Journal of Applicable Chemistry 2015, 4 (6): (International Peer Reviewed Journal)

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1 Available nline at ISS: Jurnal f Applicable Chemistry 215, 4 (6): (Internatinal Peer Reviewed Jurnal) Chemical Educatin Article fr Graduate Students Demnstratin f n-linear Resnance Free Energy Relatinship in the S 2 Reactins f rth- Substituted Benzylamines with Benzyl Brmide V. Jagannadham 1*, R. Ravi 1 and R. Sanjeev 2 1. Department f Chemistry, Osmania University, yderabad-57, IDIA 2. Department f Chemistry, Ashka Institute f Engineering and Technlgy, yderabad-58252, IDIA jagannadham195@yah.cm Accepted n 9 th vember 215 ABSTRACT The rates f reactins f rth-substituted benzyl amines (BA) with benzyl brmide (BB) were measured using cnductivity technique in methanl medium. The reactin fllwed a ttal secnd rder path. The end prduct f the reactin is identified as di-benzyl alkyl amine (C 6 5 C 2 (R)C 2 C 6 5 ). The rates decreased with increase in resnance cntributin r with increase in the σ R value f electrn dnating substituents () like methxy (σ R = -.43), Cl (σ R = -.24), at rth psitin f the amine till σ R value becmes -.15 fr methyl (σ R = -.15) and then the rates decreased with decrease in resnance cntributin r with increase in σ R value f electrn withdrawing substituents () like and CF 3 (σ R f =. and σ R f CF 3 =.1). The lcus f the resnance free energy relatinship (RFER) has a maximum near the pint fr rth-methyl benzyl amine shwing that there is a sharp change in the rate determining step. A mechanism invlving frmatin f an S 2 type transitin state between the amine nuclephiles and the benzyl brmide and its subsequent decmpsitin is prpsed. Activatin parameters were calculated and discussed. Keywrds: n-linear resnance free energy crrelatin, reactins f benzyl amines, reactins f benzyl brmide, and change in rate determining step. ITRODUCTIO Thugh the ammett and Taft equatins are nearly mre than seventy five years ld, the interest even tday in their applicatin fr predicting the rganic reactin mechanisms is still cntinuing. Ever since the ammett [1] and Taft [2,3] equatins were develped, there were several hundreds f reactins with metaand para- substituted benzene derivatives in the literature, fr which the ammett and Taft reactin ( ) cnstants were reprted. There were sme reactins which deviate frm linearity f ammett plt and dcumented in detail by Schreck [4]. There were several reprts n nn-linear ammett relatinships. An example f nnlinear ammett plt is that reprted by Crwell et.al. [5] in the reactin f armatic aldehydes with n-butyl amine. The prducts are substituted benzylidene-n-butyl amines. This was 1579

2 V. Jagannadham et al Jurnal f Applicable Chemistry, 215, 4 (6): interpreted in terms f the change in the rate determining step frm reversible additin f amine t aldehyde t the subsequent dehydratin step which is favred by electrn dnating substituents. ancck et al had tried t explain the V-shaped ammett lcus based n the same explanatins f change in mechanisms f Wlf-Kishner reactin f hydraznes [6] and U-shaped ammett lcus fr the rates f slvlysis and thisulfate reactins f α-chlr tluenes [7] fr tw sets f electrn dnating and electrn withdrawing f substituents. One f the recent examples in this directin is the reactins f alkaline hydrlysis f O-aryl thinbenzates [8]. Anther ntable example f this kind is frm ur labratry in the reactins f -substituted benzylamines with benzyl brmides [9]. DISCUSSIO The cnductivity f the reactin mixture increases with time as the reactin prduces Br as ne f the end prducts. Under the cnditins [-BA] > > [BB], the plt f lg (C - C t ) versus time (eq. 1) was linear until three half-lives with crrelatin cefficient f.999, indicating the rder in [BB] was unity. lg (C - Ct) = lg (C - C) - (k/2.33) t (1) ere C, C and C t are the cnductivities f the reactin mixture at infinite time, initial and at regular intervals f time respectively. Frm slpes f such plts, the pseud first rder rate cnstants (k bsd /min) were calculated fr all the -substituted benzylamines. The plt f lg (k bsd /min) versus lg [BA] was again fund t be linear with a crrelatin cefficient f.995 indicating the rder in [BA] was unity. The pseud-first-rder rate cnstants (k bsd ) fr all reactins beyed eq. (2) in methanl. The clean secndrder rate cnstants, k was btained as the slpe f the plt f k bsd against [benzylamine]. The secndrder rate cnstants (k) are summarized in table 1. k bsd = k [benzylamine] (2) Therefre the simple rate equatin f the reactin wuld be: Rate = k [benzylamine] [benzyl brmide] where k is the secnd rder rate cnstant. Under therwise similar cnditins the slvlysis reactin f benzyl brmide in the absence f benzylamine was checked and the rate was fund t be 1 times less than the rate f the secnd rder reactin. ence the rates f backgrund reactin f benzyl brmide were neglected in these studies. Table-1: Effect f temperature and substituent n k and activatin parameters in rth-substituted benzylamines - benzyl brmide reactin [BA]=.1 ml dm -3, [BB] =.1 ml dm -3, Slvent: methanl, Temp = K in BA σ R k x 1 2 (dm 3 ml -1 min -1 ) 298K 33K 38K 313K Ea k J ml -1 k J ml -1 2-C 3 O Cl C CF S J (ml K) -1 Reactin mechanism and substituent effects: Since the ttal rder f the reactin is tw, a bimlecular type f reactin is prpsed as shwn in the fllwing mechanism via an S 2-type transitin state (Scheme 1). www. jac.inf 158

