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Isomerization of Neopentyl Chloride and Neopentyl Bromide by a 1,2-Interchange of a Halogen Atom and a Methyl Group Carmen E. Lisowski,† Juliana R. Duncan,† Anthony J. Ranieri,† George L. Heard,† D. W. Setser,‡ and Bert E. Holmes*,† Department of Chemistry, UniVersity of North Carolina-AsheVille, One UniVersity Heights, AsheVille, North Carolina 28804-8511, and Department of Chemistry, Kansas State UniVersity, Manhattan, Kansas 66506 ReceiVed: May 22, 2010; ReVised Manuscript ReceiVed: August 14, 2010
The recombination of chloromethyl and t-butyl radicals at room temperature was used to generate neopentyl chloride molecules with 89 kcal mol-1 of internal energy. The observed unimolecular reactions, which give 2-methyl-2-butene and 2-methyl-1-butene plus HCl, as products, are explained by a mechanism that involves the interchange of a methyl group and the chlorine atom to yield 2-chloro-2-methylbutane, which subsequently eliminates hydrogen chloride by the usual four-centered mechanism to give the observed products. The interchange isomerization process is the rate-limiting step. Similar experiments were done with CD2Cl and C(CH3)3 radicals to measure the kinetic-isotope effect to help corroborate the proposed mechanism. Density functional theory was employed at the B3PW91/6-31G(d′,p′) level to verify the Cl/CH3 interchange mechanism and to characterize the interchange transition state. These calculations, which provide vibrational frequencies and moments of inertia of the molecule and transition state, were used to evaluate the statistical unimolecular rate constants. Matching the calculated and experimental rate constants, gave 62 ( 2 kcal mol-1 as the threshold energy for interchange of the Cl atom and a methyl group. The calculated models also were used to reinterpret the thermal unimolecular reactions of neopentyl chloride and neopentyl bromide. The previously assumed Wagner-Meerwein rearrangement mechanism for these reactions can be replaced by a mechanism that involves the interchange of the halogen atom and a methyl group followed by HCl or HBr elimination from 2-chloro2-methylbutane and 2-bromo-2-methylbutane. Electronic structure calculations also were done to find threshold energies for several related molecules, including 2-chloro-3,3-dimethylbutane, 1-chloro-2-methyl-2-phenylpropane, and 1-chloro-2-methyl-2-vinylpropane, to demonstrate the generality of the interchange reaction involving a methyl, or other hydrocarbon groups, and a chlorine atom. The interchange of a halogen atom and a methyl group located on adjacent carbon atoms can be viewed as an extension of the halogen atom interchange mechanisms that is common in 1,2-dihaloalkanes. I. Introduction The thermal unimolecular reactions of neopentyl chloride and neopentyl bromide can be summarized by eq 1 for experimental conditions such that radical reactions are suppressed.1-4
transfers a CH3 group and eliminates HCl (or HBr) by 1,1- and 1,3- processes, see reaction 2.
(CH3)3CCH2Cl (or Br) f HCl (or HBr) + CH2dC(CH3)C2H5 (1a) f HCl (or HBr) + (CH3)2CdCHCH3
(1b)
The thermal pyrolysis studies agree that 2-methyl-1-butene and 2-methyl-2-butene are the major products in an approximate ratio of 2:1, and that the Arrhenius parameters for the neopentyl chloride reaction are 1013.8(0.6 s-1 and Ea ) 62 ( 2 kcal mol-1. Since elimination of HCl or HBr by a conventional four-centered mechanism is not possible from neopentyl chloride or bromide, investigators adopted a Wagner-Meerwein rearrangement mechanism that involves formation of an ionic (or, at least, a very polar) intermediate that subsequently (or simultaneously) * To whom correspondence should be addressed. E-mail: bholmes@ unca.edu. † University of North Carolina-Asheville. ‡ Kansas State University.
Experiments3 with (CH3)3CCD2Cl were consistent with the essential aspects of the mechanism, since the products were 2-methyl-1-butene-d2 and 2-methyl-2-butene-d1, see reaction 2. This ionic mechanism of reaction 2 was proposed1,2 at the time when the HX elimination mechanism also was considered to proceed by a highly polar transition state. According to modern views5-7 of the transition state for HX elimination, the partial charges on the carbon atoms are not large. We wish to propose an alternative mechanism for neopentyl chloride (or bromide) that involves the unimolecular interchange of the chlorine atom (or bromine atom) and a methyl group to give 2-chloro-2methylbutane (or 2-bromo-2-methylbutane), which subsequently
10.1021/jp1047166 2010 American Chemical Society Published on Web 09/01/2010
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eliminates HCl (or HBr) by the conventional four-centered mechanism. Reactions 3 and 4 summarize the mechanism for neopentyl chloride-d2. The polar nature of the interchange transition state will be considered in the Discussion section.
