COMMTJNICATIOKS TO THE EDITOR
Aug. 5, 1961
3349
such pronounced electrophilic reactivities as the rates of solvolysis of t-cumyl chlorides, rates of protonolysis of phenyltrimethylsilanes, and rates of bromination of mesitylenes and durenes, as well as such strongly nucleophilic reactivities as the ionization of phenols and thiophenols (for listings of reactions cf. Table IT, ref. 2a).8 In contrast, in three radical reaction series (for which data are available) the m-OCHa substituent shows enhanced reactivity over that predicted (uop) for the Ar-Y inductive effect (benzoyl perCIS oxide reaction series F-1, F-3, and F-4 of Table XI, ref. 2a), the enhancement (Ar-Y resonance effect) amounting in each case to 0.3 kcaZ./mole. While this figure is not impressive in magnitude, it comes from reactions with relatively small p values and conI # ,' -1.m sequently is clearly discernible in the B value.4 In the radical reactivities listed in Table I1 of reference 2a, small apparent Ar-Y resonance enhancements in rate are also noted for m-alkyl The intermediate might be considered as a very and phenyl substituents. A much more impressive enhancement (Ar-Y tight ion pair. It seems highly unlikely that the resonance effect) is obtained for the m-OCH3 reaction proceeds by simple nucleophilic attack substituent in the formation of the radical ion by on the double bond without some prior ionization, especially since the attack cannot be intramolecular. polarographic oxidation of m- and p-substituted Support for this position is gained by the prelimi- anilines.6 Here the effect on oxidation potential nary observation that the addition of water in- over that estimated by aop amounts to a free energy creases the rate of reduction but does not change decrease of 1.1 kcal./mole. It is further to be the product ratio. This observation is consistent noted that an extensive investigation of m-substiwith that of Eliel'O who found that yields were tuents has been made and substantial Ar-Y resomaterially improved in the lithium aluminum hy- nance effects are noted for other -R meta substidride reduction of a chlorohydrin, involving a tuents (e.g., -NH2, -NHCOCH3 and -CH3). tertiary chloride, by the addition of water or The resonance enhanced effects reach a maximum alcohols. This was attributed to the greater free energy decrease of 3.0 kcal./mole for the macidity of the oxygen substituted aluminum which NH2 substituent. The enhancement of the resonance effect of was presumed to coordinate with the chlorine. The chemistry of the tricyclic olefin (111) and meta substituents may be accounted for in terms the mechanism and scope of the reaction are of acceptable quinoid isovalent structures,h e.g. presently under further investigation.
coordinates with the chlorine thereby providing impetus to ionization which is strongly assisted by the proper double bond to give a non-classical intermediate. Probably, ionization does not proceed to a great extent before reduction occurs.
&p1
u-
(IO) E.L. Eliel and T. J. Prosser. J. Am. Chem. Soc., 18, 4045 (1956).
BELLTELEPHONE LABORATORIES, Ixc. PAULR. STORY MURRAY HILL,NEWJERSEY RECEIVED JUNE 29, 1961
(3 forms)
( 3 forms\
It is well known that the analog interaction structures for -Y:- or -Yf (systems with an even REGARDING ENHANCED RESONANCE EFFECTS FROM META POSITION IN RADICAL REACTIVITIES'
Sir: A valuable tool for the characterization of the transition state of a radical reaction is provided by a measure of enhanced resonance effects of meta-substituents. The Ar-Y inductive effects of a meta methoxy substituent in reactivities of benzene derivatives (in the general formula for such compounds, X-ArY, X refers to the substituent and Y to the reaction center) are precisely described (to a standard error of f 0.03 sigma unit) by the modified Hammett equation2: log (k/ko) = aop. Included are (1) This work was supported in part by the O 5 c e of Naval Research. (2) (a) R. W. Taft, Jr., and I. C. Lewis, J. Am. Chcm. Soc., 81. 6343 (1959); (b) R. W.Taft. Jr., S. Ehrenson, I. C. Lewis, and R. E. Glick. ibid., 81, 6352 (1959); (c) R. W.Taft, Jr., J . Pkys. Chcm., 14, 1805
(1960).
