Theoretical Study of theMechanism for theGas-Phase PyrolysisKinetics of 2-MethylbenzylChlorideEDGAR MARQUEZ,1 JOSE R. MORA,2 TANIA CORDOVA,3 GABRIEL CHUCHANI,1

1Departamento de Quımica, Escuela de Ciencias, Universidad de Oriente Nucleo Sucre, Cumana, Venezuela2Centro de Quımica, Instituto Venezolano de Investigaciones Cientıficas (I.V.I.C.), Apartado 21827, Caracas 1020.A,Venezuela3Department of Medicinal Chemistry, College of Pharmacy, University of Florida. P.O Box. 100485. Gainesville, FL 32610

Received 4 February 2011; revised 12 April 2011; accepted 6 May 2011

DOI 10.1002/kin.20571Published online 19 August 2011 in Wiley Online Library (wileyonlinelibrary.com).

ABSTRACT: The study of the kinetics and mechanism of dehydrochlorination reaction of 2-methyl benzyl chloride in the gas phase was carried out by means of electronic structure calcu-lations using ab initio Møller-Plesset MP2/6-31G(d,p), and Density Functional Theory (DFT)methods: B3LYP/6-31G(d,p), B3LYP/6-31++G(d,p), MPW1PW91/6-31G(d,p), MPW1PW91/6-31++G(d,p)], PBE/6-31G(d,p), PBE/6-31++G(d,p). Investigated reaction pathways comprise:Mechanism I, a concerted reaction through a six-centered cyclic transition state (TS) geometry;Mechanism II, a 1,3-chlorine shift followed by beta-elimination and Mechanism III, a single-step elimination with simultaneous HCl and benzocyclobutene formation through a bicyclictype of TS. Calculated parameters ruled out Mechanism III and suggest the elimination reactionmay occur by either unimolecular Mechanism I or Mechanism II. However, the TS of the formeris 20 kJ/mole more stable than the TS of the latter. Consequently, the Mechanism I seem tobe more probable to occur. The rate-determining process is the breaking of C-Cl bond. Theinvolvement of π-electrons of the aromatic system was demonstrated by NBO charges andbond order calculations. The reaction is moderately polar in nature. C© 2011 Wiley Periodicals,Inc. Int J Chem Kinet 43: 537–546, 2011

Correspondence to: Gabriel Chuchani; e-mail: chuchani@ivic.gob.ve

Supporting Information is available in the online issue atwww.wileyonlinelibrary.com.

Contract grant sponsor: Consejo de Desarrollo Cientıfico yHumanıstico (C.D.C.H.).

Contract grant number: PG-03-00-6499-2006.c© 2011 Wiley Periodicals, Inc.

INTRODUCTION

The gas-phase elimination kinetics of alkyl halidesare usually known to occur through a four-centeredcyclic transition state (TS) type of mechanism(1). The products of the reaction are the corre-sponding olefin and hydrogen halide (reaction (1))[1–5].

538 MARQUEZ ET AL.

R1R2C CR3R4

H X

R1R2C CR3R4

H X

R1R2C CR3R4 + HX

(1)

The presence of a β-hydrogen at the adjacent tothe C H bond is required for molecular elimination.However, the absence of a Cβ H bond in some alkylhalides pyrolysis generally proceeds through a differ-ent type of reaction (1) and sometimes undergo via afree radical mechanism [1,2,5].

Several types of mechanisms for these gas-phaseelimination reaction have been proposed such as thepolarization of the C X bond (Cδ+ · · ·Xδ−), with theassistance of the adjacent C H bond (2) [6]. The verypolar structure in terms of an intimate ion-pair inter-mediate (3) [7], and finally, the semi-ion pair TS mech-anism (4) [8].

Among the mechanisms of gas-phase pyrolysis ofalkyl halides that calls to an attention is the molec-ular dehydrochlorination of 2-methylbenzyl chloride.This substrate without the presence of a β-hydrogenadjacent to the C X bond was found to give benzocy-clobutene and HCl (reaction (2)) [9].