Reaction 3 is exothermic by about 2 kcal mol-1; thus, 2-methyl-2-chlorobutane acquires an additional 2 kcal mol-1 of energy during the interchange in reaction 3. The threshold energy, E0, for the interchange reaction is ≈60 kcal mol-1, based on our electronic structure calculations and the thermal activation studies of neopentyl chloride, whereas those for HCl elimination, reactions 4a and 4b, are ≈45 kcal mol-1 according8 to pyrolysis studies of 2-methyl-2-chlorobutane. Thus, the rearrangement process in 3 is the rate-limiting step for all forms of activation of neopentyl chloride (or bromide) molecules. Supporting Information has a Figure illustrating the energy profile for the formation and decomposition of neopentyl chloride via reactions 3 and 4. According to the interchange mechanism, the kinetic isotope-effect for reaction 3 would just be the statistical secondary effect associated with the two D atoms in neopentyl chloride-d2. The product branching ratios in reaction 4 would have different isotope effects, but they would be small. In contrast, the single-step Wagner-Meerwein mechanism of reaction 2 would have different kinetic-isotope effects for the formation of 2-methylbutene-1 and 2-methylbutene-2, with the latter being larger by a factor of 1.5, at least, since it is a primary isotope effect.15-17 Measuring the experimental H/D kineticisotope effect for the 2-methylbutene products would be an additional test of whether the mechanism consisted of reaction 2 or reactions 3 and 4. In the present work we provide chemical activation data for neopentyl chloride and neopentyl chloride-d2 formed by recombination of t-butyl and chloromethyl (CH2Cl and CD2Cl) radicals plus electronic structure calculations using density functional theory (DFT) in support of reaction 3. The experiments consist of measurement of the ratios of collisionally stabilized product, neopentyl chloride, to the decomposition products, 2-methyl1-butene and 2-methyl-2-butene, as a function of pressure. The 2-methyl-2-chlorobutane molecules with 91 kcal mol-1 of vibrational energy are not stabilized at the pressures of these experiments. The radicals were generated by the cophotolysis of (CH3)3CI and CH2ClI (or CD2ClI). The Cl/F, Br/Cl, and Br/F interchange reactions for haloalkanes with halogen atoms located on adjacent carbon atoms have been demonstrated and characterized in a series of recent studies from this laboratory.9-17 These interchange reactions, which often are in competition with hydrogen halide elimination reactions, have threshold energies for dihaloethanes ranging from
Figure 1. Diagram of the transition state for interchange of a methyl group and a chlorine atom from neopentyl chloride. The distances are in Å; the angles, 16.2° and 13.5°, denote the inclination of the triangular planes of the CH2 and C(CH3)2 groups with the C-C axis. Calculations were done by the B3PW91 method with the 6-31G(d′,p′) basis set. The partial charges on each atom, calculated by the atoms-in-molecules method, are listed. The large negative charge on the Cl atom should be noted. The transition state for neopentyl bromide is very similar to the diagram for neopentyl chloride; the C-Br distances increase to 2.83 and 3.10 Å for C6 and C5, but the C6-C1 and C5-C1 distances remain the same, that is, 1.80 and 1.95 Å.
43 (Br/Cl)7 to 60 (Cl/F)8-17 kcal mol-1. The Cl/methyl group and Br/methyl group interchange reactions are members of this genre of unimolecular reaction. The presence of two methyl groups on one carbon of the transition state in neopentyl chloride (bromide) serves to lower the E0 of reaction 3, and the interchange reaction becomes viable at modest temperatures. Electronic structure calculations were done using DFT at the B3PW91 level with the 6-31G(d′,p′) basis set. Some calculations also were done with the 6-311+G(2d,p) basis set, as well as with the B3LYP method, for comparison. The structures of the molecule and the transition state were very similar for all the calculations. However, the E0 for the rearrangement does depend somewhat on the method and basis set. Our collective experience from studies of numerous HX elimination and X/Y interchange reactions (X, Y are halogen atoms) is that the 6-31G(d′,p′) basis set with the B3PW91 method most closely matches experimentally based threshold energies. The structure of the Cl/CH3 interchange transition state, E0 ) 62 kcal mol-1, is shown in Figure 1. The nearly planar nature of the two carbon atoms in the bridged structure is evident. This also is a characteristic feature of the transition states for halogen atom interchange.7,13-17 Another noteworthy feature is the very low torsional frequency, 73 cm-1, of the CH3 group in the bridged structure, which implies that the bonding is with the π-orbital between the two carbon atoms. The calculated frequencies and moments of inertia of neopentyl chloride and its transition state that were used to evaluate the statistical rate constants for the interchange reactions of neopentyl chloride-d0 and -d2 are given in the Supporting Information. Matching the calculated and experimental rate
