Furanones - ACS Publications - American Chemical Society

Article pubs.acs.org/JPCA

Thermal Decomposition of 2(3H) and 2(5H) Furanones: Theoretical Aspects Judith Würmel,* John M. Simmie, Michelle M. Losty, and Cathal D. McKenna Combustion Chemistry Centre & School of Chemistry, National University of Ireland, Galway, Ireland S Supporting Information *

ABSTRACT: The thermal decomposition reactions of 2(3H) and 2(5H) furanones and their methyl derivatives are explored. Theoretical calculations of the barriers, reaction enthalpies, and the properties of these and intermediate species are reported using the composite model chemistry CBS-QB3 and also the functional M06-2X allied to the 6-311+ +G(d,p) basis set. Thus, the bond dissociation enthalpies, ionization energies, and unimolecular chemical kinetic rate constants in the high-pressure limit were computed. We show that flow reactor experiments that intimated that heating the 2(3H) furanone converts it to the isomeric 2(5H) furanone occurs via a 1 → 2 H-transfer reaction to an open ring ketenoic aldehyde. The latter can then ring close to the other isomeric structure. The final products acrolein and carbon monoxide are only formed from 2(3H), and acrolein will further decompose to ethylene and CO. Comparable channels explain the interconversion of 5-methyl-2(3H) furanone to its 2(5H) isomer and to the formation of methyl vinyl ketone and CO. The influence of the methyl group at other positions on the ring is hardly of significance except in the case of 5-methyl-2(5H) furanone where a hydrogen atom transfer from the methyl group leads to the formation of a doubly unsaturated carboxylic compound, 2,4-pentadienoic acid. Studies of the UV photolysis of the parent compounds in both low-temperature inert argon matrices and in solution are broadly in accord with the thermal findings insofar as product formation is concerned and with our theoretical calculations. The dominant features of the early decomposition chemistry of these compounds are simple hydrogen transfer and simultaneous ring opening reactions, which do however result in some quite unusual species.

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interaction of 2(5H) furanone with α-hydroethyl radicals generated in the radiolysis of deaerated ethanol has been studied, and it was shown that addition of CH3Ċ HOH occurs directly across the CC double bond.14 The pyrolysis of acyclic ethyl and higher esters is well-known to yield an alkene and a carboxylic acid,15 but in the case of cyclic esters, or lactones, the difficulty in forming a six-centered transition state involving the β-hydrogen atom renders this unlikely. Hence, the course of pyrolysis is certain to illustrate new aspects of the chemistry of these species. In the case of furanones, it is known that thermolysis of 5methyl-2(3H)-furanone, commonly known as α-angelica lactone, yields 3-buten-2-one (methyl vinyl ketone or MVK). 16 Xu et al. followed the interconversion and decomposition of both 2(3H) and 2(5H) furanones and showed that interconversion occurs at ∼300 to 400 °C, while decomposition takes place at ∼600 °C yielding acrolein (2propenal) and carbon monoxide.17 They suggested that decarbonylation is the dominant process with the 2(3H) form as the common precursor. They made similar

INTRODUCTION 2(xH) furanones, a set of unsaturated cyclic esters, are interesting compounds1,2 whose derivatives have applications in liquid crystals,3 microbial quorum sensing,4 and marine chemical ecology;5 they can be found in raw6 and cooked foodstuffs,7 etc. Recently it has been shown that the 2(5H) isomer induces cellular DNA damage and is cytotoxic in leukemia and cancer cells.8 It is formed in the bio-oil during the fast pyrolysis of palm kernel cake 9 and unsurprisingly in the thermal decomposition of vitamin C.10 The heterocyclic ring-opening dynamics of 2-furanone have been probed by ultrafast transient infrared spectroscopic methods in acetonitrile solution by Murdock et al.11 Following irradiation at 225 nm they confirmed ring-opening by the appearance of a typical ketene-like stretching vibration at ∼2150 cm−1. The more fundamental physicochemical properties of furanones are, however, little known, although very recently enthalpies of formation, ionization, and bond dissociation energies have been computed.12 In addition the rate constants for both addition and H-abstraction reactions with the methyl radical and hydrogen atoms were calculated. The kinetics of reaction of 2(3H) with the hydroxyl radical at room temperature has been measured by Bierbach et al.13 The © 2015 American Chemical Society

