Indian Journal of ChemistryVol. 23A, June 1984, pp. 466-469

Kinetics of Iodide Ion Induced Debromination of Chalcone Dibromides

R S'RAGHAVAN*, K VELAYUTHAM & V RAJENDRAN

Department of Chemistry, Pachaiyappa's College, Madras 600030

Received 24 October 1983; accepted 14 February 1984

Iodide ion induced debromination of various chalcone dibromides in isopropanol-DMSO mixture(80:20, vIv) follows totalsecond order kinetics, first order each in chalcone dibromide and iodide ion. All substituents, regardless of their electronicnature, facilitate the reaction. The isokinetic relationship holds good and fJ = 33°C. The effect of change of isopropanol contentand replacement of isopropanol by methanol, ethanol and n-propanol on the rate of debromination has been studied and thedata rationalised on the basis of varying hydrogen bonding abilities of the alcohols with the iodide ion.

The mechanism and stereochemistry of the de-bromination of vicinal dibromides with iodide ionhave been well established I. However, reports onsubstituent effects on the rate of this reaction arecontradictory. While Baciocchi and Schiroli? andNasielski and Limbourg ' who studied the iodideinduced debromination of meso-stilbene dibromidereported that substituents, regardless of theirelectronic nature, accelerated the debromination,others" observed that in the debromination ofcinnamic acid dibromides by iodide ion in aq. aceticacid (80%, vIv) electron-withdrawing substituents suchas p-nitro and m-methoxy retarded the rate and theelectron-releasing p-methoxy group increased the rateconsiderably. In the case of debromination of chalconedibromide in acetone, it was reported" that m-chloroand p-nitro groups accelerated the rate whereas the p-methoxy group had such an accelerating effect as topreclude rate measurements. A recent report" on theiodide induced debromination of cinnamic aciddibromides in isopropanol points that all m- and p-substituents facilitate the reaction but the effect israther small and that the electron-withdrawing andelectron-releasing o-substituents retard and acceleratethe reaction, respectively. These divergent reportsprompted. us to undertake a detailed kinetic study ofthe iodide-induced debromination of a number ofchalcone (phenyl styryl ketone) dibromides inisopropanol-OM SO (80:20, v/v) and the results arereported herein.

Materials and MethodsThe choice of the solvent, isopropanol-OMSO

(80:20, vIv) was dictated by the solubility of thesubstrates. Reproducible rate data were obtainedwhen [substrate]: [iodide] ratio was kept either 1:4or 1:5. The chalcone dibromides were prepared andpurified as described in the literature 7• The reactionwas followed almost upto completion by withdrawing

466

suitable aliquots of the reaction mixture at regular timeintervals, quenching the reaction with ice-water andtitrating the liberated iodine against standardthiosulphate solution.

Isopropanol, OMSO and acetone all of AR gradewere used as such. Ethanol, methanol and n-propanolwere purified by standard procedures. Sodium iodideused was of GR, Loba grade.

The product of the reaction was isolated and foundto be the chalcone as revealed by direct comparison(m.p., m.m.p. and IR) with an authentic sample.

Results and DiscussionConstant values of rate constants are obtained only

when the rate data were fitted into the second orderrate equation (1)

k_ 2.303 a(b - 3x)- loglo~---

t(b - 3a) b(a - x)... (1)

where a = concentration of chalcone dibromide and b= concentration of iodide ion. The excellent fit of thedata into Eq. (1) testifies to the overall second ordercharacter of the reaction with first order dependenceeach on the substrate and iodide ion, and also points toa stoichiometry of 1:3for substrate to iodide. The ratioof 1:5used for substrate to iodide in the present study isquite conducive to this stoichiometry. The stoi-chiometric equation for the reaction is given by Eq. (2)

oII e

Y-C6H4-C-CHBr-CHBr-C6H4- X + 31 -

oII

Y C e e- 6 H4 - C - CH = CH - C6 H4 - X + 13 + 2 Br

..... _(21One of the three iodide ions is consumed in theformation of 13-, It is found that the reaction is notcomplicated by any competing or concurrent

RAGHAVAN et al.: I--INDUCED DEBROMINATION OF CHALCONE DIBROMIDES

solvolysis since the conductivity of the reactionmixture remains constant for well over 2 hr, the periodwithin which the debromination is complete.

