Oct. 5, 1962
ACETONITRILE-NITROMETHANE IN AROMATIC CHLORINATION
[CONTRIBUTIOS FROM
THE
3633
DEPARTMENT OF CHEMISTRY, UNIVERSITY OF CALIFORNIA, DAVIS,CALIF.]
Acetonitrile-Nitromethane Mixtures as Media for Aromatic Chlorination and Chlorine Addition Reactions BY R. M. KEEFERAND L. J. ANDREWS RECEIVED APRIL 24, 1962 The rates of reaction of chlorine with toluene, 2-~hloronaphtlialene,2,3-dichloropropene, crotonic acid, acrylonitrile a n d iodobenzene ( t o form iodobenzene dichloride) have been determined using various mixtures of acetonitrile and nitromethane as solvent. T h e two aromatic chlorination reactions increase appreciably in rate with increases in t h e nitromethane cont e n t of t h e medium. T h e rates of the other reactions are relatively insensitive t o the solvent change. These reactions also have been investigated using carbon tetrachloride as solvent and trifluoroacetic acid as catalyst. T h e addition of acetic acid slows the catalyzed aromatic chlorinations markedly. The rates of the other catalyzed reactions are less subject t o inhibition by acetic acid. T h e function of the polar solvent or the polar catalyst in solvating the activated complexes in these reactions is discussed in terms of the results of t h e r a t e studies.
Trifluoroacetic acid catalyzes a variety of reactions of chlorine and other halogens in carbon tetrachloride, including aromatic substitutions, additions to double bonds and the reaction t o form iodobenzene dichloride. These reactions are presumed t o proceed via polar activated complexes of the type SX +X-, where S is the organic reactant and Xz is the halogen. The catalytic effect of trifluoroacetic acid is modified in different ways, depending on the particular type of organic reactant which is used, when acetic acid is added to the I t seems likely that the capacity of the catalyst t o solvate the negative end of the activated complexes (an electrophilic solvation process) must be reduced by interaction with acetic acid t o form the mixed dimer, CF3COOH. CH,COOH. On the other hand, the medium should be better constituted to provide for solvation of the positive end of a polar particle (nucleophilic solvation) when acetic acid is present. I n the case of a reaction which is subject t o marked rate depression b y acetic acid, e.g., the chlorination of toluene or durene, the polar additives (the carboxylic acids) are believed to serve primarily t o provide for electrophilic solvation in the activation process. Under these circumstances the organic reactant itself must a t the same time provide for the development of positive charge, with relatively limited assistance from the medium, by dispersing t h a t charge over several atoms or over the whole aromatic nucleus if the reactant is benzenoid in character. I n situations in which the addition of acetic acid causes only mild rate depression (e.g., in the reactions of iodobenzene and chlorine and of allyl chloride and bromine) or in which i t produces some rate acceleration (the addition of chlorine to crotonic acid), the need for nucleophilic solvation of the activated complexes is considered to be greater than in the aromatic chlorination reaction. To assess the plausibility of the interpretation of the different effects of added acetic acid on the rates of the catalyzed reactions, several of the reactions involving chlorine have been subjected to further rate study without a catalyst and with various mixtures of acetonitrile and nitromethane ;IS solvent. Since these two liquids have essentially (1) R. &I, Keefer and I,, J. A n d r e w , J . A m . Ciiein. Suc., 83, 376 (19131). (2) L T . \ i i ~ l r c w:~i t i i l I< 11 I i c c f e r . i ' i i / , 82, :305!l l l ! ) M l ) . (31 R. 11 Kccler and I.. J Aiidre\%s.z " i d 8 2 , i i l i ( l ! ~ t i O ~ . ~
the same dielectric ~ o n s t a n t the , ~ free energies of activation and consequently the rates5of the various polar reactions of chlorine should not be subject to a dielectric effect as the composition of the mixed solvent is changed. Acetonitrile is a considerably stronger donor in molecular complex formation with iodine monochloride than is nitromethane.G It is, therefore, reasoned that acetonitrile will function more effectively than nitromethane as a nucleophilic solvating agent and that, of the two solvents, the latter will be the better electrophilic solvating agent.7 If the capacity of the medium to solvate the activated complex, rather than the reactants, is the rate-controlling factor, then the reactions of chlorine should fall into like behavior groups (in their responses to a change in solvent from acetonitrile t o nitromethane) which are similar to those established on the basis of the influence of acetic acid on the trifluoroacetic acidcatalyzed reactions in carbon tetrachloride. Experimental Materials.-The snurces and methocly of purification of the solvents and most of the reagents have been indicated in earlier publications.1,8 Samples of acrylonitrile (Eastman Organic Chemicals) and 2,3-dichloropropene (Columbia Organic Chemicals, Co .) were redistilled before use. Eastman Organic Chemicals white label 2-chloronaphtlialene was used without further purification. T h e Rate Runs.-The rates of the substitution and addition reactions of chlorine in nitromethane, acetonitrile or their mixtures were determined spectrorihotometrically using 1-cm. glass stoppered silica absorption cells as reaction vessels. T h e chlorine concentrations of the reaction mixtures during the course of the runs were calculated from t h e optical densities of t h e rate samples, measured a t snme fixed wave length (375-300 mp) against the solvent a s a blank. T h e extinction coefficients of the lxilogen in tlic various solvent mixtures were established bj- sep:irute measurement of solutions, t h e chlorine concentration5 of which were determined iodometrically. Further det:iils ( i f the experimental procedures are t o be found in t h e t l e s c r i l ~ tion of the methods used in earlier work on toluene chlorination.8 The rate studies of the reaction of iodobenzene and chlorine were also conducted spectrophotometrically (390 m p ) in much the same !ray as was a n earlier investigation _____ (-1) 4.A . h l a r y u t t a n l E. R . S m i t h , "Table of Ilielectric Constants of P u r e Iicluids." Xatl. Bur. S t a n d d r d s Circ. 514 (1951). ( 5 ) A. A . F r u s t and R. G Pearsou, "Kinetics and Mechanism," Second Edition, j v h n Wiley and Sons, In'.,, New York, iY,Y., 1961, pp. 137-112. ( 6 ) U'. B. Pers!Jti, K . E IIumiJhre)., \V. 4 ljeskin and .4 I. Pdlrov, .I. . Z i i i ( ' h e m . .Sot-., 8 0 , 2019 (1Y:X). ( 7 ) S i t r v compounds in general s h u w a specilic capacity fur interactidn with basic substances. See f o r example, (a) 1,. J. Andrews, S . nayliss and C. J. Brackenridge,
( 8 ) I.. J. Andrews a n d I