Received: May 8, 2015 Revised: June 6, 2015 Published: June 8, 2015 6919

DOI: 10.1021/acs.jpca.5b04435 J. Phys. Chem. A 2015, 119, 6919−6927

Article

The Journal of Physical Chemistry A

ΔfH⊖(298.15 K). The enthalpies of formation of the species encountered in this work were determined by using the appropriate furanone parent compound that we had previously computed12 and adding the reaction enthalpy (together with the formation enthalpy of well-known species such as CO as and if required); these are then catalogued in Table 1. Thus, for

observations about the decomposition of the methylated furanones α-angelica lactone and 3-methyl-2(5H)-furanone. In recent work Urness and colleagues18 have used a resistively heated silicon carbide microtubular reactor19 to investigate the pyrolysis of 2(3H), 2(5H), and 5-methyl-2(3H) furanones using fixed frequency photoionization time-of-flight mass spectrometry and infrared spectroscopy in a cryogenic matrix to analyze the products exiting the reactor. Typically residence times of ∼150 μs and temperatures of 1500 K are employed with control experiments at 300 K in all cases with the reactants highly dilute in either helium or argon as carrier gases. For a review of the general considerations that govern the properties of such extreme flow reactors see the work of Daily et al.20 It is the objective of this work to address the possible unimolecular reactions that might be responsible for the flow reactor observations.16−18

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Table 1. Formation Enthalpies and Ionisation Energies

COMPUTATIONAL METHODS

The composite method CBS-QB3,21,22 embedded in the application Gaussian 09,23 was used to determine the geometries, vibrational frequencies, and zero-point corrected electronic energies of each species and transition state. Thus, both the geometries and the harmonic frequencies are determined via economical density functional theory at B3LYP/6-311G(2d,d,p), which is the built-in optimizer in CBS-QB3. The connection between reactant, transition state, and product was established by intrinsic reaction coordinate calculations also at B3LYP/6-311G(2d,d,p).24,25 In addition, the performance of a newer functional, M06-2X,26 with the basis set 6-311++G(d,p), was tested; a scale factor of 0.983 was adopted in this case.27 The computation of partition functions and derived values for the entropies, specific heats, and enthalpy functions was performed with the Thermo code of Multiwell, which also was used to calculate chemical kinetic rate constants in the highpressure limit.28 The harmonic oscillator rigid rotor approximation was employed for the determination of the partition functions apart from those cases where hindered rotors were considered when relaxed torsional potential energy scans were done at 10° intervals to determine the rotational barriers. In appropriate unsymmetrical cases, a Fourier series was used to represent the potential energy:

∑ [cos(nσv{χ + φ})] n=1

9

+

∑ [sin(nσv{χ + φ})] n=1

ΔfH⊖, kJ mol−1

IE, eV

2(3H)-furanone 2(5H)-furanone 5-methyl-2(3H)-furanone 5-methyl-2(5H)-furanone 3-methyl-2(5H)-furanone 2-furanol 4-oxo-3-butenal cis-2-propenal trans-2-propenal 3-methyl-4-oxo-3-butenal 4-oxo-2-pentenal 2-butenedial 2-oxiran-2-yl-ethenone 2-(3-methyloxiran-2-yl)-ethenone 1-pentene-1,4-dione 1-hydroxy-4-oxo-1,3-butadiene 1-hydroxy-1-methyl-4-oxo-1,3-butadiene 2,4-pentadienoic acid 3,4-pentadienoic acid 1,2,4-pentatriene-1-one 5-methyl-2-furanol 2H-oxete (oxetene) 2-methyl-2H-oxete 4-methyl-2H-oxete

−251 −261 −302 −302 −305 −204 −167 −55.2 −63.8 −187 −209 −156 −72.3 −115 −220 −140 −184 −223.4 −190 202 −247 54.8 18.7 10.9

9.33 10.26 8.94 10.11 10.11 8.21 8.67 10.09 10.08 8.26 9.97 9.71 8.93 8.70 8.26 7.76 7.47 9.57 9.40 8.72 7.80 9.01 8.36 8.77

example, the listed ΔfH⊖(298.15 K) for 4-oxo-3-butenal is estimated from a ΔfH⊖(298.15 K) of 2(3H) furanone of −251.0 kJ mol−1 and a reaction enthalpy change of 84.0 kJ mol−1. In some cases two or more channels lead to the same species, and in those cases the listed values are the average of those possibilities. Exact agreement with our previous work is unlikely since multiple composite methods were used before, whereas in this work we are relying on a single model chemistry, namely, CBS-QB3. Independent checks are rarely possible, but in the case of the 2H-oxete (3-oxacyclobutene) G3(MP2)//B3LYP calculations by Taskinen29 indicated values of 51.3 and 53.1− 54.0 kJ mol−1 from atomization and isodesmic approaches, respectively (the reported mean value of −53.8 is clearly a misprint). More recent work by the Green group30 recorded 52.7 ± 3.8 kJ mol−1 from RQCISD(T)/cc-pV∞QZ//B3LYP/ 6-311++G(d,p) calculationsall of which are in satisfactory agreement with our value of 54.8 kJ mol−1. Ionization Energies. Since many of the experiments on these species have been conducted by photoionization methods it is useful to compile the calculated adiabatic ionization energies, Table 1, for the compounds in the various schemes. Bond Dissociation Energies. The bond dissociation energies (BDEs), D(X−Y), for a number of reactant and product species were computed directly from the enthalpy change at 298.15 K for the reaction XY = Ẋ + Ẏ utilizing the model chemistries M06-2X/6-311++G(d,p) and CBS-QB3, Figure 1. Some of the radicals fail Lee’s T1 diagnostic test, for

9

V (χ ) = V0 +

species

(1)

where χ is the dihedral angle, σv is the symmetry number for the scan, and φ is the phase angle. The rotational constant or reduced moment of inertia was also fitted as a function of the dihedral but only in cos θ terms. More symmetrical hindered rotor scans as exemplified by methyl groups only require the rotational barrier and the threefold symmetry number to evaluate the contribution that they make to the partition functions.