CH2Cl

CH3

CH2

CH2

+ HCl (2)

The reaction in a static system was reported to befirst order, homogeneous, unimolecular, and the Arrhe-nius equation was expressed as log k1(s−1) = 11.47–(46.95 kcal/mole)/2.303 RT. According to the exper-imental results of this work [9], the authors have ar-gumented that an ortho effect was obtained. Alongthis line of thought, they suggested that the activa-tion energy is lower than the C Cl bond dissocia-tion energy of benzyl chloride (66.0 kcal/mole) [10]and therefore a radical nonchain process, that is, abiradical mechanism is to be ruled out. This means,that 2-methylbenzyl chloride decomposed by a molec-ular mechanism. Consequently, the mechanism wasthought to undergo through a six–membered cyclicTS as a homolytic (5) or heterolytic (6) in nature as

described below.

C

C

Cl

H

H H

H HC

C

Cl

H

H H

H H δ+

δ-

5 6

In addition to this interpretation, the degree of po-larity was discussed from the facts that log A = 11.47and Ea = 46.95 kcal/mole are rather low thus implyinga concerted TS of little polarity.

A decade later, Morello and Trahanovsky [11] car-ried out the flash vacuum pyrolysis of 2-methylbenzylchloride-α,α-d2 at 750◦C and 0.05 Torr. The reactionproducts are benzocyclobutene-α, α-d2, and HCl gas(reaction (3)).

CD2Cl

CH3

CD2

CH2

+ HCl750 °C

(3)

This result was believed to be consistent witha δ-elimination (Mechanism A, reaction (4)) or by1,3-sigmatropic shift of the chlorine atom followedby β-elimination of HCl (Mechanism B, reaction (5)).

Mechanism A:δ-elimination

(4)

CH2Cl

CH3

CH2

CH2

Mechanism B:1,3-sigmatropic

shift

CH2

CH3

Cl

β-eliminationof HCl

CH2

CH2

(5)

To consider a reasonable mechanism for the de-hydrochlorination of 2-methylbenzyl chloride to yieldbenzocyclobutene, the present work aimed at examin-ing the potential energy surface (PES) at the ab initioand Density Functional Theory (DFT) levels of the-ory and thus obtains the kinetic and thermodynamicparameters of this reaction in the gas phase. Our inter-est is whether this thermal decomposition undergoes

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STUDY OF THE KINETICS OF 2-METHYLBENZYL CHLORIDE 539

the unusual six-membered cyclic TS type of mecha-nism (Mechanism I), by a 1,3-sigmatropic shift of theCl atom followed by a four-membered cyclic structurefor HCl elimination (Mechanism II), or the simulta-neous HCl and benzocyclobutene formation througha bicyclic-type TS described in the following section(Mechanism III). The estimation of the kinetic andthermodynamic parameters in the elucidation of thisreaction mechanism was followed as described in theprevious theoretical calculations [12–15].

COMPUTATIONAL METHODSAND MODELS

The electronic structure calculations were carriedout at Møller-Plesset perturbation theory of sec-ond order MP2/6-31G(d,p) [16,17], and DFT ofBecke’s three-parameter formulation from functionalLee, Yang and Parr [B3LYP/6-31G(d,p), B3LYP/6-31++G(d,p) [18–20]. Perdew-Wang 1991 correlationfunctional [MPW1PW9/6-31G(d,p), MPW1PW91/6-31++G(d,p)] [21], and Perdew, Burke, and Ernser-hof [PBE/6-31G(d,p), PBE /6-31++G(d,p)] [22] havebeen used to elucidate a reasonable mechanism of theelimination kinetics of 2-methylbenzyl chloride in thegas phase. The calculations have been performed inGaussian 03 [23].

The Berny analytical gradient optimization routineswere used, with convergence on the density matrix was10−9 atomic units, threshold value for maximum dis-placement 0.0018 A, and maximum force was 0.00045Hartree/Bohr. The nature of the stationary points wascarried out by using frequency calculations for the min-imum energy structures and for the TS. The QuadraticSynchronous Transit (QST) protocol was used to locatethe TS structures. The identification of the TS struc-

tures was obtained through the normal-mode analysisby having a single imaginary frequency and the cor-responding transition vector (TV). To confirm the TSstructures associating the reactant and products in theminimum energy path, the Intrinsic Reaction Coordi-nate (IRC) calculations were made.

The thermodynamic values of zero-point vibrationalenergy (ZPVE), temperature corrections (E(T)), andabsolute entropies (S(T)), were achieved from fre-quency calculations. Temperature corrections and ab-solute entropies were obtained when considering idealgas behavior from the harmonic frequencies and mo-ments of inertia by standard methods [24] at averagetemperature and pressure values within the experimen-tal range. Scaling factors for frequencies and zero pointenergies were cited from the literature [25,26].