3 lg k V. Jagannadham et al Jurnal f Applicable Chemistry, 215, 4 (6): Structure reactivity: The rates decreased with increase in resnance cntributin r with increase in the σ R value f electrn dnating substituents () like methxy (σ R = -.43), Cl (σ R = -.24), at rth psitin f the amine till σ R value becmes -.15 fr methyl (σ R = -.15) and then the rates decreased with decrease in resnance cntributin r with increase in R value f electrn withdrawing substituents () like and CF 3 (σ R f =. and σ R f CF 3 =.1). The lcus f the resnance free energy relatinship (RFER) has a maximum near the pint fr rth-methyl benzyl amine shwing that there is a sharp change in the rate determining step (Figure 1). σ R values are frm reference 1. A mechanism invlving frmatin f an S 2 type transitin state between the amine nuclephiles and the benzyl brmide and its subsequent decmpsitin is prpsed. 2 C Br step-1 C 2 Br -Br step-2 = EWG r EDG C 2 C 2 Scheme 1 = rth-oc 3, rth-c 3,, rth-cl, rth-cf 3-1 Figure 1: Plt f lg k versus R C R 2 =.9954 R = Cl R 2 =.991 R = CF C 3 O R www. jac.inf 1581

4 V. Jagannadham et al Jurnal f Applicable Chemistry, 215, 4 (6): Mechanistic implicatins f S : The frmatin f the transitin state invlves the cmbinatin f tw reactants t give ne species. This wuld result in a lss f several translatinal degrees f freedm f the reactants, leading t high negative entrpy f activatin. Effect f temperature: The rate cnstants (k) fr the reactin f benzylamines were fund t increase with increase in temperature. The activatin energy (E a ) is evaluated frm Arrhenius plts f ln k versus 1/T and activatin parameters were evaluated frm Eyring plts f ln k/t versus 1/T and these values are presented in the table-1. The negative S values indicate that the lss f entrpy during the frmatin f unstable inic transitin state, which prmtes a high degree f slvatin and hence lss in entrpy. Iskinetic relatinship: An iskinetic plt is btained in the present wrk when is pltted against S accrding t the equatin = β S + cnstant, the slpe f which crrespnds t an iskinetic temperature β f 281 K, which is well belw the experimental temperature range ( K) used in the present reactins t study the effect f temperature n reactin rates. Accrding t iskinetic thery [11], this means that the effect exerted by the rth- substituted f benzyl amines, bth the prcesses i.e. frmatin and the heterlysis f the transitin state are cntrlled predminantly by entrpy. It has been asserted that apparently linear crrelatins f with S are smetime misleading and the evaluatin f β by means f the equatin = β S + cnstant, lacks statistical validity [12]. Exner [13] describes an alternative methd fr the treatment f experimental data. If the rates f several reactins in a series have been measured at different temperatures and lg k 2 (at T 2 ) is linearly related t lg k 1 (at T 1 ), i.e., accrding t the equatin, lg k 2 = a + b lg k 1, s that β can be evaluated using the equatin, β = T 1 T 2 (1 b)/(t 1 - T 2 b). We have calculated the iskinetic temperature β as 275K by pltting lg k 2 at 313 K versus lg k 1 at 33 K as the value f β (275) is lwer than the experimental temperature 298 K. This indicates that the rate is gverned by entrpy f activatin [14]. COCLUSIO The reactins f rth- substituted benzylamines with benzyl brmide fllw a S 2 type reactin. The reactins are entrpy cntrlled. ACKOWLEDGMETS One f the authrs RR is grateful t UGC fr a fellwship. REFERECES [1] L. P. ammett, Sme Relatins between Reactin Rates and Equilibrium Cnstants, Chem. Rev., 1935, 17, 125. [2] C. K. Ingld, Mechanisms f acid and base catalyzed hydrlysis f esters, J. Chem. Sc., 193, 132. [3] R. W. Taft, J. Am. Chem. Sc., 1952, 74, [4] J. O. Schreck, J. Chem. Edn., 1971, 48, 13 and references cited therein. [5] G. M. Santerre, C. J. Jr. anrte, T. I. Crwell, J. Am. Chem. Sc., 1958, 8, [6].. Szmant, C. armuth, J. Am. Chem. Sc., 1964, 86, 299. [7] R. Fucks, D. M. Carlt, J. Am. Chem. Sc., 1963, 85, 14. [8] I.. Um, J. Y. Lee,. T. Kim, S. K. Bae, J. Org. Chem., 24, 69, [9] R. Ravi, R. Sanjeev and V. Jagannadham, Int. J. Chem. Kinet., 213, (DOI 1.12/kin.2818), 45,83-81 [1] J. March, Advanced Organic Chemistry, 4 th Editin, Jhn Wiley & Sns, 1992, p www. jac.inf 1582

5 V. Jagannadham et al Jurnal f Applicable Chemistry, 215, 4 (6): [11] J. E. Leffler, E. Grunwald, Rates and Equilibria f Organic Reactins, Wiley: ew Yrk, [12] E. S. Lewis, Investigatins f Rates and Mechanisms f Reactins, 3 rd Ed. Wiley: ew Yrk, [13] O. Exner, Cllect Czech. Chem. Cmmun., 1972, 3, [14] J. E. Leffler, J. Org. Chem., 1955, 2, V. Jagannadham Department f Chemistry, Osmania University, yderabad-57, IDIA 2. R. Ravi Department f Chemistry, Osmania University, yderabad-57, IDIA 3. R. Sanjeev Department f Chemistry, Ashka Institute f Engineering and Technlgy, yderabad-58252, IDIA AUTORS ADDRESSES www. jac.inf 1583