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RESULTS All the results presented below are based solely upon CBS-QB3 calculations; later, a comparison will be made with results obtained from M06-2X/6-311++G(d,p) computations. 6920


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Figure 1. Bond dissociation energies, kJ mol−1.

example, the allenyl radical CH2CĊ H, for which T1 = 0.050; those BDEs are therefore not wholly reliable.31 Those for the five primary reactants have already been reported.12 2(3H)-Furanone. Xu et al.17 pyrolyzed the furanones in a 45 cm long quartz reactor of 1.2 cm diameter at a pressure of 1 Torrthe residence times were not, unfortunately, reported. They used photoelectron spectroscopy to follow the course of reaction, which they found was influenced by the reactor surface. In the case of 2(3H) they noted the appearance of the isomeric 2(5H) at 300 °C and that by 550 °C the interconversion was 80% complete. At higher temperatures 2propenal (acrolein) and carbon monoxide were formed, and by 800 °C only these products were evident. The latter observation is evident from the theoretical calculations, which indicate that cis-acrolein arises directly from the decarbonylation of 2(3H); 1,2-H transfers (which can be regarded formally as 1,5-sigmatropic shifts) and ring-bond scissions leading to 4-oxo-3-butenal and 2-butenedial have higher barriers as does enolization to 2-furanol, Figure 2; decarboxylation to cyclopropene is much higher again at ≥410 kJ mol−1. In Figures 2−12, reaction arrows are numbered according to the listing order in Table 2 and labeled with the zero-point corrected electronic energy difference or barrier height E‡ at 0 K in kJ mol−1. At first sight the production of the isomeric 2(5H) furanone in equilibrium quantities, 2(3H) ⇌ 2(5H), prior to the

Figure 2. Molecular decompositions of 2(3H) furanones.

formation of the final products, seems counterintuitive. The calculated equilibrium constant, K = [2(5H)]/[2(3H)], at 825 K, based on the frequencies, rotational constants, and the energies of each isomer, is 3.57, which implies [2(3H)] = 0.22 and [2(5H)] = 0.78very close to the 20:80 ratio reported by Xu et al.17 There are no convincing channels for the semidirect conversion to the 2(5H) furanone on a singlet surface except through the intermediate 4-oxo-3-butenal, Figure 3, a littleknown compound. An alternative route through intermediate 6921


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The Journal of Physical Chemistry A Table 2. High-Pressure Limit Rate Constant Parameters No.

reaction

(1)

2(3H)-furanone →2propenal + CO 2(3H)-furanone →4-oxo-3butenal 2(3H)-furanone →2butenedial 2(3H)-furanone →2furanol 4-oxo-3-butenal →2propenal (trans) + CO 4-oxo-3-butenal →2propenal (cis) + CO 4-oxo-3-butenal → methylketene + CO 2-butenedial →2-propenal (trans) + CO 2(5H)-furanone →4-oxo-3butenal 2(5H)-furanone →1hydroxy-4-oxo-1,3butadiene 2(5H)-furanone →2oxiran-2-yl-ethenone 2(5H)-furanone → CH2O + C2H2 + CO 3-methyl-2(5 H)-furanone → MVK 3-methyl-2(5H)-furanone →3-methyl-4-oxo-but-3enal 3-methyl-4-oxo-but-3-enal → methacrolein + CO 5-methyl-2(3H)-furanone → MVK + CO 5-methyl-2(3H)-furanone →1-pentene-1,4-dione 5-methyl-2(3H)-furanone →4-oxo-2-pentenal

(2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18)

A, s−1

n

EA, kJ mol−1

No.

13

4.15 × 10

0.360

222.6

(19)

7.27 × 1010

0.879

235.9

(20)

1.44 × 1011

0.742

242.3

(21)

9.38 × 1002

3.043

275.6

(22)

1.55 × 1011

0.739

197.4

(23)

1.38 × 1011

0.856

285.0

(24)

6.81 × 1010

1.156

324.0

(25)

9.73 × 1009

1.456

339.4

2.58 × 1010

0.992

215.4

4.34 × 1008

1.615

298.8

(26) (27) (28)

13

1.56 × 10

0.397

317.2

1.01 × 1014

0.889

409.1

(29)

2.20 × 1012

0.840

493.6

(30)

5.26 × 1010

0.864

229.0

(31) (32)