The values of the second order rate constants for thedebromination of several chalcone dibromides at20°C, presented in Table 1, show that all substituents,regardless of their electronic character, increase therate of debromination. However, the increase in rate isnot marked, possibly indicating that there is nosubstantial charge on the p-carbon atom in thetransition state. The increasing trend may be due toconjugative stabilisation of the developing doublebond in the E2 transition state for the reaction (Scheme1) by the substituents. This could be true, however,only of the 0- and p-substituents in the styryl moiety orof the substituents in the phenyl nucleus.In order to evaluate activation parameters, thedebromination reaction was studied at three differenttemperatures and from the Arrhenius plots E. valuesand hence !J..Ht, !J..St and !J..Gt were calculated. Thedata are presented in Table 1. The constancy of !J..Gtvalues indicates that the debromination of all thechalcone dibromides occurs by the same mechanism.For attempting a rational explanation of the trend inAHt and Ast values, the concerted E2 mechanism inwhich the e-bromine is attacked by the iodide ion canbe invoked. The nucleophilic attack by the iodide ionon the c-bromine is probable in view of the - I and- M effects wielded by the carbonyl group and thelarge size of the phenyl group. In this E2 transitionstate there is partial cleavage of the C, - Br bond,loosening of the CfJ - Br bond and incipient formationof the C, - CfJ double bond. The observed variation inAHt values can be due to the following factors: (i)easeof nucleophilic attack by I - at the n-bromine; (ii)mesomeric stabilization of the incipient double bond in

the transition state; and (iii) ease of rupture of the Cil- Br bond and the exit of the p - Br as Br -. Thesignificantly low value of AHt for the 2-methylstyrylisomer must be due to the factors (ii) and (iii), Here thenucleophilic push provided by the p-CH3 group andthe conjugative stabilization by this group of thedeveloping double bond in the transition state greatlyfacilitate the reaction. The low AHt values for the 0-

chlorostyryl and p-chlorostyryl isomers must be due tothe factor (ii). The low !J..Ht value for the m-chlorostyryl isomer in which the mesomericinteraction is not possible defies explanation. Thehigher value of !J..Ht for p-Cl relative to o-Cl is alsohard to explain. The considerably low value of AHt forthe p-fluorostyryl isomer is almost certainly due to theeffect of factor (ii) but the much lower value for the m-fluorostyryl isomer runs counter to expectations.Comparing the p-Cl with the p-F group, the lattercontributes to higher tJ.Ht, apparently owing to itshigher electronegativity making it more difficult forthe p - Br to depart as Br -. The low values of !J..Ht forthe p-chlorophenyl isomer is possibly due to factor (i).The much lower AHt value for the p-chlorophenyl, p-

1._. !ItI .

I E2 T.a •• m•• Stat. )

oII

8r + lB. + Y - C& H4 - C - CH. CH - C& H4 - X

SCHEME-I.

Table I-Second Order 'Rate Constants and Activation Parameters for the Debromination of Chalcone Dibromides withIodide Ion in Isopropanol-DMSO (80:20, v/v) Mixture[Chalcone dibromide] =0.02 mol dm -3; [I -]=0.1 mol dm ?

