Onishi, Ichiro

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TitleThe effect of pressure on the rate of the benzidinerearrangement VII : substitution effects and related kineticdiscussionsAuthor(s)Onishi, IchiroCitationThe Review of Physical Chemistry of Japan (1972), 41(1/2):52-59Issue Date1972-03-31URL http://hdl.handle.net/2433/46962RightType Departmental Bulletin PaperTextversionpublisherKyoto UniversityThe Review of Physical Chemistry of Japan Vol. 41 No. 1&2(1971)THE RENEW OP PHYSICAL CII Elt35TiY OF JAPAN, VOI. 4I, N0. 1 At Z, 1971THE EFFECT OF PRESSURE ON THE RATE OF  THE BENZIDINE REARRANGEMENT VIISubstitution Effects and Related Kinetic DiscussionsBy Icxiao Ovrsxt   The benzidine r arrangement is a general term for [he acid-catalyzed rearrangement of aromatic hydrazo-compounds (B) to benzidine (P), diphenyline (D) or semidine (S).    From the facts that he only form of benzidine obtained from [he rearrangement of unsymmetrical hydrazo-Compound (ab form) is unsymmetrical ( h form), and [hat the products obtained from the rearrangement of the mixture of two symmetrical hydrazo-compounds (aa form and bb form) are sym-metrical, it is believed that he so-called benzidine r arrangement is an fntramolecular rearrangemend>.     Over a 38-year period ewer since the intramoletularity of the benzidine r arrangement was con-firmed, active controversies for[he reaction mechanisms, especially for the transition states, have been repeated.     For some kinds of hydrazo-compounds, the disproportionation reaction, which has nothing to do with the rearrangement, concurs with the rearrangement. a d kinetically i[ can t>e seen that the rate of [he disproportionation reaction isof first order with respect to the concentration of hydrazo-compound and acid, just as in the case of the rearrangement?131.     In order to obtain some information about he reaction mechanism and the transition state by studying the effect of pressure on the rate of the rearrangement, the reactions of 2, 2'-dimethoxy~>, 2-methoxy-2'-methyl-sl, 1, 2'-dimethyl•sl, 2-chloro-2'-methyl-7> and 2, 2'-dibrmm~hydrazohenzenael were studied under ordinary and high pressures.   The reaction order in acid     Although it is common i all cases that the rate of the rearrangement is of first order with respect to the concentration of hydrazo-compound (B), the reaction order in acid varies with the substituents introduced into the aryl ring. The rate of the rearrangement of unsubstituted hydrazobenzene is ofsecond order with respect to [he concentration f acid. However, it bas been found that, though the ratio of the tan products ofbenzidine (P) to diphenyline (D) is independent of he acid concentration, the yield of benzidine slightly decreased from 7630 to 72%91 by changing the reaction soh•ent from       (h:aceiwd Augusf 20, 1971)        1) C. K. Ingold andH.V. Rid$ J. Clrtnr. Sot., 1933, 9g4       2) D. V. Banthorpe and A. Cooper. ibid., (B), 1968,60i        3) D. \'. Banthorpe, A. Cooper and C. K. Ingold, ibid., (B), t96g. 609       4) J. Osugi. \I. Sasaki and L Onishi, Thit Journal, 40, 39 (1970)       5) J. Osugi and I. Onishi, Pippon liagaku Zarshi (l. Chem.Sot.Japan, Pure Chern. Sect.), 92, 702 (19i 1)       b) J. Osugi, DL Sasaki and I. Onishi, This Jonrnai, 36, l00 (1966)        7) J. Osugi and I. Onishi, Nippon i gaku Zauhi (J. Chem.Sot.Japan, Pure Chem. SetfJ. 92, l31 (1971)        g) J. Osugi, :tf. Sasaki and I, Onishi, TMs Jwvnal, 39, s7 (t969)The Review of Physical Chemistry of Japan Vol. 41 No. 1&2(1971)              The Effect of Pressure on the Rate of.the Beazidiae Rearrangement VII 53 60% aqueous dioxane [0 100% ethanol. This fact indicates that the productforming step is influenced by the dielectric constant of the medium.    Since Carlin and Odioso discovered in 1954 the unintelligible fad that the rate of the rearrange-ment o[ 2, 2'-dimethylhydrazobenzene is L6 order with respect o the acid concentrationtol, attention has been paid to the problem of the benzidine rearrangement and various interpretations to tbis fact have been offered by Carlin and Odioso, Hinshelwoodttl and Dewadz>.    