Kinetics and mechanisms of gas-phasedecarbonylation of α-methyl-trans-cinamaldehyde and E-2-methyl-2-pentenalunder homogeneous catalysis of hydrogenchlorideLibia L. Julioa, Jesus Lezamaa, Alexis Maldonadoa, José R. Moraa

and Gabriel Chuchania*

The kinetics of the gas-phase elimination of α-methyl-trans-cinamaldehyde catalyzed by HCl in the temperature range of399.0–438.7 °C, and the pressure range of 38–165 Torr is a homogeneous, molecular, pseudo first-order process andundergoing a parallel reaction to produce via (A) α-methylstyrene and CO gas and via (B) β-methylstyrene and CO gas. Thedecomposition of substrate E-2-methyl-2-pentenal was performed in the temperature range of 370.0–410.0 °C and thepressure range of 44–150 Torr also undergoing a molecular, pseudo first-order reaction gives E-2-pentene and CO gas. Thesereactions were carried out in a static system seasoned reactions vessels and in the presence of toluene free radical inhibitor.The rate coefficients are given by the following Arrhenius expressions:

Products formation from α-methyl-trans-cinamaldehydeα-methylstyrene: logk′1 s�1lmol�1� � ¼ 12:67±0:02ð Þ � 183:3±0:31ð ÞkJmol�1 2:303RTð Þ�1

β-methylstyrene:logk ′1 s�1lmol�1� � ¼ 13:19±0:03ð Þ � 183:0±0:45ð ÞkJmol�1 2:303RTð Þ�1

Products formation from E-2-methyl-2-pentenalE-2-pentene: logk ′1 s�1lmol�1� � ¼ 12:79±0:06ð Þ � 174:5±0:80ð ÞkJmol�1 2:303RTð Þ�1

The kinetic and thermodynamic parameters for the thermal decomposition of α-methyl-trans-cinamaldehyde suggest that via(A) proceeds through a bicyclic transition state type of mechanism to yield α-methylstyrene and carbon monoxide, whereas via(B) through a five-membered cyclic transition state to give β-methylstyrene and carbon monoxide. However, the elimination ofE-2-methyl-2-pentenal occurs by way of a concerted cyclic five-membered transition state mechanism producing E-2-penteneand carbon monoxide. The present results support that uncatalyzed α-β-unsaturated aldehydes decarbonylate through athree-membered cyclic transition state type of mechanism. Copyright © 2014 John Wiley & Sons, Ltd.

Keywords: kinetics; gas-phase elimination; α-methyl-trans-cinamaldehyde; E-2-methyl-2-pentenal

INTRODUCTION

Aldehydes are organic molecules difficult to study in homoge-neous, unimolecular gas-phase thermal decomposition kineticsin the gas phase. This type of compounds are liable to be oxidizedin the presence of oxigene and to undergo a complex free radicalreactions.[1] In this sense, the thermal decomposition of low molec-ular saturated aldehydes, formaldehyde,[2–4] acetaldehyde,[5–20]

propionaldehyde,[21–27] and at temperatures above 450 °C, haveshown to undergo complex free radical processes. However, thearomatic aldehyde, benzaldehyde[28–32] at 550–1400°C decomposedvia a radical reaction yielding benzene and CO gas, and smallamounts of biphenyl and hydrogen gas.

2,2-Dimethyl-3-butenal, the only β-γ-unsaturated aldehyde,studied in gas-phase kinetic decomposition[33] (282–302 °C),was found to produce 2-methyl-2-butene and carbon monoxide.The kinetic isotope effect of this reaction gave a kH/kD = 2.8 at296.9 °C (i.e., 7.2 at 25 °C), suggesting that the aldehyde

hydrogen migrated during the slow step of the reaction. Theseresults imply a unimolecular reaction and obeying a first-orderrate law, thus suggesting a five-membered cyclic transition statetype of mechanism as described in reaction (1).

