Experimental and Theoretical Studies of the Homogeneous

J. Phys. Chem. A 2010, 114, 4203–4209

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Experimental and Theoretical Studies of the Homogeneous, Unimolecular Gas-Phase Elimination Kinetics of Trimethyl Orthovalerate and Trimethyl Orthochloroacetate Edgar Marquez,† Rosa M. Domıńguez,† Jose´ R. Mora,† Tania Co´rdova,‡,§ and Gabriel Chuchani*,† Centro de Quı´mica, Instituto Venezolano de InVestigaciones Cientı´ficas (IVIC), Apartado 21827, Caracas 1020-A, Venezuela, and Escuela de Quı´mica, Facultad de Ciencias, UniVersidad Central de Venezuela, Apartado 1020-A, Caracas, Venezuela ReceiVed: January 19, 2010; ReVised Manuscript ReceiVed: February 25, 2010

The rates of gas-phase elimination of trimethyl orthovalerate and trimethyl orthochloroacetate have been determined in a static system, and the reaction Pyrex vessels have been deactivated with the product of decomposition of allyl bromide. The reactions are unimolecular and follow a first-order rate law. The working temperature and pressure ranges were 313-410 °C and 40-140 Torr, respectively. The rate coefficients for the homogeneous reaction are given by the following Arrhenius expressions: for trimethyl orthovalerate: log k (s-1) ) [(14.00 ( 0.28) - (196.3 ( 1.7) (kJ/mol)] (2.303RT)-1, (r ) 0.9999); and for trimethyl orthochloroacetate: log k (s-1) ) [(13.54 ( 0.21) - (209.3 ( 1.9)(kJ/mol)](2.303RT)-1, (r ) 0.9998). The theoretical calculations of the kinetic and thermodynamic parameters were carried out by using B3LYP, B3PW91, MPW1PW91, and PBEPBE methods. The theoretical results show reasonably good agreement with the experimental energy and enthalpy of activation values when using the B3PW91/ 6-31++G** method for trimethyl orthovalerate and PBEPBE /6-31++G** for trimethyl orthochloroacetate. These calculations suggest a molecular concerted nonsynchronous mechanism where C-OCH3 bond polarization, in the sense Cδ+ · · · δ-OCH3, is the rate-determining step. The increase in electron density of the oxygen atom at OCH3 eases the abstraction of the hydrogen of the adjacent C-H bond in a four-membered cyclic structure to give methanol and the corresponding unsaturated ketal. The electrondonor substituent enhances decomposition rates by stabilizing the positive charge developing in the transition state at the carbon bearing the three methoxy groups, whereas the electron-withdrawing substituent destabilizes this charge, thus retarding the reaction. I. Introduction Little information on the gas-phase pyrolysis of orthoesters has been described. In this respect, the thermal decomposition of some of this type of compounds, under different conditions, is reported. Several triethyl orthoesters1 in the presence of nickel at 250-260 °C yielded ethyl ether and the corresponding ethyl ester (reaction 1). Moreover, in this work,1 distillation of

triethyl orthophenylacetate simultaneously produced ethyl phenylacetate and the corresponding phenylketene acetal (reaction 2). The latter compound when heated in a high bomb pressure at 260-270 °C was found to give mainly ethyl phenylacetate, ethylene gas, and an unidentified solid. * Corresponding author. † Instituto Venezolano de Investigaciones Cientı´ficas (IVIC). ‡ Universidad Central de Venezuela. § Present address: Department of Medicinal Chemistry, College of Pharmacy, University of Florida, P.O. Box 100485, Gainesville, Florida 32610.

The substrate diethylbenzyl orthoacetate2 heated in a steel bomb at 200 °C produced o-tolylacetate from rearrangement of the ketene ethylbenzyl acetal intermediate. Additionally, the pyrolysis of triethyl orthobenzoate with no R-hydrogen gave ethyl benzoate and diethyl ether (reaction 3).

