Nov. 20, 1957
BIMOLECULAR DISPLACEMENT OF CYCLOHEXYL ~-TOLUENESULFONITES 5995
[COSTRIBUTION FROM THE CHEMICAL
LABORATORIES OF THE UNIVERSITY
OF
NOTREDAME]
Conformational Analysis. IV. Bimolecular Displacement Rates of Cyclohexyl p-Toluenesulfonates and the Conformational Equilibrium Constant of the p-Toluenesulfonate Group1 BY ERNESTL. ELIELAND ROLLAND S. Ro2 RECEIVED FEBRUARY 16, 1957 Rates of displacement of sec-butyl tosylate, cyclohexyl tosylate and the cis- and trans-4-t-butylcyclohexy1,-4-methylcyclohexyl and -3-methylcyclohexyl tosylates with thiophenolate ion have been measured. The products of the reaction have been isolated in three instances. The displacement reaction is second order and is accompanied, in the case of the cyclohexyl, cis-4-t-butylcyclohexyl, cis-4-methylcyclohexyl and trans-3-methylcyclohexylcompound, by a surprisingly fast second-order elimination. The ratio of displacement rates of equatorial tosylate : axial tosylate :acyclic secondary tosylate is 1: 19:80. The geometry of the transition state for the displacement reaction is discussed The conforinational equilibrium constant for tosylate is calculated t o be 3.2 corresponding t o a free energy difference of 0.7 kcal./molc between equatorial and axial tosylate. The consistency of the data is examined.
Introduction Nucleophilic substitution reactions of the S N 2 type involving cyclohexyl halides have been found to be quite slow as compared to similar reactions involving acyclic secondary halides. The difference (by a factor of about 100) has been explained as being due to steric interference of the ring to backside attack4 or as being a manifestation of “Istrain.”5 One weakness of the theoretical arguments is that, for lack of information, the axial or equatorial nature of the displaced halide substituent could not be taken into account. Although many reactions of cyclohexyl compounds, such as esterification, ionic elimination, rearrangements, etc., have been studied as a function of the conformation of the substituent affected,6 no complete study of conformational factors in bimolecular substitution is available. Difficulties might be expected in such a study due to the incursion of the unimolecular mechanism and of elimination reaction^.^ I n the present work these difficulties were minimized by using thiophenolate, a highly nucleophilic reagent,8 as substituting agent. The reactions of cyclohexyl and substituted cyclohexyl tosylates with sodium thiophenolate in S7Y0 ethanol were found to be clean-cut second order in the reagents; moreover, the substitution reactions of the 4-t-butylcyclohexyl tosylates were found to give the inverted thioethers, so that both the kinetic and stereochemical criteriag indicated this reaction to be (1) (a) Paper 111, E. L. Eliel and R. S. Ro, THISJOURNAL, 79, 5992 (1957). (b) A preliminary account of t h e present work has appeared in Chemistry & I n d u s t r y , 251 (1956). (2) Texas Co. Fellow, 1954-1956. From t h e P h . D . thesis of R. S. Ro (3) For a summary see E . L. Eliel in M. S. Newman’s “Steric Effects in Organic Chemistry,” John Wiley and Sons, Inc., New York, N . Y.. . 1956.. D. _ 121 ff. (4) P. D. Bartlett a n d L. J. Rosen, THISJ O U R N A L , 64, 543 (1942); also discussed b y S. J. Angyal a n d J. A. Mills, Revs. Pure A p p l . Chem., a, 185 ( i 9 j z ) . ( 5 ) H. C . Brown, I