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constants using E0 as a variable permitted assignment of an experimentally based threshold energy for reaction 3. The models of the transition states were also used to obtain Arrhenius pre-exponential factors for comparison to thermal activation studies1-4 of neopentyl chloride and bromide. Exploratory calculations also were done for other molecules to identify and illustrate the class of molecules for which reactions are expected to interchange Cl or Br atoms with a methyl or other organic group. II. Experimental Methods A given experiment consisted of photolysis at room temperature of measured quantities of CH2ClI (or CD2ClI) and (CH3)3CI followed by gas chromatographic (GC) analysis for the ratios of 2-methyl-1-butene/neopentyl chloride and 2-methyl2-butene/neopentyl chloride. Pyrex vessels with volumes ranging from 34.7 to 4900 cm3 containing 0.540 µmol of CH2ClI (or CD2ClI) and 0.180 µmol of (CH3)3CI were irradiated with the output of a high pressure Oriel 6137 arc lamp. A small amount of mercury(I) iodide was added to the vessels to aid in the formation of the radicals and to remove the HCl product.18 The use of Pyrex vessels limits the photolysis to wavelengths longer than 290 nm. Irradiation times of 2-15 min gave approximately 10% conversion of the reactants to products. Gas samples were measured on grease-free vacuum lines and pressures were measured with a MKS 270 Signal Conditioner. The measured samples of CH2ClI and (CH3)3CI were quantitatively transferred from standard volumes, that were attached to the vacuum line, to the photolysis vessels by cryogenic pumping. The entire photolyzed sample was transferred to the inlet vacuum system of the gas chromatographs for analysis of the products. Products were identified from their retention times and mass spectra by comparison to authentic samples from analysis with a Shimadzu QP5000 gas chromatograph with a mass-spectrometer detector. A Rtx-VMS 120 m column with 0.25 mm diameter was used in the analysis. The temperature program began by holding the column at 30 °C for 20 min, followed by heating at a rate of 2 °C per min until the column reached 100 °C, then the rate was increased to 4 °C per min until the temperature was 200 °C. The main products were 2-methyl-1-butene, 2-methyl-2-butene, and neopentyl chloride. No evidence was found for 1,1-dimethylcyclopropane or 3-methyl-1-butene, which were minor products in some of the thermal pyrolysis studies. An authentic sample of 1,1-dimethylcyclopropane was available for confirmation. Photolysis of t-butyl iodide and CD2ClI (CDN Isotopes) gave CH2dC(CH3)CD2CH3, (CH3)2Cd CDCH3, and (CH3)3CCD2Cl as products. Analysis of the reaction mixtures to obtain decomposition to stabilization (D/S) product ratios (2-methyl-1-butene/neopentyl chloride and 2-methyl-2-butene/neopentyl chloride) were done with a Shimadzu GC-14A gas chromatograph with a flame ionization detector and a Shimadzu CR5A Chromatopac integrator. A Rtx-VGC column of 105 m length and 0.53 mm diameter was used. The temperature program consisted of 20 min at 35 °C, followed by a heating rate of 12 °C per min until a final temperature of 190 °C was reached. The typical retention times were 2-methyl-1-butene (15.6 min), 2-methyl-2-butene (17.4 min), neopentyl chloride (32.5 min), chloroiodomethane (36.7 min), and t-butyl iodide (45.1 min). Deuterated compounds had similar retention times as nondeuterated compounds. Response factors for the products were measured by comparing four samples from each of three independently prepared mixtures containing CH2dC(CH3)CH2CH3, (CH3)2CdCHCH3, (CH3)3CCH2Cl, and CH2ClI. The response factors were 0.92 ( 0.05
Figure 2. Plot of the decomposition to stabilization ratios vs pressure-1 for neopentyl chloride-d0: O CH2dC(CH3)C2H5/(CH3)3CCH2Cl with slope and intercept of 0.000 516 ( 0.000 081 and 0.0355 ( 0.0122 respectively, 0 (CH3)2CdCHCH3/(CH3)3CCH2Cl with slope and intercept of 0.000 249 ( 0.000 033 and 0.0141 ( 0.0051, respectively, [ [(CH3)2CdCHCH3 + CH2dC(CH3)C2H5]/(CH3)3CCH2Cl with slope and intercept of 0.000 765 ( 0.000 112 and 0.0496 ( 0.0171 respectively.