* Correspondence to: Gabriel Chuchani, Centro de Química, Instituto Venezolanode Investigaciones Científicas, Apartado 21827, Caracas, Venezuela.E-mail: chuchani@ivic.gob.ve

a L. L. Julio, J. Lezama, A. Maldonado, J. R. Mora, G. ChuchaniCentro de Química, Instituto Venezolano de Investigaciones Científicas,Apartado, 21827, Caracas, Venezuela

(1)

Research Article

Received: 4 October 2013, Revised: 5 December 2013, Accepted: 18 December 2013, Published online in Wiley Online Library: 5 February 2014

(wileyonlinelibrary.com) DOI: 10.1002/poc.3282

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450

Grela and Colussi[32], besides reporting the pyrolysis kineticof benzaldehyde in a flow system, included in the work thethermal decomposition of 2-furaldehyde and 2-butenal. The 2-furaldehyde gave a biradical mechanism, whereas 2-butenalproved to be a molecular process. The evaluation of themechanism of 2-butenal was performed by a copyrolysis of 2-butenal in a 1:3 mixture of normal and 2-butenal-O-d, where itwas found a nearly the same ratio in the products C3H5D andC3H6, with a negligible kinetic isotope effect of 0.97 ± 0.1. Basedfrom this result, it was proposed the mechanism of 2-butenal toproceed through a tight concerted three-membered cyclictransition state structure with an A factor of 1013.5 s�1 as depictedin reaction (2).

Chuchani and collaborators[34] studied the gas-phase thermaldecarbonylation of two α-β-unsaturated aldehydes: E-2-butenaland E-3-phenyl-methylpropenal. The kinetics was determinedin a static reaction system at the temperature range of 430–480 °C (703.15–753.15 K). These reactions showed to be homoge-neous, unimolecular, and followed a first-order law. Thedecomposition products of 2-butenal are propene and CO gas,whereas 3-phenyl-2-methylpropenal yielded α-methylstyrene,cis-trans-β-methylstyrene, indan, and CO gas. The reportedkinetics and thermodynamics data suggested these eliminationreactions to proceed through a three-membered cyclic transitionstate type of mechanisms, thus supporting the proposal of Grelaand Colussi[32] for α-β-unsaturated aldehydes decomposition.However, a two-steps mechanism for the formation of a carbenetype of intermediate through a four-membered cyclic transitionstructure was not contemplated.The proposed three-membered cyclic transition state type of

mechanism from few works in gas-phase thermal decompositionof α-β-unsaturated aldehydes appears to require verification orsupport of this type of transition state by using a catalysis suchas hydrogen chloride. In this respect, undertaking an experimen-tal task of decarbonylation of α-β-unsaturated aldehydes byincorporating the effect of a catalyst such as HCl gas underhomogeneous conditions is a challenge. Consequently, thepresent work aimed at examining the gas-phase elimination kinet-ics of α-methyl-trans-cinamaldehyde and E-2-methyl-2-pentenalcatalyzed by HCl gas.

EXPERIMENTAL SECTION

The starting materials α-methyl-trans-cinamaldehyde (99.1%) and E-2-methyl-2-pentenal (98.0%) were acquired from Aldrich. The purityof the substrates and products were determined by GC/MS/MS (Saturn2000, Varian). Capillary column DB – 5MS, 30 × 0.250mm. id. 0.25μm.The quantitative chromatographic analyses of α-methylstyrene, β-methylstyrene, and E-2-pentene were determined by using a gaschromatograph Hewlett Packard 5710-A with a Chromosorb 103,80–100 mesh 1m and internal diameter of 2mm. The identification ofthe products was made by comparing chromatogram of the reactionproducts with true authentic samples bought from Aldrich and in aGC–MS (Saturn 2000, Varian 3600X with a DB-5MS capillary column30× 0.25mm. i.d. and 0.25μm).

Kinetics

The thermal decomposition experiments were performed in cylindricalvessels seasoned or deactivated by the products of decomposition ofallyl bromide in a static system,[35–37] and the rate coefficients weredetermined manometrically (mercury manometer). The temperaturewas controlled within ±0.2 °C with a SHINKO DIC-PS (Shinko,Senbahigashi, Osaka, Japan) 23TR resistance thermometer controller witha calibrated Iron Constantan thermocouple. The temperature readingwas measured within ± 0.1 °C with the Iron-Constantan thermocouple at-tached to a Digital Multimeter Omega 3465B. The reaction vessel showedno temperature gradient at different points, and the substrates wereinjected directly into the reaction vessel through a silicone rubberseptum. HCl gas was added into the reaction vessel to an amountmeasured manometrically from a reservoir of the system. The volumeof substrate used for each reaction was ≈0.05–0.1ml.