Substituent 103 k2 E. flHt -flst flGtX Y (dm+mol "! s -I) (leJ mol :') (kJ mol ") (J mol " K -I) (kJ mol -I)

20° 25° J00

H H 2.44 5.43 7.39 80.65 78.12 28.13 86.642-CI H 3.48 4.42 7.20 54.20 51.68 115.50 86.723-CI H 2.84 4.30 7.07 66.71 64.24 74.46 86.764-Cl H 4.18 5.41 10.20 64.24 61.73 79.64 85.803-F H 2.68 2.77 3.72 24.82 22.30 218.00 88.354-F H 4.70 7.13 11.60 64.98 62..44 76.03 85.494-Me H 7.07 10.50 16.20 63.81 61.32 77.00 84.672-NOl H 3.38 3.69 4.01 12.39 9.84 258.40 88.163-N02 H 2.65 4.44 6.94 71.81 69.31 57.72 86.80H 4'-CI 4.93 5.37 8.40 41.27 38.76 157.00 86.30H 3'-NOl '7.44 11.60 15.50 53.53 50.97 111.40 84.764-CI 4'-Cl 6.40 8.28 10.50 35.53 33.02 174.00 85.74

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INDIAN J. CHEM., VOL. 23A, JUNE 1984

chlorostyryl isomer is due to factors (i) and (ii). Thelowest value of I1Ht for the o-nitrostyryl isomer isobviously due to factor (ii), thereby demonstrating theeffectiveness of the o-nitro group in entering intoconjugative interaction with the incipient double bondin the transition state. in the case of the m-nitrostyrylisomer, the lower value of I1Ht relative to that for theunsubstituted chalcone dibromide cannot be ex-plained. The low value in the case of the m-nitrophenylisomer is possibly due to the factor (i).

To account for the change in I1St values, we have toconsider the interplay of four contributing factors: (I)the E2 transition state has an antiperiplanarconformation with all the four reacting centres lying inthe same plane. This entails loss of freedom of rotationaround the C - C single bond. (2)The iodide ion, whichis free prior to reaction gets linked to the substrate inthe transition state, thus losing some of itstranslational entropy. (3) Prior to reaction, the iodideion bears a full negative charge and is solvated with ashell of solvent molecules surrounding it. On reachingthe transition state, the charge on the iodide getsdispersed over several atoms resulting in defreezing theshell of solvent molecules which earlier surrounded theiodide ion. (4) In the transition state the Cp- Br bond isbeing broken and the leaving of Br - might be expectedto freeze the solvent molecules surrounding it. Thecontribution by factors (1) (2) and (4) is negative whilethat by the factor (3) should be positive. The variationin I1St can be rationalised best by considering factor(4) as the dominant one. We should expect that anyelectron-releasing substituent in the styryl moietyshould provide a nucleophilic push, facilitating thedeparture of fJ - Br as Br -, and this should result in anincrease in the number of frozen solvent molecules inthe transition state and hence in a higher negativeentropy of activation. The high negative L\st noted forthe p-methylstyryl isomer relative to the unsubstitutedchalcone dibromide is in accord with this expectation.On the other hand, any electron-withdrawingsubstituent in the styryl portion should hinder thedeparture of the fJ - Br as Br -, resulting in a lessernegative I1St. The experimental data showed exactlythe reverse trend. The electron-withdrawing sub-stituents seem to facilitate the exit of the fJ - Br as Br -.A possible rationalisation of this unexpected trend isthat the electrostatic repulsion between the fJ - Br andthe electron-withdrawing phenyl group of thesubstituted styryl moiety forces out the former as Br -.

The reaction obeys isokinetic relationship and theplot of L\Ht versus L\St is linear with fJ (isokinetictemperature) = 33°C. The operation of the isokineticexpression also signifies that the same mechanismoperates in all the cases studied. The isokinetictemperature in the present case is very close to the

468

temperature range in which the debrominationkinetics has been studied (200-30~C).

The effect of variation in solvent composition on therate of the reaction has been studied and it is found thata decrease in the isopropanol content leads to anincrease in the rate of the reaction. For example, underthe conditions, [chalcone dibromide] =0.01 moldm -3; [I -] = 0.05 mol dm -3 and temp = 30°C, 103K2increased from 7.49 to 16.50 dm? mol " s -1 when %isopropanol (v/v) was decreased from 80 to 60. This isattributable to decreased hydrogen bonding betweenisopropanol and the iodide ion, resulting in a increasein the nucleophilic efficiency of the latter.

