual alkylating agent, it will be shown that the seemingly anomalous results can be explained best on the assumption of the intermediate formation of esters. SULFURIC ACID CATALYZED REACTIONS
Alkylation by olefins. The recent study (lh) made in this laboratory on the isomerization accompanying the alkylation of benzene with 31 Presented before the Division of Organic Chemistry of the American Chemical Society at Milwaukee, Wisconsin. Sept. 1938. 263
254
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N. IPATIEFF, HERMAN PINES, AND LOUIS SCHMERLING
methylbutene-1 showed that olefins do not always react with benzene to yield the product that would be obtained if the phenyl radical added to the least hydrogenated carbon atom of the double bond and a hydrogen atom added to the other. I n all cases the reaction presumably proceeds by way of the alkyl hydrogen sulfate. The ester reacts with benzene to yield the alkylbenzene, the reaction being catalyzed by concentrated sulfuric acid. With
1 Olefins Pentene-1 3-Methylbutene-1 Alcohols n-Propyl alcohol Isoamyl alcohol Naphthene Cyclopropane
TABLE I ALKYLATION PRODUCTS
1
8UU"UBICACID
ALUMINUX CELOBIDE
2- and 3-Phenylpentane 2-Methyl-3-phenylbutane t-Amylbenzene n-Propylbenzene No alkylation
Isopropylbenzene t-Amylbenzene
n-Propylbenzene at 0" n-Propylbenzene regardless of Isopropylbenzene at 65" temperature
Alkyl Halide n-Propyl chloride
n- & ieopropyl-
Mixture of benzene
weak acid at low temperatures esters are formed but do not react with benzene (IC). Isomerization of the intermediate ester may occur prior to alkylation, resulting, with 3-methylbutene-1, in the formation of l-amylbenzene rather than of 2-methyl-3-phenylbutene. (CH~)~-CH--CH=CHS
+
(CH3)zC-CHzCHs
a
*
(C&)ZCH-CH-CH~
tl
I
OSOSH [ (CHs)2C=CHC&]
+
bSO3H With pentene-1, the primary reaction-product is more stable, isomerization is slower, and a mixture of 2- and 3-phenylpentanes is formed. Here, the rate of alkylation is almost the same as that of isomerization. Alkylation by alcohols. In the presence of sulfuric acid the primary product of the reaction is again the monoalkyl sulfate. As before, this may or may not undergo isomerization, depending on the conditions. Thus, n-propyl alcohol reacts with benzene in the presence of SOY0 sulfuric acid at 65" to yield isopropylbenzene. Similarly, isoamyl hydrogen sulfate undergoes isomerization even a t 0" in the presence of 96% sulfuric
ALKYLATION O F BENZENE
255
acid (isoamyl alcohol yields t-amylbenzene). On the other handj n-amyl hydrogen sulfate undergoes only partial isomerization (n-amyl alcohol yields a mixture of 2- and 3-phenylpentanes). It may be expected, then, that n-propyl alcohol would yield n-propylbenzene in the presence of sulfuric acid at 0'. However, no alkylation takes place at this low temperature. Alkylation by naphthenes. The fact that the alkylation of benzene with cyclopropane in the presence of sulfuric acid yields isopropylbenzene when the reaction is carried out at 6 5 O , and n-propylbenzene when it is carried out at 0'(2b), may be considered to show that the alkylation takes place via the ester. The alkyl hydrogen sulfate, which is more stable a t the lower temperature, is isomerized at higher temperatures, a result that was predicted on the basis of the mechanism discussed above. Recently Simons and co-workers (li, 2c, 3k, 4f) have shown that the alkylation of aromatic hydrocarbons is catalyzed by hydrogen fluoride. The unusual catalytic behavior of hydrogen fluoride, compared with that of other hydrogen halides, probably lies in the fact that hydrogen fluoride reacts with olefins to form alkyl acid esters similar to those obtained with sulfuric acid. Thus, the mechanism of hydrogen fluoride catalyzed reactions may be formulated as shown in I and 11. I. RCH=CH2 HFnHn-l RCH-CHI
+
I
FnHn-1
CH3
11.
