~vi th water, and dried to leaye 0.12 g ( 2 4 5 ) , m.p. 262-273". This material n a s identical n i th that obtained in procedure *A n ith respect to infrared and ultraviolet spectra and paper chronlatographic beha~ io r .

This worli was carried out under the auspices of the Cancer Cheniotherapy Xational Service Center, Kational Cancer Institute, Sational Institutes of Health, Public Health Service, Contract No. S-A-43-ph-1892. The opilliolls expressed in this paper are those of the authors and are not necessarily those of the Cancer Chemotherapy National Service Center.

The authors are indebted to Dr. Peter Lim for infrared interpretations, to his staff for ultraviolet and paper chromatographic data, and to AZr. 0. P. Crews for the large-scale preparation of intermediates.

1. J. DEGRAW, L. GOODMAN, B. \\-EINSTEIK, and B. R. BAKER. J . Org. Chein. 27, 576 (1962). 2. C. KOELSCH. J. Am. Chem. Soc. 65, 2458 (1943). 3. R. BERITETTI, F. MANCINI, and C. PRICE. J. Org. Chem. 27, 2863 (1962). 4. L. MIKESKA. U.S. Patent Yo. 2,461,336 (February 8, 1949): Chem. Abstr. 43, 4689 (1949).

THE EFFECT OF ALKYL GROUP VARIATION ON THE RATES OF SOLVOLYSIS OF ALKYL SULPHONIUM HALIDES

Tertiary butyl and amyl dimethylsulphonium salts have been the standard examples of the sulphonium salt series used in neutral solvolytic rate studies of this type of conlpound in the past (1). Apart from these cases the apparent hygroscopic nature of the sulphonium salts has limited the study of their solvolytic-behavior. In the course of our studies of sulphonium salt solvolysis we have prepared and characterized a-phenethyldimethyl- sulphonium and benzyldimethylsulphonium bromides. The solvolytic rate behavior of these two compounds together with comparative data on the t-butyldimethylsulphonium salt are reported here.

RESULTS

Approximately M solutions of the various sulphoniu~n salts in the desired solvent media were prepared by weight. Solvolytic kinetics were followed using the conductimetric technique described previously (2) arid due to Robertson (3). While the iodide salt was used in the t-butyl case the bromides of both the a-phenethyl and benzyl sulphonium ions were employed. Since 0.325 mole fraction ethanol in water was the lowest polarity solvent used the anion does not enter the rate determining step as was shown previously (2, 4). Consequently the change from iodide to bromide does not invalidate the comparison of rates. In both the a-phenethyl and benzyl cases difficulty was encountered in obtaining reproducible results in pure water. In the benzyl case only those rates measured in higher ethanolic compositions were of satisfactory reproducibility. In Table I the rates for the

Canadian Journal of Chemistry. Volume 41 (1963)

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CANADIAh- JOURNAL O F CHEMISTRI'. VOL. 41, 1963

TABLE I Rates of solvolysis of sulphoni~~rn salts i11 EtOH-H?O illixtures

three sulpho~lium salts studied under various solvent and temperature conditions are recorded.

DISCUSSION

The availability of the rates of solvolysis for the three sulphorlium salts in Table I enables a cornparison to be made with the rate behavior of the corresponding alkyl chlorides. Using the date of Winstein and Fainherg (5) and Hyne, Wills, and Wonkka (6) the relative rates of solvolysis of the t-butyl, a-phenethyl, and benzyl chlorides and di- methylsulphoniurn salts may be compared. Conditions chosen for the comparison were 0.204 mole fraction ethanol (approx. 45% ethanol by volume in water) a t 50" C for the chloride and 78.4" C for the sulphonium salts. These conditions were dictated largely by the availability of results. The temperature difference is unlikely to have a significant effect since the con~parisons are internal within a given set a t the same temperature. Use of a mixed solvent medium is perhaps not the best condition in view of the established selective solvation effects (6, 7). Since free energies of activation are being compared, however, -the likelihood of gross errors being introduced by such effects is minimized. The rate comparisons are shown in Table 11.

