Mechanism of Acid-Catalyzed Decomposition of Dicumyl Peroxide in

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The Mechanism of the Acid-Catalyzed Decomposition of Dicumyl Peroxide in Dodecane: The Intermediacy of Cumene Hydroperoxide Mark Lewis Conley, Fiaz S. Mohammed, Charles Winslow, Harris Eldridge, Jeffrey M. Cogen, Bharat Indu Chaudhary, Pamela L. Pollet, and Charles L. Liotta Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00975 • Publication Date (Web): 02 May 2016 Downloaded from http://pubs.acs.org on May 10, 2016

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Industrial & Engineering Chemistry Research

The Mechanism of the Acid-Catalyzed Decomposition of Dicumyl Peroxide in Dodecane: The Intermediacy of Cumene Hydroperoxide Mark Conley1,3, Fiaz S. Mohammed1,3, Charles Winslow1, Harris Eldridge1, Jeffrey M. Cogen4, Bharat I. Chaudhary4, Pamela Pollet2,3, and Charles L. Liotta*1,2,3 1

School of Chemical and Biomolecular Engineering; Georgia Institute of Technology; 311 Ferst St; Atlanta, GA 30332 (USA) 2 School of Chemistry and Biochemistry; Georgia Institute of Technology; 911 Atlantic Dr; Atlanta, GA 30332 (USA) 3 Specialty Separations Center; Georgia Institute of Technology; 311 Ferst St; Atlanta, GA 30332 (USA) 4 The Dow Chemical Company; 400 Arcola Road, Collegeville, PA 19426 *Corresponding Author: Fax (+1) 404-385-3210 Email: [email protected] Abstract The acid-catalyzed decomposition of dicumyl peroxide (DCP) in dodecane from 60 to 130 oC produces αmethyl styrene (AMS) and phenol as the major products. Pseudo-first-order rate constants were determined as a function of temperature for the reaction of DCP with dodecylbenzene sulfonic acid (DBSA) in dodecane and resulted in an Arrhenius plot exhibiting two distinct kinetic regimes with differing activation energies: 76.9 kJ / mol at low temperatures (measured from 60 to 90 °C) and 8.50 kJ/mol at higher temperatures (measured from 90 to 130 °C). Employing a combination of kinetics, product analysis, and trapping experiments, evidence is presented to show the intermediacy of cumene hydroperoxide – a reactive intermediate absent from previous mechanistic descriptions of this process. The yield of cumene hydroperoxide production is discussed and the mechanistic pathways for the formation of the observed products are presented. Introduction Dicumyl peroxide (DCP) is widely used as a radical initiator in the cross-linking of many unsaturated and saturated polymer systems, such as polyethylene, ethylene vinyl acetate and rubbers.1 This cross-linking process results in improved physical properties of the base resin, thus promoting their application in architectural materials, electronics, electrical and thermal insulation, plastic foams, and composites. The process itself is initiated by the thermally driven homolytic cleavage of the peroxy bond to form two cumyloxy radicals. These radicals can abstract hydrogen atoms from polymer chains to form cumyl alcohol (CA) and a carbon-centered radical on the polymer which can subsequently react with another polymer radical to produce a cross-link. The cumyloxy radical can also undergo beta-scission to form a methyl radical and acetophenone (ACP) as shown in Figure 1.2,3 The methyl radical can subsequently proceed to abstract hydrogen from the polymer to produce methane.

Figure 1. Thermal degradation of DCP.3 Initial homolytic cleavage of the peroxide, followed by formation of the products cumyl alcohol (CA) and acetophenone (ACP). RH represents a hydrocarbon polymer such as polyethylene.

In contrast to the thermal decomposition of DCP, the acid-catalyzed decomposition as reported in literature is suggested to follow an ionic mechanism resulting in the formation of 1 mole each of AMS, phenol, acetone, and water as shown in Figure 2a.1,4 It is postulated that AMS is produced through the dehydration of cumyl alcohol 3 while the phenol and acetone are derived from phenyl migration via the incipient oxenium ion to form the oxocarbonium ion (OCC). A related process employed for the industrial

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quantification of reaction products using nuclear magnetic resonance and liquid and gas chromatographic techniques; (3) cumene hydroperoxide trapping experiments, and (4) a quantitative estimation of cumene hydroperoxide produced from the acidcatalyzed decomposition of DCP. Finally, a complete mechanistic description for this acid-catalyzed reaction in dodecane is presented.

preparation of phenol and acetone is the acid-catalyzed decomposition of cumene hydroperoxide5,6 (CHP) (Figure 2b). As in the acid-catalyzed decomposition of DCP, the reaction sequence proceeds by phenyl migration via an incipient oxenium ion to produce the identical oxocarbonium ion (OCC). Phenol and acetone are the products of this reaction pathway.

Materials and Experimental Materials Dicumyl peroxide (DCP, 98%), dodecylbenzenesulfonic acid solution (DBSA, 70% in isopropanol), cumene hydroperoxide (CHP, 85%), tetrahydrothiophene (THTP, 99%), and dodecane (≥99%) were obtained from Sigma-Aldrich. The received CHP contained cumene and trace amounts of acetophenone as impurities. Dodecane was stored at room temperature in a resealable container under an air atmosphere and used without further purification; it contained approximately 50 ppm of water. The solution of 70% DBSA in isopropanol was purified using a Büchi Rotavapor R-210 and Vacuum Controller V-850. The solution was kept at approximately 30 psi and 50 °C for approximately 2 hours. The density of the resulting liquid phase was measured until it matched the reported value for DBSA (0.992 g/mL). The purified DBSA was promptly transferred to a dark freezer to prevent decomposition. The cold DBSA container was continuously purged with nitrogen while coming to room temperature to prevent condensation of water.