The work of triethyl orthophenyl acetate3 was revisited by slow distillation of the substrate. The results were similar1 with the formation of phenylketene diethyl acetal and ethyl phenylacetate. Otherwise, trimethyl orthophenylacetate3 heated at 250-260 °C produced phenylketene dimethyl acetal and methyl phenylacetate (reaction 4). Because of the vigorous formation of methanol, the pyrolysis was thought to occur by the interaction of two molecules of trimethyl orthophenylacetate.

10.1021/jp1005296  2010 American Chemical Society Published on Web 03/08/2010

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Marquez et al. TABLE 1: Ratio of Final (Pf) to Initial Pressure (P0) of the Substrate substrate

T (°C)

P0 (Torr)

Pf (Torr)

Pf/Po

av

trimethyl orthovalerate

313.6 322.6 332.3 342.6 351.4 370.3 379.8 390.8 400.3 410.0

60 70 50 90 110 70 67 75 129 140

117 135 105 176 215 132 128 148 260 290

1.9 1.9 2.1 1.9 1.9 1.9 1.9 2.0 2.0 2.1

2.0

trimethyl orthohloroacetate

2.0

TABLE 2: Stoichiometry of the Reactions

Most recently, the experimental results and the kinetic parameters of the homogeneous unimolecular, gas-phase elimination kinetics of triethyl and trimethyl orthoesters4,5 suggested the mechanisms of these elimination reactions as described in reactions 5 and 6.

substrate

T (°C)


332.2

trimethyl orthochloroacetate

379.8

time (min)

% reaction (pressure)

% methanol (GC)

1 3 5 7 12 3 5 8 10 15 20

7.0 19.0 29.8 39.0 57.2 11.0 17.0 21.0 30.0 41.0 51.0

6.4 18.8 29.1 38.0 56.0 10.6 16.4 20.2 28.6 40.2 50.3

II. Experimental Section

In a later work, DFT theoretical calculations on the thermal decomposition of the ethyl and methyl orthoesters6 showed to be in good agreement with the experimental values. Apparently, the estimated results supported the fourmembered cyclic transition-state type of mechanism depicted in reactions 5 and 6. The unsaturated ketal intermediate from ethyl orthoesters (reaction 5) is unstable and under experimental conditions rapidly decomposed through a sixmembered cyclic transition state to give ethylene and the corresponding ethyl ester. Further investigations and more experimental data appear to be needed to gain more insight into the mechanisms of the gasphase thermal elimination of organic orthoesters, especially methyl orthoesters. Consequently, the present work considered it interesting to examine the homogeneous, gas-phase elimination kinetics of trimethyl orthovalerate and trimethyl orthochloroacetate in combination with theoretical calculations.

Both reagents, trimethyl orthovalerate and trimethyl orthochloroacetate, were acquired from Aldrich and distilled several times until the fraction was >98% purity. The purity of the substrates and products and their identifications were determined by GC/MS (Saturn 2000, Varian; capillary column DB-5MS, 30 mm × 0.250 mm, i.d. 0.25 µm). The products methanol and the corresponding methylketene acetal were quantitatively analyzed using a chromatograph Varian Star 3600 CX with a thermal conductivity detector (capillary column: GSQ, 30 m long and 0.53 i.d., helium gas carrier). Kinetics. The kinetics runs were performed in a static reaction system as described before.7-9 The reaction vessel was deactivated with allyl bromide decomposition, and the rates were followed manometrically. The temperature was maintained to better than (0.1 °C with a thermopar of iron-constantan attached to a Digital Multimeter Omega 3465B. Different points along the reaction vessel showed no temperature gradient. The starting materials were injected directly into the reaction vessel through a silicone rubber septum. The amount of substrate used for each reaction was ∼0.05 to 0.1 mL. III. Results and Discussion The stoichiometry of reaction 7

in a static system and deactivated with allyl bromide indicates that the final pressure, Pf, should be twice the initial pressure, P0. The average experimental results for Pf/P0 values at five different temperatures and 10 half-lives were 2.0 (Table 1).

