Benzene alkylation, base catalyzed

Thus the reactions of cyclic or acyclic enamines with acrylic esters or acrylonitrile can be directed to the exclusive formation of monoalkylated ketones (3,294-301). The corresponding enolate anion alkylations lead preferentially to di- or higher-alkylation products. However, by proper choice of reaction conditions, enamines can also be used for the preferential formation of higher alkylation products, if these are desired. Such reactions are valuable in the a substitution of aldehydes, which undergo self-condensation in base-catalyzed reactions (117,118). Monoalkylation products are favored in nonhydroxylic solvents such as benzene or dioxane, whereas dialkylation products can be obtained in hydroxylic solvents such as methanol. The difference in products can be ascribed to the differing fates of an initially formed zwitterionic intermediate. Collapse to a cyclobutane takes place in a nonprotonic solvent, whereas protonation on the newly introduced substitutent and deprotonation of the imonium salt, in alcohol, leads to a new enamine available for further substitution. 

Acid sites were shown to be located in the three-pore system of protonated samples (HMWW), and methods were recently proposed for determining the distribution of these sites as well as their respective role in o-, m-, and p-xylene transformations. While xylene transformation was shown to occur in the three locations, benzene alkylation with ethylene was catalyzed by the acidic sites of the external hemicups only. Indeed, the activity for this reaction is completely suppressed by adding a base molecule (collidine) to the feed that is too bulky to enter the inner micropores. Moreover, adsorption experiments show that collidine does not influence the rate of ethylbenzene adsorption, so that the suppression of alkylation activity was not caused by pore mouth blocking. ... 

Acid-catalyzed reactions of aromatics with monoolefins result in nuclear alkylation. But the base-catalyzed reactions of aromatics with olefins do not result in nuclear alkylation as long as benzylic hydrogens are available. This is true even with aromatics, such as cumene, which have deactivated benzylic hydrogens resulting in facile metalation of the ring. Apparently phenyl carbanions do not readily add to olefins. Pines and Mark (20) found that in the presence of sodium and promoters only small yields of alkylate were produced at 300° in reactions of benzene with ethylene and isobutylene and of t-butylbenzene with ethylene. With potassium, larger yields may be obtained at 190° (24)-... 

Cumene is an important intermediate in the manufacture of phenol and acetone. The feed materials are benzene and propylene. This is a Friedel-Crafts alkylation reaction catalyzed by solid phosphoric acid at 175-225 °C and 400-600 psi. The yield is 97% based on benzene and 92% on propylene. Excess benzene stops the reaction at the monoalkylated stage and prevents the polymerization of propylene. The benzene propylene ratio is 8-10 1. 

The diastereoselectivity is reversed in the alkylation of the enolate derived from the structurally very similar bicyclic lactam, tetrahydro-3-phenyl-l//.577-pyrrolof 1,2-c]oxazol-5-one (3). Thus, the major diastereomer 4 produced has the tram relationship between the newly introduced substituent in the pyrrolidine ring and the fused oxazolidine ring residue11,12. Only active electrophiles such as iodomethane, 3-halopropenes or (halomethyl)benzenes react11,12. Base-catalyzed equilibration of the product obtained by reaction with 3-bromocyclohexene gives a 50 50 mixture of the cis- and rra s-diastereomers11. 

The reaction pathway of benzene alkylation with propylene catalyzed by acids is very similar to that already reported for EB. The main difference is represented by the tendency of cumene to isomerize to n-propylbenzene, which is thermodynamically more stable at increased temperature. Also, cumene can undergo further alkylation to diisopropylbenzene (DIPB), which could be recovered by transalkylation with benzene to give cumene. The transalkylation reaction requires a higher temperature than the related alkylation. In addition, not all of the alkylation catalysts are suitable for transalkylation. Beta or dealuminated mordenite are suitable catalysts for transalkylation. The first industrial demonstrations of cumene technologies based on zeolite catalysts were started-up in 1996 by Mobil-Raytheon, EniChem and UOP, independently. In 2001, worldwide, 14 cumene units were already operating with zeolite catalysts. Around 98% ofcumene is used to produce phenol and expected world production of cumene in 2008 is around 9 million tons. For cumene, among the 40 units in the world (2004), 14 cumene plants were in operation with zeolite catalysts [222]. Today over 70% of cumene plants use a zeolite as the catalyst. 

Base catalyzed reaction has been covered with reference to transesterification to make biodiesel. A classical case of considerable practical importance is the side chain alkylation of toluene with propylene to give isobutyl benzene, apart from conversion of acetone to diacetone alcohol and isophorone and even here heterogenerous versions are in vogue or under consideration. Microwave assisted reactions are duly covered and it would have been useful to cover ultrasonically assisted reactions, particularly when solid reactant/ catalyst is involved. 

Cumene capacity topped 9.5 million metric tons in 1998 and is projected to reach 10.4 million metric tons by the end of 2003 (19). Like ethylbenzene, cumene is used almost exclusively as a chemical intermediate. Its primary use is in the coproduction of phenol and acetone through cumene peroxidation. Phenolic resins and bisphenol A are the main end uses for phenol. Bisphenol A, which is produced from phenol and acetone, has been the main driver behind increased phenol demand. Its end use applications are in polycarbonate and epoxy resins. The growth rate of cumene is closely related to that of phenol and is expected to be approximately 5.1% per year worldwide over the next five years. Process technologies for both chemicals have been moving away from conventional aluminum chloride and phosphoric acid catalyzed Friedel-Crafts alkylation of benzene, toward zeolite-based processes. 

The application of the ideas of Lewis on acids, which correlate a wide range of phenomena in qualitative fashion, has as yet led to very few quantitative studies of reaction velocity but has led to detailed speculations as to mechanisms (Luder and Zuffanti, 118). Friedel-Crafts reactions are considered to be acid-catalyzed, the formation of a car-bonium ion being the first step. The carbonium ion then acts as an acid relative to the base benzene which, upon loss of a proton, yields the alkylated product. Isomerizations of isoparaffins can be explained in similar fashion (Schneider and Kennedy, 119). An alkyl halide yields a carbonium ion on reaction with acids such as boron trifluoride, aluminum chloride, and other metal halides. 

---