July 5 , 1959
TRANSMISSION OF ELECTRONIC EFFECTS IN -N=KC6H4-
tive to substituent effects (i.e., that the reaction constant p~ = 0), and t h a t the scatter of p-compounds is due to a failure of Sawicki's approximation. The p-arylazo-N,N-dimethylanilines are being investigated further. The value p~ = 0 derived in the preceding paragraph is of considerable interest. We had a t one time argued that the structure of the conjugate acid of p-dimethylaminoazobenzene must be the ammonium form I11 because of the small p-value for the second b a ~ i c i t y . ~ bIt now appears on the basis of two lines of evidence that our argument was fallacious. We believe that we have now shown conclusively t h a t a tautomeric equilibrium between the two possible foIms exists, and if, as proposed, p~ = 0, i t follows that no appreciable difference can exist in susceptibility of the equilibria I H + $ I1 and I H+ I11 to substituent effects. Alternately, we can estimate the p-value for the second of these processes from the correlation between the p-values of the basicities of series of com-
+
+
pounds with constant Y and the U-values of the apropriate Y.lra Such an estimate leads to PIII = 1.0, not quite twice the magnitude expected for the p-value of the corresponding ammonium ion d i s s o c i a t i ~ n . ~Hence ~ ~ ' ~ again PT = PIII - PI is quite small. It then only remains to ask ourselves why the p-value for protonation of the azo group is so low in these cases. If one remembers, however, t h a t the Hammett equation is really concerned with electronic effects, i t becomes apparent t h a t the factor affecting p most strongly is not the site of attachment of the proton, but the energy required to localize the positive charge. Since i t seems very likely t h a t the quinoid structure IIIb makes a very important, if not the predominant contribution to 111, ~ I I then I measures not so much the effect (10) S:J.
Yeh and H. H. Jaff6, THISJ O U R N A L , 81, 3287 (1959).
[CONTRIBUTION FROM
THE
;Mi.
of substituents on the ease of addition of a proton a t the nitrogen atom alpha to the substituent, but on the ease of placing a positive charge on the amino group. T h a t this quantity is of a similar magnitude as p11 for the protonation of the amino group is not surprising since in both processes the charge is placed on the same atom. These arguments seem to provide an alternate explanation of the variability of p values with Y in the series of compounds VI. I n a preceding paper we have proposed that this magnitude of p is dependent on the asymmetry of the proton position in the parent compound of each series. It now seems very likely that the value is p is more importantly dependent on the extent to which the positive charge is displaced from the azo group to the end of the ring or to Y by the intervention of quinoid structures.
Experimental p-Dimethy1aminoazobenzene.-A
product of the HiltonDavis Chemical C0.I' was recrystallized twice from 95yo ethanol; m.p. 119-120" (uncor.), lit.3a m.p. 119-120'. p-Phenylazo-N,N,N-trimethylaniliniumMethylsulfate.Ten grams of p-dimethylaminoazobenzene, recrystallized from ethanol, was placed in a 500-ml. round-bottomed flask fitted with a reflux condenser. After 100 ml. of dimethyl sulfate was added to the flask, the mixture was refluxed in a water-bath for 5 hours. On standing, the red p-phenylazo-N,N,N-trimethylanilinium methylsulfate crystallized out. T h e crystals were washed with anhydrous ether until the filtrate was clear, and dried under vacuum. Anal.12 Calcd.: C, 54.66; H, 6.03; N, 11.97; S, 9.13. Found: C, 54.41; H, 6.26; N, 11.74; S, 9.03. pK Determinations.-All pK's were determined by a standard spectrophotometric method using a Beckman DU quartz spectr~photometer.'~The general procedure for the determination of concentration ratios, CB/CBH+ and CBH+/CBH~++ and the calculation of pK's were described elsewhere.a The pH of the solutions was measured by a Beckman model G pH meter calibrated with standard buffer solutions. (11) We are grateful t o the Hilton-Davis Chemical Co. for a generous gift of this compound. (12) Microanalyses were performed b y A. Bernhardt, Miihlheim (Ruhr), Germany. (13) L. A. Flexser, L. P. Hammett and A. Dingwall, THISJOURNAL, 67, 2103 (1935).