Gas-Phase Elimination Kinetics

J. Phys. Chem. A, Vol. 114, No. 12, 2010 4205 TABLE 3: Homogeneity of the Reaction substrate trimethyl orthovalerate trimethyl orthochloroacetate a

temp (°C)

S/V (cm-1)

104k (s-1)a

104k (s-1)b

332.2

1 6 1 6

52.3 ( 4.1 62.7 ( 8.7 29.5 ( 5.1 53.3 ( 7.3

11.8 ( 0.4 11.9 ( 0.2 6.0 ( 0.3 5.9 ( 0.2

379.8

Clean Pyrex vessel. b Vessel seasoned with allyl bromide.

TABLE 4: Effect of Toluene on Rates (P0: substrate; Pi: toluene) Figure 1. Kinetic profile for gas-phase elimination of trimethyl orthovalerate at 332.2 °C

temp (°C)

P0 (Torr)

Pi (Torr)

Pi/P0

104k (s-1)


332.3


379.8

80 90 74 60 80 88 82 62 60

0 92 172 182 0 91 130 160 190

0 1.0 2.3 3.0 0 1.0 1.6 2.6 3.2

11.80 ( 0.43 11.52 ( 0.51 11.83 ( 0.48 11.71 ( 0.39 6.21 ( 0.31 5.90 ( 0.28 6.19 ( 0.33 6.31 ( 0.35 5.92 ( 0.28

substrate

TABLE 5: Variation of the Rate Coefficient with Initial Pressure temp (°C)

P0 (torr)

104k1 (s-1)


332.3


379.8

40 56 62 80 110 67 80 100 120 140

11.82 ( 0.41 11.57 ( 0.35 11.64 ( 0.57 11.78 ( 0.61 11.84 ( 0.28 6.21 ( 0.31 5.92 ( 0.28 5.98 ( 0.21 6.12 ( 0.23 5.93 ( 0.19

substrate Figure 2. Kinetic profile for gas-phase elimination of trimethyl orthochloroacetate at 379.8 °C

For additional verification of the stoichiometry, up to 60% reaction, it was found that the percentage decomposition of the substrate calculated from pressure measurements was in good agreement with the quantitative chromatographic analyses of the product methanol (Table 2). The kinetic profiles of the elimination process of each substrate are shown in Figures 1 and 2. The homogeneity of these elimination reactions were examined by carrying out several runs in a vessel with a surface-to-volume ratio of 6.0 as compared with that of a normal vessel, which is equal to 1 (Table 3). The packed and unpacked clean Pyrex vessels showed a significant effect on the rates. However, when the packed and unpacked seasoned vessels are seasoned with allyl bromide, no effect on the rate coefficients was obtained. The absence of the free radical chain reaction was observed by carrying out several runs in the presence of different proportions of toluene as inhibitor (Table 4). No induction period was observed. The rates are reproducible with a standard deviation not greater than 5% at a given temperature. The rate coefficients for the orthoesters, determined in seasoned vessel, are calculated from k1 ) (2.303/t) log[(2P0 Pt) - P0)], have been found to be independent of the initial pressure (Table 5). The first-order plots of log(2P0 - Pt) against time, t, are satisfactorily linear to at least 60% decomposition. The variation of the first-order rate coefficient shown in Table 6 and Figure 3 leads to the following Arrhenius equations, where 95% confidence coefficients from a least-squares method are used:

TABLE 6: Temperature Dependence of the Rate Coefficient substrate trimethyl orthovalerate


temperature (°C)

104k1 (s-1)

313.6 322.3 332.3 342.6 351.4 360.2 360.0 370.3 380.1 390.8 400.3 410.4

3.33 ( 0.11 5.91 ( 0.13 11.64 ( 0.57 22.89 ( 0.91 39.88 ( 2.08 60.92 ( 3.50 2.12 ( 0.09 3.50 ( 0.12 6.02 ( 0.17 11.12 ( 0.35 20.13 ( 1.13 36.89 ( 2.05

