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1%'. GREIZERSTEIN AND J . -4. BRIEUX
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Both inductive and hyperconjugative effects group slightly enhances the reactivity from the mfor these substituents are positive causing them to position (R.R. = 1.05); a greater increment is obbe deactivating with respect to hydrogen. served from the p-position (R.R. = 2.1). The inductive effect should not be very different Berliner and co-workers have obtained a similar when the substituent is in m- or p-position, but result in the reaction of 3-bromo-3-nitrobiphenyl hyperconjugation determines the order k5-R > with piperidine in which this compound reacts 2 . 2 k 4 - ~because i t can only act directly when the times as fast as o-bromonitrobenzene. alkyl group is in the p-position. Brown, Okamoto and I n u k a P found that the The ratio kln,'kp for t-butyl, significantly greater phenyl substituent, in the solvolysis of phenylthan unity, suggests that for this substituent as dimethylcarbinyl chlorides system in which no well as for methyl, hyperconjugation plays an im- nitro group is present, increases the rate by six times portant role. These results agree with those of from the #-position while i t is deactivating from Capon and Chapman." the m-position (R.R. = 0.32). These results can The effects of the phenyl group cannot be inter- be compared with those of Berliner and Shieh74 preted for both positions of the substituent on the for the solvolysis of a-phenylethyl chlorides in same basis of negative inductive and positiv e meso- which they find that forcing both rings of the bimeric effects. phenyl moiety to be coplanar, by introduction of Ingold7" pointed out the variable capacity of the an ortho-ortho methylenic bridge (fluorenyl derivaphenyl group to act as an electron acceptor or tive), increases the rate of reaction by a factor of donor, depending on the reaction requirements, seven hundred. and the general structure to which it is attached. Finally we wish to record from experiments De la Mare and Ridd71 have recently discussed made in this Laboratory by Miss Hebe B. Belthe influence of several factors on electrophilic sub- monte that it has not been possible to obtain 3stitutions in diphenyls and structurally related chloro-4-nitroazobenzene b y condensation of nitrocompounds. Berliner and c o - ~ o r k e r s have ~ ~ ~ ~ sobenzene ~ with 3-chloro-4-nitroaniline and hence to established that transmission of polar effects of sub- study the polar effects of the m-phenylazo substitustituents through the diphenyl moiety is less in- ent in the reaction. tense than through benzene itself. In the activaAcknowledgments.-This research was supported tion of the chlorine atom by the phenyl group in 4in part by a grant from the Council for Scientific chloro-3-nitrobiphenyl, the negative inductive ef- and Technical Research of Argentine (C.S.I.C.T.). fect of the substituent is more powerful than the The authors are grateful to Professor Ti. Deulofeu opposing mesomeric effect; but in 3-chloro-4- for his interest and to Professor J. F.Bunnett and nitrobiphenyl, where the phenyl group is pura to Dr. C. A. Bunton for helpful discussions. They the nitro group, mesomeric interaction by para- also wish to thank hIiss H. Belmonte and Mr. T. conjugation operates with its consequent elec- H. Suarez for their assistance in some of the preptronic release balancing the electron-attracting arations. They appreciate as well the Xaumee effect of the phenyl group. Chemical Co., Toledo, Ohio, for their generosity in In the conditions of our experiments the phenyl providing them with a sample of 4-chloro-3-nitro(70) C. K. Ingold, Chem. Reus., 16, 225 (1934). benzotrifluoride. (71) P. B. D. d e l a M a r e a n d J. H. Ridd, "Aromatic Substitution" (Nitration a n d Halogenation), B u t t e r n o r t h s Sci. Puhl., London, 1959, C h a p t e r 12. ( 7 2 ) E. Berliner a n d L. H. Liu, J . A m . Chem. SOC.,76, 2417 (1953).
( 7 3 ) H. C . Brown, J. Okamoto a n d T. Inukai, i b i d . , 8 0 , 4005 (1958). (74) E. Berliner and S . Shieh, ; b i d , , 79, 38-18 (1957)
[CONTRIBUTION FROM THE LABORATORIO DE QUIMICAORGAXICA, FACULTAD DE CIENCIAS EXACTAS T SATURALES, UNIVERSIDAD SACIOXAL DE BUEXOS AIRES, BUESOSAIRES, ilRGESTISA]
The ortho:para Ratio in the Activation of the Nucleophilic Aromatic Substitution by the Nitro Group BY IT. GREIZERSTEIN AND J. A. BRIEUX~ RECEIVED MAY26, 1961 The amount of substitution of the chlorine atoms of 2,4-dichloronitrobenzene-4-C13" by piperidine in benzene and methanol has been determined by a radiochemical method. The results have been compared with the reactions of 0- and p-chloronitrobenzene with piperidine in the same solvents and in ethanol. I t is concluded that while the change from benzene to methanol lowers the over-all rate of substitution, it enhances selectively the reactivity of the chlorine atom pura to the nitro group. Reaction kinetics are tentatively explained and drrhenius parameters presented.
Bunnett and Zahler2 and more recently Bunnett and Morath3 have reviewed the extensive litera(1) Inquiries should be addressed t o Laboratolio de Quimica orghnica, Facultad d e Ciencias Exactas y Naturales, Per6 272, Buenos Aires, Argentina. (2) J. F. B u n n e t t a n d R. E. Zahler, Chem. Revs., 49, 273 (1951). ( 3 ) .J+ P. Bunnett a n d R . J. Morath, J. Am. Chrm. So
