Studies on BF3-Complex Catalysts in the Alkylation Reaction of Benzene Homologues

Part 2. Competitive Alkylation of Benzene and Alkyl Benzenes*

Norihiko Yoneda**, Kazuo Aomura** and Hiroshi Ohtsuka**

Summary: Competitive experiments in the alkylation of benzene and its homologues with olefins and alcohols were carried out in the presence of boron trifluoride and its complex catalysts. Considering the results obtained, the substance attacking the benzene ring directly was in the form of a carbonium ion (R+), an ion-pair (R+…Cat.-) or

a polarized intermediate (Rδ+…Cat.δ-), according to the type of alkylating

reagents and the nature of the catalysts. Generally speaking, the alkylating reagents producing a stable carbonium ion such as tertiary butyl cation or the complex catalysts making a homogeneous reaction phase had the tendency to give a carbonium ion as the attacking substance. While, in the presence of the alkylating reagents (which produced an unstable carbonium ion and formed a stable ester with acid or the catalysts, giving a heterogeneous reaction phase), the attacking substance seemed to be in the form of an ion-pair or a polarized intermediate. The form of the attacking substance exerted a great influence on the relative reactivity of benzene homologues. In the case of the attack by a carbonium ion, the inductive effect of the alkyl groups on the benzene ring was predominant. Hence, toluene and alkyl- benzenes were more reactive than benzene in the tertiary butylation or in the propylation, with the catalysts giving a homogeneous reaction phase. In the case of the attack by an ion-pair, or a polarized intermediate having a less positive charge than a carbonium ion, the inductive effect was surpassed by the stereochemical and probability effects. Hence, benzene showed a higher reactivity than toluene and other alkyl benzenes in the ethylation and the propylation, with the catalysts giving a highly heterogeneous reaction phase. The reactivity of benzene decreased with the decrease of the heterogeneity of the reaction phase.

ly, in the presence of either boron trifluoride Introduction etherate or aluminum chloride-nitromethane. It is well known that the substitution of Olah2) reported that the relative rates of alkylbenzenes with the electrophilic reagents tertiary butylation of 1, 2, 3-trimethyl benzene, is greatly influenced by the alkyl groups on o-xylene, m-xylene and toluene versus benzene the benzene ring. In geneneral, alkylbenzenes at 25℃ were 110-170, 44.3-47.8, 2.54-3.82 are alkylated more readily and under milder and 15.2-16.6, respectively. Hence, as far as conditions than benzene due to the inductive the inductive effect of the alkyl group is effect of their alkyl groups. concerned, the following order may be general- Condon1) found that the relative rates of ly indicated as regards the relative reactivity propylation of toluene, ethylbenzene, cumene of benzene homologues: polyalkylbenzenes> and tertiary butylbenzene versus benzene at toluene>ethylbenzene>cumene>butyl and 40℃ were about 2.1, 1.8, 1.7 and 1.4 respective- higher alkylbenzenes>benzene. However, on the other hand, Volkov and * Received October 3 , 1966 Zavgorodnii3) suggested that the stereochemi- Part 1, Bull. Japan Petrol. Inst., 8, 19 (1966) cal and the probability effects of alkyl groups ** Chemical Engineering Department , Hokkaido Univer- sity, Sapporo, Japan on the benzene ring (which do not favor the

