The rates of reaction of anthracene and dimethylanthracene with maleic anhydride and several of its substitution products have been followed by colorimetric procedures. In the presence of a large excess of dienophile the reactions in chloroform are first order with respect to diene, but the first-order rate constants are not directly proportional to dienophile concentration. This kinetic behavior can be explained quantitatively through consideration of the fact that a substantial fraction of the diene in the reaction mixtures is bound in a molecular complex with the dienophile. The mechanism of the reaction is discussed in the light of these observations and in terms of the effects of changes in temperature, solvent and substituents in the reactant molecules on the reaction rates. The possibility that Diels-Alder adduct may form by reaction of the dienedienophile complex with a second dienophile molecule has been ruled out on the basis of rate studies.
Detailed information concerning the argentation of aromatic hydrocarbons is lacking. The results of studies of the phase relationships in the systems silver perchlorate-benzene-water and silver perchlorate-toluene-water2 suggest that the tendency for such complex formation is appreciable.In the present investigation, which represents an extension of the work reported previously,3 the solubilities of several aromatic hydrocarbons in aqueous silver nitrate solutions have been measured. The data have been interpreted on the assumption that from each hydrocarbon two watersoluble complexes, AgAr+ and Ag2Ar++, are formed.4 5Equilibrium constants for reactions leading to their formation have been calculated. ExperimentalThe Aromatic Hydrocarbons.-All of the hydrocarbons used were of the best grade obtainable from Eastman Kodak Co. The benzene and toluene were washed successively with concentrated sulfuric acid, water and dilute sodium hydroxide and, after drying, were fractionated. Cuts of b. p. 80.10 (benzene) and 110.4°( toluene) were used in the solubility experiments. The xylenes were
mole of haloalkyl ether. The required volume of phenyllithium solution was added from a calibrated dropping funnel, over a period of 1.5 hours, to the haloalkyl ether in solution in 50 ml. of diethyl ether. Mechanical stirring was employed, and addition was at room temperature without cooling. The reactions proceeded smoothly, with mild refluxing. The Michler ketone color test8 was negative a the end of the addition (except in the case of l-ethoxy-3chloropropane), indicating absence of phenyllithium. Occasional color tests made during the course of the addition indicated that reaction was immediate, since such tests were always negative. The reaction mixture was cooled in an ice-bath and hydrolyzed by the addition of aqueous ammonium chloride. After separation, washing with water, and drying on Drierite, the organic layer was distilled. A 42-cm. Fenske column (Win. glass helices) was used to separate the diethyl ether, benzene, and other low-boiling products. The higher-boiling portion was then distilled under reduced pressure through a 25-cm. Vigreux column.For certain reactions, data not apparent from Table I are given below.
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