The Transition State Theory (TST) [10] permittedto estimate the first order rate coefficient k(T) and as-suming that the transmission coefficient is equal to 1.The kinetic and thermodynamic parameters such as�G‡, �H‡ and �S‡ were determined as previouslydescribed [27,28].

RESULTS AND DISCUSSION

The dehydrochlorination of 2-methylbenzyl chloridein the gas phase was studied to help and to under-stand the reaction mechanism and determine whetherMechanism I, a cyclic unimolecular elimination in-volving the aromatic ring; Mechanism II, 1,3 sig-matropic chlorine migration followed by eliminationof HCl; or Mechanism III, a single step elimina-tion with simultaneous HCl and benzocyclobuteneformation through a bicyclic type of TS. The cal-culated parameters for Mechanism I, Mechanism II,and Mechanism III are given in Tables I, II, and III

Table I Thermodynamic and Arrhenius Parameters Calculated for Gas-Phase Decomposition of 2-MethylbenzylChloride at 425.4◦C: Mechanism I

Ea log A �H�= �S �= �G�=Method 104 k1 (kJ/mol) (s−1) (kJ/mol) (J/mol K) (kJ/mol)

Experimental 2.93 196.4 12.22 190.2 −26.29 208.3B3LYP/6–31G(d,p) 274.5 190.2 13.67 184.4 1.29 183.5B3LYP/6–31 ++ G(d,p) 705.8 185.3 13.70 179.5 2.00 178.1B3PW91/6–31G(d,p) 191.1 202.2 13.40 196.4 −3.69 199.0B3PW91/6–31 ++ G(d,p) 385.1 198.6 13.43 192.8 −3.12 194.9MPW1PW91/6-31G(d,p) 48.1 211.2 13.48 205.4 −2.23 207.0MPW1PW91/6-31 ++ G(d,p) 99.1 207.4 13.51 201.6 −1.74 202.8PBEPBE/6-31G(d,p) 941.9 173.9 13.29 168.0 −5.91 172.2PBEPBE /6–31 ++ G(d,p) 319.7 170.5 13.25 164.7 −6.62 169.3PBE1PBE1/6–31G(d,p) 39.1 212.6 13.49 206.8 −2.01 208.2PBE1PBE1/6–31G++(d,p) 81.4 208.7 13.51 202.9 −1.59 204.0MP2/6–31G(d,p) 0.014 268.9 13.80 263.1 4.1 266.0Fast step 2: B3PW91/6-31 ++ G(d,p) 4,152,500 104.7 12.45 98.93 −5.42 114.3

International Journal of Chemical Kinetics DOI 10.1002/kin

540 MARQUEZ ET AL.

Table II Thermodynamic and Arrhenius Parameters Calculated for Gas-Phase Decomposition of 2-MethylbenzylChloride at 425.4◦C: Mechanism II

Ea log A �H�= �S �= �G�=Method 104 k1 (kJ/mol) (s−1) (kJ/mol) (J/mol K) (kJ/mol)

Experimental 2.93 196.4 12.22 190.2 −26.29 208.3B3LYP/6-31G(d,p) 124.3 207.7 13.61 201.7 0.22 201.5B3LYP/6-31++G(d,p) 0.5 238.7 13.53 232.9 −1.20 233.8B3PW91/6-31G(d,p) 6.1 224.2 13.56 218.5 −0.77 219.0B3PW91/6-31++G(d,p) 15.7 219.4 13.61 213.7 0.18 213.5MPW1PW91/6-31G(d,p) 0.3 237.9 13.28 232.1 −5.98 236.3MPW1PW91/6-31++G(d,p) 2.0 229.8 13.48 224.1 −2.25 225.6PBEPBE/6-31G(d,p) 377.9 201.2 13.62 195.4 0.47 195.1PBEPBE /6-31++G(d,p) 1286.2 193.4 13.57 187.6 −0.47 187.9PBE1PBE1/6-31G(d,p) 0.1 234.2 12.39 228.4 −23.07 228.8PBE1PBE1/6-31G++(d,p) 7.0 225.9 13.74 220.1 2.79 218.2MP2/6-31G(d,p) 0.0022 276.4 13.60 270.6 −0.50 276.8Fast step 2: B3PW91/6-31++G(d,p) 976,167,104 57.3 13.28 51.54 −6.11 55.81Fast step 3*: B3PW91/6-31++G(d,p) 4,152,500 104.7 12.45 98.93 −5.42 114.3