Thereafter, in the re-examination made by Iagold in 1962ta> or by the present author in 196661, and also in the present author's recent studies for the rearrangement of 2, 2'-dibromo-al nd 2-chloro-2'-methylhydrazobenzene7l, it was observed that the reaction order in acid increased continuously with the increase of the acid concentration, and further, a good linearity was obtained by plotting the ratio of the apparent rate constant o the acid concentration against he concentration of [he acid. Therefore, it was confirmed from these facts that two different reactions concurred in the rearrangement; one was of first order with respect o the acid concentration (one-proton mechanism) and the other was of second order (two-proton mechanism).    Figs. 1 and 2 illustrate the dependency of the apparent rate constants k.pp of the rearrangement of 2-chloro-2'-methylhydrazobenzene upon the concentration of the acid._,0 m 0Pig._~                 _, e         log /HCI] l The dependency of the apparent   rate constants of the rearrange   ment of 2-chloro-2'-methylhydra~   zobenzene upon the acid concent•   ration at 20'C, under ordinary   pressure in 85 vo1~ aqueous   ethanolic solutionISIOSaE O e lJ X x'Fig. 2/d/ d,d ie /i   0 0.1 O.i 0.6 0.8        iBCli, molnf'~ The dependency of the apparent rate constants of the rearrange-ment of 2-chloro•2r•methylhydra-zobenzene upon the acid concenb ration in 83vol%aqueous ethano. ]ic solution Temperature: 30'C, 23'C, 20'C,  13'C, from top to bottom Pressure: - 1 kg/cmr,           •••~•~2,000 kg/cma9) l0) 11) Il~ l3)ll. V. Banthorpe, E. D. Hughes, C. R. Ingald and j. Roy, J. Cheer. Snc., 1962, 3294 R. B. Cadin and R. C. Odioso, J. Arn. CLero. Soc.. 75, t00 (1933) U. V. Blackadder and S. C. Rinshelwood, J. Cherx. Soc.. 1957. 2898 Af. J. S. Dewaq "Theoretical Organic Chemistry, $ekule Symposium", p. I93. Bulterworth, London (1959) D, V. Banthorpc, C. K. Ingold and J, Roy,J. Clrem. Sa., 1962, 2436The Review of Physical Chemistry of Japan Vol. 41 No. 1 & 2 (1971)54 I, Onishi                         • H' H• k'                        B BH' BHs'--*                              P P                  -d[B]Idt=kane[B7=CkdH']+ks[H']'}[B]                          kr=k,*Kr, kz=ks*R,Kz or k1=k'K„ k'/k'_f=K,.     In the above scheme and equations. k, refers to one-proton mechanism and k, to two-proton mechanism. Table I illustrates the reaction order in acid for the rearrangement a d [he rate constants kr (1•mole '•min ') and ks (ls•mole'=•min'') at 25-C, under the ordinary pressure in85 vol% aqueous ethanolic solution-Table 1 The reaction orders in acid and the rate constants of the rearrangementSuhstituents Reaction order     kt (!•mole ~•min tl     kZ (d2•mole'2•min-q2 OCH3, 2 OCHa, 2 CHs, 2 C1, 2 Bq 2 I,2' OCH3 2' CA3 2' CHa 2' CHa 2' Br 2' I1 1 1~2 1~.2 1~2  I *)270 72  3.i1  0.0235  O.OOOIfi70 0.127 0.00023a) Cited from Banthorpe and O'Sullivan's data in 60% aqueous dioxanelo, The transition states and the volumes of actuation     The reactions of one-proton mechanismoccur in two opposite cases: one is when the electroo donative substituent (methosy or methyl) is introduced into the aryl ring, and the other is when the electron attractive substituent of halogen (bromine or iodine) is introduced. This means [hat the reactions of one-proton mechanism are strongly induenced by the electrostatic haracter of [he sub-stituents. As shown in Table 1, the stronger the electron donating effect, the greater the rate of the rearrangement, and the stronger the electron-attracting effect, the slower the reaction.    Affording to the polaz transition state theory by Ingo1d151, in the case ofone-proton mechanism, after the first protonation on [he a-ring, the stronger the electron donation of the substituent of the h-ring and the stronger the electron affinity of NH:* of the a-ring, the easier the electron displacement from NH to NHr*. It is considered that the heterolytic and ionic fission is facilitated without any help of the second protonation.     