Srinivasan, V. S.

Venkatasubramanian, N.

English

NOPR

Indian Journal of ChemistryVol. 15A, February 1977, pp. 115-117Oxidation of Hydrazine, Phenylhydrazine & Substituted Phenyl-hydrazines by Thallium (III) - Substituent Effects & MechanismV. s. SRINIVASAN & N. VENKATASUBRAMANIANDepartment of Chemistry, Vivekananda College. Madras 600004Received 18 December 1975; revised 28 July 1976; accepted 25 August 1976The kinetics of Tl(III) oxidation of hydrazine have been studied at different [substrate],[oxidant], acidity, ionic strength and temperatures in the presence of sodium chloride. The re-action exhibits a first order dependence on [TI(III)] and [Hydrazine] and an inverse first orderdependence on [CI-] and [H+]. TI(OH)2+ is shown to be the active species under these conditions.Also the substituent effects in the TI(III) oxidation of phenylhydrazine are reported. Hammettplot shows considerable scatter. Both bond formation and bond breaking are shown to be slgnl-ficant in this reaction. A suitable mechanism is proposed for the TI(III) oxidation of phenyl-hydrazine.ALTHOUGH the oxidation of hydrazine withthaUium(III) has been investigated]. 2 thekinetic aspects of this reaction have not beenstudied fully. We have, therefore, investigated thisreaction at different [substrate], [oxidant], acidity,ionic strength and temperature. Also, the kineticsand mechanism of oxidation of phenylhydrazineand substituted phenylhydrazines by thallium(III)do not seem to have been investigated so far. Itis, however, known that this reaction proceedselegantly to give the corresponding hydrocarbons.In view of the fact that TI(III) behaves as a two-electron oxidant in its reaction with hydrazine",it was of interest to study the reaction betweenTl(III) and phenylhydrazine from the kinetic pointof view.Materials and MethodsHydrazine sulphate and thallium(III) oxide (BDH,99% pure) were used as such. Phenylhydrazineand substituted phenylhydrazines (Koch-LightjFlukaAG) were of extra pure variety which were usedeither after a redistillation or recrystallizationfrom suitable solvents. The reaction was monitor-ed by estimating the unreacted [Tl(III)] by aniodometric procedure to a starch end-points. Asthe reactions between Tl(III) and the substrateswere generally very fast, the reaction rate wascontrolled by the initial addition of NaCl. Thereactions were followed for over 3 half-lives andthe data reported are the average of at least tworuns in each case. The second order constantsreported have been calculated from integrated secondrate equations.Results and DiscussionDependence on thallium (III) , hydrazine sulphateand phenylhydrazine - The oxidation of hydrazinesulphate by TI(III) was studied in 2M sulphuricacid in the presence of 0·2M Na2S04 and 0'005MNaC!, between 25° and 40°. The second orderrate constants at different [hydrazine sulphate]in 2M sulphuric acid in the absence of chloride ion(at a conveniently low temperature) and also in thepresence of O'005M NaCl are summarized in Table 1.These data show that the oxidation of hydrazines~lphate by Tl(III) is governed by the rate expres-SIOn-d[Tl(III)] .---,dt:----- = k2 [Tl(III)] [hydrazine sulphate]The reaction exhibits total second order depen-dence - first order in [Tl(III)] and first in [phenylhydrazine] as shown by log (a-xjb-x) versus timeplot (Fig. 1). The rate law for the oxidation ofphenylhydrazines in 2M H2S04 in the presence of0'25M NaC! is also of the form-d[Tl(III)] .