∗Step 3, Mechanism II is the same as step 2, Mechanism I

Table III Thermodynamic and Arrhenius Parameters Calculated for Gas-Phase Decomposition of 2-MethylbenzylChloride at 425.4◦C: Mechanism III

Ea log A �H�= �S �= �G�=Method 104 k1 (kJ/mol) (s−1) (kJ/mol) (J/mol K) (kJ/mol)

Experimental 2.93 196.4 12.22 190.2 −26.29 208.3B3LYP/6-31G(d,p) 0.000000004 346.6 13.53 340.8 1.21 339.2B3LYP/6-31G++(d,p) 0.0000002 333.1 14.21 327.3 11.76 319.1B3PW91/6-31G(d,p) 0.000000005 349.2 13.82 343.3 4.33 340.3B3PW91/6-31++G(d,p) 0.00000004 337.8 13.96 332.0 6.90 327.2MW1PW91/6-31G(d,p) 0.0000000005 364.1 13.93 358.3 6.28 353.9MW1PW91/6-31++G(d,p) 0.000000002 350.5 13.60 344.7 0.08 344.7PBEPBE/6-31G(d,p) 0.000001 316.5 14.91 310.7 25.11 293.1PBEPBE/6-31++G(d,p) 0.000008 310.7 14.17 304.9 11.00 297.2PBE1PBE1/6-31G(d,p) 0.00000001 351.6 14.46 345.8 16.60 334.2PBE1PBE1/6-31++G(d,p) 0.000000009 345.6 13.81 339.8 4.13 336.9

respectively. The enthalpies and energies of activa-tion for the slow step of both mechanisms are similar.For Mechanism I (Table I), good agreement betweencalculated and experimental enthalpy and energy ofactivation was found using B3PW91/6-31++G(d,p);the inclusion of polarization functions gave smaller,improving the calculated parameters; MPW1PW91/6-31++G(d,p) also gave higher calculated Ea, �H pa-rameters. MP2/6-31G(d,p) gave over-estimated values.For Mechanism II (Table II), we found good agreementfor enthalpy and energy of activation using PBEPBE/6-31++G(d,p); other functional gave higher activationparameters. In the case of Mechanism III (Table III),the calculated enthalpy and consequently the energy ofactivation are much higher than the experimental val-ues, deviating by over 100 kJ/mol. These results implyMechanism III is very unlikely. Almost every methodgave erroneous entropy values. However, in the case of

PBE1PBE1/6-31G(d,p) for Mechanism II, a better en-tropy value was obtained. For both Mechanisms I andII, the first step is rate-determining. The experimentalentropy of activation −26 implies the loss of degreesof freedom in the TS. Since both Mechanisms I and IIappear to be possible based on the calculated kineticand thermodynamic parameters, we have examined theNBO charges and bond orders for further evidence tosupport a given mechanism.

Transition State and Mechanisms

The optimized structures of reactants, TS, and prod-ucts from B3PW91/6-31++G(d,p) calculations for theMechanisms I, II, and III in the dehydrochlorination of2-methlbenzylchloride are shown in Figs. 1–5. Mech-anism III can be disregarded because the calculatedenthalpy and energy of activation are much larger, and

International Journal of Chemical Kinetics DOI 10.1002/kin

STUDY OF THE KINETICS OF 2-METHYLBENZYL CHLORIDE 541

Cl Cl

CH2

H+ HCl + HCl

2slow

1fast

Mechanism I

+ HCl

+ HCl

fast

Cl

CH3 CH3

3

slow

1

f astCl Cl2

Mechanism II

Cl Cl

CHH

+ HCl

Mechanism III

Figure 1 Optimized structures for reactant R, transition state TS, and product P, in the gas-phase elimination of 2-methylbenzylchloride at B3PW91/6-31++G (d,p) theoretical level. Mechanism I, step 1. [Color figure can be viewed in the online issue,which is available at wileyonlinelibrary.com.]

consequently deviated from the experimental values(Fig. 5).