After the 4eterolytic fission, each ring is stabilized by resonances and both rings come near to each other in consequence of the localized positive and negative charges appearing on the rings, and      14) D. V. Banthorpe and M. O'Sullivaq !. Chem. Soc., (B) 1968, 62i      IS) D. V. $aothorpe, E. D. Hughes and C. R. Ingold, ibid., 1964,2864The Review of Physical Chemistry of Japan Vol. 41 No. 1 & 2 (1971)The Effcct of Pressure on the Rate of [he Benzidine Rearrangement VII SS         H, ft o ~               A"~NH: - NH       R. b -yb• NH;Ra-a+ •4H      8. na+a-> rthe rearrangement reaction comes to an end to produce benzidine and diphenyline.    As shown in Table 2, in which the volumes of activation of one-proton a d two-proton mechanisms obtained by the author are summarized, the negative values of the volumes of activation for one-proton mechanisms are obtained. As the volume changes ofpre-equilibrium of the pro[onation are to be about -5 cm'(moleto>, these transition states are presumed tobe so strongly solvated and polar as to sufficiently overcome the volume increase r sulting from the NH_ : A'H bond fission alter proto-nation. Therefore, it is concluded that Inogld's `'polar transition state" must be supported.     For the two-proton mechanisms, a  the second pro[onation breaks the asymmetry of NHS :NH, the possibility of the homalytic fission is caused. Since the volume Changes ofpre-equilibrium of Che addition of two protons are presumed tobe about -IOcm'(moletsl, it may be considered that the transition states of two-proton mechanisms are less olvated and less polar than postulated byIngold.Tahlc 2 The volumes of activation of [he rearrangement in 85 vol$o e[hanolic solution a[ 15'C under ordinary pressureSubstituents JP*tcros/mole tone-proton) dP*zcros/male (lwo-proton)Z OCHs, 2 OCHs, 2 CHa, 2 CI, 2 Br,2' OCHs 2r CHs 1' CHs 2' Ctfs 2' Br-12  -8 .5  - 2.5  -6 .8 -I0.7- 7.2 - 3.2 -0 -4 The product forming step and the intermediate    As for the elimination of the ring hydrogen atoms, since no hydrogen isotopic effect was observed for the rearrangement of hydrazo-compounds, it is obvious that the hydrogen atoms of the ring still keep the stable bonds in its transition statet7>. That is, [he hydrogen atoms of the rings have to be eliminated in the step after passing the transition state and this fact is consistent with the result of      l6) E, Nhalley, Traes, Faraday Soc. 55, T98 (t9i9)      17) D. V. Banthorpe. E. D. Hughes and C. 1;.Ingold, J. Chern. Soc.. 1962, 3199 ; 1962, 3308The Review of Physical Chemistry of Japan Vol. 41 No. 1&2(1971)i I56 I, Onishi [he solvent effect on the rearrangement as described above91.     As previously reported in the rearrangement of 2, Y-dimetboxyhydrazobenzene+I, the dispro-portionation reaction which produced 2,2'-dimethoxyazobenzene and 2-methoryaoiline occurred con-currently. At that time, two experimental ( cts were observed; one is the fact that the rate of the dis-proportionation reaction isof first order with respect [othe concentration of hydrazo-compound (B)and acid, respectively asin the case of the rearrangement, a d the other is the fact hat the ratio of the rate of benzidine formation (d~P]/dt) tothat of aniline formation (d[An]/dt) were keptconstant. Therefore, from the viewpoint of the stoichiometry, in the disproportionation reaction, it is considered as follows: the former indicates the fact that the second hydrazo-molecule must participate in [he fast step after passing the rate-determining step, and the latter suggests hat the disproportionation reaction and the rearrangement reaction pass through the separate rate-determining steps each other, and one intermediate has to exist after [he rate-determining step in the disproportionation reaction.     Various controversies on the benzidine r arrangement are entatively separated in ta•o pinions; one is Ingold's polar transition state theory in which the reaction proceeds in one step without any intermediate, where [he product's skeleton isalmost wnstructed inthe transition state, and the other is Dewar's a-complex theory which includes the a-complex intermediate capable of the free rotation ( the both rings, although is early r.