CINCINNATI, OHIO
DEPARTMENT O F CHEMISTRY, UNIVERSITY
OF
CINCINKATI]
Transmission of Electronic Effects through the -N=NC6H4-
Group1
BY SI-JUNG YEHAND H. H. JAFFB RECEIVEDDECEMBER 20, 1958 The acid dissociation constants in 20% ethanol of a series of p-arylazophenols are reported, and shown to obey the Hammett equation with p = 0.516. Comparison of this value with a n interpolated value for phenols gives a p-ratio of 0.16, in fair agreement with a theoretical estimate of 0.14, and an experimental value of 0.13 from the p-arylazo-N,N-dirnetliylmilines.2
Some years ago we have proposed that the reaction constants ( P ) ~ for identical reactions occurring (1) This work has been jointly supported by the Office of Ordnance Research, U. S. Army, and by the American Cance; Society through a Bonita Geho Memorial Grant for Cancer Research. This support is gratefully acknowledged. ( 2 ) H . H. JaffB, J . Chem. P h y s . , 23, 415 (1953). Organic Chemistry," (3) (a) L. P. Hammett, "Physical Hill Book Co., Inc., New York, N. Y.,1940, Chapter V I I ; (b) H . H. JaffC, Chem. R E V S .SS, , 191 (1953).
a t a reaction center Y under identical conditions in two series of compounds I and I1 can be related by the equation PIIIPI
=
D X - Y
Q-l
R
R
I
in which the constant
(1)
7r'
T'
I1
is independent of the group
11.\'. GEORGEAND HENRY CILMilN
XhS
W,of the reaction a n d the conditions under which it occurs, as long as these are the same for p1 and ~ I I . ? I n connection with work on the basicities of substituted azo compounds and on an IIo function in 20So ethanol,4 we have had occasion to prepare a series of p-arylazophenols. K'e have now determined the acid dissociation constants of the phenolic function of these compounds in an attempt to gain information concerning the transmission of electronic effects through the group X = --N=KC&-. Such information was of special interest for two reasons; it provided an additional test of the proposed method of calculating the constant (K') of eq. 1 by molecular orbital theory, and it provided an interesting comparison with the data available from the consideration of the P-arylazoN,S-dimethylanilines.2 3 4 d Experimental Compounds.-The preparation and physical properties of the p-arylazophenols have been reported in a previous ~aper.4~ pK Determinations.-The pK's of the P-arylazophenols mere determined in about AI solution in 2OY0 ethanol by a standard spectrophotometric method.6 Standard phosphate and borate buffers made in 20% ethanol were used,e and no ionic strength corrections were made. pK's were determined with a Beckman model G $H meter, and spectroscopic measurements were made with a Beckman DU quartz spectrophotometer.
Results and Discussion T h e pK's of the p-arylazophenols in 20y0 EtOH are listed in Table I. They are well correlated with the g - ~ a l u e of s ~the ~ substituents in the aryl group with a p = 0.515 f 0.019, s = 0.011,r = 0.998, n = 5 log ko = 8.39.'3.7 (1) (a) H . H. Jaffe and R. W. Gardner, THISJ O U R N A L . 80, 319 !1953); (b) S -J. \'eh and H . H . Jaff6, ibid.,EO, 3271 fc) 3279, ( d ) 3283 I1'3.39! (.i) L . A . I'lexser, I,. P. Hammett and A Dingwall, i b i d . , E l , 37, 2103 (1935) f(i) \V,R l . Clark, "The Determination of Hydrogen Ions." Williams and TLWkins, Baltimore, 11d , 3rd. ed., 1928, p . 200. ' 7 ) AI! symbols have their i i w a l significance; 4.ref. 2b.