Figure 2. Current literature mechanism for (a) the acidcatalyzed decomposition of DCP1,4 and (b) the acidcatalyzed decomposition of cumene hydroperoxide to form phenol and acetone.7

According to Figure 2a, the aromatic products should be AMS and phenol in a ratio of 1:1 assuming that (1) – all the cumyl alcohol undergoes an acidcatalyzed elimination of water and (2) – all ketal intermediates derived from OCC, if formed, are hydrolyzed. However, our preliminary investigations of the acid-catalyzed decomposition of DCP in solution with dodecane revealed that this was not the case; far more AMS was produced than phenol. In light of this large excess of AMS, we hypothesize that the literature mechanism for the acid-catalyzed decomposition of DCP does not completely describe the reaction system in dodecane and that a major reaction pathway actually proceeds through the formation of CHP as a reactive intermediate. Herein, we discuss the experimental evidence for this assertion. Homogeneous solutions of dodecane were chosen for the current studies as a model system to mimic the chemical environment of polymers such as polyethylene that are commonly crosslinked with peroxides. Specifically, we present (1) kinetics studies of both thermal and acid-catalyzed decomposition reactions of DCP; (2) identification and

Reactions of DCP and additives in dodecane Reactions typically contained 2 wt% DCP in dodecane (0.0589 M). Reaction solutions were prepared in cylindrical glass vials with stir bars. Generally, reactions were conducted under an air atmosphere except where noted. Vials were immersed in well-stirred baths of silicon oil set at the desired reaction temperature (110 °C to 150 °C for thermal decomposition studies and 60 °C to 130 °C for acidcatalyzed decomposition studies). Samples were removed at designated time intervals and worked up for analysis with liquid chromatography and/or nuclear magnetic resonance. 2


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Gas chromatography (GC) Studies

Determination of DBSA room temperature critical micelle concentration by UV-Visible spectroscopy

Samples prepared for GC were diluted at exactly 1 part sample to 3 or 4 parts solvent (either acetone or isopropanol). GC spectra were gathered using a Shimadzu GCMS-QP2010S with a Supelco PTA-5 column. The inlet temperature was 250 °C and a constant velocity method was used with a velocity of 40 cm / second. The oven used a variable temperature method starting at 150 °C for 5 minutes and ramping at 25 °C per minute to 250 °C for 6 minutes. The mass spectrometer ion source and interface were set at 225 °C. GC spectra aided in product identification and quantification.

In the reaction system with dodecane as the solvent, we expected DBSA to form inverse micelles wherein the long greasy tails orient out toward the bulk liquid phase. Therefore, we conducted a study to determine the critical micelle concentration (CMC) of DBSA in dodecane. Solutions of 0.00025 M to 0.04 M DBSA in dodecane were prepared and analyzed using an Agilent Technologies Cary Series UV-Vis Spectrophotometer with quartz cuvettes of 1 cm path length. The UV-Vis absorption spectrum of 0.002 M DBSA in dodecane is available in the Supporting Information as Figure S1. The maximum absorbance of each DBSA spectrum was plotted against concentration as shown in the Supporting Information as Figure S2. We determined that the CMC of DBSA in dodecane ranges from 0.0025 M to 0.006 M DBSA (which corresponds to 0.042 and 0.10 moles per mole of DCP, respectively) at room temperature in dodecane. The details of this assessment are highlighted in the Supporting Information. 1

Results and Discussion Thermal Decomposition of DCP in Dodecane: Kinetics and Product Analysis In order to provide a baseline for the acidcatalyzed decomposition of DCP, the kinetics and product analysis for the corresponding thermal decomposition were investigated. Solutions of 10 wt% DCP in dodecane (0.321 M) were prepared and reacted at temperatures between 110 °C and 150 °C for periods of time up to 8 hours. The rate of disappearance of DCP was followed using HPLC. The concentration of DCP as a function of time and temperature is plotted in Figure 3.

H Nuclear Magnetic Resonance (NMR) Studies

Aliquots were diluted at approximately a 1:3 ratio with CDCl3. Proton NMR spectra were gathered using a Brüker 400 MHz NMR instrument and Topspin processing software. Spectra were taken using 16 scans with 2-second relaxation time. NMR was used for product identification and quantification of THTP oxidation and sealed mass balance studies.

[DCP] (M)

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High Pressure Liquid Chromatography (HPLC) Studies Samples prepared for HPLC were generally diluted at exactly 1 part sample to 2 parts solvent (either acetone or isopropanol), measured using a micropipette. HPLC spectra were gathered using a Shimadzu LC20AD Liquid Chromatograph with SPD-20A UVVisible detector set at 210nm. The column was a Shimadzu C18 4.6 mm x 50 mm column with a 5 µm particle size. The mobile phase was 85% methanol / 15% water at a flow rate of 1 mL / minute. LC was used primarily for tracking the concentration of DCP and reaction products in the reaction mixture.

0.06

110 °C

0.05

120 °C 130 °C

0.04

150 °C 0.03 0.02 0.01 0 0

2

4

6

8

10

Time (h) Figure 3. Thermal decomposition of 10 wt% (0.321 M) DCP in dodecane at various temperatures. (Samples were diluted by a factor of 6 prior to HPLC analysis – see experimental.)

