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Partial Rate Factors for the Nitration of Cyclopropylbenzene Derivatives. Influence of Geometry on the Stability of 3- and 4-Cyclopropylbenzenonium Ions’ Leon M. Stock* and Paul E. Young2 Contribution f r o m the Department of Chemistry, The University of Chicago, Chicago, Illinois 60637. Received November 18, 1971 Abstract: Relative rates and isomer distributions for the nitration of cyclopropylbenzene and several derivatives with acetyl nitrate in methylene chloride at -25” have been determined. The results for cyclopropylbenzene, 2,6dimethylcyclopropylbenzene, syn- and anti-(cis-2,3-dimethylcyclopropyl)benzene,spiro(cyc1opropane-1,1’-indan) and 3’,3’-dihydrospiro(cyclopropane-l,1’(2H’)-naphthalene) and related compounds indicate that 3f(cyclopropyl) ranges from 2 to 5 for both the perpendicular and bisected conformations and that 4r(cyclopropyl) is small, probably no more than 10, for the perpendicular conformation, but about 1800 for the bisected conformation. The relationship between these results and the relative reactivities inferred from calculations based on the CNDO model is discussed. factors, 3r ‘v 2 and 4r N 950, for the nitration of cyclolectrophilic aromatic substitution reactions3 and the propylbenzene.6 Hahn and his students and we, in conelectrophilic reactions of side-chain derivatives of junction with an investigation of the electron spin resobenzene4 have long served as testing ground for thenance spectra of 4-nitrocyclopropylbenzene anion radories concerning the relationship between molecular icals,1° have independently extended the investigation structure and reactivity. Reactions of this kind have of the nitration reaction to other aromatic compounds been used by Brown, Hahn, Martin, and Shechter and t o assess the influence of perpendicular and bisected their associates to study the influence of the cyclopropyl group on the stability of electron-deficient molecyclopropyl groups. cules.5-8 T o illustrate, an investigation of the solResults volysis of 2-chloro-2-(3- and 2-chloro-2-(4-cyclopropylFeatures of the Nitration Reaction. We attempted pheny1)propane established that the 4-cyclopropyl t o use the highly selective noncatalytic bromination group stabilized the incipient carbonium ion imporreaction, p+ = -12.1, to define the relative reactivity tantly and that the stabilization was diminished significantly by methyl groups in the 3 and 5 p o ~ i t i o n s . ~ ~ 6of the cyclopropylbenzenes. Unfortunately, spiro(cyThese results meshed beautifully with other emerging clopropane-I, 1’-indan) isomerized in acetic acid in the presence of bromine and hydrogen bromide. The product, 1-ethylindene, was isolated and identified by its nmr spectrum. Under the mild experimental conditions of the bromination reaction, this olefin brominated to yield a complex mixture of products. l 1 The successful nitration of cyclopropylbenzene by several investigators prompted us t o explore the use of observations and theories which implied that the maxIn particular, Hahn, Corbin, and this imum stabilization was achieved when the cyclopropane Shechter reported that nitric acid in acetic anhydride substituent was in the bisected conformation rather yielded 95 2- and 4-nitrocyclopropylbenzene.6 Prethan in the perpendicular c o n f ~ r m a t i o n . ~Hahn, liminary experiments revealed that the nitration of Corbin, and Shechter also estimated the partial rate spiro(cyc1opropane- 1,1 ’-indan) at 25 O produced substantial amounts of I-ethylindene. However, the nitra( I ) This research was supported by the National Science Foundation. tion of spiro(cyc1opropane-l,1 ’-indan) in acetic anhy(2) Fellow of the New York State Board of Regents, 1968-1969; Fellow of the Petroleum Research Fund, 1969-1970. dride at - 2 5 ” proceeded at a reasonable rate without (3) (a) C. I