ALUMINUM CHLORIDE CATALYZED REACTIONS
Any mechanism advanced for the interpretation of the catalytic action of aluminum chloride in the alkylation of aromatic hydrocarbons must explain the following facts: 1. The presence of hydrogen chloride is essential for the alkylation of aromatics with olefins or naphthenes when aluminum chloride is used as a catalyst. 2. Alkylation of benzene with alcohols in the presence of aluminum chloride is not accompanied by isomerization. 3. Alkylation of benzene with cyclopropane in the presence of aluminum chloride-hydrogen chloride yields n.-propylbenzene and not isopropylbenzene. 4. Alkylation of benzene with alkyl halides in the presence of aluminum chloride is often accompanied by isomerization, especially a t higher temperatures.
256
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Two mechanisms are proposed for the alkylation of aromatic hydrocarbons: the first applies to the alkylation of aromatics with olefins, naphthenes, or alkyl halides; the second, to the alkylation by alcohol. Alkylation by olefins. The first step in the alkylation of aromatic hydrocarbons with olefins in the presence of aluminum chloride-hydrogen chloride is assumed to be the formation of a complex similar to sodium aluminum chloride. A1C13
+ HC1 + AlC13HCl
r :cl:
i-
or L:Cl:f:Cl:] :c1:
H+
It may be assumed that hydrogen aluminum tetrachloride forms esters, analogous to those obtained with sulfuric acid, and that these react with the aromatic hydrocarbons. RCH==CH2
CeHa
+ HA1C14
+ RCH-CHa I
ClAlCIs
4
Le.,
RCH-C&,
4
+ mcI,
CeH6CH
\
R
However, unlike the alkyl sulfates, the alkyl tetrachloraluminates apparently isomerize slowly, if at all. Hence, for example, isopropylethylene reacts with benzene in the presence of aluminum chloride-hydrogen chloride to yield 2-methyl-3-phenylbutane and not t-amylbenzene. Alkylation by naphthenes. By reacting cyclopropane with benzene in the presence of aluminum chloride-hydrogen chloride catalyst, n-propylbenzene is obtained; this is true whether the reaction is carried out at ' 0 (2a) or a t 71". The fact that no isopropylbenzene is found in the reaction-product excludes the possibility that n-propyl chloride was an intermediate in this reaction, since the latter on reacting with benzene yields both n- and iso- propylbenzene. The mechanism of the alkylation of benzene with naphthenes involves the formation, not of alkyl halides but of alkylaluminum tetrachloride, which, as deduced above, does not undergo isomerization. CH2-CH2
\ /
CHZ CHs-CH2-CHzClAlCla
+ HAlClr + CH3-CH2-CH2-ClAlCla + CsHo + C~HE.CH~-CH~.--CH~+ HAlC4
ALKYLATION OF BENZENE
257
That sulfuric acid and aluminum chloride do not bring about the formation of the same end-product in the alkylation of benzene with naphthenes is shown also in the results obtained with methylcyclobutane. In the presence of aluminum chloride (2a), isoamylbenzene and other isomers were formed, and t-amylbenzene was very probably not one of these. On the other hand, when the reaction was catalyzed by sulfuric acid (2b), only t-amylbenzene was obtained. Its formation was explained on the basis of the isomerization of the intermediate amyl hydrogen sulfate. Alkylation by alkyl halides. Konowalow (4b, 4c), in investigating the alkylation of benzene with alkyl halides in the presence of aluminum chloride, showed that isomerization often accompanied the alkylation, especially at higher temperatures. Our own experiments have indicated that n-propyl chloride, on reacting with benzene in the presence of aluminum chloride a t -6', yields monopropylbenzene consisting of 60% of n-propylbenzene and 40% isopropylbenzene; when the reaction is carried out a t +35', 40% of n-propyl- and 60% of isopropyl-benzene is obtained. In the alkylation of aromatics with alkyl halides, aluminum chloride being a dehydrohalogenatingcatalyst, two competitive reactions take place. At lower temperatures, reaction A predominates; a t higher temperatures, B. A. CH3-CH2-CHzCI AlCIi * CH3CH2CHzClAlCl3 7 CH3CHCICHs
+
AlC1 B. CHsCHsCHsCl --S [CH&H=CH2]
9
+ HC1
\
Alcl3
The recent observations of Bowden (6c) on the alkylation of benzene by various esters (formates, acetates, sulfates, etc.) in the presence of aluminum chloride may be explained in a similar manner. The fact that the n-propyl ester yielded n-propylbenzene while the n- and iso- butyl compounds yielded sec.- and t-butylbenzene, respectively, finds analogy in the fact that n-propyl chloride yields a mixture of n-propyl- and isopropyl-benzeneat even as high a temperature as 35', whereas with isobutyl chloride only t-butylbenzene is formed even a t a temperature as low as -18'. On the other hand, McKenna and Sowa (6b) state that in the presence of boron fluoride, n-propyl formate condenses with benzene to yield isopropylbenzene.