3



The first-order plots at each of the temperatures are shown in Figure S3 and the corresponding Arrhenius plot is shown in Figure 4. The activation energy for the thermal decomposition of DCP in the solvent dodecane is calculated to be 154.7 kJ / mol. This value is in reasonable agreement with the reported activation energy of 144 kJ / mol in the solvent cumene.2,8,9 Using 1H NMR, the two major products from the thermal decomposition of DCP were identified as acetophenone and cumyl alcohol. These products are formed via the radical mechanism illustrated in Figure 1. The 1H NMR spectra of the thermal decomposition of DCP is shown in the Supporting Information (Figures S4i and S4ii).

60 °C 0.020

0.0026

0.5

1.0

1.5

2.0

2.5

[DCP] (M)

Time (hours) Figure 5. Acid-catalyzed decomposition of DCP in the presence of 3 mol% DBSA in dodecane.

The first-order plots for the rate data at each of the investigated temperatures in Figure 5 and associated standard errors of the slopes are shown in Figure S5. Figure 6 shows the corresponding Arrhenius analysis for the acid-catalyzed decomposition of DCP. The data gathered at 102 °C, 120 °C, and 130 °C have been adjusted take into account the competing thermal decomposition of DCP at these temperatures (using the kinetic data gathered in Figure 4). At the lower temperatures, the influence of thermal decomposition is vanishingly small. It was surprising to see that the Arrhenius plot exhibited two distinct kinetic regimes with differing activation energies: 76.9 kJ / mol at low temperatures (measured from 60 to 90 °C) and 8.50 kJ/mol at higher temperatures (measured from 90 to 130 °C). Both of these activation energies are considerably lower than that of the thermal decomposition of DCP (154.7 kJ/mol). These observations suggest that there may be a change in the rate-controlling step in the acid-catalyzed decomposition mechanism of DCP. The interpretation of this Arrhenius plot will be discussed in a later section of this report.

y = -18610x + 44.852 R² = 1.00 0.0025

120 °C

0.0

-2

0.0024

102 °C 0.010

0.000

0

-5 0.0023

90 °C

130 °C

-1

-4

0.015

0.005

1

-3

70 °C 80 °C

2

ln(k)

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0.0027

1/T (K-1) Figure 4. Arrhenius plot for the thermal decomposition of 0.321 M DCP in dodecane.

Acid Catalyzed Decomposition of Dicumylperoxide in Dodecane: Kinetics and Product Analysis The rate of reaction of DCP dissolved in dodecane in the presence of catalytic quantities of dodecylbenzenesulfonic acid (DBSA) was investigated. DCP (2 wt%, 0.0589 M) was reacted with of 3 mole% DBSA (0.001767 M) at temperatures ranging from 60oC to 130oC.10 The data are graphically shown in Figure 5. In contrast to the thermal decomposition of DCP, the acid-catalyzed reaction takes place much more rapidly and at significantly lower temperatures. The thermal reaction is known to proceed via a radical pathway; the acid-catalyzed reaction is conjectured to proceed via an ionic pathway.

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1.0

0.020

0.0 -1.0

0.00 eq DBSA

y = -9,254x + 25 R² = 0.99

y = -1,022x + 2.6 R² = 0.94

0.015

[DCP] (M)

ln k

-2.0 -3.0 -4.0 0.0024

0.0026

0.0028

0.003

0.01 eq DBSA 0.02 eq DBSA

0.010

0.03 eq DBSA 0.10 eq DBSA 0.15 eq DBSA

0.005

0.0032

1/T (K-1) 0.000

Figure 6. Arrhenius plot for the acid-catalyzed decomposition of DCP with 3 mol% DBSA in dodecane.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Time (hours)

In order to gain insights regarding the mechanistic role of DBSA in the acid-catalyzed decomposition of DCP, the rate of reaction as a function of acid concentration was explored. Figure 7 shows the rate data for the acid-catalyzed decomposition of DCP at 120 °C over a course of 2 hours with varying amounts of DBSA Also included in Figure 7 is a baseline data set (shown in black) for the thermal decomposition of DCP at this temperature in the absence of added acid. Reactions were conducted with 0.01, 0.02, 0.03, 0.10, and 0.15 equivalents of added DBSA.11 It is clear that the rate of decomposition of DCP increases as the concentration of added acid increases. To determine the kinetic order with respect to DBSA, the initial slopes of the rate data in Figure 7 were determined by application of the finite difference three-point formula at t = 0 hours. The initial slopes were taken as the initial rates of reaction. The log of the initial rates were plotted against the log of initial DBSA concentration as shown in Figure 8 and the slope of the line was taken as the reaction order with respect to DBSA. The results indicate that the kinetic order with respect to DBSA is calculated to be 1.1, which is interpreted to be unity. Note that despite the fact that the experiments with 0.10 eq and 0.15 eq DBSA are above the calculated CMC of DBSA in dodecane, Figure 8 still displays a linear trend. We conjecture that the CMC of DBSA in dodecane is significantly higher at 120 °C than at room temperature; the kinetics of this reaction therefore appear to be minimally affected by micellar formation.

Figure 7. Concentration of DCP during reactions of 0.0589 M DCP with DBSA at 120 °C.

-0.5

log(ratei)

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y = 1.0932x + 1.3532 R² = 0.9712

-1 -1.5 -2 -2.5 -3.5

-3

-2.5

-2

-1.5

log([DBSA]i) Figure 8. log (rate) plotted against log([DBSA]) to determine the reaction order with respect to DBSA for the reactions of DCP with DBSA.