Trimethyl orthovalerate:

log k (s-1) ) [(14.00 ( 0.28) (196.3 ( 1.7) (kJ/mol)](2.303RT)-1 ; r ) 0.9999 Trimethyl orthochloroacetate:

log k (s-1) ) [(13.54 ( 0.21) (209.3 ( 1.9) (kJ/mol)](2.303RT)-1; r ) 0.9998 According to the experimental results of the kinetic and thermodynamic parameters of the two orthoesters examined in

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Figure 3. Graphic representation of the Arrhenius plot for the gasphase elimination of trimethyl orthovalerate (9) and trimethyl orthochloroacetate (2).

the present work (Table 7), their values of log A > 13.2 and positive entropy of activations suggest a polar concerted fourmembered cyclic transition-state-type of mechanism, as depicted in reaction 2. In addition to this fact, the difference in rate coefficient of the orthoesters (Table 7) implies that the electronreleasing groups at the carbon containing the methoxy groups enhance the rate, whereas the electron-withdrawing group decreases the elimination of methanol. This consideration leads to the reasonable assumption about the elimination of these orthoesters, where the breaking of the C-OCH3 bond in the direction of Cδ+ · · · δ-OCH3 in the transition state is the ratedetermining step (reaction 2). To find some support or ponder this interpretation, theoretical calculations have been undertaken to elucidate a reasonable mechanism of elimination of trimethyl orthovalerate and trimethyl orthochloroacetate. IV. Computational Methods and Model Electronic structure calculations were carried out on the reaction path for the gas-phase decomposition reaction of trimethyl orthov-

alerate and trimethyl orthochloroacetate. The two substrates decompose to methanol and the corresponding unsaturated ketal. We used the functionals B3LYP, B3PW91, MPW1PW91, and PBEPBE with basis sets 6-31G** and 6-31++G** implemented in Gaussian 03W.10 The Berny analytical gradient optimization routines were used with the default convergence criteria for the density matrix, maximum displacement, and maximum force. Transition-state searches were performed using the quadratic synchronous transit protocol. The nature of stationary points was established by calculating and diagonalizing the Hessian matrix (force constant matrix). The optimized structure for reactants, products, and TS structures was characterized by means of normalmode analysis. Intrinsic reaction coordinate (IRC) calculations were performed to verify transition-state structures along the minimum energy path (MEP). The unique imaginary frequency associated with the transition vector (TV), that is, the eigenvector associated with the unique negative eigenvalue of the force constant matrix, has been characterized. Thermodynamic quantities such as zero-point vibrational energy (ZPVE), temperature corrections, E(T), and absolute entropies, S(T), were obtained from frequency calculations, and consequently, the rate coefficient can be estimated assuming that the transmission coefficient is equal to 1. Thermal corrections and absolute entropies were obtained assuming ideal gas behavior from the harmonic frequencies and moments of inertia by standard methods11 at average temperature and pressure values within the experimental range. Scaling factors for frequencies and zero-point energies for B3LYP methods used are taken from the literature.12 The first-order rate coefficient k(T) was calculated using the transition-state theory (TST)13 and assuming that the transmission coefficient is equal to 1, as expressed in the following expression (eq 1)

k(T) ) (kBT/h) exp(-∆G# /RT)

(1)

∆G# is the Gibbs free-energy change between the reactant and the transition state and kB and h are the Boltzmann and Plank constants, respectively.

TABLE 7: Kinetic and Thermodynamic Parameter at 330 °C

trimethyl trimethyl trimethyl trimethyl

substrate

104k1 (s-1)

Ea (kJ/mol)

Log A

∆Hq (kJ/mol)

∆Sq (J/mol · K)

∆Gq (kJ/mol)

orthoacetate orthobutyrate orthovalerate orthochlorooacetate

5.25 11.43 11.20 0.26

194.7 ( 1.2 195.3 ( 1.6 196.3 ( 1.7 209.3 ( 1.9

13.58 ( 0.11 13.97 ( 0.37 14.00 ( 0.28 13.54 ( 0.21

189.7 190.3 191.3 204. 3

0.89 8.36 8.93 0.12

189.6 185.3 185.9 204.2

TABLE 8: Calculated Kinetic and Thermodynamic Parameters of the orthoesters at 330 °C substrate

method

Ea (kJ mol-1)