RESULTS AND DISCUSSION

The elimination reaction of α-methyl-trans-cinamaldehyde andE-2-methyl-2-pentenal catalyzed by HCl gas, in a staticdeactivated reaction vessel with allyl bromide decompositionand in the presence of toluene free radical suppressor, producesCO gas and the corresponding olefin as described in reaction (3)and (4):

The experimental stoichiometry for reactions (3) and (4) waschecked by carrying out the decomposition process until no fur-ther pressure increase was observed. These stoichiometries forproduct formations requires that, for long reaction times, Pf = 2P0, where Pf and P0 are the final and initial pressure, respectively.The average experimental results for Pf/P0 values at five differenttemperatures and 10 half-lives were 1.95 for α-methyl-trans-cinamaldehyde and 1.90 for E-2-methyl-2-pentenal (Table 1).

The stoichiometries of (3) and (4) are shown in Table 2. Forboth unsaturated aldehydes, up to about 40% reaction, there isa reasonable agreement between the extents of decompositionof the substrate as predicted from pressure measurements andform the chromatographic analyses of products formation.

Product distributions in the thermal decomposition of α-methyl-trans-cinamaldehyde as described in reaction (3) at 40%reaction at 420 °C and at different temperatures are specified inTables 3 and 4. The quantitative chromatographic analysis ofthe styrene products indicates a kinetic control in the processof elimination. The ratio %β-methylstyrene/%α-methylstyreneshows a ratio kβ/kα of 3.54 in the temperature range, where β-methylstyrene is favored by the kinetic control and the morethermodynamic stable isomer.

(2)

(3)

(4)

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The homogeneity of these elimination reactions, in the pres-ence of the chain inhibitor toluene, was examined by carryingout several runs in a vessel with a surface-to-volume ratio ofabout 6.0 relative to that of the normal vessel, which is equalto one. The packed and unpacked clean Pyrex vessel gave a

significant effect on the rates. However, when the packed andunpacked vessels are seasoned or deactivated with allyl bromidedecomposition products, no effect on the rate coefficients wasobtained (Table 5). These results suggest that the polymeric layerformed in the thermal decomposition of allyl bromide does notaffect the rate coefficient, and consequently, the homogeneityis confirmed.

Table 1. Ratio of final (Pf) to initial pressure (P0)a

α-methyl-trans-cinamaldehyde

Temperature P0(Torr) Pf(Torr) Pf/P0 Average

399.2 53 104 1.96 1.95 ± 0.02410.2 65 124 1.91421.0 47 92 1.97430.0 48 95 1.97437.4 84 165 1.96

E-2-methyl-2- pentenal371.1 64 121 1.90 1.90 ± 0.03380.7 60 113 1.88390.6 59 111 1.88399.6 74 140 1.89409.3 76 149 1.96aSeasoned vessel with presence of toluene inhibitor of ≈2P0and threefold of HCl gas with respect to the substrate.

Table 2. Stoichiometry of the reactions a

Substrate Temperature(°C)

Time(min)

%Productb

(GC)

% Reaction(pressure)

α-methyl-trans-cinamaldehyde

420.0 °C 4 7.8 8.05 12.7 13.57 22.5 23.7

10 36.1 37.0E-2-methyl-2-pentenal

390.7 ° C 4 14.3 13.06 21.3 20.0

10 32.0 31.015 42.9 43.0

aSeasoned vessel with presence of twofold toluene inhibitor,and threefold of HCl gas with respect to the substrate.bThese values are the total percentage of products.

Table 3. Olefin distribution of α-methyl-trans-cinamaldehydedecomposition at different % reaction.

Time(min)

%Reaction

% α-Methyl-styrene

% β-Methyl-styrene

4 7.8 22.1 77.95 12.7 22.0 78.07 22.5 21.8 78.210 37.0 22.0 78.0

Temperature 420.0 °C.Seasoned vessel with presence of twofold toluene inhibitorand threefold of HCl gas with respect to the substrate.