The major products resulting from the acidcatalyzed decomposition of DCP were determined to be α-methylstyrene (AMS) and phenol - a dramatically different product suite compared to the thermal decomposition. No cumyl alcohol was observed. The NMR spectra of the reaction mixture after 15 minutes, 30 minutes, and 45 minutes are shown in the Supporting Information (Figure S6). By far the major product was α-methyl styrene with the accompanying production of some phenol. One potential source of the phenol is the acid-catalyzed Hock rearrangement of DCP involving an incipient oxenium ion accompanied by phenyl migration, analogous to the well-established Hock rearrangement in cumene hydroperoxide.7 There are several possible sources of AMS. It could have been derived from cumyl alcohol by a rapid acid-catalyzed dehydration pathway or from the protonation of DCP 5



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followed by a β-elimination process to form AMS and cumene hydroperoxide (CHP). CHP could subsequently form AMS either via an analogous elimination reaction to form hydrogen peroxide and another molecule of AMS, and/or via Hock rearrangement. At this juncture, however, it should be emphasized that cumene hydroperoxide was not directly observed in the acid-catalyzed decomposition of DCP; it is just a hypothesized intermediate.

All reactions involving THTP were carried out under an argon atmosphere. Reaction vials were sealed and thoroughly sparged with argon gas for 30 minutes prior to reaction. Vials were kept under a positive pressure of argon throughout the reactions. The following experiments were conducted and the corresponding 1H NMR spectra for these experiments are shown in Figure 10. (i) 0.0589 M DCP was reacted with 0.25 molar equivalents of THTP at 102°C in dodecane for 3 hours. (ii) 0.0589 M CHP was reacted with 1 molar equivalent of THTP at 102°C in dodecane for 30 minutes. (iii) 0.0589 M DCP was reacted with 0.25 molar equivalents of THTP and 0.03 molar equivalents of DBSA in dodecane at 102°C for 60 minutes. (iv) The reaction mixture from iii spiked with tetrahydrothiophene-1-oxide (THTP-1-oxide). In each of the four experiments, acetophenone is one of the prodcts as indicated by the singlet at 2.52 ppm - an absorption assigned to the methyl group of acetophenone. An additional control experiment was conducted of 0.0589 M THTP with 0.03 equivalents of DBSA at 120°C for 2 hours (Figure S7); no reaction was observed. The results of the four experiments are summarized as follows:

Evidence for the formation of cumene hydroperoxide: Oxidation studies of tetrahydrothiophene (THTP) In experiments conducted in our laboratory, we had previously observed that organic hydroperoxides can rapidly oxidize sulfides to their corresponding sulfoxide or sulfone derivatives. In contrast, however, under the same conditions, DCP does NOT oxidize sulfides at a rate comparable to that of hydroperoxides. These experimental facts provide a basis for determining if cumene hydroperoxide is indeed an intermediate in the acid-catalyzed decomposition of DCP. Specifically, we will demonstrate that THTP rapidly reacts with cumene hydroperoxide to form both the sulfoxide and the sulfone while reacting only slowly or not at all with DCP (Figure 9).In order to probe for the presence of CHP, THTP was added to the reaction mixture as a readily-oxidizable sulfide that would remain intact after oxidation and whose oxidation product (sulfoxide) could easily be observed by 1H NMR. The proton NMR chemical shifts of THTP and its corresponding sulfoxide are shown in the Supporting Information (Figures S7 and S8).

OH Ph

O O

S

S d

b a

c

O Ph

Ph

O

Figure 9. Reaction of tetrahydrothiophene with cumene hydroperoxide (CHP) and dicumyl peroxide (DCP).

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observation of multiple absorptions is due to the enantiotopic protons located at the α and β positions of THTP-1-oxide. Thus, it is clearly shown that CHP is capable of rapidly oxidizing THTP. It should be noted that the proton chemical shifts associated with THTP-1oxide are concentration dependent - especially the protons at the α position. (iii) 0.0589 M DCP was reacted with 0.25 molar equivalents of THTP and 0.03 molar equivalents of DBSA in dodecane at 102 °C for 60 minutes. The 1 H NMR clearly indicates the formation of THTP-1oxide. Since it has already been demonstrated that DCP does not oxidize THTP over a period of 3 hours, these results provide strong evidence for the intermediacy of CHP.12 It appears from a comparison of spectra (ii) and (iii) that peaks ‘c’ and ‘d’ can vary in chemical shift and appearance as a function of THTP-1-oxide concentration. In particular, the shifts of peaks ‘d’ change dramatically in chemical shift. Nonetheless, the peak labelled ‘c’ at approximately 2.4 ppm is relatively constant in both shift and appearance and therefore serves as confirmation of the presence of THTP-1oxide. (iv) The reaction mixture from (iii) was spiked with tetrahydrothiophene-1-oxide (purchased from Sigma Aldrich) demonstrating the correct assignment of the sulfoxide absorptions. Integrating the absorptions in spectrum (iii) of Figure 10 allow us to estimate the extent of the THTP oxidation reaction with respect to the extent of acidcatalyzed DCP decomposition. The baseline of the NMR spectrum was first smoothed using the Whittaker smoother function in the MestreNova software13. The ratio of the absorption integrals of ‘d’ (the α protons of THTP-1-oxide) to ‘a’ (the β protons of THTP) is 1.00:2.00. Since ‘a’ corresponds to 2 protons and each part of peak ‘d’ corresponds to 1 proton, the ratio of sulfoxide to sulfide is therefore estimated to be 1.00:1.00. It can be concluded that 50% of the initial sulfide was oxidized to sulfoxide. The extent of acidcatalyzed decomposition of DCP was determined by measuring the final DCP concentration via HPLC. The conversion of the starting material in this reaction was 19.1%.14 Thus 19.1% of the initial DCP led to oxidation of 50% of the initial 0.25 equivalents of THTP in the reaction mixture. In other words, for every mole of DCP which underwent acid-catalyzed decomposition, 0.65 moles of THTP were oxidized by the intermediate CHP.

