Indian Journal of ChemistryVol. 28A, January 1989, pp. 12-15

Linear Free Energy Relationships in Solid State Reactions:Part III-Reactions of Zinc Acetate with Aniline Salts

P S BASSI* & G S CHOPRADepartment of Chemistry, Guru Nanak Dev University, Amritsar 143005

Received 20 January 1988; revised and accepted 6 April 1988

Reactions between anhydrous zinc acetate and a series of substituted aniline hydrochlorides have been studiedin the solid state. The products, characterized by elemental analyses and spectral data are ZrrCl.: 2(XCfiH~NH~).The kinetics of these reactions have been studied by the mass loss method. Energy of activation values are 26.9,38.9, 57.7, 144.7, 92.8, 117.4,97.4, 162.2 and 186.5 kJ mol-I for X=2-Cl, 3-Cl, 4-Cl, 4-1, 3-0CH" 4-F, H,4-CH, and 4-0CH, respectively. A plot of energy of activation with Hammett 0 constants of the substituents islinear. Unsubstituted aniline salt and 4-iodoaniline hydrochloride show deviations which have been interpreted interms of differences in crystal structure and size of these reactants. The reactions proceed by the formation of anadduct, which then decomposes with the loss of gaseous acetic acid.

The Hammett equation I has been successfullyused for the correlation of the reactivities of dis-ubstituted benzene derivatives with the positionand nature of substituents. The equation, extens-ively used in organic reactions, has recently beenshown to correlate the reactivities of inorganic com-plexes containing benzene derivatives with thesubstituent parameters". The applicability of therelationship to solid-solid reactions is expected togive valuable information about the nature andmechanism of these reactions also. In the reactionsof aniline hydrochlorides with cobalt acetate in thesolid state, studied by the capillary and mass lossmethods+" the Hammett equation applies success-fully. Presently we have examined the applicabilityof Hammett equation to the solid-solid reactionsof zinc acetate with aniline hydrochlorides.

In order that the kinetic data may be correlated,it is necessary that the structure, composition andbonding of the reactants throughout the series issimilar and the same holds for the products. Alarge number of zinc halide complexes with aro-matic amines, such as ZnY2.2(XC6H4NH2) whereY= CI, Br, I and X = 2-CH3' 4-CH3' 2-0CH3'4-0CH3' 2-CI, 3-CI, 4-CI, 2-Br, 3-Br and 4-Bretc. have been reported in the literatures - 8. Allthe salts are centro symmetric except that with ani-line hydrochloride, which is non-centrosyrnmetric.The products as characterized by spectral and ele-mental analyses show the same composition ZnCI2'2(amine) and are tetrahedral in which Zn - Nbond is predominantly covalent". X-ray studies ofZnCI2'2(C6HsNH2) and ZnCI2'2(4-CH3C6H4NH2)and data on heats of coordination of the com-plexes show that they are isostructural'".

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Materials and MethodsZinc acetate (AR) was dried at 383K overnight

(Found: Zn, 35.1. Calc. Zn, 35.7%). Aniline hy-drochlorides, prepared by passing dry hydrogenchloride gas through solutions of amines in dryethanol, were purified by repeated recrystalliz-ations and the purity checked by elemental ana-lyses and melting points. All the reactants werepowdered and sieved to collect of fixed size (44fi). The solution phase reaction products wereprepared by known methods.

KineticsThe measurement of kinetic data for solid state

reactions is difficult. The reactants were chosen insuch a manner that they either give a colouredproduct or that one of the products was volatile.Acetic acid was the only gaseous product, and sothe mass loss gave a measure of the extent ofreaction. In a typical kinetic run zinc acetate(0.001 mol) and aniline hydrochloride (0.002 mol)were separately weighed and therrnostated forabout 10 min. They were mixed with special careto get the best possible homogeneity while exert-ing minimum of pressure on the reactants. Thismixture was transferred to a glass bucket, whichwas connected to a five decimal electrical balance.The change in weight with time was monitored us-ing fixed particle size of all the reactants (44 u.).

Results and DiscussionTo determine compounds present in the gase-

ous products in the solid-solid reactions betweenanhydrous zinc acetate and substituted aniline hy-drochlorides, the volatalized material was success-

BASSI et al: SOLID STATE REACTIONS OF ZINC ACETATE WITH ANILINE SALTS

ively passed through three towers contammg (i)anhydrous copper sulphate (to detect water), (ii)freshly prepared lime water (for carbon dioxide)and (iii) silver nitrate solution (for halogen). Thesetests gave negative results for the presence of wa-ter, halogen and carbon dioxide in the gasesevolved on the reaction.