11

8.46 × 10

0.565

193.2

1.24 × 1014

0.248

226.5

4.16 × 1010

0.901

236.1

2.86 × 10

0.649

11

(33) (34) (35) 237.5

reaction 5-methyl-2(3H)-furanone →5-methyl-2-furanol 5-methyl-2(3H)-furanone →3,4-pentadienoic acid 1-pentene-1,4-dione → MVK + CO 4-oxo-2-pentenal → MVK + CO 5-methyl-2(5H)-furanone →1-pentene-1,4-dione 5-methyl-2(5H)-furanone →2,4-pentadienoic acid 5-methyl-2(5H)-furanone →2-(3-methyl-oxiran-2yl)ethenone 5-methyl-2(5H)-furanone →1-hydroxy-1-methyl-4oxo-1,3-butadiene 2,4-pentadienoic acid → butadiene + CO2 2,4-pentadienoic acid →1,2,4-pentatriene-1,1diol 2,4-pentadienoic acid →1,2,4-pentatriene-1-one 2,4-pentadienoic acid →3,4-pentadienoic acid 3,4-pentadienoic acid → butadiene + CO2 3,4-pentadienoic acid → methyl allene + CO2 2-propenal (trans) → ethylene + CO2 2-propenal (cis) → ethylene + CO2 2-methyl-2-propenal → propene + CO2

A, s−1

n

EA, kJ mol−1

03

1.82 × 10

2.958

278.6

2.13 × 1009

1.640

325.9

1.57 × 1012

0.243

289.9

4.65 × 1009

1.632

342.5

1.58 × 1011

0.716

203.3

5.54 × 1006

2.078

270.3

1.18 × 1013

0.342

312.1

1.95 × 1009

1.366

298.9

3.70 × 1009

1.149

238.2

4.14 × 1008

1.323

278.4

4.72 × 1009

1.636

311.1

1.67 × 1011

0.858

319.9

1.52 × 1007

1.407

156.5

4.46 × 1003

2.893

269.4

3.32 × 1009

1.630

359.5

1.75 × 1009

1.482

350.2

4.44 × 1009

1.588

356.2

Figure 3. Interconversion of 2(3H) and 2(5H) furanones, barriers: forward/reverse reactions.

1,4-dicarbonyl, 2-butenedial only leads to trans-acrolein and CO via barriers of 244 and 346 kJ mol−1, respectively. Bierbach et al.13 have shown that photolysis of cis-butenedial (maleic dialdehyde) leads to an intramolecular rearrangement or cyclization to 2(3H) furanone, while photolysis of cis-4-oxo-2pentenal similarly gives rise to 5-methyl-2(3H) furanone or αangelica lactonethe converse of the thermal reactions outlined above. But-2-enedial exists in two isomeric forms, namely, the cis or Z and the trans or E forms, commonly known as malealdehyde and fumaraldehyde.32 Both are formed during the OH-initiated gas-phase oxidation of furan, while similarly 4-oxo-2-pentenal has been reported as a product in the room-temperature photooxidation of 2-methylfuran.33,34 More generally these unsaturated 1,4-dicarbonyls are formed from the OH-initiated oxidation of aromatics and go on to react further with Ȯ H to yield 2(3H) and 5-methyl-2(3H) furanones.13 In other work12 we have shown that the presence of a catalytic amount of hydrogen atoms can interconvert the

isomers via a sequence of low activation energy addition and elimination reactions: 2(3H) + Ḣ ⇄ 2(5H) + Ḣ . The microtubular reactor experiments of Urness et al.18 utilize photionisation mass spectrometric and infrared spectroscopic detection of the reactants and products. They observed the formation of acrolein, both cis and trans conformers, and CO as the primary products from both 2(3H) and 2(5H) furanone decompositions. With residence times of ∼100 μs they observed acrolein beginning at ∼1000 K, whereas the same species was seen at some 150−200 K higher in the case of 2(5H) pyrolysis. The rate of trans ⇔ cis isomerization is sufficiently fast that these isomers are effectively present in equilibrium amounts under these conditions. There is some evidence from the IR studies that a ketene-like vibrational mode seen in an argon matrix is due to 4-oxo-3-butenal, whose adiabatic ionization energy we calculate at 8.70 ± 0.04 eV. Acrolein or 2-propenal is found in the atmosphere primarily from combustion sources and is quite toxic.35 It is produced during shock-heating of propargyl alcohol, HCCCH2OH.36 The ignition and chemical kinetics of acrolein−oxygen−argon 6922


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The Journal of Physical Chemistry A mixtures has been recently explored by Chatelain et al.37 behind reflected shock waves at 1200−1600 K and 170−420 kPa. They postulated two principal initiation reactions: ̇ + Ḣ CH 2CH−CHO → CH 2CH−CO (2) ̇ + CHO ̇ CH 2CH−CHO → CH 2CH

