5347
Nov., 1950
COMMUNICATIONS T O T H E EDITOR INTERACTION OF FRIEDEL-CRAFTS CATALYSTS WITH ALKYL HALIDES AND AROMATIC HYDROCARBONS
MeBr, MeI, EtCI) form stable 1: 1 addition compounds at -46 to -80': RX GaC& = RX:GaC&. Thus, a t -78.5' gallium chloride Sir: dissolves in excess methyl chloride to form a clear The activity of Friedel-Crafts catalysts is solution. As excess methyl chloride is removed, usually attributed to the formation of strong two pressure plateaus are observed: 26.0 mm. complex acids, HC1 hlCl3 = HAIC14, or to between 5.4:1 and 1 : l mole ratio MeCI/GaCls ionization of the alkyl halides, RC1 AlCl, = and 3.2 mm. between 0.9:l to 0.1:l. The first R+AlC14-. For several years we have been study- plateau corresponds to a saturated solution of ing interaction of aluminum and gallium chlorides addition compound in methyl chloride; the second with hydrogen chloride, alkyl halides, and aro- to the dissociation of solid addition compound. matic hydrocarbons in order to attain a better Similar data were obtained for the other halides. understanding of these catalysts. The paper by Molecular weight determinations support the Van Dyke [THISJOURNAL, 72; 3619 (1950)l pre- existence of MeCl: GaCls in solution. Vapor sents results with aluminum bromide that are in pressure a t -78.5' of solution containing 0.447 marked contrast to our own findings and prompts mmole of gallium chloride in 3.067 mmole methyl us to summarize our results and conclusions. chloride was 28.94 mm. (34.00 for pure solvent). No reaction could be observed between hydro- Assuming free GaCI3 in solution, mol. wt. 147; gen chloride and aluminum or gallium chloride assuming 0.447 mmole solvent effectively reunder a variety of conditions (300 to -120'). moved to form MeC1:GaCh in solution, mol. wt. Since no combination can be detected at -120°, 172. The latter is in good agreement with the it is improbable that appreciable concentrations actual molecular weight 176 for monomeric galof HAlC4 can exist a t room temperature. lium trichloride. Dissolved gallium chloride exchanges very Aluminum chloride and toluene do not react at -80'. However, hydrogen chloride causes the slowly with methyl bromide at -80' (25% in aluminum chloride to dissolve, forming a brilliant 10 days). Evidently the addition compound ingreen solution. One mole of hydrogen chloride volves a covalent carbon-halogen bond and is absorbed per mole of aluminum chloride in ionization, Me-Br :GaC& = Me+ [BrGaCbI-, must be a relatively slow process under these H 4. conditions, if it occurs a t all. Originally our observation of 1 : l comCHI \ poun& was in good agreement with the report by Van Dyke [Abstracts, A. C. S. Meeting, HCI AlCls = * AICLAtlantic City, 19491 that aluminum bromide \ forms similar addition compounds. However, A in this recent paper in THISJOURNAL, he I-l 11 H H states that such compounds do not exist. He etc. finds no compound MeBr:AlBr3 at -56'. I In view of our results, this observation is solution. Removal of the hydrogen chloride, highly surprising and warrants reinvestigation. precipitates aluminum chloride; the hydrocarbon We are continuing our studies in the field and is unchanged. The reaction is attributed to the hope to resolve the discrepancy. formation of an ionized salt of toluene and DEPARTMENT OF CHEMISTRY HERBERTC.BROWN HA1C4 (I) [Brown and Pearsall, Abstracts, PURDUEUNIVERSITY HOWARD PEARSALL LAFAYETTE, INDIANA LOWELLP. EDDY A. C. S. Meeting, New York, 19471. RECEIVED AUGUST28,1950 Gallium chloride and hydrogen chloride react with toluene and mesitylene similarly. Therefore the free acids HAIC14 and HGaC14 do not exist TRE HuTTIG MULTILAYER ADSORPTION independently, but form stable salts with weak ISOTHERM' organic bases, such as aromatic hydrocarbons and Sir: conjugated olefins. The oily complexes which inhas derived a new adsorption isotherm variably accompany the use of Friedelcrafts byHtittig2 a very fundamental modification of the wellcatalysts must be organic salts of this kind. known B.E.T. kinetic argument. Namely, he These complexes must be essential to the reaction The opinions expressed herein are those of the author and do in furnishing a polar medium in which the ionic not(1)necessarily reflect the viewe of the Navy Department. Articlr intermediates can form and react. no0 copyrighted. Gallium chloride and alkyl halides (MeC1, (P) zaattlr, YOUW., ~ 8 , 1 7 7(1~s).
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