Kinetics and mechanism of unimolecular ...

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Kinetics and mechanism of unimolecular decomposition of 3-methyl-1-p-tolyltriazene KEITHVAUGHAN Department of Chemistry, Saint Mary's University, Halifax, N.S., Canada B3H3C3 AND

MICHAELT. H. LIU Department of Chemistry, University of Prince Edward Island, Charlottetown, P.E.I., Canada CIA 4P3 Received June 23, 1980 KEITHVAUGHAN and MICHAEL T . H. LIU. Can. J. Chem. 59,923 (1981). Kinetic parameters (E, and AS") for the thermolysis of 3-methyl-1-p-tolyltriazene(1) in tetrachloroethylene have been determined. The near-zero value for AS" suggests a transition state with minimal stretching of the N-N bonds in tautomers l a or l b . 'The activation energy (E, = 29.2 kcal/mol) and AS* for the decomposition of 1 are very similar to those of the azoarylalkanes (Ar-N=N-R), suggesting a similar mechanism of degradation. The major products of the thermolysis of 1 are p-toluidine, N-methyl-p-toluidine, and p-chlorotoluene, which are rationalized in terms of homolytic breakdown of both tautomers. KEITHVAUGHAN et MICHAELT. H. LIU. Can. J. Chem. 59,923 (1981). On a determine les parametres cinetiques (E, et AS*) de la thermolyse du methyl-3 p-tolyl-1 triazene (1) dans le chlorure de methylene. La valeur du AS* qui est voisine de zero suggke un Ctat de transitionavec uneklongation minimale des liaisons N-N des tautomeres l a ou l b . L'knergle d'activation (E. = 29,2 kcallmol) et le AS" de la d6composition de 1sont trbs voisines de celles des azoarylalkanes (Ar-N=N-R) suggerant ainsi que leurs mecanismes de degradation sont analogues. Lesproduits principauxde la thermolyse du compose 1sont lap-toluidine, la N-methylp-toluidine et lep-chlorotoluene dont on peut rationnaliser la formation en fonction d'un clivage homolytique des deux tautomeres. [Traduit par le journal]

,

Introduction Alkyl aryl triazenes (Ar-N=N-NHR) have been widely studied and their synthesis, tautomerism, degradation, and biological properties have recently been reviewed (1). These monoalkyltriazenes exist as a tautomeric mixture of forms A and B (2, 3):

The kinetics of the benzoic acid-catalyzed decomposition of 3-alkyl-1-aryltriazenes in aprotic solvents has been studied (3, 4) and it has been suggested that proton transfer and departure of the alkyl cation are synchronous and rate determining during the decomposition to give ArNH,, N,, and products from the alkyl fragment. The thermal decomposition of methylphenyltriazene in apolar hydrocarbon solvents follows first order kinetics (9, and the formation of methyl radicals was clearly demonstrated. In addition, a CIDNP investigation of the thermolysis of monoalkyltriazenes has demonstrated the formation of the aminyl radical pair (ArNH R), which is presumed to arise via homolysis of the unconjugated tautomeric form B (6). The analogous tautomeric form has also been implicated in the decomposition of monoalkyltriazenes in aqueous solution; when Ar contains electron-withdrawing groups, a simple unimolec-

ular heterolysis occurs involving the alkane diazonium ion (7): We report here the measurement of the kinetic parameters for the unimolecular decomposition of 3-methyl-1-p-tolyltriazene (1) in tetrachloroethylene and an interpretation of the elementary processes involved in the degradation of this class of compounds. Experimental Materials 3-Methyl-1-p-tolyltriazene (I), obtained from Aldrich Chemical Company, was recrystallized from petroleum ether before use, and had mp 81-82°C. Tetrachloroethylene was spectroscopic grade. Kinetics Kinetic measurements were carried out by following the characteristic absorption of the triazene (1) with a Unicam SP 800A uv spectrophotorneter. In all experiments, the decompositions were conducted in sealed Pyrex tubes containing an 0.1 M solution of 1 in tetrachloroethylene. Kinetic parameters were determined by following the change of absorbance with time at the wavelength of maximum absorption (337nm in C,CI,). First-order rate constants were determined graphically from the plot of log (A, - A,) against time. Product Analysis 3-Methyl-1-p-tolyltriazene (1) in tetrachloroethylene (0.1 M solution) was heated at 100°C until starting material could not be detected (after approximately 2 h) by tlc on silica gel F-254 (Merck) with chloroform as eluent (the triazene 1had Rf 0.32).