Volume 9-March 1967 Catalysts in the Alkylation Reaction of Benzene Homologues 27 alkylation) become appreciable when the kB, kT, kE, kC: Alkylation rate constant attacking reagent does not have a significant of B, T, E and C, respectively positive charge. Under the circumstances, where the true It was also known4) that the miscibility alkylation reaction is accompanied by the of the hydrocarbon phase with the catalyst transalkylation (disproportionation) such as phase exerts some influence on the relative reactivity of benzene homologues in the alkylation reactions. In this paper, the competitive alkylation of benzene, toluene, ethylbenzene, cumene the true relative reactivity of the aromatic and xylenes with the boron trifluoride complex hydrocarbon can not be determined. The catalysts was studied to observe the influence alkylation of aromatic hydrocarbons with of the nature of the catalysts and the aluminum chloride catalyst is such a case as alkylating reagents on the relative reactivity mentioned above. of benzene and its homologues from the In the case of the alkylation of aromatic standpoint of inductive, stereochemical and hydrocarbons having side chains up to C3 probability factors. with boron trifluoride complex catalysts, it was confirmed by the experimental results Experimental Procedure mentioned hereafter that these catalysts had The alkylation experiments were conducted no appreciable activity for the dealkylation chiefly in a four-necked flask and partly in (transalkylation) of alkylbenzenes under a 300 cc Autoclave with a magnetic stirrer. reaction conditions, although they had some Details of the experimental apparatus were activity for the intramolecular isomerization reported in the previous paper.5) of alkylbenzenes. Ethylene, propylene and isobutylene were All the reactions were conducted under used as alkylation reagents. Secondary and more severe conditions than those applied to tertiary alcohols were also used. the competitive alkylation experiments as The catalyst boron trifluoride complexes mentioned below. were prepared from water, orthophosphoric 1) 2 grams of the mixture of benzenene, acid (85-87%) and normal alcohols by saturat- toluene, ethylbenzene and cymenes were mixed ing them with boron trifluoride gas at room with 0.01 mole of the boron trifluoride com- temperature. plex catalyst and well stirred for 60-70 The composition of the alkylate was deter- minutes at 20-30℃. No appreciable change mined by means of gaschromatography. in the composition of the aromatics was observed before and after this treatment. In Experimental Results the presence of BF3-H2O and BF3-H3PO4 com- The relative reactivity of the aromatic plex catalysts, the increase of m-cymene hydrocarbon in the alkylation reaction was content and the decrease of o-cymene content calculated by the formula: in the cymene fraction were observed pre- sumably due to the intramolecular isomeriza- tion activity of the catalysts. This tendency became more and more appreciable with the A0 and A, B0 and B are the initial and final increase of the reaction time. quantities of the substances used for the 2) 0.1 mole of the benzene-diisopropyl benzene reaction ; ka and kb are the respective alky- mixture was mixed with 0.1 mole of the lation rate constants. boron trifluoride complex catalyst and well The following symbols were used almost stirred for 240 minutes at 30℃. No appre- throughout this paper: ciable change was observed in the composi- B: Benzene T: Toluene tion of the aromatic mixture and nor was E: Ethylbenzene C: Cumene any cumene formation noted. Ar: Aromatics (T,E & C) other than benzene 3) The boron trifluoride catalysts were

Volume 9-March 1967 28 Yoneda, Aomura and Ohtsuka: Studies on BF3-Complex observed to have some dealkylation activity Table 2. Influence of the amounts of the catalyst on for the mixture of benzene and alkylbenzenes the relative reactivity of benzene and toluene having tertiary butyl side chains under com- paratively severe reaction conditions (reaction temperature: 30-80℃, reaction time: 90-180 minutes, aromatics/catalyst mole ratio: 2:1). However, it was also observed that under the same reaction conditions as applied to Benzene: 0.5 mole the competitive alkylation experiments re- Toluene: 0.5 mole Cat.: BF3-H2O ported in this paper (reaction temperature: Propylene: 0.07-0.08mole 30℃, reaction time: 20 minutes, aromatics React. temp.: 30℃ React. time: 10 mina /catalyst mole ratio: 2:0.1), the following reactions 1)-4) did not occur and reaction Table 3. Relation between the initial composition of 5) occurred very slightly. the aromatic mixtures and their relative reactivity

