AromaticC-nitroso compounds. Thermodynamics and kinetics of the equilibrium between 2,6-dimethylnitrosobenzene and itstrans-dimer

摘要

J.C.S. Perkin I1 Aromatic C-Mitroso Compounds. Thermodynamics and Kinetics of the EquiIibr iu m between 2,6-Dimet hyln itrosobenzene and its trans-Dimer By Michel Azoulay," Richard Lippman, and Gunnar Wettermark, Department of Physical Chemistry, The Royal Institute of Technology S-1 00 44,Stockholm 70, Sweden Using i.r., n.m.r. and visible spectrophotometry, the system 2,6-dimethylnitrosobenzene (D MNB) 2,2',6,6'-tetramethylazodioxybenzene (TMAB) was investigated. In the solid state, the stable species is trans-TMAB. In acetonitrile solution, the following thermodynamic and activation parameters were derived for the reaction trans-TMAB q2z 2 DMNB AH" -54.0 kJ mol-l, ASo -1 29 J K-l mol-I, AHX84.5 kJ mol-l, and AS1 -6 J K-mol-l. There was no evidence for the existence of cis-TMAB in solution, THE dimerization-dissociation reaction in nitroso-taining these species can yield the pure cis-isomer in the azodioxy systems t has been extensively discussed in the form of single crystals, but conversion of the solid cis-literature.l- However, thermodynamic and kinetic parameters related to this reaction have been deter-mined only in a few cases.2.10,11,18,21-24 This is apparently the result of the unfavourable position of the equilibrium prevailing for a great nurnbcr of nitroso-azodioxy systems.For instance, in the aliphatic class the equilibrium lies far in favour of the dimeric species with the trans-configuration.21 For aromatic nitroso- azotlioxy-systems, the equilibrium may be substantially shifted towards the nitroso-form depending on the type of substitution on the aromatic ring^.^^-^' ortlzo-Substitution is known to affect the equilibrium in favour of tlie azodioxy-form.' Thus, the equilibrium constant for the dissociation-dimerization is ca. lo2, 1, and lop2 at ambient temperatures for nitrosobenzene, o-nitroso- toluene, and nitrosomesitylene. Tt is noteworthy that cis-trans-isomerization in aromatic nitroso-azodioxy-systems has been observed only in a few systems, in particular, in o-nitrosotoluene. azodroxy-compound 2 R-N=O 2M monomer nitroso -compound FIGURE Reaction scheme for the dissociation and1 cis-trans-isornerization of azodioxy-compounds The beliaviour of this system has been recently investi- gated in detail both in the solid state and in solu-tio11.18.28.29 (i) The azodioxy-species undergo cis-tram-isomeriz-ation in solution analogously to the aliphatic species (see Figure 1).(ii) Slow evaporation of solutions con- TXitroso and azodioxy refer to the C-nitroso rind climcric nitroso species, respectively, thronghout this papcr. compound leading to the trarts occurs readily at room temperature. (iii) Internal rotation about the Ar-N bond is sterically hindered in the cis-form; whereas tliis type of sterical hindrance is likely to be absent in the tram-configuration. (iv) The cis-dimer is more stable energetically (i.e. by AH") than the trans-dimer but entropically less favoured. (v) The rate of the dis-sociation of the dimer is considerably faster as coinpared to aliphatic systems.In an attempt to test the extendibility of these experi- mental findings to other closely related nitroso-azodioxy- systems, we have investigated the behaviour of 2,6-dimethylnitrosobenzene (DMBA) +2,2',6,6'-tetra-mcthylazodioxybenzene (TMAB). EXPERIMENTAL Materials.-The solvents CH,CN,, ClICI,, and CL),CN were spectroscopic grade and used without further purific- ation (Ciba-Geigy; 99.5 ; isotopic purity). Sawz@es.