for (CH3)2CdCHCH3/CH2dC(CH3)CH2CH3, 0.90 ( 0.06 for CH2dC(CH3)CH2CH3/(CH3)3CCH2Cl, and 0.98 ( 0.11 for (CH3)2CdCCHCH3/(CH3)3CCH2Cl. III. Experimental Results A. Rate Constants. The chemical activation rate constants for the interchange reaction were obtained from the usual measurement of the decomposition (D) to stabilization (S) product ratio at various pressures; D/S ) kexp/kM[M] where kM is the collision rate constant for neopentyl chloride with the bath gases. In this simple formulation we are assuming strong collisions, that is, sufficient internal energy is removed from neopentyl chloride per collision such that further reaction has negligible probability. The experimental results for neopentyl chloride-d0 and -d2 are shown in Figures 2 and 3; the pressure range is from 70 to 3.4 mTorr. The D/S range is limited to e0.3 because of the small rate constants and the low-pressure limit of our technique. The appropriate decomposition product for obtaining kexpt for the interchange reaction is the sum of 2-methyl-1-butene and 2-methyl-2-butene, since all of the (CH3)2C(Cl)CH2CH3* molecules will decompose at the pressures of these experiments because of the low E0 of 45 kcal mol-1 for HCl elimination. The slopes of the pressure-1 plots in Figures 2 and 3 give the average high pressure rate constant, kexpt, which is equivalent to k〈E〉. The values of the slopes for the interchange reaction are (7.65 ( 0.41) × 10-4 and (5.76 ( 0.69) × 10-4 Torr for neopentyl chloride-d0 and -d2, respectively. The rate constants in pressure units of Torr were converted to units of s-1, see Table 1, using the factor of 1.80 × 107 s-1/ Torr, which was obtained using collision diameters and (/k)19 values of 5.8 Å (425°), 5.1 Å (400°), and 5.3 Å (400°) for neopentyl chloride, chloroiodomethane, and t-butyl iodide, respectively in the standard formula kM ) πd2(8kT/πµ)1/2 Ω(2,2)(T*). The resulting rate constants are (1.4 ( 0.2) × 104
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Figure 3. Plot of decomposition to stabilization ratios vs pressure-1 for neopentyl chloride-d2; O CH2dC(CH3)CD2CH3/(CH3)3CCD2Cl with slope and intercept of 0.000 419 ( 0.000 046 and 0.0174 ( 0.0075, respectively, 0 (CH3)2CdCD(CH3)/(CH3)3CCD2Cl with slope and intercept of 0.000 157 ( 0.000 022 and 0.006 79 ( 0.003 77, respectively, [ [(CH3)2CdCDCH3 + CH2dC(CH3)CD2CH3]/(CH3)3CCD2Cl with slope and intercept of 0.000 576 ( 0.000 066 and 0.0242 ( 0.0109, respectively. -1
and (1.0 ( 0.2) × 10 s for neopentyl chloride-d0 and -d2. The slopes of the plots in Figures 2 and 3 give an overall kineticisotope effect of 1.33 ( 0.20, which is a statistical secondary effect since the D atoms are not involved in the reaction coordinate of the interchange reaction. The (CH3)2C(Cl)CH2CH3 molecules decompose by 1,2- and 3,2-HCl elimination pathways with reaction path degeneracies of 6 and 2, respectively. RRKM calculations for E0 ) 45 kcal mol-1 were done for 1,2-HCl elimination from 2-methyl-2chlorobutane with 91 kcal mol-1 of energy. The rate constant was 1.5 × 108 s-1 and that for 3,2-HCl elimination would be similar for the same E0. The unimolecular reactions of 2-methyl2-chlorobutane are 4 orders of magnitude faster than the interchange rate, and the 2-methyl-2-chlorobutane molecules will decompose at all feasible pressures employed in experiments for the study of neopentyl chloride. The branching between 1,2and 3,2-HCl elimination in reaction 4 depends on the relative rate constants. The experimental branching ratios can be obtained from the average ratio of 2-methylbutene-1 to 2-methylbutene-2 from each experiment or from the ratio of the slopes of the plots in Figure 2 and 3. Of course, they are very similar, but we have chosen the former and they are 2.3 and 2.6 with the 1,2-HCl channel being favored for neopentyl chloride-d0 and -d2, respectively. The branching ratio in the pyrolysis experiments2 of neopentyl chloride was similar. The reaction path degeneracies favor 1,2-HCl by a factor of 3, but the 3,2-HCl elimination channel must have a slightly lower E0. In fact, 3,2-HCl elimination to give 2-methyl-2-butene was favored in pyrolysis of 2-chloro-2-methyl-butane,8 which also suggests that the E0 is lower for 3,2-HCl elimination than for 1,2-HCl elimination. This is supported by the DFT calculations of the threshold energies that give E0(1,2-HCl) ) 41.4 kcal mol-1 and E0(3,2-HCl) ) 40.4 kcal mol-1, although the absolute values are too low, see the Supporting Information. The kineticisotope effect is larger for 3,2-DCl elimination than for 1,2HCl elimination from (CH3)3C(Cl)CD2CH3, and that is why the branching ratio increased to 2.6, see Figure 3. 4