The effects of the experimental conditions on the relative reactivity of aromatic hydro- carbons (kAr/kB etc.) were observed at 30℃ by using propylene as the alkylating reagent. Cat.: BF3-H2O 0.1 mole As shown in Table 1-Table 3, the initial React. temp.: 30℃ composition of the aromatic mixtures Table React. time: 20 mins. 3 and the amount of propylene reacted Table Propylene: 0.14 mole 1 had no appreciable effect on the relative the apparent kT/kB value decreased slightly reactivity. However, as observed in Table 2, with the increase of the catalyst amount. Table 1. Influence of the propylene reacted on the The results of the propylation of the equi- relative reactivity of benzene and toluene molar mixture of benzene, toluene and ethyl- benzene with the BF3-H2O complex catalyst at 30℃ were summarized in Table 4. The kT/kB and kE/kB values in this experiment were almost the same as those obtained in the experiments shown in Table 1-Table 3. By this evidence, it may be considered Benzene: 0.5 mole that each aromatic hydrocarbon in the aromatic Toluene: 0.5 mole mixture acts quite independently in the alkyl- Cat.: BF3-H2O 0.1 mole ation reaction. React. temp.: 30℃ Propylene feed rate 0.08 mole/10 mins. The competitive ethylation, propylation and

Bulletin of The Japan Petroleum Institute Catalysts in the Alkylation Reaction of Benzene Homologues 29

Table 4. Competitive propylation of the equimolar aromatic mixture

Table 5. Competitive ethylation of the equimolar aromatic mixture (Autoclave)

Starting material Benzene 0.3 mole, Toluene 0.3 mole, Cumene 0.3 mole, Cat. 0.1 mole

Table 6 Competitive tert. butylation of the equimolar aromatic mixture

Starting material Benzene 0.5 mole Toluene 0.5 mole Ethylbenzene 0.5 mole Cumene 0.5 mole Isobutylene 0.14 mole Cat. 0.1 mole tertiary butylation of the aromatic mixture marized in Table 4-Table 6. were conducted with various boron trifluoride In the ethylation and propylation with the complex catalysts. The results were sum- BF3-H2O, BF3-H3PO4, BF3-CH3OH and BF3

Volume 9-March 1967 30 Yoneda, Aomura and Ohtsuka: Studies on BF3-Complex

Table 7. Competitive alkylation of aromatic mixture with various alcohols

-C2H5OH complex catalysts , the order of catalyst shown in Table 4-Table 6. the reactivity of each aromatic hydrocarbons was B>T>E>C. All the reactions were Discussion of the Results conducted in heterogeneous liquid phase and The alkylation experiments reported in this the order of the heterogeneity of the reaction paper were conducted in a manner which systems was BF3-H2O>BF3-H3PO4>BF3- avoided complication reactions such as trans- CH3OH>BF3-C2H5OH. The difference between alkylation and disproportionation. As men- the respective reactivity of each aromatic tioned above, transalkylation did not occur hydrocarbons as mentioned above became appreciably, although intramolecular isomeri- more and more evident with the increase of zation occurred very slightly with the excess the heterogeneity of the reactions systems. amount of the catalyst. In the case of the tertiary butylation, the The boron trifluoride complex catalysts are reactivity order of each aromatic hydrocarbons generally considered to be protonic acid was always T>B and T>E>C at the same catalysts. When the alkylating reagents, time. However, the reactivity of benzene, such as olefins, come into contact with these ethylbenzene and cumene varied with the catalysts, three types of reaction may occur catalyst used. The reactivity of benzene according to the type of olefins and the nature increased with the increase of the hetero- of the catalysts. geneity of the reaction system. Those are: In the presence of the BF3-normal alcohol 1) Carbonium ion formation (R+) having 4 and more carbon atoms complexes, 2) Ion-pair formation (R+… Cat.-) the reactions proceeded in homogeneous phase 3) Polarized intermediate formation (Rδ+…

and the reactivity of toluene was always Cat.δ-) greater than that of benzene (T>B). And R+, R+… Cat.- or Rδ+… Cat.δ- thus By using secondary and tertiary alcohols generating a direct attack on the aromatic as alkylating reagents, the competitive alkyl- nucleus. Thus, the alkylation reaction pro- ation experiments of the aromatic mixture ceeds as follows: were conducted in the presence of boron tri- fluoride. The results were shown in Table 7.