The stoichiometry of the reactions was esta-blished by mixing the reactants in molar ratios of1:1, 1:2 and 1:4 and allowing them to go to com-pletion in a thermostat (373 K), till no furthermass loss was detected. As a representative casethe reaction of zinc acetate with 2-chloroanilinehydrochloride is reported. After the completion ofreaction, it was found that 1:1 mixture containedunreacted zinc acetate whereas 1:4 mixture con-tained unreacted aniline salt. TLC of the 1:2 mix-ture showed that none of the reactant was presentin the product and that it was a single compound.The amount of acetic acid evolved was estimatedacidirnetrically. The evolved gas was bubbledthrough a solution of sodium hydroxide using drynitrogen as the carrier gas. The loss of acetic acidwas 17.7, 22.8 and 13.9% in 1:1, 1:2 and 1:4 stoi-chiometric mixtures. The percentage of Zn and CIdetermined for these mixtures was also in accord-ance with the formation of a 1:2 complex. Thereactions thus can be written as:Zn(CH3COO)2 + 2XC6H4NH3CI-+

(s) (s)ZnCI2'2(XCt;H4NH2)+ 2CH3COOH

(s) (g)(X =2-Cl, 3-CI, 4-CI, 3-0CH3' 4-0CH3' H, 4-1,4-F and 4-CH3)

The products obtained from the solid-solidreactions of the 1:2 stoichiometric mixtures wereanalysed and results are consistent with the forma-tion of ZnCI2' 2(XC6H4NH2) (Table 1).

The reflectance spectra of the products were al-most identical and displayed peaks in the regionof 265-285 nm. The IR spectra of the solid phasereaction products exhibited a split hand at 3300

em -1, assignable to vasNH2 and vsNH2' ThevZn - Cl and vZn - N modes appeared at 300and 400 em - 1 respectively. The characteristicasymmetrical and symmetrical stretching frequen-cies of acetate and bands due to free - NH2 and- NH; were absent in the spectra of the products.

The X-ray pattern of the solid phase reactionand solution phase reaction products were exactlyidentical in terms of band positions and intensities.The X-pattern did not contain any lines character-istic of the reactants.

The TLC of the reaction products showedsingle spots indicating the formation of a singleproduct in each case. No spots due to reactantswere present. The R, values for the solid and so-lution products were identical indicating that thetwo methods gave identical products.

KineticsThe only volatile product of these reactions is

gaseous acetic acid, so the extent of chemicalchange was followed by measuring the mass lossdue to acetic acid at any time. In each case thereactants were taken in the molar ratio of 1:2, sothat the measured mass loss not only gave a mea-sure of the extent of reaction, but also a directmeasure of a, the fraction of the reactants con-sumed. Plots of a versus time for different reac-tions at different intervals of time and at differenttemperatures for fixed particle size of the reactantswere drawn. These plots may be divided into twogroups. One group showed an induction periodfollowed by an acceleratory region and decay nearthe completion (X =H, 2-CI, 3-CI, 4-Cl and3-0CH3) (see Fig. 1). The other group showed adeceleratory behaviour throughout (X =4-1, 4-F,4-CH3 and 4-0CH3) (see Fig. 2).

The interpretation of measured isothermal kin-etic data has frequently been based on comparisonof excellence of fit of such measured values withthe requirements of rate equations derived fromdifferent geometric models. In the reactions re-

Table 1- Microanalytical Data of Solid Phase Reaction Products, ZnCl~' 2(XC(,H~NH~) Found (calc), (0;',)X C H N Zn CI

2-CI3-CI4-CI4-14-1-'

H3-0CH,4-CH,4-0CH,

301(37.0)3S.0(37.0):17.5(37.0)25.:;(25.2)4IX(404)45.3i 45.0)43.A(43lJ)50.2( 4K.2)43.4!·U,lJ)

3.613.1 142(3.1 )4.3(31 )2.7(2.1 )2.XIJA)40(4A)5.1(4.7)44(5.2).''i.3( 4.X)

,,

7.0(7.2)7.1\(72)7.0(7.2)50(4.lJ)Xo(X.I)1'.7:X.Xl6.lJ(7.X)XJ,(X.O)(,.lJ!7.X)

loA( 10.X)170( lli.X)lli.X(IIi.X)11.5( IIA)IX.2( IX.3)20.0(20.4)17.5( 17.1)IX.5( IX.X)17.3( 17.1)

IX.5( IX.2)l!\.O( I R.2)IRA( IR.2)12.3( 12A)IlJ.5( IlJ.lJ)22.0(22.2)IR.R(IX.o)IlJ.3(20A)IX.2( 11'.0)

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INDIAN J CHEM,JANUARY 1989

,·00r----------------:-----,

0-66

033

t(min.)