Lisowski et al. B. Average Energy of Neopentyl Chloride. Under our experimental conditions, the CH2Cl and C(CH3)3 radicals will collide with bath molecules before recombining; thus, the radicals are in thermal equilibrium. The average energy of the molecules formed by the recombination of CH2Cl and t-butyl radicals is given by the C-C bond dissociation energy at 0 K plus 3RT plus the thermal vibrational energy of the radicals. The 3RT arises from the 3 translational and 3 rotational motions that become vibrations in the neopentyl chloride molecule. Since the enthalpy of formation for neopentyl chloride does not seem to be readily available, we decided to use the bond dissociation energy of neopentane, (CH3)3C-CH3, as a model. The enthalpies of formation at 298 K of CH3, C(CH3)3, and (CH3)4C are 35.0,20 12.3,20,21 and 39.722 kcal mol-1, which give D(CH3-C(CH3)3) ) 87.1 kcal mol-1 at 298 K. Converting to 0 K gives 85.5 kcal mol-1. Adding the thermal energy of the radicals that contribute to the internal energy of the neopentane molecules raises the energy to 89.6 kcal mol-1. The bond dissociation energy of propyl chloride is about 0.3 kcal mol-1 lower than that of propane. Therefore, we adopted 89 kcal mol-1 as the average thermal energy of neopentyl chloride formed by the recombination of (CH3)3C and CH2Cl radicals at room temperature. The uncertainty is (3 kcal mol-1. The 2-methyl-2-chlorobutane molecules will contain 91 kcal mol-1 of internal energy following reaction 3. IV. Computational Results A. Models of Molecules and Transition States. Models for the vibrational frequencies and moments of inertia of the molecules and transition states are needed to calculate statistical (RRKM) rate constants at a specific energy and Arrhenius preexponential factors at a given temperature. The computed threshold energies are of general interest, but they are not sufficiently reliable to be used in the RRKM rate constant calculations. Electronic structure calculations were done with the suite of codes in the Gaussian package.23 We have continued to use DFT with the B3PW91 method and the 6-31G(d′,p′) basis set. Exploratory calculations24 also were done with the B3LYP method and the 6-311+G(2d,p) and aug-cc-pVTZ basis sets. However, the pre-exponential factors for the interchange reaction from these calculations24 were nearly identical, as is usually the case,25-27 and we adopted the 6-31G(d′,p′) basis set for this work. The neopentyl chloride (or bromide) molecules have four torsional modes and the transition states have three torsional modes. We treated these modes as hindered internal rotations. Our assignments of the reduced moments of inertia and the barrier heights are given below. The computed vibrational frequencies and moments of inertia are given in the Supporting Information. The barriers to internal rotation for the CH2Cl and CH2Br groups in neopentyl chloride (bromide) were calculated with the 6-31G(d′,p′) basis set as 5.8 and 6.1 kcal mol-1, and the reduced moments were 25.5 and 31.8 amu Å2, respectively. We have continued26 to employ the method of Pitzer to calculate the reduced moments for internal rotation. The barriers for methyl rotation in the t-butyl group were assigned as 4.8 kcal mol-1 from the calculation for neopentane by Baudry;28 the reduced moment was 3.1 amu Å2. The structure and the charge distribution for the transition state for neopentyl chloride are shown in Figure 1. An intrinsic reaction coordinate calculation confirmed that Figure 1 is the transition state for the interchange reaction 3. The carbon atoms in the backbone of the transition state have nearly sp2 geometry and the structure of the CH3 groups at the (CH3)2C end should
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TABLE 1: Comparison of Experimental and Calculated Results property
(CH3)3CCH2Cl-d0 (-d2)
kexpt; s k〈E〉; s-1 E0 ) 62 kcal mol-1 E0 ) 61 kcal mol-1 AArra; s-1 Acalcb; s-1 T ) 700 K Ea (expt)a; kcal mol-1 Ea (calc from best E0); kcal mol-1
1.38 ( 0.2 × 10 (1.04 ( 0.2 × 10 ) 1.51 × 104 (0.94 × 104) 2.68 × 104 (1.70 × 104) 1013.8(0.60 1014.83 62 ( 2 64 ( 2
-1
4
(CH3)3CCH2Br-d0 4
1014.2(0.3 1014.87 59 ( 1 ( c)
a The Arrhenius pre-exponential factors from pyrolysis experiments, refs 3 and 4. The Arrhenius parameters from ref 2 for neopentyl chloride are 1013.26(0.36 s-1 and Ea ) 60.0 ( 1.6 kcal mol-1. b The DFT calculated pre-exponential factor at 700 K are from our models. c If the calculated E0 of 61.1 kcal mol-1 is accepted, the Ea would be 63.3 kcal mol-1.
resemble that of (CH3)2CdCH2. The reported29 barrier for internal rotation in isobutene is 2.1 kcal mol-1. We suspect that this barrier may be a lower limit for the transition state, and we selected 3.1 kcal mol-1 as the barrier for the two methyl groups; the reduced moments are 3.18 amu Å2. The calculated torsional frequency for the CH3 group in the bridge is 73 cm-1. This low value indicates that the barrier to internal rotation is very low. We assigned a barrier of 1.0 kcal mol-1; the reduced moment is 3.03 amu Å2. The description of the internal rotations for the neopentyl bromide transition state is the same as for neopentyl chloride. The lower barriers for internal rotation of the CH3 groups in the transition state relative to those in the molecule should be noted. Statistical rate constants for neopentyl chloride-d0 and -d2 were calculated using the standard equation given below.