--d-t ~ = k2 [Tl(III)] [phenylhydrazine]Effect of acidity on the reaction rate - The effectof varying sulphuric acid on the rate was studiedTABLE1 - RATE DEPENDENCEON [Tl(III)] AND[HYDRAZINESULPHATE]IN THE ABSENCEAND PRESENCE OF NaCI{[R.SO.]=2·OM; [Na.SO.]=0·2M; temp.=300}[Reactant]Mk x 10 litremole'? see"!(A) VARYING[Tl(III)] (A)0·001398 2-400·002070 2·200·002705 2·07(E) VARYING[HYDRAZINESULPHATE](B)0·003056 74·00·003820 78·4(C)0·0022700·0024060·0046750·0057920·0069512·202·062·022·092·36(D)0·00250 5·180·005640 5·05(A) [Tl(III)J/[NaCl]=1: 2·~.(Bl In the absence of NaC!.(C) In the presence of O·OOSM NaCI at 2·0M H.SO •.(D) In the presence of O·OOSM NaCI at 1·0M R.SO •.l1SINDIAN J. CHEM., VOL. 15A, FEBRUARY 1977assumed that the whole of the Tl(III) species isinitially present as unreactive T1C12+. However,a partial hydrolysis of this will give the more reactivespecies Tl(OH)2+ (Eq. 1). The rate of hydrolysisM••Ia•..•.'"IoUt7I.9',8o0·' TABLE 2 - EFFECT OF ACIDITY ON TI(III) OXIDATION OFHYDRAZINESULPHATEo·!!0'30·\',2Time, seeFig. 1 - Second order plot for the oxidation of 2,4·dinitro·phenylhydrazine by TI(III); [NaCI] =O·lM; and temp. =40°0'6at different [H+] in the range 0·5 to 5·OM. Thereaction rate drops with increasing acidity up to2·OM. Beyond this acidity, there is only smallincrease in the rate (Table 2). It is observed thata plot of log [H+]against log k2 is linear with a slopeequal to -1 up to 2M.Effect of added chloride ion on reaction rate-Added chloride ion reduces the reaction rate con-siderably (Table 3). A plot of log k2 versus log[CI-) is linear with a slope equal to -1.Dependence on ionic strength - The effect ofadded Na2S04and KHS04 in the range 0·2 to 2'OMon the oxidation rate shows that the reaction rateincreases with an increase in Na2S04 concentration;the rate is unaffected by changes in added KHSO.(Table 4).Substituent effect - The effect of suitably position-ed substituents in the phenyl ring of phenylhydrazinehas been studied for the following compounds,viz. 4-bromo, 4-chloro, 4-nitro, 2-,nitro and 2,4-dinitro between 25° and 40°. Table 5 gives acomparative picture of the effect of substituentson the reactivity. It is evident from the magni-tude of these changes in the rate constants thatthe reaction is susceptible to polar effects. Therate data (Table 5) have been cast into a Hammettplot in Fig. 2. The points are considerably scatter-ed. All substituents except the 2,4-dinitro groupsincrease the rate. This type of scatter in Hammettplots is not quite uncommon and is encounteredwith reactions in which either a change of mechanismoccurs or when both bond formation and bondbreaking are significant.Mechanism of oxidation of hydrazine and phenyl-'hvdrazine - From the experimentally observed in-verse dependence on the rate of the reaction on[H+] and [CI-] , certain conclusions can be drawnon the nature of the active species in this reaction.In the presence of added chloride ion, it can beH6{[Hydrazine sulphate]=0·002M; [NaCl]=0·005M; [TI(III))=0'002M; [Na.SO.]=0·2M; temp.=300}k. x 10 (litremole"! see-I)0·51'01·52·03·04·05·08·115·183·572·203'002·723'32TABLE 3 - EFFECT OF ADDED CHLORIDE ION ONREACTIONRATE{[Hydrazine sulphate]=0·002M; [TI(III)]=0·002M; [H2SO.]=2M; [Na.SO.]=0·2M; temp.=300}[NaCl]M0·0050·00750·010·0250·050·1k.xl0a (litremole'? sec'")22014510038'610·06·82TABLE 4 - RATE DEPENDENCE ON ION STRENGTH([Hydrazine sulphateJ=0·002M; [H,SO.]=2M; [TI(I1I)]=0·002M; [NaCI']=0'005M; temp.