Log A

∆Sq (J mol-1 K-1)

∆Hq (kJ mol-1)

∆Gq (kJ mol-1)

CH3(CH2)3C(OMe)3

experimental B3LYP/6-31G** B3LYP/6-31++G** B3PW91/6-31G** B3PW91/6-31++G** MPW1PW91/6-31G** PBEPBE/6-31G** PBEPBE/6-31++G** experimental B3LYP/6-31G** B3LYP/6-31++G** B3PW91/6-31G** B3PW91/6-31++G** MPW1PW91/6-31G** PBEPBE/6-31G** PBEPBE /6-31++G**

196.3 203.2 190.9 203.9 195.1 208.7 181.8 170.4 209.3 256.9 252.3 257.7 255.2 258.0 224.6 210.1

14.00 14.26 14.37 14.43 14.53 14.48 14.04 14.38 13.54 14.31 14.77 14.11 14.40 14.04 14.11 14.25

8.93 14.02 15.93 17.10 19.04 18.30 9.66 16.25 0.12 14.92 23.7 10.90 16.60 14.01 11.10 13.70

191.3 198.2 185.9 198.9 190.0 203.7 176.8 165.3 204.3 251.9 247.5 252.9 250.2 253.0 219.6 205.1

185.9 189.7 173.7 192.5 177.5 192.7 170.9 |155.5 204.2 242.9 233.2 246.1 240.2 247.2 212.9 196.8

ClCH2C(OMe)3


J. Phys. Chem. A, Vol. 114, No. 12, 2010 4207

Figure 4. Optimized structures for reactant trimethyl orthovalerate (R), transition state (TS), and products, unsaturated ketal and methanol (P), at the B3LYP/6-31G level of theory.

Figure 5. Optimized structures for reactant trimethyl orthochloroacetate (R), transition state (TS), and products, unsaturated ketal and methanol (P), at the B3LYP/6-31G level of theory.

SCHEME 1

∆G# was calculated using the following relations (eqs 2 and 3)

∆G# ) ∆H# - T∆S#

(2)

∆H#)V# + ∆ZPVE + ∆E(T)

(3)

Where V# is the potential energy barrier and ∆ZPVE and ∆E(T) are the differences of ZPVE and thermal corrections between the TS and the reactant, respectively. Entropy values were calculated from vibrational analysis. Theoretical Results. Kinetic and Thermodynamic Parameters. The decomposition reaction trimethyl orthovalerate and trimethyl orthochloroacetate to give methanol and the corresponding unsaturated ketal was studied at different theory levels. The TS characterizing these reactions are four-centered cyclic geometries. Essentially the same structure for TS was obtained for all theory levels. The transition states were verified using vibrational analysis and IRC calculations (Supporting Information). The calculated parameters are given in Table 8. The experimental parameters in each case were compared with the experimental results. In the case of trimethyl orthovalerate, good agreement for enthalpy and energy of activation was obtained with the B3PW91/6-31++G** method, whereas the trimethyl orthochloroacetate PBEPBE/6-31++G** method gave param-