kE ) s† /h(I† /I)1/2
∑ P†(E - E0)/N*(E)
(5)
The density of states, N*(E), for the molecules and the sums of states for the transition state, ΣP†(E - E0) were obtained from the Multiwell code of Barker.30 The reaction path degeneracy is s†, and I†/I is the ratio of the three overall moments of inertia. The reaction path degeneracy arises naturally from the symmetry of the internal rotation of the t-butyl group. For neopentyl chloride s† ) 3 and the square root of the ratio of the moments of inertia was 1.22. The threshold energy was varied to obtain agreement with k〈E〉 and kexpt. Due to the rather small 〈E〉 - E0, about 28 kcal mol-1, and the large number of vibrational frequencies, the dependence of kE on 〈E〉 and E0 is strong; a 2 kcal mol-1 increase in 〈E〉 gives a factor of 1.8 increase in kE, and a 2 kcal mol-1 decrease in E0 gives a factor of 3.1 increase. The models also can be used to calculate the Arrhenius preexponential factor (s†kT/h exp(1 + ∆S†/R)) or the preexponential factor in partition function form (s†kT/h) (Q†/Q). B. Comparison of Experimental and Calculated Results for Neopentyl Chloride. The experimental rate constants for the chemical activation experiments of neopentyl chloride-d0 and -d2 and the experimental pre-exponential factors of neopentyl chloride (and bromide) are compared in Table 1. The E0 needed to match k〈E〉 and kexpt for 〈E〉 ) 89 kcal mol-1 is 62 ( 1 kcal mol-1 for both neopentyl chloride-d0 and neopentyl chloride-d2. At 91 kcal mol-1 the calculated isotope effect (Table 2) is 1.55, which agrees with the upper range of the experimental result of 1.33 ( 0.20. The threshold energy is invariant with isotopic substitution. Considering the uncertainties in 〈E〉, in the experimental measurement of kexpt, and in the harmonic models for the calculations, the actual uncertainty in E0 is probably (2 kcal mol-1. The E0 from the thermal activation experiments is 60 ( 2 kcal mol-1. Thus, the thermal and chemical activation experiments are in fundamental agreement
TABLE 2: Computed Threshold Energies for the Halogen-Methyl or Phenyl (Ph) Interchange Reaction using B3PW91/6-31G(d′,p′)a
reaction
Me-Cl interchange in (CH3)3CCCl3 Me-Cl interchange in (CH3)3CCHCl2 Me-Cl interchange in (CH3)3CCH2Cl Me-Br interchange in (CH3)3CCH2Br Me-F interchange in (CH3)3CCH2F Me-Cl interchange of (CH3)3CCH(CH3)Cl Me-Cl interchange of (CH3)2(OCH3)CCH2Cl Me-Cl interchange in (CH3)2PhCCCH2Cl Me-Cl interchange of (CH3)2(CH)CH2)CCH2Cl Ph-Cl interchange in (CH3)2PhCCHCl2
threshold energy (kcal/mol) 70.0 66.7 62.0 61.1 87.2 58.1 57.8 58.0 61.0 53.3
a The DFT calculated E0 values for these reactions should be similar if the Cl-atom is replaced by a Br-atom based on the comparison between neopentyl chloride and neopentyl bromide.
for the threshold energy of the CH3/Cl interchange reaction in neopentyl chloride. The calculated pre-exponential factor in Arrhenius form is 6.7 × 1014 s-1, which is 2.5 times larger than the upper limit of the experimental result,1-4,8 A ) 2.5 × 1014 s-1. The large pre-exponential factor is a consequence of the low bending vibrational frequencies associated with the weakly bound Cl and CH3 groups of the transition state. In this context it should be noted that the transition state still has three methyl hindered internal rotors. The electronic structure calculations gave E0 values of 62.0 kcal mol-1 at the B3PW91/6-31G(d′,p′) level and 59.8 kcal mol-1 at the B3LYP/6-31G(d′,p′) level for the CH3/Cl interchange reaction of neopentyl chloride. The larger basis set, 6-311+G(2d,p), gave a somewhat lower, ∼ 3 kcal mol-1, threshold energy for each method of calculation. The aug-ccpVTZ basis set gave the same result as the 6-311+G(2d,p) basis set. Thus, the calculations also support a threshold energy of 60-62 kcal mol-1. V. Discussion Chemical activation studies of neopentyl bromide have not been done, but comparison can be made with a thermal activation study,4 which reported Arrhenius parameters of 1014.2(0.3 s-1 and Ea ) 59.0 ( 1.2 kcal mol-1. The model, which is presented in the Supporting Information, from the 6-31G(d′,p′) calculations gave a threshold energy of 61.1 kcal mol-1 and a pre-exponential factor of 1014.9 s-1. The calculated and experimental threshold energies and pre-exponential factors have the same degree of agreement as for neopentyl chloride, and
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Figure 4. (A) Diagrams of the transition states for interchange of a phenyl group and a chlorine atom and (B) interchange of a methyl group and a chlorine atom from 1-chloro-2-methyl-2-phenylpropane. The distances are in Å; the two identified angles are the angles of inclination of the planes of the end groups, e.g., the CH2 group, with the C-C axis. Calculations were with the B3PW91 method and the 6-31G(d′,p′) basis set. The partial charges calculated by the atoms-in-molecules method33 are given for each atom in the tables. The similarity to the transition state for neopentyl chloride in Figure 1 should be noted.