0008-40421811060923-04$01.00/0 01981 National Research Council of CanadalConseil national de recherches du Canada


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CAN. J. CHEM. VOL. 59, 1981

The solution was evaporated in vacuo to afford a dark red oil. The 60 MHz IH nmr spectrum of this residue was recorded in deuteriochloroform solution with a Varian EM360 spectrometer; assignments of 'H signals were made by comparison with spectra of pure, authentic compounds and with published chemical shift data (6). The product analysis was further refined by column chromatography on silica gel 30-70 mesh (Merck-Brinkman) with carbon tetrachloride a s the initial eluent, followed successively by ether/CCI, (1:1), ether alone, chloroform, and finally 95% ethanol.

Results and Discussion Table 1 lists the first-order rate constants for the decomposition of 3-methyl-1-p-tolyltriazene (1) in tetrachloroethylene. These data gave an excellent Arrhenius plot from which eq. [I] was obtained by least-squares analysis. R is taken as 0.00198 kcal mol-I K-I .

[I] k = 1013.8T0.s exp (-29.2 T 1.4lRT) s-I An A factor of 1013.8corresponds to a AS* of 2.2 cal mol-I K-I at 104"C, the mean of the temperature range over which these measurements were made. The activation energy (E,) and AS* for the decomposition of the triazene (1) are very similar to those of the arylazoalkanes (Ar-N=N-R) (8) (see Table 2), suggesting a similar mechanism of degradation of these compounds. Unsymmetrical azoalkanes decompose with unequal stretching of the two C-N bonds; in the extreme case of arylazoalkanes the evidence (9) is overwhelmingly in favour of one-bond scission to give the radical pair ~ r - N = N R (10). Lorandand co-workers (11) have shown that Ar-N=N is longer lived than the solvent cage. The similarity in the kinetic parameters of triazene and azo-compounds suggests that triazene fragmentation occurs by one-bond scission to give either of the radical pairs A ~ N = N NHCH, or A ~ N HN=N-CH, (Scheme 1). However, the kinetic parameters do TABLE1. First-order rate constants for the decomposition of 1 in tetrachloroethylene t C'C)

k x lo4 (s-I)

TABLE2. Comparison of kinetic parameters of the triazene (1) and the azoarylalkanes (Ar-N=N-R)

E, (kcal/mol) AS' (cal/mol-1 Triazene (1) Azoarylalkanes* (Ar-N=N-R)

29.2 f 1.4 27.0-34.0

K-I)

2.2 f 3.0 3.9-5.2

'Reference 8.

not distinguish the pathways of decomposition from tautomer l a or tautomer lb. The near zero AS* value for the decompositionof 1may indicate that the stretching of the N-N bond in l a or 16 is minimal in the transition state. However, there may be other reasons for the small AS', which may contain a contribution from loss of rotational freeGom. For example, resonance stabilization of ArNH (from 16) may freeze rotation about the C-N bond more fully than in the reactant; this may also be a factor in arylazoalkane decomposition. Analysis of the product mixture suggests that decomposition of triazene (1) proceeds mainly via homolysis of 16. The major component of the product mixture had C-methyl chemical shift at 6 2.18 and was identified as p-toluidine by comparison with authentic material; the yield ofp-toluidine from the thermolysis of 1was estimated to be 42%. A second component of the product mixture had C-methyl6 2.33 as well as N-methyl chemical shift 6 2.68; these chemical shifts correlate with those reported for N-methyltoluidine (6). However, the yield of N-methyltoluidine from 1was only 5%. A third component, which was the major compound in the first fraction from column chromatography, had a C-methyl chemical shift at F 2.28; this component has a typical aromatic AA'BB' pattern for a p-disubstituted benzene. The coupling constant (J = 8 Hz) in the AA'BB' pattern is identical with that in authentic p-chlorotoluene, which has a measured C-methyl chemical shift of 6 2.27. The yield ofp-chlorotoluene from the decomposition of 1was less than 10%. These nmr assignments agree entirely with those made by Albert et al. (6). The moderate material balance (57% total recovery) in the product analysis reflects considerable tar formation in the reaction mixture. The 43% of radicals not accounted for probably underwent addition to tetrachloroethylene leading to some polymerization. Scheme 1 shows the most probable mechanistic pathways consistent with these observations. Onebond scission of tautomer 16 gives the radical pair 3, which can give rise to the major product,


VAUGHAN A N D LIU

Ar-N=N'

Ar' Solvent

I

'NHCH,

Volatile products

Ar-NH'