The slow step in Mechanism I, in Fig. 1, illustratesa six-centered TS structure comprising atoms H6 (inmethyl group), Cl1, C2 (chloromethyl group), C3, C4,C5 (methyl). Step 2 in Mechanism I, the isomerizationelectrocyclic reaction occurs through four-centered TSgeometry, shown in Fig. 2. For Mechanism II, Fig. 3shows the structure of reactant, TS, and product for thefirst step, the 1,3 chlorine migration. The TS is four-centered geometry with Cl1 partially linked to C2 andC4. The second step in this mechanism is the elim-ination through four-centered TS geometry shown inFig. 4. Step 3 in this mechanism is the same as step 2 ofMechanism I. Further analysis of geometrical param-eters, NBO charges, and bond order is presented for

C4

C3

C2Cl1

C5

H6

H H

Scheme 1

C4

C3

C2

CH3

Cl1

Scheme 2

Mechanism I and II. Drawings of the TS geometriesfor the rate-determining step of Mechanism I and IIare shown in Schemes 1 and 2, respectively. Atom

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542 MARQUEZ ET AL.

Figure 2 Optimized structures for reactant R, transition state TS, and product P, in the gas-phase elimination of 2-methylbenzylchloride at B3PW91/6-31++G (d,p) theoretical level. Mechanism I, step 2. [Color figure can be viewed in the online issue,which is available at wileyonlinelibrary.com.]

Figure 3 Optimized structures for reactant R, transition state TS, and product P, in the gas-phase elimination of 2-methylbenzylchloride at B3PW91/6-31++G (d,p) theoretical level. Mechanism II, step 1. [Color figure can be viewed in the online issue,which is available at wileyonlinelibrary.com.]

Figure 4 Optimized structures for reactant R, transition state TS, and product P, in the gas-phase elimination of 2-methylbenzylchloride at B3PW91/6-31++G (d,p) theoretical level. Mechanism II, step 2. [Color figure can be viewed in the online issue,which is available at wileyonlinelibrary.com.]

Figure 5 Optimized structures for reactant R, transition state TS, and product P, in the gas-phase elimination of 2-methylbenzylchloride at B3PW91/6-31++G (d,p) theoretical level. Mechanism III. [Color figure can be viewed in the online issue, which isavailable at wileyonlinelibrary.com.]

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STUDY OF THE KINETICS OF 2-METHYLBENZYL CHLORIDE 543

Table IV Structural Parameters of Reactant (R), Transition State (TS) and Products (P) for Gas-Phase Elimination2-Methylbenzyl Chloride at B3PW91/6-31++G(d,p) Theoretical Level: Mechanism I, Step 1

Interatomic Length (A)

Cl1 C2 C2 C3 C3 C4 C4 C5 C5 H6 H6 Cl1

R 1.832 1.490 1.410 1.510 2.141 2.981TS 3.440 1.371 1.460 1.421 1.711 1.711P 3.450 1.350 1.492 1.351 2.491 1.311

Angle (degrees)

Cl1 C2 C3 C4 C2 C3 C4 C5 C3 C4 C5 H6 C4 C5 H6 Cl1 C5 H6 Cl1 C2 H6 Cl1 C2 C3

TS 36.367 8.953 −66.285 97.061 −45.877 −39.754Imaginary frequency (cm−1)

198.64

Table V Structural Parameters of Reactant (R), Transition State (TS) and Products (P) for Gas-Phase Elimination of2-Methylbenzyl Chloride atB3PW91/6-31++G (d,p) Theoretical Level: Mechanism II, Step 1

Interatomic Length (A)

Cl1 C2 C2 C3 C3 C4 C4 Cl1

R 1.825 1.496 1.410 3.518TS 3.075 1.371 1.455 2.802P 3.277 1.348 1.527 1.876

Angle (degrees)

Cl1 C2 C3 C4 C2 C3 C4 Cl1 C3 C4 Cl5 C2 C4 Cl1 C2 C3

TS 60.511 −66.722 27.596 −30.743

Imaginary frequency (cm−1)267.36

numbers are displayed for clarity in the discussion ofthe TS structural parameters, charges, and bond orderanalysis.

The TS for the rate-determining step of Mecha-nism I is a six-centered geometry comprising atomsH6, Cl1, C2, C3, C4, C5; atoms C3 and C4 are part of thearomatic system. The TS configuration is nonplanar asseen in dihedral angles; H6 and Cl1 atoms are off-planerespect to the four carbon atoms. The imaginary fre-quency is associated with the transfer of the hydrogenH6 to Cl1 to form HCl. Calculated geometrical param-eters illustrate important elongation of Cl1 C2 bondin the TS, 3.4 A (Table IV). Both C2 C3 and C4 C5

distances shorten indicating the double bond character;however, the double bond formation has not advancedto the same extent in the TS; C2 C3 and C4 C5