(3)

with associated rate constants of k−2 = 1.0 × 1014 cm3 mol−1 s−1 and k3 = 2.48 × 1024T−2.513exp(−51 360/T) s−1. Originally they attributed the 20% higher contribution of k3 over that of k2 to the relatively small differences in BDEs for saturated aldehydes of ∼20 kJ mol−1 reported by da Silva and Bozzelli.38 However, our results indicate that the differences in BDEs are some 50% higher in the case of the unsaturated aldehyde acrolein, Figure 1. On the basis of their value of k−2 = 1.0 × 1014 cm3 mol−1 s−1 and with the correct thermochemistry in JetSurf39 this implies a dissociation rate40 for k2 of 4.42 × 1016T−0.66 exp(−42 920/T) s−1. Therefore k2 is substantially greater than k3 by ≥2 over the range of 1200−1600 K. The rate constants for the molecular decarbonylation reaction of cis- and trans-acrolein were computed using the composite method CBS-QB3, Figure 4 and Table 2, and were

Figure 5. Molecular reactions of 2(5H) furanone.

decarbonylation to crotonaldehyde or methacrolein being detected in the flow reactor experiments by Xu et al.17 who speculated that isomerization to 3-methyl-2(3H) furanone precedes the decarbonylation based partially on the fact that MVK or 3-buten-2-one was found as a minor product. Our work shows that the 3-methyl-2(5H) furanone ring opens with a barrier of 232 kJ mol−1 to a ketenoic aldehyde, 3methyl-4-oxo-but-3-enal, which can then eliminate CO, barrier 194 kJ mol−1, to form trans-methacrolein; in other words, it is very similar to the path traced by the unmethylated parent 2(5H) furanone. The formation of traces of MVK is somewhat harder to explain theoretically since it is unlikely that the methyl group will migrate around the ring. A higher energy path via an oxete intermediate is however feasible, Figure 6.

Figure 4. Decarbonylation reactions of trans- and cis-acrolein and 2methyl-2-propenal.

Figure 6. Formation of MVK from 3-methyl-2(5H) furanone.

found to proceed at comparable rates as the radical initiation reactions above. The decarbonylation of methacrolein (No. 35), similar to that of acrolein (No. 34), is slowed by ∼1/3 due to the presence of the methyl group. 2(5H)-Furanone. The pyrolysis of 2(5H)-furanone, or γcrotonolactone, followed a very similar course to that of the 2(3H) via conversion to the isomer at 350 °C and a stationary state of 20% 2(3H) and 80% 2(5H) by 550 °C; the final products at even higher temperatures being acrolein and CO as before. In UV-induced photolysis of 2(5H)-furanone Breda et al. had shown that an aldehydic ketene is formed, 4-oxo-3-butenal, from a consideration of the IR spectra obtained in an argon matrix together with theoretical calculations at the B3LYP/6311++G(d,p) level of theory.41,42 The presence of a second hydrogen atom at position 5 opens other possibilities; ring scission of the O−CO bond and formation of an oxirane, 2-oxiran-2-yl-ethenone, and a 1,2-Htransfer to the heterocyclic oxygen to form a substituted butadiene, 1-hydroxy-4-oxo-1,3-butadiene, Figure 5. Murdock and colleagues determined these barriers at 315 (313) and 311 (308) kJ mol−1, respectively, from MP2/6-311+G(d,p) calculations with their lowest barrier at 224 (218) kJ mol−1 leading to the 4-oxo-3-butenalall of which are in good agreement with our computations, which are shown in brackets. 3-Methyl-2(5H)-furanone. The thermal decomposition of 3-methyl-2(5H) furanone followed a similar pattern with

5-Methyl-2(3H)-furanone. Skorianetz and Ohloff 16 showed that α-angelica lactone decomposes principally to 3buten-2-one (MVK), unspecified higher boiling point products, and presumably CO, although its detection was not reported. They passed the vapor through a 1 m long reactor heated to 475 °C at a pressure of 15 Torr but for an undetermined residence time. The main finding, that is, decarbonylation to MVK, was confirmed by Xu et al., who additionally report that isomerization to the 5-methyl-2(5H) furanone preceded the formation of MVK and CO. This is not the case for the microtubular reactor study, which only found MVK and CO at higher temperatures MVK itself becomes thermally unstable. The decarbonylation is indeed the lowest barrier at 220 kJ mol−1, 1,2-H shifts and ring bond scissions lead to 1-pentene1,4-dione and 4-oxo-2-pentenal at 238.9 and 240 kJ mol−1 respectively, while enolization and decarboxylations have much higher barriers of ca. 390 kJ mol−1, Figure 7. In all these cases the methyl group is simply a spectator and has little impact on the course of reaction, compare Figures 2 and 7. However, the methyl group does participate directly in a ring-opening reaction by transferring a hydrogen atom, leading to the formation of the unsaturated carboxylic acid 3,4pentadienoic acid with a barrier of 331 kJ mol−1, Figure 8. This particular type of reaction is very important in the decomposition of the analogous saturated furanone γvalerolactone43,44 where the isomerization to 4-pentenoic 6923


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2,4-Pentadienoic Acid. Decarboxylation of 2,4-pentadienoic acid to trans-1,3-butadiene is the most favored route at 237 kJ mol−1 followed by enolization at 285 kJ mol−1 to 1,2,4pentatriene-1,1-diol, dehydration at 307 kJ mol−1 to 1,2,4pentatriene-1-one and isomerization via a 1,3-H shift at 321 kJ mol−1 to 3,4-pentadienoic acid, Figure 11.