substituting a bromide atom for a chloride atom does not seriously affect the unimolecular interchange reaction. The methyl/halogen interchange mechanism is consistent with all the kinetic data for neopentyl chloride and bromide. In particular the experimental kinetic-isotope effect of 1.33 for neopentyl chloride-d0 and -d2 is the expected statistical effect for the interchange rate constant and agrees with the computational findings. However, it is not the result expected for a one-step Wagner-Meerewein mechanism. For the later, a small statistical effect would hold for the formation of CH2dC(CH3)CD2CH3; however, a primary effect would augment the statistical effect for formation of (CH3)2CdC(D)CH3 and the combined isotope effect would be g2 for this channel. Such a difference was not experimentally observed, since the ratios are 1.23 (1,2-HCl) and 1.58 (2,3-HCl), based upon the slopes of the plots in Figures 2 and 3. In the interchange mechanism, the intermolecular isotope effect for the two competing HCl elimination channels is given by the ratio of interchange rate constants for the d0 and d2 molecules multiplied by the ratios of branching fractions for the formation of CH2dC(CH3)C2H5 and (CH3)2CdCHCH3. The isotope effect on the branching fractions is small as illustrated by the intramolecular branching ratios of 2.3 and 2.6 for d0 and d2, respectively. In addition to the experimental results, the electronic structure calculations gave plausible transition states for structures representing CH3/Cl or Br interchange, whereas no structures could be identified that would support a Wagner-Meerwein type transition state. We conclude that the CH3/halogen atom interchange mechanism adequately explains the unimolecular reactions of neopentyl chloride and neopenthyl bromide. Exploratory calculations, see the summary of threshold energies in Table 2, were done for molecules with CHCl2 and
CCl3 groups in place of the CH2Cl group. The threshold energies were raised by about 4 kcal mol-1 for each additional chlorine atom. However, the interchange reaction still should be the unimolecular reaction with the lowest E0 for these two molecules. Calculations for CH3/F interchange in neopentyl fluoride gave a threshold energy of 87.2 kcal mol-1, and the interchange reaction of methyl or other groups with a fluorine atom will have very high activation energies and interchange will not be important. Chuchani and co-workers31 have suggested that the small (10%) yield of 2,3-dimethyl-1-butene and 2,3-dimethyl2-butene from pyrolysis of 2-chloro-3,3-dimethylbutane (pinacolyl chloride) arises from a Wagner-Meerwein rearrangement. However, a CH3/Cl interchange mechanism would be preferred by analogy to neopentyl chloride. Adding a third methyl group to the interchange transition state shown in Figure 1 should lower the threshold energy, and the interchange pathway becomes competitive with 1,2-HCl elimination, which gives (CH3)3CCHdCH2 from (CH3)3CCHCl(CH3). The reported31b activation energy based on experiments over only a 55 K range was 47.2 kcal mol-1 for the total decomposition rate constant of pinacolyl chloride; however, the pre-exponential factor seems to be too small and the experimental Ea probably is a lower limit. Our DFT calculations confirm the effect of the third CH3 group and the calculated threshold energies for 1,2-HCl elimination and interchange were 44.3 and 58.1 kcal mol-1, respectively; however, calculated E0 values for HCl elimination by this method frequently are too low. How general is the interchange reaction between a Cl or Br atom and an adjacent methyl (or other organic group)? Chuchani and Dominguez32 have studied the unimolecular reactions of 1-chloro-2-methyl-2-phenylpropane. The principal products (96%) were 2-methyl-3-phenyl-1-propene and 2-methyl-3-
Isomerization of Neopentyl Chloride and Bromide phenyl-1-propene. The authors32 proposed a Wagner-Meerwein rearrangement mechanism to explain the products. However, the interchange of a phenyl group and a Cl atom to give 1-phenyl-2-chloro-2-methylpropane followed by HCl elimination is a more likely mechanism. Our DFT calculations using B3PW91/6-31G(d′,p′) identified a transition state, see Figure 4A, for the interchange of the phenyl group and the Cl atom with a threshold energy of 53 kcal mol-1, which is in excellent agreement with the experimental activation energy of 54 ( 2 kcal mol-1. In addition, the calculations identified a transition state for CH3/Cl interchange, see Figure 4B, to give 2-chloro2-phenylbutane with a threshold energy of 58 kcal mol-1, which is consistent with the products (4%) from this pathway. Similar calculations also were done for 1-chloro-2-methyl-2-vinylpropane, and the threshold energy for methyl interchange with the Cl atom was 61.0 kcal mol-1. Electron donating groups are expected to lower the threshold energy for the interchange reaction, and this possibility was tested by calculations for 1-chloro-2-methoxy-2-methylpropane; the threshold energy for CH3/Cl interchange was 57.8 kcal mol-1, which is 3 kcal mol-1 lower than for neopentyl chloride. Electron attracting groups, such as halogen atoms, increase the threshold energy for CH3/ Cl interchange reactions as demonstrated by the (CH3)3CCHCl2 and (CH3)3CCCl3 examples in Table 2. On the basis of the examples in Table 2, there should be a sizable group of molecules for which interchange of methyl (or other) groups and Cl or Br atoms is an important unimolecular gas-phase reaction, provided that the competing HCl or HBr elimination reactions are either blocked or have high threshold energies. As a final part of the Discussion, we wish to consider the structure of the CH3/Cl (or CH3/Br) interchange transition state and its relation to the transition states for halogen atom interchange reactions.7,11-17 One clear difference is the larger entropy of activation for the CH3 case, because of the weakly bound CH3 group with low torsion and bending frequencies. The Cl (or Br) atom is further from the carbon atoms for the methyl-interchange transition state than for the Cl/Br-interchange transition state,7 and the carbon backbone is less planar in the Cl/CH3 transition state. A second major difference is the asymmetric charge distribution for the CH3 case, as illustrated by the very negative Cl (or Br) atom in Figures 1 and 4, whereas, the charge distribution for halogen atom interchange tends to be quite symmetric7 with both halogen atoms sharing the negative charge. The negative Cl atom causes a strong polarization of the C-H bonds of the two CH3 groups that are directed toward the Cl atom as shown in Figures 1 and 4. In fact, these H-atoms in the CH3 groups are the most positive atoms of the structures of Figures 1 and 4A. In summary, the negative charge is mainly localized on the halogen atom, but the positive charge is dispersed over all the other atoms, except the carbon atom in the CH3 group of the bridge. The charge distributions were obtained by the atoms-in-molecule approach.33 The structures in Figures 1 and 4 have some interesting features. The methyl and phenyl groups, as well as the Cl atom, are nearer to the CH2 end than to the C(CH3)2 ends of the transition state. The carbon atom of the phenyl group in the phenyl/Cl transition state is closer to the bridgehead carbon atom than is the carbon atom of the CH3 group for either CH3/Cl transition state. Despite lowering E0 by 4 kcal mol-1 for CH3 interchange, the structures in Figures 1 and 4B are very similar; however, the positive charges on the bridgehead carbon atoms (C5 and C6) are slightly lower with the phenyl substituent.
