BROMOETHYLATION OF AROMATIC HYDROCARBONS

Abstract: An aromatic hydrocarbon is bromoethylated to a 1-bromo-1-arylethane with minimal co-formation of diaryl­ alkane by-product by reacting it with hydrogen bromide and acetaldehyde at a temperature in the range of +10°C to -35°C in the presence of at least about one mol of hydro­ gen sulfate per mol of the aromatic hydrocarbon and in the absence of more than bout 15% by weight of water, based on the weight of the hydrogen sulfate. The process is of particular advantage in the bromoethylation of the less reactive aromatic hydrocarbons, such as isobutylbenzene and other monoalkylaromatic hydrocarbons, which have previously yielded an undesirable amount of diarylalkane when bromoethylated with acetaldehyde and hydrogen bromide.

Description: This invention relates to a process for bromoethylating aromatic hydrocarbons to form 1-bromo-1-arylethanes. As disclosed in March, Advanced Organic Chemistry , Second Edition, McGraw-Hill, New York, 1977, pp. 501-502; Olah, Friedel-Crafts and Related Reactions , Volume 2, Interscience Publishers, New York, 1963-1964, pp. 659-784; U. S. Patent 2,516,971 (Galitzenstein et al.); and the references cited therein, it is known that aromatic compounds can be haloalkylated by reacting them with a hydrogen halide and an appropriate aldehyde in the presence of a Lewis acid or a proton acid as a catalyst, most commonly in the presence of zinc chloride. The chloroalkylations utilizing formaldehyde as the aldehyde have been successfully employed in providing fairly high yields of 1-chloro-1-arylalkanes; reasonably high yields of 1-chloro-1-arylalkanes have also been obtained from chloroalkylations utilizing higher aldehydes in some cases, e.g., when the aromatic compound has had an appropriate functional substituent or a plurality of alkyl substituents; and reasonably acceptable, although lower, yields of 1-halo-1-arylalkanes have been obtained in comparable bromoalkylation reactions. However, when the aromatic compound has been a less reactive compound, e.g., an unsubstituted aromatic hydrocarbon or a monoalkyl-aromatic hydrocarbon, it has not been found possible to provide commercially acceptable yields of 1-halo-1-arylalkane, even when the haloalkylation has been a chloroalkylation rather than a bromoalkylation. There has been too much co-formation of diarylalkane by-product, especially in the bromoalkylation reactions. Another disadvantage of the known haloalkylation processes utilizing the higher aldehydes has been their providing too much o-isomer in processes performed to provide 1-halo-1-(4-alkylphenyl)alkanes, such as the compounds which have been synthesized by other techniques to provide intermediates for ibuprofen, related pharmaceuticals, or detergents. It would be desirable to find a way of increasing the para/ortho ratio obtainable from such processes to provide a more economical method of preparing the 1-bromo-1-(4 alkylphenyl)alkanes which can be used in known processes, such as those of U. S. Patent 4,536,595 (Gardano et al.), Canadian Patent 1,197,254 (Francalanci et al.), British Patent 1,560,082 (Dynamit Nobel), Czechoslovakian Certificate of Authorship 219,752 (Palecek et al.), and Japanese Kokai 47-39050 (Miyatake et al.) and 52-111536 (Tokutake). The present invention provides a process for bromoethylating aromatic hydrocarbons with hydrogen bromide and acetaldehyde to form 1-bromo-1-aryl-ethanes. The new process reduces co-formation of diarylalkane by-product even when the aromatic hydrocarbon starting material is a monoalkylaromatic hydrocarbon. In addition, when the aromatic hydrocarbon is a monoalkylbenzene, the new process gives a high para/ortho ratio in the product. The process of the present invention comprises reacting an aromatic hydrocarbon with hydrogen bromide and acetaldehyde at a temperature in the range of +10°C to -35°C in the presence of at least one mol of hydrogen sulfate per mol of the aromatic hydrocarbon and less than 15% by weight of water, based on the weight of the hydrogen sulfate. The aromatic hydrocarbon employed in the practice of the invention may be an unsubstituted aromatic hydrocarbon, such as benzene, naphthalene, anthracene, or phenanthrene. Also, it may be a polyalkylaromatic hydrocarbon, such as xylene, pseudocumene, or mesitylene. However, because of the commercial interest in their bromoethylated products and the difficulty that has previously been encountered in preparing 1-bromo-1-arylethanes from them, the preferred aromatic hydrocarbons are monoalkylaromatic hydrocarbons, such as 1-methylnaphthalene, 2-methylnaphthalene, 9-methylanthracene, 9-butylanthracene, 9-dodecylanthracene, and the various monoalkylbenzenes, e.g., the methyl-, ethyl-, propyl-, isobutyl-, sec-butyl-, t-butyl-, isopentyl-, t-pentyl-, and hexylbenzenes. The most preferred aromatic hydrocarbons are the monoalkylbenzenes wherein the alkyl group contains 1-5 carbons. The hydrogen bromide which is reacted with the aromatic hydrocarbon and acetaldehyde is preferably anhydrous or at least substantially anhydrous. However, some water in the hydrogen bromide can be tolerated as long as it is not an amount sufficient to raise the total amount of water in the reaction mixture above 15% by weight of the hydrogen sulfate, although it is preferred to keep the total amount of water at a concentration not higher than about 10% by weight of the hydrogen sulfate. The hydrogen bromide may be incorporated into the reaction mixture per se or as a salt, such as sodium bromide, which reacts with sulfuric acid to form hydrogen bromide under the reaction conditions. The acetaldehyde may be employed per se or may be introduced in the form of a substance, such as paraldehyde, which decomposes to yield acetaldehyde under the reaction conditions. The aromatic hydrocarbon, hydrogen bromide, and acetaldehyde are normally employed in substantially equimolar amounts, but the proportions do not appear to be critical. Thus, amounts of any of the reactants which are smaller or larger than the equimolar amounts may be used if desired. In order to avoid the presence of an excess of water in the reaction mixture, the hydrogen sulfate is introduced in the form of 85-98% sulfuric acid, preferably sulfuric acid having a concentration of 90-98%, most preferably 93-98%. The amount employed is such as to provide at least one mol, preferably at least 5 mols, per mol of aromatic hydrocarbon. There does not appear to be any maximum to the amount of hydrogen sulfate that may be used other than any maximum that might be imposed by economic constraints. The reaction is conducted at a temperature in the range of +10°C to -35°C, preferably 0°C to -35°C, in order to achieve the advantages of the invention. The process of the invention is exothermic, so the reactants should be combined at a rate that permits control of the reaction temperature. In conducting the process it is preferred to add a mixture of the aromatic hydrocarbon and acetaldehyde to a sulfuric acid solution saturated with hydrogen bromide and to add additional hydrogen bromide during the reaction. However, alternatively, the acetaldehyde and hydrogen bromide can be prereacted, or the aromatic hydrocarbon can be the first charge to the reaction vessel. The invention is useful as an alternative method of preparing 1-bromo-1-arylethanes from aromatic hydrocarbons that are known to be capable of providing reasonably acceptable yields of such products by known bromoethylation techniques. However, it is particularly advantageous as a method of preparing 1-bromo-1-arylethanes from the less reactive aromatic hydrocarbons, such as monoalkylbenzenes and other monoalkylaromatic hydrocarbons, that have not previously been found to be capable of providing acceptable yields of such products by bromoalkylation processes utilizing acetaldehyde. The process is of especial interest in the bromoethylation of monoalkylbenzenes, where it has the advantage of not only minimizing the co-formation of diarylalkane by-product but of also increasing the para/ortho ratio in the product. As is known, the products obtained by the process are useful as internal standards or intermediates for the preparation of monomers, detergents, or pharmaceuticals. When they are used as chemical intermediates, they may be subjected to the same reactions as have previously been used to convert them to desired products. For example, the 1-bromo-1-arylethanes can be dehydrobrominated in any known manner to provide styrenes which can then be polymerized by known techniques. A particularly interesting application of the 1-bromo-1-(4-alkylphenyl)ethanes which are prepared in a preferred embodiment of the invention is as intermediates for the preparation of ibuprofen and related pharmaceuticals. When they are used in such applications, they may be converted to the desired products in any suitable manner. For example, they may be reacted with carbon monoxide in the presence of a carbonylation catalyst and then acidified to the corresponding propionic acids as in Gardano et al., Francalanci et al., or Dynamit Nobel; or they may be reacted with an alkali metal cyanide or a tetraalkylammonium cyanide and then hydrolyzed to the corresponding propionic acids as in Palecek et al. or Tokutake. Another useful synthesis involves reacting the compounds with magnesium, carbonating the resultant Grignard reagent with carbon dioxide, and hydrolyzing the carbonated product to the propionic acid as in Miyatake et al. The following example is given to illustrate the invention and is not intended as a limitation thereof. EXAMPLE A suitable reaction vessel was charged with 60 mL of 93% sulfuric acid, which was cooled to -3°C and saturated with anhydrous hydrogen bromide. A solution of 7.8g of acetaldehyde and 21.3g of isobutylbenzene was fed to the reaction vessel over a period of 50 minutes at -3°C with hydrogen bromide bubbling into the reaction mass. The reaction mass was stirred for one hour at -3°C and then poured into ice water. Analysis showed a 64/36 molar ratio of 1-bromo-1-(isobutylphenyl)ethane to 1,1-di(isobutylphenyl)ethane.