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HERMAN PINES, AND LOUIS SCHMERLING
It is worth while to point out that there are exaggerated reports in the literature concerning the ability of aluminum chloride to cause isomerixation during alkylation. Thus, for example, Calloway (7) states : “Propyl, butyl, and amyl halides react to yield, largely, branched alkyl substances. Generally, regardless of the configuration of the alkyl halide, the final product contains an alkyl group of the highest possible branching.” Calloway gives the following equation: “C6H6 n- or iso-C4HgCl -+ tert.-C4HoCeHa” However, none of his references describes the preparation of t-butylbenzene from n-butyl chloride. Indeed, except with neopentyl chloride (8), there has been no evidence for the branching ( i e . , the migration of a methyl group) of the alkyl radical of the halide during alkylation of benzene in the presence of aluminum chloride. Alkylation by alcohols. A consideration of the experimental evidence indicates that the mechanism of the alkylation of benzene with primary alcohols in the presence of aluminum chloride is similar to that proposed by Tzukervanik (3e) for secondary alcohols, in contrast to that for tertiary alcohols. Thus, n-C3H,OH AlC13 + n-C3H,OAlC12 HC1 nGH.rOA1Clz CeH6 Alcla’ W C ~ H ~ C ~ HOAlClz H~
+
+
+
+
+
Recently, Norris and Ingraham (3i) have studied the alkylation of benzene with methyl and ethyl alcohols. They state that in the study of the mechanism of the condensation which they will report in detail later, it was shown that the alcohols react with aluminum chloride to form compounds having the formula ROAICh, which decompose when heated t o produce RCI and AlOC1. In the presence of aluminum chloride, the usual Friedel-Crafts synthesis then takes place. This mechanism can scarcely be considered satisfactory for n-propyl alcohol, since the formation of propyl chloride by the decomposition of propoxyaluminum chloride would result in the formation of isopropylbenzene, as well as of n-propylbenzene. For similar reasons, the mechanism proposed by Txukervanik for alkylation by tertiary alcohols (3d) (ie., the decomposition of the alkoxyaluminum chloride to yield an olefin and hydroxyaluminum chloride) cannot be applied to the reaction with primary alcohols. EXPERIMENTAL
Materials. All the chemicals with the exception of the pentenes were obtained from commercial sources and, when necessary, were purified by the usual methods. Pentene-1 was prepared according to the directions of Hurd and Goldsby (9). The preparation of the 3-methylbutene-1 is described in a previoue paper (lh).
ALKYLATION OF BENZENE
259
260
V. N. IPATIEFF, HERMAN PINES, AND LOUIS SCHMERLING
Sulfuric acid catalyzed reactions. The alkylations were carried out following procedures described by Ipatieff, Corson, and Pines for the reaction with olefins (IC) and with naphthenes (2b), and by Meyer and Bernhauer for the reaction with alcohols (3b). In general, a mixture of benzene and sulfuric acid was stirred in a flask equipped with a mercury-sealed stirrer, a reflux condenser, and either a dropping-
0
10 20 30 40 SO 60 70 BO 90 LOO %BmlNILPLNTAN* k-RIVATIV6
FIGURE I. MELTINGPOINT CURVE FOR MIXTURESO F THE MONACETAMINO DERIVATIVES O F 2- AND %PHENYLPENTANE 156
IS2 ,j I48 b 144
I
8 140 f 136 b
g
I3Z
lZ4o
IO
-
20 30 40 SO 60 70 80 9o loo
%SPHCNYLPfiNTbNC DLRIVATIVC.