-Dimeric 2,6-dimethylnitrosobenzenewas pre-pared essentially according to The reaction mixture was mechanically stirred at -2 to 5 "C for 6 h at pH 6.8-7.0. An adjustment of pH was made with acetic acid and I,CO,. The nitroso-dimer precipitated from the mixture and was filtered, washed with a little water, 10 HCI, and cold methanol.Recrystallization three times from ethanol- light petroleum produced small flakes. Directly before spectroscopic measurements further purification was to obtain by bulb-to-bulb, high vacuum sublimation at 140 "C. Upon recrystallization from ethanol, the trans-dimer was obtained and identified by i.r. In one instance, a slow crystallization experiment lasting several days at ambient temperature resulted in 3 nim flakes which appeared predominantly to consist of the cis-isomer. However, attempts to reproduce this selective crystallization were unsuccessful. At higher temperatures osidation oi the nitroso-compound caused the solutions to turn yellow. This oxidation was inhibited by flushing the samples with argon.The m.p. of tlie brans-isomer was 133.5-134 "C, RF (CHC1,) 0.704. I.r. nleasuret7ze.lzts.-l.r. spectra were recorded using a Perkin-Elmer 180 or a Perkin-Elmer 130 spectrometer. The solid samples were examined at rooni temperature in the range 1 700-400 cni-1 using the IiBr disc technique (pressure 7.6 kg cn-2; 13 mm, concentration 16 nig per C$ 300 mg KHr). Wave numbers are estimated to be accurate to A2 cm-l. The presence of cis-and trans-isomers in the solid state was indicated by well established characteristic peaks at for1 400 and 050 cm-l for the cis-form and at 1 279 c~n-~ the tvans-form. For i.r. studies in solution of the dimeric species, 0.05~solutions were examined at room temperature where the dimer-monomer equilibrium is shifted far to the left in favour of the dimer. For studies of solutions contain- ing predominantly the mononicr, diluted solutions (5 x 10-4~)have been used.Here, i.r. spectra were recorded on a IXgilab Fourier i.r. spectrometer (number of scans 100). In all solution studies, the cell path lengths were 1 nim. Visible Spectra.-All tlie visibie spectrophotometric measurements were carried out on a Shimadzu MpS-5OL double-beam instrument. At temperatures below ambierit, the spectrophotorneter was purged contirruvusly with nitrogen. The cell holders were thermostatted by circnlat-ing etlmnd from a constant temperature bath. The teni-perature uf the solutions was measured continuously by means of a copper-constantan therinocouple conmcted to a digital voltmeter.Quantiktive evaluation of opticamp;l densities of the specific band of the monomer in the wave-length region 400-800 iim were made in the equilibration stndies of the dimer-monomer equilibrium. The concen- tration range was 5 x IW3-ll x lo-". Optical cells of path length 1, 2, 5, and 10 cni were used. Samples were prepared immediately prior to the experiments by weighing TMBA together with the solvent (ca. 50 or 100 nil) in E-flasks. The equilibration measurements were carried out with 1cm cells (ca. 3 nil) with the addition of the azodioxy- compound (20 mg) in a pre-thermostatted cell which was shaken vigorously. Dilution experiments were carried out using an Oxford rapid-dispensing syringe equipped with polyethylene tips of 10 ml.The agitation following the injection was sufficiently vigorous in all experiments to yield homogeneous solutions. In all equilibration measure- ments the absorption of the monomer was followed. N.m.r. Spectra.-lH Fourier transiorni n.m.r. spectra were taken on a Bruker IVY-200 spectrometer operating at 200 MHz in tlie deuterium-lock mode and equipped with its variable temperature unit. In most experiments accumulation of 4 f.i.d. was sufficient. The spectra of the methyl protons were recorded with a frequency scale factor of 0.021 Hz cm-l. Temperature nieasureinents were made b;~means of a copper-constantan thermocouple before and after each experiment.The average of two temperatures was taken ; the accuracy is estimated at ca. f1 "C. RESULTS In the solid state, TMBA showed N=O stretching at 1472, 1 258, and 1 250 cin-l and C-n' stretching at 777 cm-l. These bands corresponded to typical trans-spectra of other nitroso-aromatic dimers. There was no indication of any of the characteristic cis-bands which are known to exist for this class of compounds (e.g. at 1 400 and 950 cm-l). Also in solution, no cis-bands were observed, but there were strong trans-bands in concentrated solutions. As expected, there is a great similarity between these spectra and those observed by Luttke for trans-dimeric nitro~omesitylene.