Table 4. Olefin distribution at different temperatures of α-methyl-trans – cinamaldehyde at 35% decomposition

Temperature (°C) % α-Methylstyrene % β-Methylstyrene

410.0 22.2 77.8420.0 22.0 78.0430.0 21.9 78.1

Seasoned vessel with presence of twofold toluene inhibitorand threefold of HCl gas with respect to the substrate.

Table 5. Homogeneity of the reaction

S/V (cm�1) 106 k1(Torr�1 s�1)(a) 106 k1(Torr

�1 s�1)(b)

α-Methyl-trans-cinamaldehyde at 420.0 °C1.0 9.90 ± 5.30 7.80 ± 0.236.0 12.80 ± 7.00 7.84 ± 0.10

E-2-methyl-2-pentenal at 391.7 °C1.0 7.56 ± 4.50 2.85 ± 0.976.0 9.70 ± 8.70 2.88 ± 1.90

S, surface; V, volume.Presence of twofold toluene inhibitor, and threefold of HClgas with respect to the substrateaClean Pyrex vessel.bVessel deactivated with allyl bromide.

Table 6. Effect of toluene inhibitor on rates

P0(Torr)

Pi(Torr)

Pi/P0 106 k1(Torr�1 s�1)

k1(cm3mol�1 s�1)

α-Methyl-trans-cinamaldehyde at 420.0 °C61 — — 6.90 ± 0.20 298.53 ± 8.6555 56 1.02 7.40 ± 0.01 320.16 ± 0.4341 60 1.46 7.79 ± 0.01 337.20 ± 0.4346 92 2.00 7.79 ± 0.36 337.10 ± 15.5727 62 2.30 7.80 ± 0.20 337.46 ± 8.65

E-2-methyl-2-pentenal at 389.7 °C47 — — 2.64 ± 0.07 109.19 ± 2.8973 85 1.16 2.76 ± 0.05 114.16 ± 2.0674 112 1.51 2.87 ± 0.06 118.70 ± 2.5056 114 2.04 2.87 ± 0.10 118.50 ± 4.1350 120 2.40 2.87 ± 0.10 118.20 ± 4.13

P0, pressure of the substrate; Pi, pressure of the inhibitortoluene inhibitor of ≈2P0.Vessel deactivated with allyl bromide. Presence of threefoldof HCl gas with respect to the substrate.

L. L. JULIO ET AL.

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The kinetic runs of these substrates were performed in thepresence of at least twice the amount of the radical chainsuppressor toluene in order to inhibit any possible radicalchain reactions. The effect of different proportions of toluenein the process is shown in Table 6. The absence of toluene in-hibitor yields a small decrease in the rate coefficient withvalues of 6.90 × 10�6 (Torr�1 s�1) and 2.64 × 10�6 (Torr�1 s�1)for α-methyl-trans-cinamaldehyde and E-2-methyl-2-pentenalelimination, respectively. When the reaction is carried out inthe presence of toluene in ratios Pi/P0 from 1.0 to 2.6, the ratecoefficient is constant, and any radical chain reaction issuppressed.In the pseudo first-order rate law described in Equation (1), the

rate coefficient kobs is not independent of the catalyst HCl pres-sure, which indicates that the variation of PHCl gives differentvalues of kobs-values. Therefore, the second-order rate coeffi-cient is obtained by dividing kobs into PHCl. Thus, Equation (1)changes into Equation (2). In Table 7 are shown the values ofthe kobs with different values of PHCl from 74 to 249 Torr

Table 7. Effect of HCl gas catalyst on the rate of decarbonylation

PHCl(Torr) 104kobs(s�1) 106 k1(Torr

�1 s�1)

α-Methyl-trans-cinamaldehyde at 420.2 °C219 17.00 ± 0.70 7.76 ± 0.31110 8.60 ± 0.30 7.82 ± 0.27100 7.80 ± 0.40 7.80 ± 0.4085 6.60 ± 0.30 7.76 ± 0.3574 5.70 ± 0.85 7.70 ± 0.11

E-2-methyl-2-pentenal at 390.5 °C249 7.10 ± 0.07 2.85 ± 0.03229 6.60 ± 0.05 2.88 ± 0.02207 5.90 ± 0.10 2.85 ± 0.05191 5.50 ± 0.05 2.88 ± 0.03175 5.00 ± 0.07 2.86 ± 0.04