Claims: A process for producing a 1-bromo-1-arylethane which comprises reacting an aromatic hydrocarbon with hydrogen bromide and acetaldehyde at a temperature in the range of +10°C to -35°C in the presence of at least one mol of hydrogen sulfate per mol of the aromatic hydrocarbon and less than 15% by weight of water, based on the weight of the hydrogen sulfate. The process of claim 1 wherein the aromatic hydrocarbon is a monoalkylbenzene in which the alkyl contains 1 to 5 carbon atoms, and the product is a 1-bromo-1-(4-alkylphenyl)ethane. The process of claim 1 wherein isobutylbenzene is reacted with anhydrous hydrogen bromide and acetaldehyde at a temperature in the range of 0°C to -35°C in the presence of at least 5 mols of hydrogen sulfate per mol of isobutylbenzene, the hydrogen sulfate being introduced in the form of 93-98% sulfuric acid, and 1-bromo-1-(4-isobutylphenyl)ethane is obtained. A process for preparing a 1-(4-alkylphenyl)-propionic acid which comprises reacting a 1-bromo-1-(4-alkylphenyl)-ethane produced by the process of claim 2 with carbon monoxide in the presence of a carbonylation catalyst and acidifying the product. The process of claim 4 wherein the starting material is 1-bromo-1-(4-isobutylphenyl)ethane produced by the process of claim 3. A process for preparing a 2-(4-alkylphenyl)-propionic acid which comprises reacting a 1-bromo-1-(4-alkylphenyl)-ethane produced by the process of claim 2 with an alkali metal cyanide or tetraalkylammonium cyanide and hydrolysing the product. The process of claim 6 wherein the starting material is 1-bromo-1-(4-isobutylphenyl)ethane produced by the process of claim 3.

Inventors: KNESEL, GEORGE ALBERT
Applicants: ETHYL CORPORATION
Application year: 1989
Main classification: C07C22
IPC codes: C07B61, C07C17, C07C51, C07C22, C07C57, B01J27