number of ?r electrons) are not acceptable, in accord with the applicability (noted above) of the modified Hammett equation to such systems. The (3) Not all -R meld substituents are as well behaved as nr-OCH,. Effective 8 values for m-SCH:, NHs and N(CH.)r are appreciably affected by the polarization from a change in formal charge a t the first atom of the side-chain reaction center. I n reactivities in which the latter factor is not involved, however, these substituents appear generally t o follow the d'scale ( c j . references 58 and b). (4) This type of rate enhancement for para -R substituents has been well recognized and documented in terms of correlations employing u + values: cf. P. D. Bartlett and C. Riichardt, J. Am. Chcm. Soc., 83, 1756 (1960). However, Bartlett and RClchardt have given an additional reason (besides t h a t considered here for mela substituents) for such rate enhancements with pard substituents. See also, G. A. Russell, J. Org. Chon., 33, 1407 (1958); E. S. Hayser, J. Am. Chem. S O C . , 83,304 (1960). (5) I. R. Fox, R. W. T a f t , Jr., and J. M. Schempf, Abst. of Papers, Am. Chem. Sac. Meeting, Atlantic City, N. J., Sept., 1959. (5a) NOTEADDED IN PROOF.--W. N. White, C.D. Slater, and W K. Fife, J. Org. Chcm., 36, 627 (1961) have similarly noted t h a t enhanced resonance effects of m-substituents in the rate of the Claisen rearrangement can be accounted for by the contribution of such structures t o the reaction transition state.
3350
COMXUSIC ATIOXS
fact that forms such as I1 are expected to be more important than 1 6 s 7 accounts for the relative magnitudes of the effects noted above. The above oxidation reaction series indicates that enhanced resonance effects of meta -R substituents will becmne
increasingly important the greater the radical cation Character o f the transition state.
TO THE
EDITOR
7'01. 8.7
EVIDENCE FOR PHENYL CATION WITH AN ODD NUMBER OF r-ELECTRONS FROM THE AQUEOUS THERMAL DECOMPOSITION O F THE DIAZONIUM ION'
Sir:
The recent suggestion2 that substantial enhancements in rate due to -R (conjugatively donor) substituents in the mefa position of benzene derivatives may be used to characterize the degree of radical cation character of a reaction transition state finds interesting application to the rates of aqueous thermal decomposition of substituted phenyldiazonium salts. Except for acknowledged radical reactivities, the diazonium decomposition reaction apparently provides the only existing bona $de example of a reaction (heretofore regarded as a direct ionic heterolytic cleavage) for which rate enhancements of the above kind are observed.2a For the phenyldiazonium ion substituted as indicated in the metaThe pleasing agreement between theory and the position, these decreases in free energy of activaresults from reactivity analysis may be taken as tion are estimated from the quantity -2.303RT. eridence for the validity of the procedures employed [log ( k , ! k ~ )- u o p ] : OCH,, 1.7kcal.,!mole; OH, in the analysis (which were deduced quite ivde- 1.3 kcal.,,mole; CsHa, 0.8 kcal./mole; CHa, 0.5 pendent of present considerations). The impor- kcal./mole; C1, 0.2 kcal./:nole. These figures tance of the reactivity interpretation procedure strongly imply a high degree of radical-cation recommended in reference 2a is emphasized, since character for the transition state of the deccmipoall of the effects reported here can be masked to sition reaction. varying degrees of approximation by forced fits The alternate interpretation that the above rate to the unmodified Hammett equation. It is ap- enhancement figures result from a favorable parent that reaction series which involve Ar-17 polarizing influence4 of the positively charged resonance effects of meta substituents present carbon generated by loss of nitrogen from the special difficulty in the determination of the re-- diazonium ion appears untenable in the present action constant p . The results reported here are case in view of the fact that the modified Hammett based upon the JaffC and Roberts procedure': equation, log ( k j k o ) = sop, is followed to high for determining p ( = P , ) , ? ~ In addition to this precision (to a standard error of + 0.03 sigma unit) generally recommended procedure, equivalent re- by the mcta-methoxy substituent in such "model" sults are expected from the judicious limitation of non-radical ion processes3 as : the ionization conthe modified Hammett equation to select substit- stants of aqueous pyridiniuni ions. the rates of uents. 2 , bromination 01 substituted durenes and mesitylenes the rates of protonolysis of phenyltrimethylsilanes, COLLEGE OF CHEMISTRY ASD PHYSICS l:$). C . K. Ingold, "Structure and 3Ipchanism in Organic Chemistry." Cornell University Press, Ithaca, s Y . , 1953, p. 799-801; D . F. DeTar and T. Kosuge. .I A m CJ2i.m So'- , 80, 1hO7? (19%) and earlier references cited therein. (6) E. S. Lewis, ibid.. 80 1371 (1958).