At first, the reaction proceeded in a homo-

geneous phase. However, in the course of When the attacking reagent is an ion-pair the reaction, the reaction phase became (R+… Cat.-) or a polarized intermediate

heterogeneous due to the formation of water. (Rδ+… Cat.δ-), its positive charge is not as Hence, in the latter stage of the reaction, developed as in the case of a carbonium ion the catalysis with BF3-H2O complex became (R+). Hence, in these cases, stereochemical appreciable. The results were very similar and probability factors became predominant to those obtained with the BF3-H2O complex and therefore benzene can exhibit greater

Bulletin of The Japan Petroleum Institute Catalysts in the Alkylation Reaction of Benzene Homologues 31

Table 8. Competitive propylation of aromatic mixture with H2SO4

TMS: tetramethyl sulfonate reactivity than its homologues. methyl sulfonate as solvent, the reaction Propylation and ethylation with BF3-H2O phase was considered to be homogeneous. He BF3-H3PO4, BF3-CH3OH and BF3-C2H5OH com- neglected the influence of sulfonation. plex catalysts shown in Table 4 and Table 5 In our experiments, sulfonation occurred at were those cases. All the reactions proceeded 10℃ and over in the alkylation with 96% in a heterogeneous liquid phase. The degree sulfuric acid, which caused the apparent of heterogeneity of the reaction phase was increase of kT/kB value. in the following order: In the heterogeneous system with the boron BF3-H2O>BF3-H3PO4>BF3-CH3OH> trifluoride complex catalysts, alkylbenzenes BF3-C2H5OH seemed to have poor solubility in the catalyst The relative reactivity of benzene increased phase (not in the case of aluminum chloride with the degree of heterogeneity. In these catalyst). This fact may also cause the cases, the direct attacking reagent seemed decrease of the reactivity of alkylbenzenes. to be in the form of an ion-pair or a polarized The ion-pair (R+… Cat.-) or the polarized intermediate. intermediate (Rδ+… Cat.δ-) has a less positive When the reaction phase was homogeneous, charge and is larger in molecular size than the as in the cases of BF3-C4 and higher alcohols carbonium ion (R+). Hence, these are less sen- catalysts, toluene was more reactive than ben- sitive to inductive effect and more susceptible zene. In the case of tertiary butylation with to stereochemical effect than the latter. This isobutylene, toluene was always the most tendency was fairly observed in the propy- reactive in spite of the heterogeneity of the lation of m-xylene as shown in Table 9. reaction phase as shown in Table 6. In this Table 9. Composition of the aromatics the produced case, the attacking reagent was considered in the propylation of m-xylene to be a tertiary butyl cation. It is well known that tertiary carbonium ions are very stable and therefore very easy to form. In the tertiary butylation, the relative reactivity of benzene as compared to those of alkyl- benzenes decreased with the increase of homogeneity of the reaction phase. References In the case of the propylation with con- 1) Condon F. E., J. Am. Chem. Soc., 70, 2265 (1948). centrated sulfuric acid, the reaction proceeded 2) Olah, G. A. et al., ibid., 86, 1060 (1964). in heterogeneous liquid phase and benzene 3) Volkov, R. N. and Zavgorodnii, S. V., Dokl. Akad. was more reactive than toluene and ethyl- Nauk U. S. S. R., 133, 843 (1960). 4) Tanabe, K., Takeshita, T., "Acid base catalysis" 349 benzene (see Table 8). Olah6) reported that (1966) Sangyo-Tosho. toluene was more reactive than benzene in 5) Yoneda, N., Aomura, K. and Ohtsuka, H., Bull. Japan Petrol. Inst., 8, 19 (1966). the alkylation with 100% sulfuric acid. 6) Olah, G. A. et al., J. Am. Chem. Soc., 86, 1046 However, as he used nitromethane or tetra- (1964).

Volume 9-March 1967