-complex theory was kinetically inconsistent with the experimental resultr and was thereafter corrected many times.    Recently it has been discovered thatp-semidine is formed in the rcarrangement of 4-methoxy-hydrazobenzene, even if no heavy metalic ion exists'-ia>. If the reaction mechanism is to be considered including the p•semidine formation, the polar transition state theory which requires the fission of N: N and simultaneous attraction through the long distance between itrogen and 4'-carbon, is not probable. And further, from the standpoint o[intramolecularity, i  seems probable that a kind oC intermediate in which both rings are held together psrallel by the weak >c-electronic force may be existent, because the hydrogen atoms of the benzene rings are kept rather fixed in the transition state. Therefore, the rearrangement reaction may pass through akind cf r,-complex intermediate after passing the polar transition state. though nn accurate evidences for this intermediate cannot be found.B f ~ Btl'slowslow(Imermedute) -~ RearrangementfastBInlermedule _~ Disproporrionatianfast   For instance, [he solvent effect on the rearrangement of hydrazobenzene described aboce91-[hat the yield of benzidine decreased by changing the reaction solvent from 60% aqueous dioxane to t00% ethanol-may be explained by the assumption [hat the repulsion between NH.* and NHz* o[ each ring of a kind of rr-complex intermediate favors the free rotation of both the rings, by decreasing [heThe Review of Physical Chemistry of Japan Vol. 41 No. 1&2(1971)              The Effect of Pressure on the Rate of the Benzidine Rearrangement VIT 57 dielectric constant of the mediums>. And the pressure ffect on the rearrangement ofhydrazobenzene -that the yield of benzidine is increazed with increasing pressure-can be in accord with the interpretation that [he pressure hinders the free rotation oI eac6 ring in rz•complex intermediate~sl.   The linear free energy relationship     The reactions of the or[ho-substituted compoucds depart from Hammett's rule. As reported pre-viouslys3~>, by taking account of the steric effect of the substituents, a good linearity of slope of L00 was obtained between the logarithms of the rate constants of the rearrangement of one-proton mechanism and the values of pK, of the corresponding ortho-substituted unilinium ions.     The rate constants of one-proton mechanism k, (!•mole '•min-') and of two-proton mechanism ka (1'. mole-a. min'') are indicated as follows,                                  k,=k,+`K,, h.:=kx*K,Kz. (1 )     Assuming that the equilibrium constants of the protonation on the hydrazo-compounds.K, and Ks, are proportional to the equilibrium constants of the protonation on the corresponding orthe-sub-stituted free aniline, K, and Ka, equation (I) becomes s follows,                                 k'=k,"PK,, k.=kx"PrK,Kx, (Z ) where p and p' are propotional constants. In one-proton mechanism, assuming that the heterolytic fission occurs after the first protonation on the a-ring, the rate constant of the rate-determining step, k,', is proportional [o the ease of the electron displacement from un-pmtooated b-ring to a-ring, that is, proportional to the electron density of NII of b-ring, or to the bazicity of h-ring, accordingly to the equilibrium constant Kx of the protonation on the corresponding ortho•subs[ituted iree aniline. There-fore, equatimr (2) becomes as follows,                                     k,=paX,Kx, (3 ) where k,•=aKa, and a is s proportional constant.    As log K, or IogK is equal to pK„ or pKoa. equations (2) and (3) are rewritten as follows,                        log k,=log (pa)+pK„+pk.a. (4 )                         logk,=lag(p'ka')+pK„+pK:.    