The effect of replacement of isopropanol by n-propanol, absolute ethanol and methanol has beenstudied and 103 k2 values are found to be 7.49, 6.03,4.19 and 1.78 dm ' mol -1 s -1 respectively under theconditions [chalcone dibromide] =0.02 mol dm -3;[I -] = 0.1 mol dm -3; solvent composition, alcohol:DMSO (80:20, v/v); and temp =30°. The rate decreaseswith an increase in the acidity of the hydroxyl protonof the alcohol component. It is apparent that hydrogenbonding between the alcohol and the iodide ion isresponsible for the observed variation in the rate of thereaction.

With a view to assessing the role played by thecarbonyl group in this reaction, the kinetics ofdebromination of chalcone dibromide, stilbenedibromide and benzalacetone dibromide have beenstudied in acetone medium at 30.5°C. The change overto acetone as solvent is necessitated by the very poorsolubility of stilbene dibromide in isopropanol-DMSOmixture. The data are given in Table 2. Clearly,incorporation of the CO group leads to a higher rate ofdebromination as revealed by the faster reactions ofchalcone dibromide and benzalacetone dibromiderelative to stilbene dibromide. This is ascribable to thegreater ease of nucleophilic attack at the e-bromineand conjugative stabilization of the developing doublebond in the transition state, due to the - I and - Meffects of the carbonyl group. The higher rate in thecase of chalcone dibromide relative to benzalacetonedibromide is evidently due to the greater degree of

Table 2-Rate Constants for the Debromination of SomeVicinal Dibromides with Iodide Ion in Acetone

[Vicinal dibromide] =0.0025 mol dm -3; [I -] =0.0125 mol dm -3;temp =30SC

Vicinal dibromide 103 k2(dm? mol :" s -I)

Ph-CHBr -CHBr- PhPh-CHBr -CHBr - CO -CH3Ph-CHBr-CHBr-CO-Ph

2.5232.0033.70

RAGHAVAN et al.: I--INDUCED D~ROMINATION OF CHALCONE D1BROMIDES

conjugative stabilization of the developing doublebond in the transition state afforded by the additionalphenyl group in the former.

p-Dimethylaminostyryl, p-methoxystyryl and 0-

hydroxystyryl isomers are found to undergodebromination extremely rapidly or spontaneously,even in the absence of added iodide ion. It is possiblethat these substituents, with their pronouncedconjugative potential, stabilize the olefin and thetransition state leading to it by conjugation. There isalso another mechanistic possibility; in these threecases the reaction may proceed via an initial ionizationto form a carbonium of the type: Ph - CO - CHBr-CH -C6H4 - NMe2(P), In view of the well knowneffectiveness of the p-NMe2' p-OMe or the o-OHgroups in stabilizing such a carbonium ion with theirpronounced mesomeric electron-releasing effecttowards the electron-deficient benzilic carbon, thismechanistic route cannot be dismissed.

AcknowledgementThe authors are indebted to Prof. Sp

Shanmuganathan for encouragement and facilities.Financial support for the project by the UGC, NewDelhi is also gratefully acknowledged.

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reactions (John Wile>,& Sons, New York) 1973, 333; IbneRasa K M & Ahmad Ashfag, J Sci Res (Lahore), 3 (1968)1-18.

2 Baciocchi E & Schiroli A, J chem Soc, (1969) 554.3 Nasielski J & Guiette Limbourg V, Bull Soc Chim Belges, 81

(1972) 351.4 lbne Rasa K M, Ahmed Ashfag & Amir Ud Din, J Sci Res

(Lahore), 3 (1968) 1-16.5 Davis T L & Heggie R, J org Chern, 2 (1937) 470.6 Mathai I M & Pratap Singh A J A, unpublished results; personal

communication from Dr. Mathai; Pratap Singh A J A, Ph.DThesis, University of Madras, Madras, 1974.

7 de la Mare P B D, Q Rev, 3 (1949) 126.

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