Figure 7. Molecular decompositions of 5-methyl-2(3H) furanone.

Figure 11. Reactions of 2,4-pentadienoic acid.

In a more comprehensive study of the unimolecular reaction pathways for carboxylic acids of relevance to biofuels Clark et al. 45 found that α,β-unsaturated acids, of which 2,4pentadienoic acid is a member, were more prone to decarboxylation than dehydration in comparison to the analogous saturated acids. Their results for senecioic acid, (CH3)2CCH−COOH, very closely match ours for 2,4pentadienoic acid. 3,4-Pentadienoic Acid. The gas-phase flash vacuum pyrolysis of unsaturated carboxylic acids has been studied by Bigley and Weatherhead46 at pressures of 0.01 Torr over the range of 600−650 K; they found that 3,4-pentadienoic acid decarboxylates to 1,3-butadiene, via a 1,5-H-shift, with an activation energy of 162 kJ mol−1, Figure 12. This is in good

Figure 8. 5-methyl-2(3H) furanone to 3,4-pentadienoic acid.

acid, 268 kJ mol−1, is the dominant process at temperatures below 1400 K. As before the interconversion of the methyl isomers can be mediated this time via 1-pentene-1,4-dione, whose ionization energy is calculated to be 8.28 ± 0.03 eV, with only 5-methyl2(3H)-furanone able to decarbonylate directly to MVK, Figure 9.

Figure 9. Interconversion of 5-methyl-2(3H) and 5-methyl-2(5H) furanones.

5-Methyl-2(5H)-furanone. There are no previously reported studies on this compound, which is more commonly known as β-angelica lactone. The primary molecular decompositions follow very similar routes, Figure 10, to those of the parent 2(5H)-furanone. The methyl-substituted oxirane, namely, cis-2-(3-methyloxiran-2-yl)-ethenone and its trans conformer, result from scission of the OC(O) bond and subsequent ring closure, whereas a substituted butadiene, 1-hydroxy-1-methyl-4-oxo-1,3-butadiene, follows from an enolization step.

Figure 12. Decarboxylation of 3,4-pentadienoic acid.

agreement with our calculated value of 167 kJ mol−1. An alternative 1,3-H shift reaction leads to methyl allene and CO2, Figure 12, but the computed barrier is much higher at 292 kJ mol−1. Entropies and Heat Capacities. A complete listing of the thermodynamic parameters for each species is given in the Supporting Information. The entropy S, specific heat at constant pressure CP, and enthalpy function H(T) − H(0) were all calculated via the Thermo module of Multiwell under the rigid-rotor harmonic-oscillator approximations with additional hindered rotor treatments as appropriate.

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KINETICS The unimolecular high-pressure limit rate constants were calculated for all the reactions considered above, and the results are displayed in Table 2 with the rate constants k fitted to an Arrhenius-type equation: k /s−1 = AT n exp{−EA /(RT )}

(4)

where A is the pre-exponential factor, n is the power dependence of the temperature T (K), EA (J mol−1) is the