J. Phys. Chem. A, Vol. 114, No. 38, 2010 10401 VI. Conclusions The interchange of a methyl group and the Cl (or Br) atom in neopentyl chloride (and bromide) has been demonstrated for chemically activated neopentyl chloride-d0 and -d2 and for thermally activated neopentyl chloride and bromide. The threshold energies are 60-62 kcal mol-1. This interchange mechanism followed by HCl (or HBr) elimination from 2-chloro (or bromo)-2-methylbutane replaces the previously proposed Wagner-Meerwein rearrangement mechanism. A small kinetic H/D isotope effect of 1.33 ( 0.2 measured for (CH3)3CCH2Cl/ (CH3)3CCD2Cl substantiates the interchange-elimination mechanism. The CH3/Cl interchange reaction seems to compete with HCl elimination in the thermal reaction of 2-chloro-3,3dimethylbutane. Other examples of interchange reactions of a methyl group and a Cl atom were discussed using DFT calculations to find the transition states and threshold energies. The interchange of a phenyl group and a Cl atom probably occurs in the thermal reaction of 1-chloro-2-methyl-2-phenylpropane. Electronic structure calculations predict that CH3/Cl interchange should be observable for (CH3)2(OCH3)CCH2Cl and (CH3)2(CHdCH2)CCH2Cl. The halogen atom/methyl group interchange reactions resemble the interchange reactions involving two adjacent halogen atoms of dihaloalkane molecules. Acknowledgment. Financial support from the National Science Foundation (CHE-0647582) and (MRI-0320795) is acknowledged. Supporting Information Available: Tables of the molecular and transition state structure vibrational frequencies, overall moments of inertia, and the reduced moments of inertia for the internal rotors calculated using B3PW91/6-31G(d′,p′) for CH2BrC(CH3)3 and CH2ClC(CH3)3. A figure of an energy profile for the formation and decay of chemically activated neopentyl chloride calculated with the same DFT method. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Maccoll, A.; Swinbourne, E. S. J. Chem. Soc. 1964, 149. (2) Shapiro, J. S.; Swinbourne, E. S. Can. J. Chem. 1968, 46, 1341– 1351. (3) Failes, R. L.; Mollah, Y. M. A.; Shapiro, J. S. Int. J. Chem. Kinet. 1979, 11, 1271. (4) Failes, R. L.; Mollah, Y. M. A. Int. J. Chem. Kinet. 1981, 13, 7. (5) Toto, J. L.; Pritchard, G. O.; Kirtman, B. J. Phys. Chem. 1994, 98, 8359 This reference gives a good summary of the various models for the transition states for HX elimination from alkyl halides. (6) Duncan, J. R.; Solaka, S. A.; Setser, D. W.; Holmes, B. E. J. Phys. Chem. A 2010, 114, 794. (7) Friederich, L.; Duncan, J. R.; Heard, G. L.; Setser, D. W.; Holmes, B. E. J. Phys. Chem. A 2010, 114, 4138. (8) Maccoll, A.; Wong, S. C. J. Chem. Soc., B 1968, 1492. (9) Beaver, M. R.; Heard, G. L.; Holmes, B. E. Tetrahedrom Lett. 2003, 44, 7265. (10) Dolbier, W. R., Jr.; Romelaeer, R.; Baker, J. M. Tetrahedron. Lett. 2002, 43, 8075. (11) Burgin, M. O.; Heard, G. L.; Martell, J. M.; Holmes, B. E. J. Phys. Chem. A 2001, 10, 615. (12) Heard, G. L.; Holmes, B. E. J. Phys. Chem. A 2001, 105, 1622. (13) Burgin, M. O.; Simmons, J. G., Jr.; Heard, G. L.; Setser, D. W.; Holmes, B. E. J. Phys. Chem. A 2007, 111, 2283. (14) (a) Zaluzhna, O.; Simmons, J. G., Jr.; Heard, G. L.; Setser, D. W.; Holmes, B. E. J. Phys. Chem. A 2008, 112, 6090. (b) Zaluzhna, O.; Simmons, J. G., Jr.; Setser, D. W.; Holmes, B. E. J. Phys. Chem. A 2008, 112, 12117. (15) (a) Holmes, D. A.; Holmes, B. E. J. Phys. Chem. A 2005, 109, 10726. (b) Zhu, L.; Simmons, J. G., Jr.; Burgin, M. O.; Setser, D. W.; Holmes, B. E. J. Phys. Chem. A 2006, 110, 1506. (16) Beaver, M. R.; Simmons, J. G., Jr.; Heard, G. L.; Setser, D. W.; Holmes, B. E. J. Phys. Chem. A 2007, 111, 8445.
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