b d

d

c

c a

(iv)

b d d

c

a

c

(iii)

b c

d

c

a

d

(ii)

b

a

(i)

Figure 10. 1H NMR spectra of reactions of THTP with peroxide species and/or acid in sealed glass ampules under inert atmosphere. The lettered peaks correspond to the letters designated in Figure 9. The spectral details are included in the text below. The singlet shown in all spectra at 2.52-2.53 ppm corresponds to the methyl protons of acetophenone. The singlet shown in the top 3 spectra at 2.06-2.07 ppm corresponds to the methyl protons of αmethyl styrene. All reaction samples were diluted in CDCl3. The peaks were assigned based on the 1H NMR spectra in Figures S7 and S8.

(i) 0.0589 M DCP was reacted with 0.25 molar equivalents of THTP at 102 °C in dodecane for 3 hours. The spectrum shows the unreacted THTP with the protons on the carbon beta to the sulfur at 1.85 ppm ‘a’ and the protons on the carbon alpha to sulfur at 2.74 ppm ‘b’. No new peaks are observed between 2.65 and 2.95 ppm indicating that DCP does not readily oxidize THTP within the time frame relevant to our study. (ii) 0.0589 M CHP was reacted with 1 molar equivalent of THTP at 102 °C in dodecane for 30 minutes. The spectrum shows a substantial oxidation of THTP to its sulfoxide derivative, tetrahydrothiophene1-oxide, as indicated by the four absorptions each centered at 1.92, 2.40, 2.72, and 2.84 ppm. The 7



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the heterolytic cleavage are shown in Figure 12. If the steady-state approximation is assumed for the hydrogen-bonded intermediate, the following differential expression for the change in concentration of DCP with respect to time can be derived (Equation 1). The complete derivation can be found in Figure S9.

Mechanism of Acid Catalyzed Decomposition of DCP in Dodecane The preceding discussion has shown that (1) the acid-catalyzed decomposition of DCP proceeds at a much greater rate than the corresponding thermal process, (2) the acid catalyzed process shows a convex Arrhenius plot indicating two distinct kinetic regimes with activation energies of 76.9 and 15.6 kJ/mol, (3) the kinetic order with respect to DBSA is approximately unity, and (4) the addition of THTP to the reaction results in the formation of the corresponding sulfoxide THTP-1-oxide. In this section, we hypothesize a mechanism to (1) account for the Arrhenius plot in Figure 6, and (2) account for the production of CHP along with the observed final products.

Equation 1. Analytical expression for the acid-catalyzed rate of consumption of DCP in dodecane.

There are two limiting cases associated with Equation 1. If k2 >> k-1 then k1 is the rate-controlling step. If, however, k-1 >> k2, then k2 is the ratecontrolling step and the overall rate is determined by a combination of the k1/k-1 equilibrium constant and k2. It is hypothesized, therefore, that at low temperatures (~60 to ~90oC) where the activation energy is 76.9 kJ/mol, k2 is rate controlling. Activation energies of this magnitude are associated with bond breakage/formation. At higher temperatures (~90 to ~130oC) where the activation energy is 8.50 kJ/mol, k1 becomes rate-controlling. The formation of hydrogen bonds becomes less favorable with increasing temperature. In the high-temperature regime, the reaction rate is limited by the formation of weak and less favorable hydrogen bonds followed by relatively rapid heterolytic bond cleavage.

Based upon the kinetic data previously presented, the postulated initial steps in the acidcatalyzed decomposition of DCP are illustrated in Figure 11. In the non-polar solvent dodecane, the DBSA reversibly forms a hydrogen bonded complex with DCP as illustrated by the k1 and k-1 steps. This is followed by a reversible heterolytic cleavage (k2 step) of the benzylic-oxygen bond which leads to the formation of the observed products. All steps after k2 are assumed to be fast. The various steps which follow

C Ph

Ph

O O

C

+

O

S R

Ph

O O

C H

OH

Ph

=

+

−

(1) Interpretation of the Arrhenius Plot

C

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k1

k2

O

O

k-1

O

S

products

k-2

O

R R=

Figure 11. Proposed initial steps in the acid decomposition of DCP.

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C12H25

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Figure 12. Proposed mechanism of acid-catalyzed decomposition of DCP.15 After formation of the acid-peroxide complex, there are several possible pathways: (i) reversal to form the starting materials (k-1, shown in Figure 11); (ii) rearrangement to form cumene hydroperoxide and α-methyl styrene (k2’); (iii) rearrangement to form cumyl alcohol and an oxocarbonium ion that proceeds to form phenol and acetone (k2’’).

per mole of DCP decomposed by liberating hydrogen peroxide(following the k2’ and k4 steps). In a detailed report on peroxide chemistry, Leffler mentions the formation of hydrogen peroxide upon reaction of tertbutyl hydroperoxide with acid.15 This is analogous to the proposed k4 step shown in Figure 12, lending support to this portion of the postulated pathway. This is a stark contrast to the current literature mechanism shown in Figure 2. As mentioned in the discussion of the reaction of DBSA with DCP, phenol was observed as a product of this reaction. Thus far, it cannot be conclusively stated whether the formation of phenol was as a result of acid-catalyzed decomposition of cumene hydroperoxide (k3’ in Figure 12) or of rearrangement of an oxocarbonium ion (k3’’ in Figure 12). As elucidated below, it is likely that the current literature route for the acid-catalyzed DCP decomposition (k2’’) competes with the novel CHP (k2’) route proposed in this report. Given the oxidation of THTP upon the acid-promoted decomposition of DCP (Figure 10iii), the mechanistic pathways shown in Figure 12 are likely the most accurate description of the acid-catalyzed decomposition of DCP.