eters closer to experimental values. Calculated entropies of activations deviate from experimental values because of the use of the harmonic approximation and the presence of lowfrequency modes, which are mostly enharmonic. The use of diffuse functions was important to obtain reasonable parameters, as expected. The method that gave good theoretical parameters was selected for the analysis of the structures in the reaction path. The enthalpies of activation and consequently the energies of activation reproduced the observed experimental tendency, trimethyl orthochloroacetate being less reactive than trimethyl orthovalerate. The entropies of activation for all of these processes are small and positive, suggesting an increase in degrees of freedom and a fairly loose transition-state configuration, being more so in the case of trimethyl orthovalerate. Transition State and Mechanism. The transition-state structure in these reactions is four-center geometry, as seen in Figures 4 and 5. The TVs linked with the imaginary frequency for the TS are associated with the hydrogen transfer from the carbon C2 to the oxygen atom O4 to form the corresponding alcohol (Scheme 1). Structural parameters and charges for reactant orthoester (R), transition state (TS), and products (P) are given in Table 9. Dihedral angles are small, implying that the TS structures are almost planar. Atom distances show similarities in the decomposition process of trimethyl orthovalerate and trimethyl orthochloroacetate as well as some differences. The elongation of the C2-H1 bond in the TS is greater for trimethyl orthochloroacetate than for trimethyl orthovalerate, whereas breaking of C3-O4 is more advanced in the case of trimethyl orthovalerate. Also, the H1-O4 distance is shorter in the TS for trimethyl orthochloroacetate indicating that the hydrogen transfer is more advanced for trimethyl orthochloroacetate as compared with trimethyl orthovalerate. For both substrates, C2 -C3 shortens in the TS because of the formation of C2dC3 double bond. Dihedral

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TABLE 9: Structural Parameters for Reactant (R), Transition State (TS), and Products of the Substrates Thermal Decomposition from B3LYP/6-31G Calculationsa substrate

atom distances (Å)


H(1)-C(2) H(1)-O(4) O(4)-C(3) C(3)-C(2)

1.09 2.45 1.41 1.54

1.18 1.54 2.46 1.45

P 2.33 0.97 3.14 1.35

-0.02 0.01 -0.03 0.04 373.26 (cm-1) 1.32 1.33 2.10 1.45

1.09 2.60 1.41 1.54 Dihedrals 48.42 -21.26 30.51 -23.68

H(1)-C(2)-C(3)-O(4) C(2)-C(3)-O(4)-H(1) C(3)-O(4)-H(1)-C(2) O(4)-H(1)-C(2)-C(3) imaginary frequency a

TS

Dihedrals -53.35 -24.66 38.39 28.17

H(1)-C(2)-C(3)-O(4) C(2)-C(3)-O(4)-H(1) C(3)-O(4)-H(1)-C(2) O(4)-H(1)-C(2)-C(3) imaginary frequency H(1)-C(2) H(1)-O(4) O(4)-C(3) C(3)-C(2)


R

2.35 0.98 3.37 1.36

-0.61 0.70 -1.06 1.33 1560.7 (cm-1)

Atom distances are in angstroms; bond and dihedral angles are in degree.

TABLE 10: NBO Charges of the Atoms Involved in Thermal Decomposition of the Substrates from B3LYP/ 6-31G Calculations substrate

atom

R

TS

P


H(1) C(2) C(3) O(4) H(1) C(2) C(3) O(4)

0.27 -0.52 0.85 -0.60 0.29 -0.47 0.83 -0.59

0.39 -0.62 0.84 -0.85 0.44 -0.58 0.76 -0.74

0.49 -0.44 0.62 -0.77 0.25 -0.36 0.56 -0.76


angles in the TS for both substrates are small, indicating almost planar topologies. Natural Bond Orbital Charges. Natural bond orbital (NBO) charges were used to study the changes in electron distribution during the reaction. We report NBO charges for reactant, TS, and products in Table 10; atom numbering are as in Scheme 1. There is an increase in positive charge in the hydrogen being transferred H1; however, this is more important for trimethyl orthochloroactetate. The presence of the chlorine atom at C2 in this substrate produces a displacement in electron density toward the more electronegative atom, as seen in reactant charges at C2, which is less negative in the case of trimethyl orthochloroacetate. Oxygen O4 increases its electron density, but differences are observed between the two substrates. O4 becomes more negative in the case

of trimethyl orthovalerate, implying greater charge separation at C3-O4 in the TS. This increase in negative charge at O4 causes the abstraction of H1 to form methanol. Changes in C3 from the reactant to the TS are very small in trimethyl orthovalerate, whereas for trimethyl orthochloroacetate, C3 becomes more positive as the reaction progress from reactant to the TS. Bond-Order Analysis. The changes in bond order along the reaction path were investigated by means of NBO bond-order calculations.14-16 Wiberg bond indexes17 were computed using the NBO program,18 as implemented in Gaussian 03W. These indexes can be used to estimate bond orders from population analysis. Bond breaking and making processes involved in the reaction mechanism are monitored by means of the synchronicity (Sy) concept proposed by Moyano et al.19 defined by the expression (eq 4) n