Seasoned vessel with presence of twofold of tolueneinhibitor. P0 substrate≈ 45.0 Torr

Table 8. Invariability of rate coefficient with initial pressureof the substrate

P0(Torr) 106 k1(Torr�1 s�1) k1(cm

3mol�1 s�1)

α-Methyl-trans-cinamaldehyde at 429.8 °C60 11.51 ± 0.03 504.88 ± 1.3257 11.58 ± 0.02 508.00 ± 0.8848 11.56 ± 0.05 507.07 ± 2.1946 11.62 ± 0.06 509.70 ± 2.63

E-2-methyl-2-pentenal at 371.1 °C65 1.05 ± 0.04 42.50 ± 1.6161 1.03 ± 0.03 41.40 ± 1.2157 1.08 ± 0.10 43.41 ± 4.0244 1.06 ± 0.02 42.50 ± 0.80

Seasoned vessel with presence of twofold of tolueneinhibitor and threefold of HCl gas with respect to thesubstrate Table

9.Th

evaria

tionof

therate

coefficien

tswith

tempe

ratures

Prod

uct

Parameters

Values

Temp.

(°C)

399.1

408.9

421.0

430.0

438.7

α-methy

lstyrene

106k 1(Torr�

1s�

1)

0.63

±0.01

0.99

±0.01

1.71

±0.02

2.55

±0.01

3.69

±0.01

k 1(cm

3mol

�1s�

1)

26.38±0.41

42.38±0.42

74.11±0.86

111.66

±0.43

163.86

±0.44

Rate

equa

tion:

logk 1(s�1Lm

ol�1)=

(12.67

±0.02

)�(183

.3±0.31

)kJmol

�1(2.303

RT)�

1r=

0.99

98Temp.

(°C)

399.1

408.9

421.0

430.0

438.7

β-methy

lstyrene

106k 1(Torr�

1s�

1)

2.23

±0.02

3.53

±0.04

6.06

±0.08

9.02

±0.03

13.1±0.02

k 1(cm

3mol

�1s�

1)

93.54±0.83

150.23

±1.70

262.50

±3.50

395.94

±1.31

581.00

±0.88

Rate

equa

tion:

logk 1(s�1Lm

ol�1)=

(13.19

±0.03

)�(183

.0±0.45

)kJmol

�1(2.303

RT)�

1r=

0.99

98Temp.

(°C)

370.0

380.7

390.7

399.6

410.0

E-2-pe

nten

e10

6k 1(Torr�

1s�

1)

1.06

±0.02

1.77

±0.06

2.87

±0.08

4.25

±0.15

6.73

±0.14

k 1(cm

3mol

�1s�

1)

42.50±0.80

72.50±2.44

118.70

±3,31

178.50

±6.30

287.00

±5.97

Rate

equa

tion:

logk 1(s�1Lm

ol�1)=

(12.79

±0.06

)�(174

.5±0.80

)kJmol

�1(2.303

RT)�

1r=

0.99

97

GAS-PHASE DECOMPOSITION OF UNSATURATED ALDEHYDES CATALYZED BY HCL

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453

obtained by the fitting of PT = P0[2-exp(�kobst)] by usingOriginPro 8.0 program (Origin Lab Co., Northhampton, MA, USA),and it was observed a constant value in the second-order rateconstant in agreement with the predicted by the Equation (2).

kobs ¼ PHCIk1 ¼ 1=tð Þ ln P02P0 � PTð Þ (1)

k1 ¼ 1=tð Þ 1=PHCIð Þ ln P02P0 � PTð Þ (2)

The rate coefficient of these decarbonylation reactions, atconstant HCl gas pressure, was found to be independent of theinitial pressures (Table 8). The second-order rate coefficient wasestimated from Equation (2).

To propose an elimination mechanism and discuss theobserved difference in the reactivity of these two compounds(Tables 6–8), it was considered the variation of the rate coeffi-cients with temperatures. For the decomposition of α-methyl-trans-cinamaldehyde (PHCl/P0 = 3.7) and E-2-methyl-2-pentenal(PHCl/P0 = 4.0), the results are given in Table 9. This table leads,by using the least-squares procedure and 90% confidence limits,to obtain the Arrhenius equations of each reaction.