distances are 1.37 and 1.42 A in the TS, respectively.For Mechanism II the TS is a four-centered cyclic

geometry comprising atoms Cl1, C2, C3, C4, shown inScheme 2 and Fig. 3. The Cl1 atom is positioned in-termediate between C2 and C4, however, the C4 Cl1distance in the TS, 2.80 A (Table V), is shorter to that

of Cl1 C2 in the TS, 3.07 A; both distances are farfrom the equilibrium distance for Cl-C single bond 1.8A, implying the Cl atom is detached in the TS. BothC2 C3 and C3 C4 bonds have double bond characterin the TS, as seen in the distances 1.37 and 1.46 A, re-spectively. Dihedral angles show the TS are not a planarconfiguration. The imaginary frequency that character-izes the TS is mainly associated with the transfer of thechlorine atom Cl1 from C2 to C4.

IRC calculations were performed to verify the TSof the rate-determining steps for Mechanism I and IIlead to the products and the reactant in the forward andreverse directions, respectively. IRC plots are givenas supplementary information (Figs. 6 and 7). Furtheranalysis of the reaction and the TS structure were car-ried out by means of NBO calculations. NBO chargesand bond indexes are discussed in the next sections.

NBO Charges

The changes in electron density distribution occurringin the reaction pathway for Mechanisms I and II are

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544 MARQUEZ ET AL.

Table VI NBO Charges for Reactant (R), TransitionState (TS) and Products (P), at B3PW91/6-31++G (d,p)Theoretical Level: Mechanism A, Step 1

Cl1 C2 C3 C4 C5 H6

R −0.091 −0.439 −0.080 −0.012 −0.702 0.257TS −0.517 −0.213 −0.145 −0.104 −0.596 0.264P −0.300 −0.462 −0.059 0.098 −0.355 0.266

Table VII NBO Charges for Reactant (R), TransitionState (TS) and Products (P), at B3PW91/6-31++G (d,p)Theoretical Level: Mechanism B, Step 1

Cl1 C2 C3 C4

R −0.091 −0.439 −0.080 −0.012TS −0.624 −0.155 −0.143 0.148P −0.093 −0.366 −0.080 −0.072

demonstrated by NBO charges, shown in Tables VIand VII. NBO charges for the elimination reactionof 2-methylbenzyl chloride, through Mechanism I(Table VI) show an increase in electron density at thechlorine atom Cl1 (from about −0.09 in the reactant to−0.52 in the TS). The hydrogen H6 that is transferredto Cl1 becomes more positive (from 0.25 to about 0.26in the TS). Atom C2 in the chloromethyl group inthe reactant becomes less negative in the TS, show-ing a change in charge from −0.44 to −0.21, Thecarbon atoms in the aromatic ring, C3 and C4 also un-dergo electron density changes in the TS; C3 becomesmore negatively charged, from −0.08 to −0.15 and C4

from −0.01 to −0.10. Carbon C5 at 2-methyl groupdecreases in electron density, as charge change from−0.70 to −0.60 in the TS.

For Mechanism II, NBO charges are shown inTable VII. The most remarkable change is the increasein negative charge at Cl1 from −0.09 to −0.62 in theTS. Both carbons C2 and C4 show less electron density;at C2 charge change from −0.44 to −0.16 in the TSand at C4 from −0.01 to 0.15 in the TS. C3 becomesmore negative from −0.08 to −0.14 in the TS. Thecharges distribution in the TS resembles an intimateion pair between Cl1 and an allyl system formed byC2 C3 C4.

Table VIII Wiberg Bond Index of Reactant (R), Transition State (TS) and Products (P) for Gas-Phase Elimination of2-Methylbenzyl Chloride at B3PW91/6-31++G (d,p) Theoretical Level: Mechanism I, Step 1

Cl1 C2 C2 C3 C3 C4 C4 C5 C5 H6 H6 Cl1 Sy

BRi 0.968 1.038 1.354 1.033 0.907 0.004

BT Si 0.136 1.621 1.119 1.269 0.542 0.354 0.78

Bpi 0.011 1.710 1.046 1.746 0.007 0.873

%Ev 86.95 86.69 76.00 33.18 40.56 40.11

Bond Order Analysis

The progress along different reaction coordinates in theminimum energy path can be followed using bonds or-ders of the reactant, TS, and products [29–31]. Wibergbond indexes [32] were computed using the naturalbond orbital NBO program [33] implemented in Gaus-sian 03W. These indexes allow estimating bond ordersfrom population analysis. Bond breaking and makingprocess involved in the reaction mechanism are moni-tored by means of the Synchronicity (Sy) concept pro-posed by Moyano et al. [34] defined by the expression

Sy = 1 −[

n∑i=1

|δBi − δBav|/δBav

]/2n − 2

where n is the number of bonds directly involved inthe reaction.