(2) Pathways to observed products The steps proposed in Figure 11 are considered to be the rate-controlling steps in the overall reaction process. As mentioned previously, the major observable products are α-methylstyrene and phenol. The pathways for the formation of these products as well as the postulated intermediates are illustrated in Figure 12. The k2 step in Figure 11 is a combination of two possible processes: a β-elimination process leading directly to α-methylstyrene and cumene hydroperoxide (k2’) and an acid promoted concerted migration of the phenyl substituent (k2’’) to subsequently produce cumyl alcohol and phenol and acetone - the latter two products via a k3’’ step. Note that the k3’’ step may itself represent a complex sequence of individual steps, all of which are combined into k3’’ for simplicity. It has been reported that DCP is formed as a side product in the phenol-acetone process, probably by the addition of cumene hydroperoxide to α-methyl styrene (shown in Figure 12 as k-2’).16 This observation provides indirect support for the proposed k-2’ step in Figure 12. It is important to point out that in this acidic medium, it is possible to form up to 2 moles of AMS 9



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Determination of the yield of cumene hydroperoxide production

Page 10 of 17

(1) Reaction of DCP with DBSA 0.0589 M DCP was reacted with 0.00177 M (0.03 equivalents) DBSA at 110 °C for 1 hour and at 70 °C for 2 hours in flame-sealed glass ampules. After reaction, the solution was diluted and analyzed for DCP, AMS, and phenol. The methods used to determine the concentrations of these species are described in the Supporting Information (Figure S11). By measuring the final concentrations of the relevant species, we obtained the values for phenol yield (the ratio of phenol produced per mole of DCP reacted). To ensure that the important species were accounted for, we employed a phenyl ring balance as the starting material and important products all contain phenyl rings. The phenyl ring balance is defined as (7 + ℎ + 2 ∗ )⁄(2 ∗ 0.0589). [DCP] is counted twice in the numerator (and the starting concentration is doubled in the denominator) because this species contains 2 phenyl rings. Acetone is expected to be formed in equimolar quantities to phenol; a balance of the species containing phenyl rings should therefore be sufficient. The results are shown in Table 1.

With the evidence for the formation of cumene hydroperoxide from Figure 10iii, we sought to determine the relative rates of cumene hydroperoxide formation (k2’ in Figure 12) or phenol formation (k2’’ followed by k3’’ in Figure 12) in an effort to quantify how much total CHP is produced. This is not a trivial exercise as CHP is a fleeting intermediate – its concentration cannot be directly measured. First, the assumption was made that the oxocarbonium cations produced from the k2’’ pathway ultimately formed phenol with no significant side products. No observations have been made to suggest that this assumption is unreasonable. From Figure 12, we can develop a mass-balance expression to determine the ratio of ⁄( + ) as shown below in Equation 2. The derivation of the equation below is presented in the Supporting Information (Figure S10a). ℎ =

& + 1 − ! "#$%

+

+

& + ' +

Equation 2. Yield of phenol produced during the acidcatalyzed decomposition of DCP. ‘Phenol yield’ is defined as (()*+, -.+/)* -0)123+1)⁄(()*+ 456 3)/,2(+1). ‘X’ represents the conversion of CHP.

To obtain relative kinetic parameters, the final product distributions for acid-catalyzed reactions of both DCP and cumene hydroperoxide were determined using 1H NMR. A mathematical proof of this technique is available in the Supporting Information (Figure S10b). A proton NMR spectrum of a typical reaction sample is shown in Figure S11. Representative chromatograms for standard solutions and reaction samples are shown in Figure S12. Since the Arrhenius plot in Figure 6 showed two distinct activation energies within different temperature ranges, we determined the product distributions at a specific temperature within each temperature regime. Two sets of experiments were conducted to quantitatively determine product formation: (1) reaction of DCP with DBSA to determine phenol yield and (2) reaction of CHP with ) DBSA to determine the & ⁄( & + ' + ratio.

10


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Table 1. Final species concentrations with standard deviations, yields (()*+, -0)123?⁄()*+ 456 3)/,2(+1), and conversion of sealed reactions of 0.0589 M DCP with 0.00177 M DBSA at 70 °C for 2 hours and 110 °C for 1 hour. The ‘phenyl balance’ is the molar balance of all species containing phenyl rings, or (@AB + -.+/)* + C ∗ 456)⁄(C ∗ D. DEFG). [DCP] is counted twice because it species contains 2 phenyl rings.