According to the results of Table 9, the activation energy forthe formation of α-methyl styrene and β-methyl styrene is similar(183.0 kJmol�1), yet the isomer β-methyl styrene is kineticallyfavored because log A value is greater than the value of the α-methyl styrene formation. Therefore, these parallel reactionsproceed through different transition states. The activation energyof E-2-methyl-2-pentenal decomposition was 174.5 kJmol�1,which is a lower value when compared with that obtained forthe elimination of α-methyl-trans-cinamaldehyde. Combining theArrhenius expression with the Eyring equation, it is possible toobtain the thermodynamic parameters of activation as ΔS‡, ΔH‡,and ΔG‡. In Table 10, the kinetic and thermodynamic activationparameters for the elimination process studied in the present workare listed.

The kinetic and thermodynamic parameters of Table 10 revealthe homogeneous molecular nature of the gas-phase thermaldecomposition of α-methyl-trans-cinamaldehyde and E-2-methyl-2-pentenal catalyzed by HCl gas. According to products formationof α-methyl-trans-cinamaldehyde, a parallel elimination takesplace. Via A proceeds through a concerted bicyclic transitionstate type of mechanism (ΔS≠=�25.5 Jmol�1 K�1). In thisrespect, the polarization of the Cδ�–Hδ+ bond of the CHOgroup is the limiting factor. This way a phenyl group mayundergo through an intramolecular Wagner–Meerwein typeof rearrangement. Because of this, the partial positive chargeat the Cα atom is then stabilized (reaction 5 (via A)). In the case

of the decomposition through via B, the formation of β-methylstyrene and CO gas appears to be the result of a concertedfive-membered cyclic transition state type of mechanism(reaction 5 (via B)). As previously described, these two parallelreactions proceed through different transition states andtherefore distinct values in the activation entropy. This differ-ence in rate coefficients of these two parallel processes isdominated by the entropic effects. The α-methyl styrene(Table 10) shows log A= 12.67 and a more negative valuein activation entropy of �25.5 Jmol�1 K�1. This transition stateis more rigid and presents a major lost in translational,rotational and vibrational degrees of freedom when com-pared with the transition state involved in the β-methylstyrene formation.With regard to E-2-methyl-2-pentenal elimination reaction, the

formation of E-2-pentene implies a mechanism similar to theprocess of β-methyl styrene formation from the α-methyl-trans-cinamaldehyde (reaction 5, via B). This means a concerted five-membered cyclic transition state (reaction 6). When via B of α-methyl-trans-cinamaldehyde elimination (β-methyl styreneformation) is compared with that of E-2-methyl-2-pentenal, asmall difference in the activation enthalpy is found. Thisdifference is due to the stabilization of the partial negative carbonatom formed in the rate determining factor Cδ�–Hδ+. This factinfers an impediment for electron delocalization in the transitionstate due to the steric interaction between the methyl group andthe aromatic nuclei (reaction 5). Hence, the sequence in rates isE-2-methyl-2-pentenal> α-methyl-trans-cinamaldehyde.

Table 10. Kinetic and thermodynamic activation parameters at 410.0 °C (683.15 K)

Product log A Ea(kJmol�1)

ΔS‡

(Jmol�1 K�1)ΔH‡

(kJmol�1)ΔG‡

(kJmol�1)

α-methyl-trans-cinamaldehydeβ-methyl styrene 13.19 ± 0.03 183.0 ± 0.31 �15.4 172.3 182.2α-methyl styrene 12.67 ± 0.02 183.3 ± 0.45 �25.5 172.6 189.0

E-2-methyl-2-pentenalE-2-pentene 12.79 ± 0.06 174.5 ± 0.80 �23.1 163.7 178.6

(5)

(6)

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The consideration of these mechanisms (reactions 5 and 6) in-sinuates that uncatalyzed α-β-unsaturated aldehydes may welldecarbonylate through a three-membered cyclic transition statetype of mechanism. Consequently, the present results appear tosupport the mechanisms of thermal decomposition of α-β-unsat-urated aldehydes in the gas phase reported in the literature.[32,34]

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