Figure 10. Molecular decompositions of 5-methyl-2(5H) furanone. 6924


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The Journal of Physical Chemistry A activation energy, and R (J mol−1 K−1) is the universal gas constant. ATn has the same units as those of the rate constant k. Analysis of the kinetics of 2(3H)-furanone concluded that the ring-opening reaction (No. 1) leading to cis-acrolein is the fastest reaction at the entire temperature range of 500−2,000 K. cis-Acrolein converts readily to the more stable conformer trans-acrolein as a result of the low barrier of 29.3 kJ/mol. Analogous to 2(3H)-furanone, its methylated counterpart 5methyl-2(3H)-furanone pyrolyses fastest by decarbonylation giving MVK (No. 16) over the entire temperature range studied. Pyrolysis of 2(5H)-furanone occurs predominantly via (No. 9), the ring-opening reaction forming 4-oxo-3-butenal, which is in agreement with findings by Breda et al.41,42 Similarly, akin to the methylated counterpart of 2(5H)-furanone, 5-methyl2(3H)-furanone predominantly ring-opens (No. 23) to give 1-pentene-1,4-dione. The fastest reaction of 2,4-pentadieonic acid is the decarboxylation to trans-1,3-butadiene at all temperatures studied, while trans-butadiene is also the favored product from 3,4-pentadionic acid. A comparison of the fastest reaction pathways of 2(3H)furanone, 2(5H)-furanone, and their methylated derivatives 5methyl-2(3H)-furanone and 5-methyl-2(5H)-furanone indicates that the methyl group has no effect on the ring-opening reaction of 2(3H)-furanone, while the presence of the methyl group in 2(5H)-furanone accelerates ring opening, particularly at lower temperatures. Pressure Dependence. The application ChemRate47 uses RRKM theory to compute rate constants as functions of temperature and pressure for unimolecular reaction systems such as those under consideration here. It solves the Master Equation and incorporates several collisional models although here we employed only the simplest biexponential model with the same energy-transfer variables for all the well species shown in Figure 3, namely, 2(3H) and 2(5H) furanone and 4-oxo-3butenal, based on a recent expression, ΔEd = 325(T/298)0.85 cm−1, given by Annesley et al.48 for the temperature dependence of the average collisional energy transfer parameter. Lennard-Jones parameters, σ and (ϵ/kB), for the furanones of 4.737 Å and 562.7 K and 5.134 Å and 483.4 K for 4-oxo-3-butenal were calculated from rules developed by Kee et al.49 based on estimated physical properties such as boiling points and molar volumes.50 The rate constants for ring-opening reactions are strongly pressure dependent, and a typical example of this is shown in Figure 13 for reaction No. 2 over a range of pressures from 0.001 to 100 atm. The high-pressure limit rate constant, computed separately via MultiWell as outlined earlier in Section 3.11, is shown in the figure as well for comparison.

Figure 13. Pressure dependence of reaction No. 2; p in atmospheres.

kJ mol−1 even in the worst case, E‡ = 150 kJ mol−1, an M06-2X rate constant is only three times faster/slower at 1000 K if all the variation arises from the exponential term in eq 4. The exceptions are a cluster of points of very high barriers, ca. 500 kJ mol−1, which is typically represented by reaction No. 13. Similarly in the case of reaction enthalpies virtually all the values are within ±10 kJ mol−1 of each other, Figure 14b. BDEs were computed with CBS-QB3, (Figure 1); however, equivalent M06-2X/6-311++G(d,p) values are in good agreement and lie within 2−4 kJ/mol−1; the maximum difference is 12 kJ/mol−1. Comparison of ionization energies computed with both methods shows excellent agreement with the CBS-QB3 tending to be slightly higher in general but not exceeding 0.2 eV.

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CONCLUSIONS It was the objective of this work to put a theoretical framework in place against which the extant flow reactor experiments can be viewed. Thus, the principal routes of decomposition of 2(3H) and 2(5H) furanones and their 5-methyl derivatives as well as 3-methyl-2(5H) furanone have been investigated. In general the products and intermediates formed can be understood from our computations, which are in good agreement with the very limited theoretical literature. More well-defined and sophisticated experiments would be required to test some of our predictions before a comprehensive detailed chemical kinetic modeling study would be worthwhile. Much of the unimolecular chemistry of these compounds results from simple elimination reactions or else from 1→ 2 hydrogen atom transfers and concomitant ring-opening processes. In particular the 2(5H) compounds exhibit lowenergy ring-opening processes, while the 2(3H) are somewhat more complex with decarbonylation competing with two

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M06-2X VERSUS CBS-QB3 New functionals are routinely tested as they are being developed usually against databases of variable quality and over a range of species and/or problems; see, for example, the review by Peverati and Truhlar.51 Here we compare the results obtained with the well-established composite method CBSQB3 against the newer functional M06-2X, constructed with a global-hybrid meta-generalized gradient approximation, paired with the basis set 6-311++G(d,p). For reaction barriers the differences between the results obtained are usually within ±10 kJ mol−1 with a few exceptions, Figure 14a. Given that the barriers are in the range of 150−500 6925



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ACKNOWLEDGMENTS Computational resources were provided by the Irish Centre for High-End Computing, ICHEC. We thank H. Wang (Stanford) for clarification of data in JetSurf mechanism and D. Murdock (Bristol) for the provision of Supporting Information.