Temperature final concentration

70 °C

yield final concentration

110 °C

yield

[DCP]

[AMS]

[phenol]

Conversion

Phenyl balance

0.0393 ± 0.0035

0.0170 ± 0.0031

0.0133 ± 0.0023

0.333

0.925

N/A

0.867

0.680

0.0275 ± 0.0014

0.0306 ± 0.0006

0.0175 ± 0.0006

0.532

0.876

N/A

0.976

0.558

Table 2. Final species concentrations with standard deviations and yields (()*+, -0)123?⁄()*+ 5H6 3)/,2(+1) for the sealed reaction of 0.0589 M CHP with 0.00177 M DBSA at complete conversion at 70 °C and 120 °C. The ‘phenyl balance’ is the molar balance of all species containing phenyl rings, or (@AB + -.+/)* + C ∗ 456)⁄(D. DEFG). [DCP] is counted twice because this species contains 2 phenyl rings.

Temperature

70 °C

110 °C

[AMS]

[DCP]

[phenol]

0.00851 ± 0.00029

0.00846 ± 0.00003

0.0290 ± 0.0007

yield

0.145

0.144

0.493

final concentration

0.0156 ± 0.0006

0.00631 ± 0.00003

0.0225 ± 0.0013

yield

0.265

0.107

0.382

final concentration

Reactions of DCP with DBSA were not run to completion. At long reaction times, formation of a viscous brown liquid precipitate was observed in the glass reaction vessels. We expect that this brown precipitate is the reason for the phenyl balance values being slightly less than 1. We hypothesize that the brown liquid results from acid-catalyzed dimerization and oligomerization of α-methyl styrene17 or other species in the reaction sample. A proton NMR of the brown precipitate in CDCl3 is shown in Figure S13.

Phenyl balance

0.924

0.862

(2) Reaction of CHP with DBSA 0.0589 M CHP was reacted in sealed vessels with 0.00177 M (0.03 equivalents) DBSA at 110 °C for 25 minutes and at 70 °C for 45 minutes (to complete conversion of CHP). Final concentrations of AMS, phenol, and ACP were determined using the methods described with Figure S11. These concentrations were ) used to determine the & ⁄( & + ' + ratio. A phenyl ring balance was once again employed, this time

11



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with more temperatures would reveal the detailed relationship between kinetic pathway and temperature. Regardless, Table 3 quantifies the extent to which our proposed mechanism of acid-catalyzed DCP decomposition (to form CHP) is prevalent. At both temperatures we examined, it is estimated that most of the decomposition (63% at 70 °C and 72% at 110 °C) occurs via formation of CHP. As discussed below, these values of 63% and 72% represent upper limits.

defined as(7 + ℎ + 2 ∗ )⁄(0.0589). The starting concentration in the denominator is no longer doubled as the starting material, CHP, contains only 1 phenyl ring. The results are shown in Table 2 Reactions of CHP and DBSA were carried out for much shorter times than reactions with DCP and still resulted in complete conversion of CHP. At these short times, a small amount of brown precipitate was again observed, which may again account for the missing portion of the phenyl ring balance. It is important to note that after acid-catalyzed reactions of both DCP and CHP, a small amount of ACP was observed. It has been suggested that ACP can form as a product of this reaction18, but the amounts we observed were so small that we could not eliminate the possibility that it was formed via the thermal radical pathway. In either case, the amount of ACP was negligible and does not substantially affect our analysis.

Estimating the hydroperoxide

Using Equation 2 and the concentration data in Table 1 and Table 2, the ⁄( + ) ratio was determined. The results are shown in Table 3. Reported error values were propagated through Equation 2 using the Variance Formula. Table 3. Relative kinetic parameters from Figure 12 as estimated from acid-catalyzed decomposition reactions of DCP and CHP. The I C ⁄(IC + IC ) values represent lower limits, as discussed in a later section. The ‘J’ and ‘K’

notations are explained later on this page.

& =L + ' +

70 °C

0.493 ± .007

0.370 ± .090

110 °C

0.382 ± .016

0.284 ± .053

&

maximum

possible

yield

of

Acid-catalyzed decomposition of CHP produced significant amounts of DCP at both 70 °C and 110 °C (already reflected in Figure 12). This discovery was verified by both proton NMR spectrum analysis and HPLC chromatogram analysis. Unfortunately, this complicates the analysis presented in Table 3. The DCP generated as a product of this reaction can further react with acid to form AMS and phenol as presented in Figure 12. As a consequence, it is uncertain how much of the AMS and phenol in Table 2 come directly from the breakdown of CHP as opposed to the breakdown of DCP (formed during reaction). Furthermore, the mass balances presented for the sealed experiments are near unity (not exactly unity), making precise calculations impossible. However, we can make an assessment regarding the minimum amount of CHP produced (or more directly, a maximum value of the ratio

⁄( + )). Equation 2 can be rewritten as shown below:

(3) Yield calculation

Temperature

Page 12 of 17

= " +

ℎ = " + (1 − ")L where phe = phenol yield of the decomposition of DCP, ) X = ⁄( + ), and L = & ⁄( & + ' + . Note that in expressing Equation 2 in this form, we’ve assigned XCHP a value of 1 (as complete conversion of CHP was observed in Table 2). Rearranging to solve for the unknown X gives the following: ℎ − L "= 1−L as a much simpler form. When the equation is expressed in this form, we can see that for a given value of phe, X decreases as L increases, down to a minimum of 0.