~0 The n.m.r. spectrum of the methyl protons of a solution of TMAB (ca.0.05~)at ambient temperature in the shift range 6 1.9-2.7, consists of two peaks labelled M and T (Snr2.620, aT2.426 at 20 "C). Signal T decreases with increas- ing temperature, whereas the M signal increases. Further-more, the ratio of the intensity of the monomer to the inten- sity of signal T increases with increasing temperature as expected for signals arising from a monomer-dimer equili-brium. Only one resonance, T, was obtained when the solid trans-dimer (identified by i.r.) was dissolved in CDCl, at -20 "C. At this temperature the monomer-dimer equilibrium is ' frozen '. On warming the solution, peak M appeared. Thus, M and T are attributed to the mono-meric and trans-species, respectively. N.m.r. Deteamp;nination of Thernzodynaiwic and h'inetic Parameters .-For computation of thernmdynamic para-meters from n.m .r.equilibrium data, the methyl proton + A' FIGURE Spectrophotometric measurements of the equilibrium 2 dimer +monomer of 2,2',6,6'-tetramethylazodioxybenzcnc in acetonitrile solution spectra of 0.036 2~ (Camp;lovalue) CD3CN solutions of DMNB were investigated over the temperature range -10.0 to +30 "C at which no disproportionation of the compound occurs. The equilibrium constants K were evaluated from the given integrals li of the monomeric and dimeric signals M and T, according to equation (1) where r = IM/IT and K = ~CM"Y~/(V+ l) Cr/r"= total concentration expressed in monomer units; CM" = Cx + 2CI,, CM and CDbeing the monomer and dimer concentration, respectively.Linear plots of In K versus 1/T yield the thermodynamic parameters AH" and AS". These plots are shown in Figure 3(a). No broadening of the peaks were observed over this temperature range. Thermodynamic parameters are presented in Table 1. Visible Spectro~hobonze~r-~.-Thedimer-monomer equili-brium was investigated spectrophotornetrically using the specific visible absorption band of the monomer. The Dz2M global equilibrium constant for the overall dissociation (2) is related to the global degree of dissociation 01 by equ- 258 ation (3) where a = CM/CM" (0 6 cc 1) is the ratio of the monomer concentration to the total concentration at equili-brium expressed in monomer units. K = 2a2CMo/(l-a) (3) Using equation (3) together with the Beer-lambert law, the linear relationship (4) is obtained between A (ICB~")-land (a1 bsol; -5.0 -7.0t -9-0-8-oL 7 -10.0h u*--11.0-1-t -12.0 's-13.0t 3FIGURE (a) Equilibrium for the reaction T 2M in acetonitrile solution as determined by visible spectrophoto- metry (open circles) and by 1i.ni.r.(filled circles). (b) Eyringplot for the reaction T ==amp;2M in acetonitrile solution A2(22CM0)-1 where A == absorbance, Z = path length, and CM = absorption coefficient of the monomer. Results (4) obtained at three temperatures are shown in Figure 2. From the intercept and the slope, values of A' and EM are found. The thermodynamic parameters obtained from this data are in Table 1 together with those obtained by 11.m .r. Equilibration of the trans-dimer was made at five temper- TABLE1 Thermodynamic parameters for the DMBA +TMAB equilibrium Method kJ mol-l AHa/ J K-l mol-l AS0/ Visible N.m.r. 54.0 f50/:,51.5 amp; 8 129 7 123h 13 atures by measuring the light absorbance of the monomer at 775 nm. In order to obtain sufficiently large absorption changes, fairly concentrated solutions had to be used (as expected from the equilibrium data). On the other hand, J.C.S. Perkin I1 the low solubility of the azodioxy-compound in CH,CN (as in most polar solvents), limited severely the concen-tration range which could be investigated. Thus, these two experimental difficulties did not allow the concentration TABLE2 Activation parameters for the DMBA +TMAB equilibrium AH$/ AS/kJ mol-l J K-l mol-' 85.4 