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(1) Hashem, A.; Senning, A. Reactions of 2(3H)-Furanones. Adv. Heterocycl. Chem. 1999, 73, 275−293. (2) Hashem, A.; Kleinpeter, E. The Chemistry of 2(5H)-Furanones. Adv. Heterocycl. Chem. 2001, 81, 107−165. (3) Luo, S. H.; Xiong, J. F.; Wang, Z. Y.; Mo, G. Z. Design and Synthesis of 2(5H)-Furanone Liquid-Crystal Compounds Based on Natural Molecules and Biphenyl Derivatives. Res. Chem. Intermed. 2013, 39, 1865−1876. (4) Raina, S.; De Vizio, D.; Odell, M.; Clements, M.; Vanhulle, S.; Keshavarz, T. Microbial Quorum Sensing: A Tool or a Target for Antimicrobial Therapy? Biotechnol. Applied Biochem. 2009, 54, 65−84. (5) Hay, M. E. Marine Chemical Ecology: Chemical Signals and Cues Structure Marine Populations, Communities, and Ecosystems. Annu. Rev. Marine Sci. 2009, 1, 193−212. (6) Mariaca, R. G.; Imhof, M. I.; Bosset, J. O. Occurrence of Volatile Chiral Compounds in Dairy Products, Especially Cheese − a Review. Europ. Food Res. Technol. 2001, 212, 253−261. (7) Slaughter, J. C. The Naturally Occurring Furanones: Formation and Function from Pheromone to Food. Biol. Rev. 1999, 74, 259−276. (8) Calderón-Montaño, J. M.; Burgos-Morón, E.; Orta, M. L.; Pastor, N.; Austin, C. A.; Mateos, S.; López-Lázaro, M. Alpha, BetaUnsaturated Lactones 2-Furanone and 2-Pyrone Induce Cellular DNA Damage, Formation of Topoisomerase I- and II-DNA Complexes and Cancer Cell Death. Toxicol. Lett. 2013, 222, 64−71. (9) Ngo, T.-A.; Kim, J.; Kim, S.-S. Fast Pyrolysis of Palm Kernel Cake Using a Fluidized Bed Reactor: Design of Experiment and Characteristics of Bio-Oil. J. Indust. Eng. Chem. 2013, 19, 137−143. (10) Juhász, M.; Kitahara, Y.; Fujii, T. Thermal Decomposition of Vitamin C: An Evolved Gas Analysis-Ion Attachment Mass Spectrometry Study. Food Chem. 2011, 129, 546−550. (11) Murdock, D.; Harris, S. J.; Luke, J.; Grubb, M. P.; Orr-Ewing, A. J.; Ashfold, M. N. R. Transient UV Pump-IR Probe Investigation of Heterocyclic Ring-Opening Dynamics in the Solution Phase: The Role Played by nσ* States in the Photoinduced Reactions of Thiophenone and Furanone. Phys. Chem. Chem. Phys. 2014, 16, 21271−21279. (12) Würmel, J.; Simmie, J. M. Thermochemistry and Kinetics of Angelica and Cognate Lactones. J. Phys. Chem. A 2014, 118, 4172− 4183. (13) Bierbach, A.; Barnes, I.; Becker, K. H.; Wiesen, E. Atmospheric Chemistry of Unsaturated Carbonyls: Butenedial, 4-Oxo-2-Pentenal, 3-Hexene-2,5-Dione, Maleic Anhydride, 3H-Furan-2-One, and 5Methyl-3H-Furan-2-One. Environ. Sci. Technol. 1994, 28, 715−29. (14) Brinkevich, S. D.; Reztsov, I. A.; Shadyro, O. I. Reactions of Cyclopentanone, Gamma-Butyrolactone, and Their Derivatives with Alpha-Hydroxyethyl Radicals. High Energy Chem. 2014, 48, 303−309. (15) Holbrook, K. A. Vapor and Gas-Phase Reaction of Carboxylic Acids and Their Derivatives In The Chemistry of Acid Derivatives; Patai, S., Ed.; Wiley: Chichester, U.K., 1992, Vol. 2, pp 703−746. (16) Skorianetz, W.; Ohloff, G. Cheletropic Reactions of Enol Lactone Vinylogs. Helv. Chim. Acta 1975, 58, 1272−5. (17) Xu, Z. P.; Mok, C. Y.; Chin, W. S.; Huang, H. H.; Li, S.; Huang, W. Interconversion and Decomposition of Furanones. J. Chem. Soc., Perkin Trans. 2 1999, 4, 725−730. (18) Urness, K. N. A Molecular Picture of Biofuel Decomposition: Pyrolysis of Furan and Selected Furanics, Chapter 5, Ph.D. thesis, University of Colorado at Boulder. 2014. (19) Friderichsen, A. V.; Shin, E. J.; Evans, R. J.; Nimlos, M. R.; Dayton, D. C.; Ellison, G. B. The Pyrolysis of Anisole (C6H5OCH3) Using a Hyperthermal Nozzle. Fuel 2001, 80, 1747−1755. (20) Guan, Q.; Urness, K. N.; Ormond, T. K.; David, D. H.; Ellison, G. B.; Daily, J. W. The Properties of a Micro-Reactor for the Study of

Figure 14. CBS-QB3 versus M06-2X/6-311++G(d,p).

different 1→ 2 H atom transfers leading to different diones, 2butenedial and 3-oxo-butenal, Figure 2. In summary thermal decomposition of these furanones displays a rich chemistry featuring many little-known yet intriguing species whose properties are largely unknown.

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ASSOCIATED CONTENT

* Supporting Information S

Geometries, energies, and frequencies of reactants, intermediates, and transition states are listed as well as entropies, specific heats, and enthalpy functions for all species. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b04435.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 6926


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Furanones - ACS Publications - American Chemical Society

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