'N=N-CH,

ArNH,

-

ArNH'

'CH,

+ N2

CH,'

I

(42%)

Volatile products

ArCl

(Ar = p-tolyl) 1 SCHEME

p-toluidine, after cage dissociation ?nd H-abstraction by the arylaminyl radical, ArNH. Alternatively, loss of nitrpgen from the methyl diazenyl radical MeN_=N within the cage affords the pair A ~ N HCH, which combine to give the observed minor product N-methyl-p-toluidine. Similar onebond scission of tautomer l a gives th? radical pair 2; since thearyldiazenyl radical ArN=N is relatively long-lived (1 I), dissociation of the radical pair will likely precede loss of N, to give the aryl radical Ar. Abstraction of chlorine from the solvent by A; accounts for the formation of the observed product, p-chlorotoluene. Since ArN=N is longlived, the pair Ar NHCH, is unlikely to form and cannot be the source of p-toluidine. Unless the relative amounts of the products are a reflection of the efficiency of H-abstraction by aminyl-radical over CI-abstraction from the solvent by p-tolyl radical, the ratio of p-toluidine to pchlorotoluene (>4 :1) suggests that the major Pathway of homolysis of 1 involves tautomer l b . This preference probably reflects a greater reactivity associated with the unconjugated tautomer ( l b ) rather than a greater concentration of l b in the tautomeric mixture in accordance with the CurtinHammett principle (12). Indeed the tautomerism in 1 has been shown, by variable temperature nmr (2, 13), to favour the conjugated form l a . The CIDNP investigation of monoalkyltriazene thermolysis (6) also showed the preferred fragmenta-

tion of the unconjugated tautomer and the formation of the arylaminyl-alkyl pair A ~ N HCH,. Further studies of the thermolysis of monoalkyl triazenes will be aimed at the elucidation of the effect of substituents in the aryl group on the kinetics and course of the reaction. Tautomerism of the type ( l a l b ) is greatly influenced by the nature of the aryl substituent and the latter might have a substantial effect on the thermal degradation. A preliminary study of a comparison of the p-tolyltriazene ( 1 ) with the p-nitrophenyl analogue suggests that the nitro-compound is much more stable to thermolysis. However, the determination of kinetic parameters for the nitrophenyltriazene by the spectrophotometric method proved impossible, due to the intense interference in the uv spectrum by the product of degradation, p-nitroaniline. Alternative methods of kinetic measurement are being examined.

*

Acknowledgements The authors gratefully acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada. We are also grateful for technical assistance to C. Hinman, P. Godreau, W. Y. Kwan, and J. Szabo. 1. K. VAUGHAN and M. F. G. STEVENS. Chem. Soc. Rev. 377 (1978). 2. K. VAUGHAN. J. Chem. Soc. Perkin Trans. 11, 17 (1977). J. Chem. Soc. Perkin 3. N. S. ISAACSand E. RANNALA. Trans. 11, 899 (1974).


CAN. J . CHEM. VOL. 59, 1981

4. N. S. ISAACSand E. RANNALA. J. Chem. Soc. Perkin Trans. 11, 902 (1974). 5. A. CSERHEGYI, G. SZENTGYORGYI, and 0. DOBIS. Magy. Chem. Foly. 77,607 (1971). 6. K. ALBERT,K.-M. DANGEL,A. RIEKER,H. IWAMURA, and Y. IMAHASHI. Bull. Chem. Soc. Jpn. 49,2537 (1976). 7. C. C. JONES, M. A. KELLY,M. L . SINNOTT,and P. J. SMITH.J. Chem. Soc. Chem. Commun. 322 (1980). 8. G. L. DAVIES,D. H. HEY,and G. H. WILLIAMS.J. Chem. Soc. 4397 (1956); Comprehensive chemical kinetics. Vol. 5. Edited by C. H. Bamford and C. F. H. T ~ p p e r .Elsevier, New York. 1972. p. 582.

9. P. S . ENGEL.Chem. Rev. 80, 102 (1980), and references

therein. 10. W. A. PRYORand K. SMITH.J. Am. Chem. Soc. 92, 5403 (1970). 11. R. G . KRYGER, J. P. LORAND,N. R. STEVENS,and N. R. HERRON.J. Am. Chem. Soc. 99,7589 (1977). 12. J. I.SEEMANand W. A. FARONE. J. Org. Chem. 43, 1854 (1978). 13. R. CURCIand V. LUCCHINI. Spectrosc. Lett. 6,293 (1973).