TABLE II Relative rates of solvolysis of alkyl halides and sulphoiliuin salts

in 0.204 mole fraction EtOH-Hz0

Alkyl group Leaving group

CH, k7s.40 sec-l 4.42X10-4 2.72X10-4 1 X 10-B / Relative

-S log k (AF*) 1 1.07 1.79 \

k50.00 sec-I a 1.32X10-2 9.SOX10-3 1.89X10-5 Relative

log k (AF") 1 1.07 3.51 - . .

"KOTE: Rates in this row interpolated from data of refs. 5 and 6.

Comparison of the free energies of activation (ratio of log k) for the two types of sol- volysis shows immediately that structural changes in the alkyl group have the sarne effect on the solvolysis rates of both the halide and the sulphonium salts. Although the two reactions have opposite charge character on activation-one involving charge delocalization while the other requires charge creation-in both cases an incipient alkyl carbonium ion develops on activation.

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NOTES

R-Cl --4 Ra- - - - - - - - - - - - - ClS-

CHI CHI +/ a3/

R-S -+ Rb ----- - - - - - - -

\a, \ CH,

The similar effects of structural change in the alkyl group must imply that the stabilization of the incipient carboniuin ion in the transition state is the dominant factor affecting the rate as a function of the alkyl group. This is particularly interesting in view of the fact that we are dealing here with two reactions having leaving groups of entirely different character. Clearly the stabilization of the transition state-or lack of i t in the sulphoniuin case-brought about by solvation of the leaving group does not appear to vary significantly as the nature of the alkyl group is varied. If it did one would imagine that the effect of the alkyl group variation in the case where leaving group solvation was important would be very much different froin that in the case where leaving group solvation was minimal.

The data in Table I also shows that the rate dependence on solvent composition is considerably less for the a-phenethylsulphoniuin salt than for the t-butyl compound. This should be compared with the very similar rate dependence on solvent exhibited by the two corresponding alkyl chlorides ( 5 ) . While the solvent dependence for all three sulphoniulns is in the opposite sense from the alkyl halides, as would be expected from the charge character of the activation process, the smaller dependence in the a-phenethyl case probably reflects a smaller change in polarity of the solute on activation. Whether this is due to a lowering of the polarity of the ionic initial state thus approaching the polarity of the transition state or to a more polar transition state closer to the ionic character of the initial state can only be resolved by studies of the relative physical proper- ties of the t-butyl and a-phenethyl initial states.

The data recorded in Table I permits the calculation of both enthalpies and entropies of activation for the t-butyl and a-phenethyl cases. These parameters are remarkedly insensitive to structural changes. Values of activation energy vary between 32.4 and 33.0 kcal/mole for both salts over the range of solvents used. The entropy values lie in the range of +15 to + l S e.u. in keeping with the charge delocalization on activation resulting in "desolvation" of the transition state cornpared with the ionic initial state.

Synthesis of Szrlphonium Halides The following refers particularly to the a-phenethyl and benzyl sulphoniunl salts; the

t-butyl salt is readily synthesized and isolated from t-butyl iodide and diinethyl sulphide in nitroinethane. The method is a inodified form of that reported by Siege1 and Graefe (8). Dimethyl sulphide and the corresponding alkyl halide were mixed in a 1:l inole ratio. The reaction vessel \?;as stoppered with an alu~ninutn foil covered rubber stopper, and was covered with aluillinuni foil to omit light. The mixture was allowed to stand a t room temperature until the bottonl of the flask was covered with crystals. The renlainillg liquid was deep red in color possibly due to brornine formation. The mixture was mashed with 30 ml anhydrous ether to reinove ally unreacted starting materials. The ether v ~ a s decanted, but enough was left so that the crystals were not exposed to the atmosphere; 5 to 10 1111 absolute ethanol was added to dissolve the crystals. Gentle heating was sonie- times necessary for solutioil to occur. LITarm tap water generally sufficed. The solution was then treated with norite A to decolorize it, after which it was filtered over a celite bed on a sintered glass funnel to reinove the norite and any undissolved residue. The clear filtrate was placed in a 30-11-11 standard taper flask, equipped with a vacuum adapter,