amp; 2.0 -6 f3 dependence of the decay times to be determined. Equili-bration experiments using dilution gave similar results.The rate constants Kf displayed in the Eyring plot in Figure 3(b) have been evaluated from least-square fitting of the decay curves equation (5)where A = absorbance at t = co (2A, + KEd)(amp; -4) __-(243 + KENwLJ + 2AmA,)exp--Kft(KEMZ 4-4Am)/(2Kc~Z)(5) and A, = absorbance at a given time t. The activation parameters are collected in Table 2. DISCUSSION In contrast to the mono-ortho-substituted case (i.e. o-nitrosotoluene) the present system exhibits in solution (i) an equilibrium lying far towards the dimer, (ii) slow kinetics of dissociation to the monomer, and (iii) no cis-trans-isomerization of the azodioxy-derivative.An explanation of these findings should be sought in the electronic and steric effects operating in the monomeric and dimeric species. As mentioned earlier in the introduction, small electronic and steric effects brought about by minor structural variations may change drastically the relative amounts of components present in nitroso-azodioxy systems. Geometrical parameters recently reported in the literature show that the bond lengths for aromatic and aliphatic azodioxy-derivatives closely approach each other.31 This is an indication that the electronic delocalization in aromatic azodioxy- species is restricted similarly within the ONNO bridge. Furthermore, low temperature n.m. r. studies indicate free rotation around the Ar-NO bond in the trans-o,o'- azodioxytoluene species.29 However, in the cis-azo-dioxy-molecule there is a barrier of ca. 48 kJ mol-l which has been attributed to steric hindrance between the aryl rings. The basis for the existence of the trans- species in this system is in fact directly related to steric hindrance in the cis-form since the cis-form is energetic-ally favoured: AH" ca. 10 kJ mol-l whereas ASo ca. 40 J K-l mol-l. The presence of two methyl groups in the ortho-position should further enhance the repulsion in the cis-isomers as indicated by the lopsided equilibrium ratios, ca. 1 :4,between the two cis-rotamers of azodioxy- toluene and make the trans-form still more favourable for TMAB.This would explain why the cis-form is not present in observable quantities. Increased ortho-substitution should also to a greater extent decrease the stability of the monomer in relation to the trans-dimer since it hampers conjugation in the monomer whereas conjugation between the aromatic rings and the ONNO moiety is almost non-existent in azodioxy-compounds (cf. n.m.r. chemical shifts for o-methyl protons for various speGies 32933). A decreased equilibrium constant for the dissociation-dimerization is thus expected in the series nitrosobenzene, o-nitroso- toluene, and 2,6-dimethylnitrosobenzene, which also is experimentally observed. This suggestion is consistent with reported literature parameters in Table 3.TABLE3 Thermodynamic parameters for nitroso +azodioxy equilibria. AH"/kJ A.S"/JCompound mol-1 K-1 mol-l Solvent Ref. 2-Methyl-45.6 163 Acetonitrile 18 nitrosobenzene 2,6-Dimethyl-54.0 120 Acetonitrile This nitrosobenzene work 2,4,6-Trimethyl-50.6 157 Benzene 14 nitrosobenzene 2,3,5,6-Tetramethyl-61.1 130 Benzene 32 nitrosobenzene 2,3,4,5,6-Pentamethyl-50.2 100 Benzene 32 nitrosobenzene Our results indicate a correlation between kinetic and thermodynamic data, AG (ca. AHf) paralleling AH". This appears to apply both to substitution as well as to solvent effects. The transition state has been proposed to be a twisted molecule around a stretched N-N bond.12 Resonance effects in the transition state could therefore be similar to those operating in the monomer.0/222 Received, 7th Febvuavy, 19801 REFERENCES 11.L. Hammick. J. Cliewi. SOC., 1931, 3105. a D. L. Hammick, R. G. A. New, and L. E. Suttun, J. Clioti. Soc., 1932, 742. D. L. Hammick, R. G. A. New, and R. B. Williams, J. Clicm. SOC.,1934, 29. J. W. Smith, J. Chem. SOC.,1957, 1124. J. W. 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