Science
Centennial symposium held for Frledel-Crafts The acid-catalyzed organic reactions found 100 years ago by Charles Friedel and James Crafts are widely used in chemical processing One hundred years ago this month, the French chemist Charles Friedel and his American collaborator James Mason Crafts discovered that aluminum chloride would catalyze the alkylation of benzene by amyl chloride. They, and others, quickly discovered that this was only the first of a whole family of reactions in cluding substitution, elimination, crack ing, and addition reactions and using many other acid catalysts besides alumi num chloride. In fact, today the term Friedel-Crafts is applied broadly to al most any acid-catalyzed organic reaction. Among them are some of the most in dustrially important reactions in chem istry as well as some that are playing key roles in the understanding of theoretical physical organic chemistry and in devel opment of new synthetic pathways. To celebrate the centennial of this fa mous reaction and to demonstrate its importance in contemporary chemistry, the Division of Organic Chemistry spon sored a three-day symposium on Frie-
Charles Friedel 28
C&EN April 11, 1977
del-Crafts reactions at last month's American Chemical Society meeting in New Orleans. Organized by Dr. George A. Olah of Case Western Reserve University and Dr. Royston M. Roberts of the Uni versity of Texas, Austin, the symposium brought together researchers from 12 countries. Although it included some historical papers highlighting the impor tance of this reaction to the development of the modern petrochemical industry, the focus of the symposium was on presentday Friedel-Crafts chemistry. The first industrial applications of Friedel-Crafts-type reactions were in petroleum refining to produce low-sulfur gasoline from heavy oil fractions, explains Dr. Alan Schriesheim, director of the corporate research laboratories at Exxon Research & Engineering Co. Although this process was replaced in the 1930's by vapor-phase cracking using solid acidic catalysts, the importance of FriedelCrafts chemistry to petrochemical reac tions was firmly established. Probably the largest-volume FriedelCrafts reaction carried out today is the ethylation of benzene for styrene manu facture using an aluminum chloride cat alyst. Some ethylbenzene is produced by other procedures, but most of it is made by this classical Friedel-Crafts reaction. Other important reactions based on con ventional Friedel-Crafts reactions include the production of ethyl chloride, an in termediate in tetraethyllead formation, production of detergents from propylene, and of butyl rubber as well as sealants and adhesives from isobutylene. Schriesheim indicates a number of di rections of industrial research being pursued today involving Friedel-Crafts chemistry. Major work is going on to im prove Friedel-Crafts catalysts aimed at making them easier to handle, less cor rosive, and easier to recover. Because of the removal of tetraethyllead from gaso line, many efforts are being made to upgrade the lighter naphthas in gasoline by increasing the branching of small hy drocarbon chains. Although this isomerization can be done using platinum-con taining catalysts at temperatures of about 150° C, there is a thermodynamic and energy incentive to lower the reaction temperature through the use of FriedelCrafts catalysts. Research along these lines is going on at many industrial labo ratories, Schriesheim says, but a break through has not yet occurred. Traditionally, Friedel-Crafts reactions are carried out with catalysts such as aluminum chloride in solutions. One area of current research is the binding of these catalysts to solids, thus allowing hetero
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geneous, gas phase reactions. Dr. Bruce C. Gates of the University of Delaware de scribes a polymeric complex containing sulfur, aluminum, and chlorine that will catalyze the conversion of η-butane into isobutane and n-hexane into isomerization and cracking products at 100° C and atmospheric pressure. A similar complex recently has been developed and patented by researchers at British Petroleum. Dr. Jean-Marc Lalancette of the Uni versity of Sherbrooke, Quebec, tells of the effect of intercalating aluminum chloride and other Friedel-Crafts catalysts into graphite lattices. Although this procedure apparently reduces reaction rates, it changes the ratio of products formed, in creasing the yield of less substituted products. Olah's laboratory at Case Western Reserve University is studying isomerization reactions of alkylbenzenes over solid Friedel-Crafts catalysts. His
James Mason Crafts
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group finds side reactions are minimized and complex formation with the catalyst eliminated under these conditions. Friedel-Crafts reactions also are a way of making organic polymers, and one area of current research focuses on under standing more specifically how these po lymerization reactions occur. Dr. Joseph P. Kennedy of the University of Akron's Institute of Polymer Science gave an overview of several of these projects. For example, Dr. P. H. Plesh is studying the initiation of isobutylene polymerization by Friedel-Crafts catalysts. Others, in cluding Dr. P. L. Magagnini and Dr. P. Giusti of the University of Pisa, Italy, are studying low-temperature polymerization of isobutylene and isobutylene-isoprene mixtures by Friedel-Crafts reactions. And Kennedy's own laboratory has been making new graft and block copolymers using Friedel-Crafts-type reactions. Dr. Peter Kovacic and Dr. Peter G. Engstrom
of the University of Wisconsin, Milwau kee, have been studying the polymeriza tion of benzene by aluminum chloridecupric chloride to determine whether the reaction goes by a cationic or free radical pathway. One of the more unusual applications of Friedel-Crafts chemistry is to the syn thesis of exotic polycyclic hydrocarbons. Dr. Paul Schleyer of the University of Erlangen-Nurnberg, West Germany, and associates have been trying to use a Friedel-Crafts-type isomerization reac tion to synthesize dodecahedrane, C20H20, a 12-sided regular polygon with pentago nal sides. Friedel-Crafts reactions are regularly used to make adamantane, a tricyclic Ci 0 Hi 6 hydrocarbon, from dicyclopentadiene, Schleyer notes. In theory, the same approach should be applicable to a larger, heterocyclic molecule. Ex periments along this line, though, have not been successful as yet. •
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Though herbicidal antidotes have been marketed for several years, scientific in terest in their existence is only slowly coming to life. Besides improving crop production, these substances are becom ing useful tools for studying plant bio chemistry. Scientists busy on herbicidal antidotes described current work in the field at the New Orleans meeting of the American Chemical Society. Herbicidal antidotes are substances that either allow increased doses of herbicides to be used on a given plant or extend a herbicide's range to different plants. For example, the widely used herbicide S-ethyl-N,N-dipropyl thiocarbamate (EPTC) injures corn and thus ordinarily can't be used to kill weeds infesting cornfields. But pesticide chem ists sought—and succeeded—to extend use of EPTC to corn. EPTC's relatively low cost plus its favorable standing with environmentalists—it's not persistent— offered incentives for the project. Several herbicidal antidotes enabling EPTC use on corn already are marketed.
Of the known antidotes, Stauffer Chem ical Co.'s Ν,ΛΓ-diallyldichloroacetamide, called R-25788, is the most attractive. According to Stauffer, several million of the nearly 100 million acres of corn planted in the U.S. per year receive treatment with formulations containing this antidote. Stauffer's Dr. Ferenc M. Pallos, who led development of the product, says it was found through "good planning and dumb luck." Regardless, R-25788 meets several criteria necessary for successful field use. For example, unlike many other anti dotes, R-25788 does not involve seed pretreatment for its use. Instead, it can be mixed directly with EPTC for field ap plication. This reduces the potential for mixing errors by farmers when making their own formulations. Also, because both of Stauffer's antidote-containing products are applied to soil and then disked (plowed) in, dependence on rain to wash them in is eliminated. According to Dr. Richard H. ShimaContinued on page 33
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C&EN April 11, 1977
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