Sy ) 1 - [

∑ |δBi - δBav|/δBav]/2n - 2

(4)

i)1

n is the number of bonds directly involved in the reaction, and the relative variation of the bond index is obtained from eq 5

δBi ) [BiTS - BiR]/[BiP - BiR]

(5)

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

TABLE 11: NBO analysis for the orthoesters thermal decomposition from B3LYP/631G calculationsa substrate trimethyl orthovalerate


a

enlace

R(Bi)

TS(Bi)

P(Bi)

(% Ev)

∆Bav

Sy

H(1)-C(2) C(2)-C(3) C(3)-O(4) O(4)-H(1) H(1)-C(2) C(2)-C(3) C(3)-O(4) O(4)-H(1)

0.90 0.97 0.91 0.001 0.89 0.95 0.94 0.001

0.61 1.14 0.19 0.18 0.46 1.18 0.35 0.30

0.02 1.71 0.01 0.71 0.01 1.70 0.01 0.72

32.9 23.2 79.7 25.6 48.6 40.9 63.0 29.9

0.40

0.68

0.46

0.85

Wiberg bond indexes (Bi) and % evolution through the reaction coordinate (% Ev) are shown for R, TS, and P. Average bond index variation (δBaV) and synchronicity parameter (Sy) are also reported.


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The evolution in bond change is calculated as in eq 6

% Ev ) (δBi)100

(6)

ent enhances decomposition rates by stabilizing the positive charge developing in the transition state at the carbon bearing the three methoxy groups, whereas the electron-withdrawing substituent destabilizes this charge, thus retarding the reaction.

(7)

Acknowledgment. T.C. is grateful to the Consejo de Desarrollo Cientı´fico y Humanı´stico (C.D.C.H.) for grant No. PG03-00-6499-2006.

The average value is calculated from eq 7 n

δBav ) 1/n

∑ δBi i)1

The reaction changes along the reaction coordinate can be followed using Wiberg bonds indexes, Bi. The indexes were calculated for the bonds involved in trimethyl orthovalerate and trimethyl orhtochloroacetate pyrolyses, that is, H1-C2, C2-C3, C3-O4, and O4-H1 (Scheme 1, Table 11); all other bond bonds remain practically unchanged during the process. Atom numbering for structural parameters (atom distances, dihedral angles) and NBO calculations are shown in Scheme 1. Bond-order analysis reveals that even though the two reactions proceed through similar TS in unimolecular fashion, there are differences in some reaction coordinates along the decomposition path. Both reactions are dominated by the breaking of C3-O4 bond; however, in the case of trimethyl orthovalerate, this reaction coordinate has advance close to 80% in the TS, compared with 63% for trimethyl orthochloroacetate. In the case of H1-C2 bond breaking, it is more advanced for trimethyl orthochloroacetate, as well as the C2-C3 double-bond formation. Changes in bond indexes illustrate that the two unimolecular processes proceed in nonsynchronous fashion. The synchronicity parameter, with values between 1 for synchronic reactions and 0 for asynchronic, stepwise processes, show that the thermal decomposition of trimethyl orthovalerate (Sy ) 0.68) is more polar and asynchronous than the pyrolysis of trimethyl orthochloroacetate (Sy ) 0.85). V. Conclusions The unimolecular homogeneous gas-phase thermal decomposition of trimethyl orthovalerate and trimethyl orthochloroacetate have been examined both experimental and theoretically. The rate coefficients for the homogeneous, unimolecular reaction are given by the following Arrhenius expressions: for trimethyl orthovalerate: log k (s-1) ) [(14.00 ( 0.28) - (196.3 ( 1.7) (kJ/mol)] (2.303RT)-1, (r ) 0.9999); and for trimethyl orthochloroacetate: log k (s-1) ) [(13.54 ( 0.21) - (209.3 ( 1.9)(kJ/mol)](2.303RT)-1, (r ) 0.9998). DFT theoretical calculation gave reasonable enthalpies and energies of activation. These calculations were useful to analyze the progress along the potential energy surface (PES). The two reactions proceed in concerted fashion through cyclic four-membered TS structures and are dominated by the breaking of the C-OCH3 bond. However, the decomposition of trimethyl orthovalerate is more polar-asynchronous (Sy ) 0.68) compared with trimethyl orthochloroacetate (Sy ) 0.85), with a TS showing more progress in the C-O breaking, close to 80% progress. The increase in electron density at oxygen O4 is also more important in trimethyl orthovalerate TS, suggesting that the abstraction of the hydrogen H1 by oxygen O4 to form methanol is favored. The presence of a chlorine atom decreases the electron density at C2 but does not increase significantly the acidity of H1. Comparison of energies of activation for the series of trimethyl orthoesters in Table 7 suggests that the electron-donor substitu-