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3142 C4XADIAY JOURNAL O F CHEJIISTKY. VOL. 41, 1963

which mas previously evacuated and weighed. The flask was then filled with dry ether to precipitate the salt and placed in the refrigerator overnight. The liquor was decanted using an aspirator, but enough was left behind to keep the crystals covered. The remaining liquid was removed under vacuum. The white salt was then dried over phosphorous pentoxide under vacuum. a -Pheneth~ ldimethylsulphoniurn bromide-reaction time 96 hours; yield 10.17,; 1n.p. 91.7-92.8" C (lit. 1n.p. 76-80" C ref. ( 8 ) ) ; analysis % C calc. 4S.58Yc, obs. 48.50y0; yo H calc. 6.12%, obs. 6.09yG; yo S calc. 12.97%, obs. 12.94%; Br calc. 32.33To, obs. 32.24%. Benzyldimethj Isulphonium bromide-reaction time 120 hours; yield 22.7y0; m.p. 100.5-101.5° C ; analysis % C calc. 46.36%) obs. 46.69%; % H calc. 5.6270, obs. 5.73%; % S calc. 13.7676., obs. 13.92%; 5% Br czlc. 34.27%, obs. 34.08%.

Copies of infrared potassiurn bromide pellet spectra of the above salts are available on request.

ACKSO\\ LEDGILIESTS

The authors acknowledge the assistance of JIr . R. E. SS7o11kka ill making certain of the rate measurements and the National Research Council of Canada for financial support.

I . I<. A. COOPER, E. D. HCGHES, C. K. INGOLD, and B. J. h I ~ c S ~ ~ ~ ~ . J. Chem. Soc. 2038-42 2. 1. R . HYNE. Can. 1. Chrm. 39. 1207 (1961). . . - . - - - - - . - . . - . a - - - \ - - --, - 3. R. E. ROBERTSON. Can. J. chi;%, 1.536 (19.55). 4. J. B. HYNE and J. 11:. ABRELL. Can. J. Chem. 39, 1657 (1961). 5. S. L~~INSTEIP ; and A. H. FAINBERG. 1. Am. Chem. Soc. 79. 1597. 5937 (19.57). 6. 1. B. HYNE. R. \\-ILLS. and R. E. L V ~ X K K A . T . -Am. Chem. Soc. 84. 2194 (1962). 7. E. McC. A R ~ E T T , P. ILlcC. DUGGLEBY, and J "J. BURICE. J. Am. Chem. Soc. 85, 1350 (1963) 8. S. SIEGEL and A. I;. GRAEFC. J. Am. Chem. Soc. 75, 4519 (1953).

N.M.R. STUDIES OF BRIDGED RING SYSTEMS. I. CARBON-HYDROGEN SPIN COUPLING CONSTANTS IN BICYCL0[2,2,1]HEPTANE DERIVATIVES

K. TORI, R. ~ I V N E Y U K I , AND H. TANIDA

The carbon-hydrogen spin coupling constants, JC~3-H, are considered as useful tools for the investigatio~l of s-character of the carbon atomic orbital participating in the C-H bond (1-4). JIuller and Pritchard ( I ) suggested that decrease of the ring size of cycloalkanes with their C-C-C angles increased JC1sPH with the percentage of s- character of the C-H bond, and, for example, the JC13-H of c>-clopropane exhibited the comparable value to that of the sp2 bond.

I t has been known that bicyclo[2,2,l]heptane derivatives have considerably smaller bond angles on their sp3-tetrahedral and spxtrigonal carbons than the respective ordinal angles due to the contribution of ring strain.* llIusher (7) proposed that the unusual behavior of the bicyclo[2,2,l]heptane ring toward the magnetic shielding and the proton coupling constant might be due to the n-character in the ring.

*Few data relating to the molecular geo?izetry of th is r ing sys tem lzaz~e appeared; how eve^, Sclzovzaker, u s ing cln electron d i f f ~ a c t i o n method, has shown the geoinetry of noibornadie?teto be (5): <C1C2C3 P 109.1°, <C,C,C: 96.b0, < C ~ C I C ~ = 102.P, <CiCiCd = 96.?', C1-Ca = 1 .6B A, Cz-Ca = 1.333 A, Cl-C; = 1.558 A. There are otlzer data based o n the dipole ~lzoi?ze?zt studies of bridged bicyclic lnolecules (6).

Canad ian Journal of Chemis t r y . Trolume 41 (1963)

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