Fig. I-Kinetic plots for the reaction between zinc acetate and2-chloroaniline hydrochloride at different temperatures

,.00 .--------------------y

0-66

0'33

o 80t(hrs)

o

Fig. 2-Kinetic plots for the reaction between zinc acetate and4-toluidine hydrochloride at different temperatures

ported, Eq. (1) fitted the data throughout therange of a, in which the reactions were studied,with a correlation coefficient greater than 0.99. Allother equations gave a poor fit. According to Gal-wey" the magnitude of E; can be accepted as avaluable method for comparing reactivities of sol-ids. Moreover, the calculated magnitudes of E; arecomparatively insensitive to the particular rateequation used for the kinetic analysis, providedthat power laws are based on (kt)" and not kt".an = kt ... (1)

The values of energy of activation (E a) have beencalculated from the Arrhenius plots. The valuesare 26.9, 38.9, 57.7, 144.7, 92.8, 117.4, 97.4,162.2 and 186.5 kJ mol- I for X = 2-CI, 3-Cl,

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4-OC~

150 04-1

'V"0E...... 110..~oLll

04·F

Ho

704·CIO

3-CI

0.2 0'0 0·2 0-4()

Fig. 3-Plot of Hammett '0' constant against energy of activa-tion

4-CI, 4-1, 3-0CH3' 4-F,H, 4-CH3 and 4"'()CH3 re-spectively. A comparison of E; values shows thatthe relative order of reactivity is: 2-Cl> 3-CI > 4-CI> 3-0CH3 > H > 4-F> 4-1 > 4-CH3 > 4-0CH3'Electron withdrawing substituents enhance thereactivity and electron donors increase the basicityof nitrogen. The greater reactivity of 2-cWoroani-line can be attributed to the ortho effect.

To test the validity of Hammett equation, theconstants 0, op and 0 I were chosen. Regressionanalysis showed that 0 R and 0 I give erratic plotswhile the best results are obtained using 0 (Fig. 3).The plot is linear with a slope of - 208.9 kJmol- I. Aniline and 4-iodoaniline hydrochloridesshowed significant deviations from the Hammettplot. The high value of E; for 4-iodoaniline hy-drochloride is due to the bulky iodo substituent,which decreases the effective contact area bet-ween the reactants. For the unsubstituted salt,much lower value of E a is attributed to a bettercontact between the reactants and also due to thepresence of polar axis which enhances reactivity'<.The negative slope shows that as the acidity of theanilium ion increases, the rate of the reaction isenhanced.

To understand the sequence of chemical stepstaking place at the zinc acetate/aniline hydrochlo-ride phase boundary, the following observationsare significant.

(i) The values of heat of addition, for reactionsgiving products ZnX2"2(amine), are almost inde-pendent of the nature of anion and the position ofthe substituent group on the benzene ring 10.

BASSI et al: SOLID STATE REACTIONS OF ZINC ACETATE WITH ANILINE SALTS

(ii) Reactions of anhydrous zinc chloride with p-toluidine and p-chloroaniline (amines which aresolid at room temperature) have also been investi-gated. The reactions are very fast and are com-plete immediately on mixing. The products havebeen identified to be the adducts by elemental an-alyses and spectral studies.

These two facts alongwith the observation thatthe IR spectra exhibit bands due to Zn - CI andR - NH; groups and no bands due to free orcoordinated NH2 groups indicate the formation ofan intermediate adduct (XC6H4NH3MZn(CH3-

COO)2CI2)' which decomposes to give the finalproduct.

To see whether these reactions are true solid-solid reactions or involve certain features of thesolid-gas reactions, the effect of presence of gasesobtained from the thermal decomposition of thereactants on the rate of reaction was studied.Reaction of zinc acetate with aniline hydrochlorideexposed to dry hydrogen chloride vapours for 5min gave a rate constant of 0.825 g h - I. In thereaction of zinc acetate exposed to vapours ofacetic acid with unexposed aniline hydrochloride,the rate constant is 0.584 g h - I, as compared to1.018 g h - 1 for unexposed reactants. Thus, both

adsorption of HCl on aniline hydrochloride and ofCH3COOH on zinc acetate has a retarding effect.This is because the adsorbed gases form an enve-lope around the reactants, and decrease the effec-tive contact between them. Thus the experimentsprovide further proof that the reactions occur inthe solid state.

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relationships (Plenum Press, New York) 1972.2 SenoffC V, Coord chem Rev, 32 (1980) 111.3 Bassi P S & Chopra G S, J Solid State Chem, 61 (1986)

103.4 Bassi P S & Chopra G S, J Therm Anal(accepted).5 Masslennikova I S, Zh Fiz Khim, 51 (1977) 1009.6 Masselennikova I S & Baranovskii V I, Zh Fiz Khim, 52

(1978) 3124.7 Ablov A V, Proskina N M & Chapurin L F, Zh neorg

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17 (1972) 132.9 Ablov A V, Burnasheva Z P & Konunova Ts B, Zh neorg

Khim, 10 (1965) 2286.10 Masslennikova I S & Smimova T N, Zh Fiz Khim, 52

(1978) 1788.11 Galwey A K, Proceedings of the seventh international con-

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