=300}[Na,SO.] [KHSO.J kl x 10 (litreM M mole"! sec'"]0·2 2·200·4 3·151·0 5·71)1·5 5'572·0 8·160·2 2·620·4 2'640·8 2·831·0 2'371·5 2·78TABLE 5 - SUBSTITUENTEFFECT IN Tl(IU) OXIDATIONSOFSUBSTITUTEDPHENYLHYDRAZINES(RCeH.-NH=NH.){[TI(III)]=0·002M; [Na.SO.]=0·2M; [H.SOJ=2·OM; [NaCl]=0·2SM; temp.=300}R k.(litre mole']sec-l)H4·Br4·CI4·NO.2·NO,2,4·Dinitro1-513-465·601·712·800·33SRINIVASAN & VENKATASU ERAMANIAN: Tl(III) OXIDATION OF HYDRAZINEwill be given by Eq. (2)H"O+ TlCF+~Tl(OHr+ +H+ +Cl- ... (1) It-C'[Tl(OH) ~+][H+][Cl-] '·8 AKh= [TICP+] ... (2) CD0Eq. (2) can be rewritten as 0.[Tl(OH)z+) = Kh[TICJ2+j ... (3) V 0[H+)[CI-] q. 4-- 8f 2-N02From the rate expression, N...0-d[Tl(III)J = kob.[TI(III)] [hydrazine sulphate] ~ 0 4-N02Hdt 1:... (4) '·0... (5)Therefore, kob. = kzKh/[H+)[CI-] ... (6)Kh value calculated from our experimental resultson the basis of kobs versus l/[H+] and koboversusl/[Cl-J plots comes to 2·63 X 10-5 (ref. 6): On thebasis of the foregoing data the mechamsm .sho~nin Scheme 1 can be formulated for the oxidationof hydrazine.+N2H4+H+~HzN-NH3H + slow[Tl(OH}t)2+ + )N -NH3 ----7 TJ++H20H k,... (7)+ ++NH-NH3 ... (8}+ + HN=N( ----+N-N+2H+HScheme 1The mechanism shown in Scheme 2 may be propos-ed for the oxidation of phenylhydrazines by TI(III).This mechanism is consistent with the substituenteffects observed with ring substituted phenylhydra-zines. First the introduction of a phenyl ring booststhe rate of oxidation by a factor of 1000. This is@T13+H@- I (i) H slow ~ + TN-N< - =NH2-tTiR I H RH + @+ 2 H··· .• · 12relatlvelY@~N=W/H ~R)(S!/-VH RScheme 20'62,4-dini1ro Go 0'4 0·8o oHIo ) ~ rr +'·2Fig. 2 - Hammett plot for substituted phenylhydrazines;[H.SO.l=2M; [Na.SO.]=O·2M; [NaCl]=0'25M; and temp.=300consistent with the stabilization that the develop-ing double bond in the slow step (Eq. 12) will getfrom the phenyl ring. Further the scatter in Ham-mett plot stresses that both bond formation andbond breakage are important (Fig. 2). A 4-bromoor a 4-chloro substituent will not stabilize the deve-loping double bond in the slow step but will alsoaid the cleavage of -N -H bond. The fact thata 4-N02 function also increases the rate is stronglyindicative of the fact that the breaking of the C-Nbond in step (13) is also relatively important. Thisduality of slow steps would account for the observedstructure-activity relationship. The increased ratefor 2-nitrophenylhydrazine can be traced. to a fieldeffect of the type (I) which would stabilize theH@:'6~HN-01/oIproduct of reaction (12). With two stronglyelectron withdrawing nitro groups, however, theremoval of hydrogen as hydride anion in step (12)is strongly retarded, leading to a lowered rate.References1. CAHN. J. W. & POWELL, R. E. J. Am. chem. Soc .• 6(1954), 1681. ., • •2. HIGGINSON. W. C. E. & STEAD. J. E .• ROSSEINKY SYKESA. G., Discuss. Faraday Soc., 29 (1960), 49.3. GUPTA, K. S. & GUPTA, Y. K.. Indian]. Chem .• 11 (1973).1285.4. HENRY, P. M.• J. Am. chem, Soc., 87 (1965), 4423.5. SRINIVASAN. V. S. & VENKATASUBRAMANIAN, N .• Curt'.Sci .• 39 (1970). 254.6. EASAL~, F. & PEARSON, R. G .• Mechanisms of inorganicreactions (John Wiley. New York). 1967.117

Oxidation of Hydrazine, Phenylhydrazine & Substituted henylhydrazines by Thallium (III) - Substituent Effects & Mechanism

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Oxidation of Hydrazine, Phenylhydrazine & Substituted henylhydrazines by Thallium (III) - Substituent Effects & Mechanism

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