FIQURE 11. MELTINGPOINT CURVEFOR MIXTURESOF DERIVATIVES O F 2-
AND
TEE MONOBENZAMINO %PHENYLPENTANE
funnel for introducing the liquid alkylating agents or an inlet tube reaching t o the bottom of the flask for introducing the cyclopropane. The reaction conditions are given in Table 11. The hydrocarbon product was separated from the acid layer, made alkaline, and steam distilled. The distillate was dried and fractionated through a vacuum-jacketed, total reflux column (10). Aluminum chloride catalyzed reactions. The general procedure used in the slkyla-
26 1
ALKYLATION OF BENZENE
tions with 3-methylbutene-1, cyclopropane, and propyl chloride was similar to that described above for the sulfuric acid catalyst, except that steam distillation of the product was unnecessary. With the first two alkylating agents, anhydrous hydrogen chloride was added during the reaction a t a rate of 300 cc. per hour. The alkylation with n-propyl alcohol was carried out according to the directions of Tzukervanik and Vikhrova (3f). In a three-liter flask fitted with a mercury-sealed stirrer and a condenser, 20 g. (0.33 mole) of n-propyl alcohol, 120 g. (1.5 moles) of benzene, and three drops of water were placed. Aluminum chloride (87 g., 0.68 mole) was added during one hour by way of the condenser. The aluminum chloride dissolved, yielding a pale yellow solution, which gradually turned golden-yellow and then brown as more catalyst was added. When all the aluminum chloride had been added, the dark brown solution was stirred for one hour and allowed to stand overnight. It was then heated under reflux a t 110-120° for ten hours, during the first part of which there was a copious evolution of hydrogen chloride. The product was worked up in the usual manner and distilled. The yield was 10.5 g. (28% of the theoretical) of monopropylbenzene.
-% S-PHlNYLPeNTANE DERIVATIVE.
FIGURE11. IELTING POINT CURVEFOR MIXTURESOF DERIVATIVES OF 2-
AND
TEE
DIACETAMINO
3-PHENYLPENTANE
The monoalkylbenzenes were identified by the preparation of their mono- and diacetamino derivatives (11). The mixtures of n- and iso- propylbenzene were analyzed by the fractional crystallization (lla) of the diacetamino derivative. The relative quantities of the two isomers were estimated by weighing the crystals; the purity of the fractions was checked by observing the crystals under the polarizing microscope. With the mixtures of 2- and 3- phenylpentanes, i t was not possible to separate the derivative into its components. The melting point curves (Figures I, 11, and 111) were, therefore, prepared and used in determining the composition of the mixtures. SUMMARY
The alkylation of benzene with olefins, alcohols, and naphthenes in the presence of sulfuric acid leads to the formation of alkylbenzenes different from those obtained when the reactions are catalyzed by aluminum chloride. In the presence of sulfuric acid, isomerization accompanies the con-
262
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N. IPATIEFF, HERMAN PINES, AND LOUIS SCHMERLING
densation of olefins with benzene; pentene-1 yields a mixture of 2- and 3- phenylpentanes ; 3-methylbutene-1 yields t-amylbenzene. In the presence of aluminum chloride no isomerization occurs ; isopropylethylene yields 2-methyl-3-phenylbutane. Isomerization occurs during the alkylation of benzene with alcohols when the reaction is catalyzed by sulfuric acid, but not when aluminum chloride is used as catalyst; n-propyl alcohol yields isopropylbenzene in the presence of the acid and n-propylbenzene in the presence of the metal halide. Isomerization does not accompany the alkylation of benzene with naphthenes in the presence of aluminum chloride ; cyclopropane yields n-propylbenzene only. I n the presence of sulfuric acid isomerization occurs, provided that sufficiently high temperatures are used; cyclopropane yields isopropylbenzene when the reaction is carried out a t 65'. The condensation of alkyl halides with benzene by aluminum chloride leads to a mixture of isomers. Even when the alkylation is made a t 35O, much n-propylbenzene results from the reaction of n-propyl chloride and benzene. The mechanism of the alkylations is discussed. RIVERSIDE, ILL.