The electron displacement results from the clectro-neutral NH and not from NHx' of the protonated b-ring. The electron displacement or the electron density of the electro-neutral NH is not proportional to the equilibrium constant Xa itself which collectively comprises the e(i ects of polazity, of resonance and of steric hindrance. Therefore, ii [be electron density is assumed [o be correlated with the value of {pK.,a+yF_.) which is obtained by deducting from the value of pK,a the steric effect thaChinders the protonation, the rate constant of one-proton mechanism is indicated ss follows.                        logkr=log lPa)+pR ,+pKez+TE„ (6 )     18) L. C. Sayder, !. Arn. CLem. Soc., S4, 340 (1962)      19) 7. Osugi and T. Hitouji, This Journal,34.88 (1964)i The Review of Physical Chemistry of Japan Vol. 41 No. 1&2(1971)SS I. Onishi where E, is Taft's steric fattor~J and r was determined, using the rate constants k, for 2, 2'-dimethoxy-and 2, 2'-dibromohydrazobenzenes. [0 6e 1.51.    Fig. 3 illustrates the plotting of the logarithms of the rate constants of one-proton and of two-proton mechanisms, log k, and logke, against the value of (pK„+pX,z+Yr-•) or (pR„+pX.r). As a result, in the case of one-proton mechanism, a good linearity of slope of 1.00 is obtained, and the co-efficient of correlation is 0.999.sy m`o ~-ti 00 0-5         3 ~        P ~ I      4 ~/ 3 5 i 45 5 6 7 8 9 10 11 12 13 pA~rFDKa+YEsor hK„+p Kmr•;g. a The linear free energy relation-ship under ordinary pressure at 25'C in Sivol?6 aqueous ethanolic SOi UIIOn - one-proton, --- two-proton I : 2, 2'-dimethoxy-, 2: 2-me[hoxy-2'-methyl-, 3: 2, 2'-dimethyl-, 4: 2-chloro-2r-m el6 yl-,5: 2, 2'-dibromohydrazobenzene   From the facts described above, Ingold's polar transition state theory that the heterolytic fission is facilitated by the electron displacement from the b-ring after the protonation on the a-ring, is supported.     For the two-proton mechanism, the mean slope of 1.22 is obtained, although this value is not reliable because of the scantiness of the data, and it slightly increases with the increase of pK,. There-fore, it is presumed that the N:N fission embraces a slightly heterolytit character, and the polarity of the transition state increases with the increase of the electron donative effect of the substituents. And these characters are consistent with the results obtained for the volumes of activation of the two-proton mechanisms.                                      Summary    Through the studies of the effect of pressure on the rate of the benzidine rearrangement, i[ was confirmed that the polar transition state will be supported, although it is somewhat less polar in the case of the two-proton mechanism. The remaining work for the overall description of the reaction mechanism of the benzidine rearrangement is to ascertain the existence and the character of an inter-     2D) J. E. Lefller and E. Grunwald, "Rates and Equilihria of Organic Reaction ", p. 268, John Wiley          and Sons, Inc., New S'ork and London (1963)                                       The Review of Physical Chemistry ofJapan Vol. 41               The Effect of Pressure on [he Rate of the Benzidiae Rearrangement VII mediate.                                 Acknowledgment    The author expresses his hearty thanks to Prof. Jiro Osugi for his kind guidance and throughout his work. The author also expresses his gratitude to Dr. Muneo Sasaki for his discussion and suggestion.                                               LaborWory of I'hysica( Chemistry                                               DeQartrnen! of Chemisby                                           Faculty of Science                                               Kyoto University                                              Kyoto,JapanNo. 1 & 2 (1971)    59 advice helpful

The effect of pressure on the rate of the benzidine rearrangement VII : substitution effects and related kinetic discussions

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The effect of pressure on the rate of the benzidine rearrangement VII : substitution effects and related kinetic discussions

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