The calculated bond index is Bi, and the relativevariation of the bond index is obtained from

δBi = [BTS

i − BRi

]/[BP

i − BRi

]where the superscripts R, TS, P, represent reactant, TS,and product, respectively.

The average value is calculated from

δBave = 1/n

n∑i=1

δBi

For Mechanism I, Wiberg bonds indexes of thespecies involved Bi were calculated for those bondsundergoing changes in the elimination reaction, that is,Cl1 C2, C2 C3, C3 C4, C4 C5, C5 H6, and H6 Cl1.Changes in other bonds are smaller and were not con-sidered in the analysis. The synchronicity parameterhad been used to explain if a concerted reaction pro-ceeds in a synchronic fashion where all events have thesame progress along the reaction coordinate (Sy = 1)or if the reaction is not synchronic (Sy = 0).

The examination of calculated bond orders given asWiberg indexes for Mechanism I, Table VIII, showsthe reaction is dominated by the elongation of Cl1 C2

bond and C2 C3 double bond formation, about 87%,

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STUDY OF THE KINETICS OF 2-METHYLBENZYL CHLORIDE 545

Table IX Wiberg Bond Index of Reactant (R),Transition State (TS) and Products (P) for Gas-PhaseElimination of 2-Methylbenzyl Chloride atB3PW91/6-31++G (d,p) Theoretical Level:Mechanism II, Step 1

Cl1 C2 C2 C3 C3 C4 C4 Cl1 Sy

BRi 0.968 1.038 1.353 0.018

BT Si 0.241 1.550 1.146 0.188 0.96

Bpi 0.051 1.794 0.988 0.901

%Ev 79.25 67.66 56.57 19.18

and in lesser extent by C3 C4 bond order change 76%.The other reaction coordinates are less advanced, thatis, C4 C5 double bond formation (33%), C5 H6 bondbreaking (41%), and H6 Cl1 bond formation (40%).The TS is highly polarized with important electrondensity at Cl1, Cl1 C2 bond almost broken in the TS,while other reaction changes are less advanced, thesynchronicity parameter Sy = 0.78 suggests a polarnonsynchronous process.

In Mechanism II, Table IX, the rate-determiningprocess is the breaking of Cl1 C2 bond (79%).Changes in bond order for C2 C3 and C3 C4 are lessadvanced 68% and 56%, respectively. The formationof C4 Cl1 shows small progress in the TS comparedto the other reaction coordinates, 19%, overall Mech-anism II illustrates a more synchronous process of Sy= 96 when compared with Mechanism I.

CONCLUSIONS

The gas-phase elimination of hydrogen chloride from2-methylbenzyl chloride has been studied using elec-tronic structure calculations to help determine the na-ture of the reaction and propose a reasonable reac-tion mechanism. Three possible mechanisms were an-alyzed: Mechanism I, a concerted reaction through asix-centered TS geometry, Mechanism II, 1,3-chlorineshift followed by beta elimination, and Mechanism III,as single step elimination with simultaneous HCl andbenzocyclobutene formation through a bicyclic type ofTS. Calculated parameters allow disregarding Mecha-nism III, which produced calculated activation param-eters 100 kJ/mol higher than the experimental values.Calculations suggest that both Mechanisms I and IIare possible in the experimental conditions, yet Mech-anism I appears to be more probable, since its TS is20 kJ/mole more stable than TS of Mechanism II. Bothprocesses are dominated by the breaking of Cl–C bond.NBO charges also suggest that the reaction through ei-ther mechanism posses high charge separation in theTS, and possibly an ion-pair character, the counter ion

in Mechanism I resembling 1,3-butadiene and allyl sys-tem in Mechanism II. Involvement of π-electrons oc-curs in both Mechanisms I and II, demonstrated byNBO charges and bond orders.

SUPPORTING INFORMATION

IRC were obtained from the gas-phase elimina-tion kinetic of 2-methylbenzyl chloride. This ma-terial is available as Supporting Information atwww.wileyonlinelibrary.com.

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