(1)
(2) (3)
(4)
REFERENCES For example: (a) KRAEMER AND SPILKER, Ber., 23, 3169 (1890); 24, 2785 (1891); (b) BROCHET, Compt. rend., 117, 115 (1893); (c) IPATIEFF,CORSON, AND PINES,J . Am. Chem. SOC.,68, 919 (1936); (d) IPATIEFF,PINES,AND KoMAREWSKY, Ind. Eng. Chem., 28,222 (1936); (e) BALSOHN, Bull. SOC. chim., (2) S1, 539 (1879); (f) BERRYAND REID, J . Am. Chem. SOC.,49, 3142 SOWA,AND NIEUWLAND, J . Am. Chem. Soc., 67, (1927); (e) SLANINA, J . Am. Chem. SOC., 1547 (1935); (h) IPATIEFF,PINES,AND SCHMERLINQ, 80, 353 (1938); (i) SIMONS AND ARCHER,J . Am. Chem. SOC.,80, 986, 2952 (1938). (a) GROSSEAND IPATIEFF, J . Org. Chem., 2 , 447 (1937); (b) IPATIEFF,PINES, AND CORSON, J . Am. Chem. SOC.,60,577 (1938); (c) SIMONS, ARCHER, AND ADAMS,J . Am. Chem. SOC.60,2955 (1938). For example: (a) BROCHET AND BOULENQER, Compt. rend., 117, 235 (1893); (b) MEYERAND BERNHAUER, Monatsh, 63-64, 721 (1929); (c) DESSEIQNE, Bull. SOC. chim., (5) 2 , 617 (1935); (d) TZUKERVANIK, J . Gen. Chem. (U. S. S. R.), 6,117 (1935); (e) TZUKERVANIK AND TOKAREVA, J . Gen. Chem. (U. S. S. R.), 6,764 (1935); (f) TZUKERVANIK AND VIKHROVA, J . Gen. Chem. (U. S. 5.R.), 7, 632 (1937); (g) HUSTONAND HSIEH,J . Am. Chem. SOC.,68, 439 J . Am. Chem. SOC. 69, 470 (1937); (i) (1936); (h) SOWAAND MCKENNA, NORRIB AND INQRAHAM, J . Am. Chem. soc., 60, 1421 (1938); (k) SIMONS, ARCHER,AND PASSINO, J . Am. Chem. SOC.,60, 2956 (1938). For example: (a) FRIEDELAND CRAFTS,Ann. chim., (6) 1, 449 (1884); (b) KONOWALOW, J . R w s . Phys.-Chem. SOC.,27, 456 (1895); (c) KONOWALOW
ALKYLATION OF BENZENE
263
AND JEQOROW, J . Russ. Phys.-Chem. SOC.,30,1031 (1898); (d) EBTREICHER, Ber., 33, 439 (1900); (e) SHOESMITH AND MCGECHEN, J . Chem. SOC.,1930, 2231; (f) SIMONSAND ARCHER,J . Am. Chem. SOC.,60,2453 (1938). (5) O’CONNOR AND SOWA,J . Am. Chem. SOC., 60, 125 (1938). (6) (a) FRIEDELAND CRAFTS,Compt. rend., 84, 1450 (1877); (b) MCKENNAAND SOWA, J . Am. Chem. Soc., 69, 1204 (1937); (c) BOWDEN, J . Am. Chem. SOC., 60, 645 (1938). (7) CALLOWAY, Chem. Rev., 17, 327 (1935). (8) PINES, SCHMERLINO, AND IPATIEFF,Reported a t the Milwaukee Meeting of the American Chemical Society, September, 1938. (9) HURDAND GOLDSBY, J . Am. Chem. SOC.,66, 1812 (1934). (10) THOMAS, BLOCH,AND HOEKSTRA, Ind. Eng. Chem., Anal. Ed., 10, 153 (1938). J . Am. Chem. SOC.,69,1056 (1937); (b) IPATIEFF (11) (a) IPATIEFFAND SCHMERLING, AND SCHMERLINQ, J . Am. Chem. SOC.,80, 1476 (1938).