[COXTRIBUTION FROX THE CHEYrCAL
IV
1701.
20rh
X
ETHA\OI AT 25" PK2
L7
4-OCHa 4-CH3 H 4-Br 3-Br
SI
-0 268
8 54 f 0 02 8 48 f 04 8 38 f 04
-0 170 0 0 232 0 391
8 27zt 8 20 +
02 02
No data appear to be available for the acid dissociation of simple phenols in the same solvent. Interpolation between reported values yields p~ = 3.10, p11!p1 = 0.16.3b This value is in satisfactory agreement with the molecular orbital value x' = 0.142 for the same group X, and with the value obtained from the basicities of the p-aryl-N,N-dixnethylanilines p11,!p1 = 0.13.2J This latter value, however, requires further comment; it was derived from the p-value for the first protonation of these bases on the assumption that this reaction involves only the amino group. 1Ye have, however, now demonstrated that the conjugate acid of the bases under discussion are tautomeric mixtures of animonium and azonium forms,4dand hence cannot expect a straightforward application of the Hainmett equation to these equilibria. Only since substituents do not appear to affect the position of tautomeric equilibrium (it.>p~ = 0) is our use of the p from the basicities of the p-aryl-N,N-dimethylanilines for a determination of p 1 1 i p 1 for X = -K=NC6H4justified, and our coniparison of this value with the value from the p-arylazmphenols and the theoretical value meaningful. (8)
Cf.XI. T.Rogers, T .
JOURNAL,
TX'. Campbell and R . W. h f a a t m a n , Tiirs
73, ,5122 (1951)
CIXCINXATI, O H I O
LABORATORY OF IOIVA
STATE COT.1,EC.E
1
Oxygen Oxidation of Triphenylsilyllithium BY 11.V. GEORGEA N D HENRYGIIAIAK RECEIVED DECEMBER 27, 1958 Oxidation of tripheri~lsilyllithiumusing nloleculnr oxygen has been studied a t different temperatures, in tetrahydrofuran a s the solvent TriphenGlsilanol, tripheiiylsilane and small amounts of hexaphenyldisilane were formed in each case, though the yields of these products varied depending on the reaction temperature. A change of the solvent medium to tetrahydropyran did not affect the nature of the reaction products. The oxidation is assumed to proceed through a n unstable hydroperoxide salt, followed by the radical decomposition of this intermediate.
Since the early investigations of Bodroux' and Bouveault,* numerous investigators have studied the reaction of Grignard reagent with molecular oxygen.3 lTuyts4 postulated that the oxidation re( 1 ) F. Bodroux, Compl r e n d . , 136, 158 (1903); Biill. T O G . ( h i m , [71 31, 3H (1904). ( 2 ) I,. Bouveault, ibid., 131 29, 103 (1903). C1) For a n extensive review on t h e oxidation of Grignard reagents \.ee, .\I. S Kharasch and 0 . Reinmuth, "Grignard Reaction? of Nonmetallic SuI).;iancei," I'rentice-Hall, I n c . , h-ew York, h-. Y , 1 9 3 1 , pp.
actions proceed through the formation of peroxide intermediates, and later Porter and Steele5 suggested the reaction scheme RMgX
+ Or +ROOMgX
RMgX
2ROMg.X
p f
Experimental evidence for this sequence was provided by b7a1ling and Ruckler,'j who in an elegant study, have isolated several intermedi:ite
I"iil-l?7. ( I ) H . Wiiyt.;, Il!i'l. ( o r , chii!!. 149, n:to (!DO!l)
I?,/,< , 36, 2 2 2 ( l < i 2 7 ) ,
~ ~ o i i i p~l ' .i i i ~ l . ,
)(;
C . W. Porter and C. Steele, THISJ O I J R N A I . , 42, 2030 C \Vidlinl: n n d S A Iliicklrr, i b i L 77, 00:32 (19.55).
(1!1~~1)