Supporting Information Available: IRC obtained from gasphase elimination reaction of triethyl and trimethyl orthoesters. This material is available free of charge via the Internet at http:// pubs.acs.org.. References and Notes (1) Staudinger, H.; Rathsam, G. HelV. Chim. Acta 1922, 5, 645–655, and references cited within. (2) McElvain, S. M.; Anthes, H. I.; Shapiro, S. H. J. Am. Chem. Soc. 1942, 64, 2525–2531. (3) McElvain, S. M.; Stevens, C. L. J. Am. Chem. Soc. 1946, 68, 1917– 1961. (4) Marquez, E.; Tosta, M.; Domıńguez, R. M.; Herize, A.; Chuchani, G. J. Phys. Org. Chem. 2008, 21, 666–669. (5) Marquez, E.; Dominguez, R. M.; Tosta, M.; Chuchani, G. J. Phys.Chem. A 2008, 112, 12140–1214. (6) Marquez, E.; Domıńguez, R. M.; Mora, J. R.; Co´rdova, T.; Chuchani, G. J. Phys.Chem. A 2009, 113, 2600–2606. (7) Maccoll, A. J. Chem. Soc. 1955, 965, 973. (8) Swinbourne, E. S. Aust. J. Chem. 1958, 11, 314–330. (9) Dominguez, R. M.; Herize, A.; Rotinov, A.; Alvarez-Aular, A.; Visbal, G.; Chuchani, G. J. Phys. Org. Chem. 2004, 17, 399–408. (10) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Calmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K. ; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (11) McQuarrie, D. Statistical Mechanics; Harper & Row: New York, 1986. (12) (a) Foresman, J. B. , Frish, A. E. Exploring Chemistry with Electronic Methods, 2nd ed.; Gaussian, Inc.: Pittsburgh, PA, 1996. (b) Scale Factors. http://cccbdb.nist.gov/vibscalejust.asp. (c) Database of Frequency Scaling Factors for Electronic Structure Methods. http://comp.chem.umn.edu/truhlar/freq_scale.htm. (13) Benson, S. W. The Foundations of Chemical Kinetics; Mc-GrawHill: New York, 1960. (14) Lendvay, G. L. J. Phys. Chem. 1989, 93, 4422–4429. (15) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985, 83, 735–746. (16) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88, 899–926. (17) Wiberg, K. W. Tetrahedron 1968, 24, 1083–1095. (18) Glendening, D. E.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO, Version 3.1; NBO, 5.0. Glendening, D. E.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.; Bohmann, J. A.; Morales, C. M.; Weinhold, F. (Theoretical Chemistry Institute, University of Wisconsin, Madison, WI, 2001) cf. http:// www.chem.wisc.edu/~nbo5/. (19) Moyano, A.; Perica´s, M. A.; Valenti, E. J. Org. Chem. 1989, 54, 573–582.

JP1005296

Experimental and Theoretical Studies of the Homogeneous

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