At the lower temperature studied (70 °C), 63% of DCP decomposition proceeds through the k2’ pathway in Figure 12 to directly form AMS and CHP. At 110 °C, 72% of DCP decomposition proceeds through the k2’ pathway. At the higher temperature, the ionic species formed via the k2’’ pathway could be relatively more stable, allowing more degradation through that pathway. A more rigorous set of studies 12


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L represents simply the yield of phenol as directly produced by acid-catalyzed decomposition of CHP. Since DCP is produced during the acid-catalyzed decomposition of CHP, the reported value of L in Table 3 is really only a maximum possible value (because some measured phenol could be derived from the intermediate DCP – any such phenol would artificially inflate L). We’re interested only in the phenol produced directly from CHP, not from the DCP that is produced during the reaction. It’s possible that less phenol is produced by the breakdown of CHP than is reported in Table 2, with the remainder produced by breakdown of the generated DCP. Since we have a maximum possible value of L, we therefore have a minimum possible value for X (the ratio ⁄( + )), which represents the portion of acid-catalyzed DCP decomposition that directly forms phenol (the portion that does NOT form CHP). As such, the values reported in mean that at 70 °C, a maximum of 63% of acidcatalyzed DCP decomposition proceeds through the CHP pathway. At 110 °C, a maximum of 72% of the decomposition proceeds through the CHP pathway. Thus far we have an estimate of the maximum portion of decomposition that proceeds through the CHP pathway. To obtain an estimate of the corresponding minimum, we can turn to the trapping experiments using THTP discussed previously in this paper. First, we assume that the inhibition of the reaction rate previously observed in the presence of THTP does not significantly change the product distribution of the acid-catalyzed decomposition of DCP. Recall that for the reaction of DCP with DBSA in the presence of THTP at 102 °C, 0.65 moles of THTP were oxidized to THTP-1-oxide per mole of DCP consumed. This implies that at a minimum, 65% of the acid-catalyzed DCP decomposition proceeded through the CHP pathway at 102 °C (as likely not all of the generated CHP is able to oxidize THTP). Since our calculated maximum at 110 °C as reported in Table 3 is 72%, we conclude that our calculations are a fairly accurate estimation. We therefore expect that at 70 °C, the true value of the ratio ⁄( + ) is not much higher than the 0.3701 reported in Table 3. With the data available, we have arrived at a reasonable estimate of the yield of the formation of cumene hydroperoxide during the acid-catalyzed decomposition of DCP.

Conclusions The major organic products formed in the acid-catalyzed decomposition of dicumyl peroxide in dodecane at temperatures ranging from 60° to 130 °C are α-methyl styrene (AMS) and phenol. Pseudo-firstorder rate constants were determined as a function of temperature and resulted in an Arrhenius plot exhibiting two distinct kinetic regimes with differing activation energies: 76.9 kJ / mol at low temperatures (measured from 60 to 90 °C) and 8.50 kJ/mol at higher temperatures (measured from 90 to 130 °C). Evidence, based on kinetics, product analysis, and trapping experiments using THTP, clearly demonstrates the formation of cumene hydroperoxide as a reactive intermediate – a reactive intermediate absent from previous mechanistic descriptions of this process. Depending on the temperature, it is estimated that between 63% and 72% of DCP reacts to form CHP. Finally, the Arrhenius plot associated with the acidcatalyzed process reveals two distinct kinetic regimes with differing activation energies. A mechanistic interpretation based on changes in the rate-controlling step was presented. List of Acronyms dicumyl peroxide acetophenone cumyl alcohol α-methyl styrene oxocarbonium cation phenol cumene hydroperoxide tetrahydrothiophene tetrahydrothiophene-1oxide

DCP ACP CA AMS OCC Phenol CHP THTP THTP-1-oxide

Supporting Information S1. UV-Visible absorption of DBSA; S2. Determination of CMC for DBSA solutions in dodecane; S3. First-order plots for the thermal decomposition of DCP in dodecane; S4. 1H NMR spectrum of thermal decomposition of DCP in dodecane; S5. First-order plot of the acid-catalyzed decomposition of DCP at various temperatures; S6. 1H NMR spectrum of the acid-catalyzed decomposition of DCP in dodecane; S7. 1H NMR spectrum of the 13



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reaction of THTP with DBSA in dodecane; S8. Experimental 1H NMR spectrum of THTP-1-oxide; S9. Derivation of the rate expression of the acid-catalyzed reaction of DCP in dodecane; S10. Derivation of the phenol yield equation; S11. 1H NMR spectrum of the acid-catalyzed decomposition of DCP with 1,4-dioxane as an internal standard; S12. Sample HPLC chromatogram of the acid-catalyzed decomposition of DCP in dodecane; and S13. 1H NMR spectrum of the brown precipitate formed during acid-catalyzed decomposition of DCP or CHP.

14


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determined at room temperature; the CMC would be expected to be at a much higher concentration at the elevated temperatures discussed in this study. (11) Some of these concentrations of DBSA are above the experimentally determined room temperature CMC. (12) Since the reactions were run under argon, atmospheric oxygen could not have produced a hydroperoxide capable of oxidizing THTP. (13) MestreLab Research: 2014; Vol. 2016. (14) According to Figure 5, approximately a 70% conversion is expected from this reaction at this temperature and time. It appears that the presence of THTP reduces the rate of the acidcatalyzed decomposition. The exact cause of this inhibition is currently under investigation. (15) Leffler, J. E. Cleavages and Rearrangements Involving Oxygen Radicals and Cations. Chem. Rev. 1949, 45, 385. (16) Zakoshanskii, V. Formation of Dicumenyl Peroxide as Side Product of Acid-Catalytic Decomposition of Isopropylbenzene Hydroperoxide. Zh. Obshch. Khim. 1989, 59, 1122. (17) Song, G.; Jin, X.; Hu, C.; Chen, X. Synthesis and Application of α-methyl Styrene Dimer. Chemical Industry and Engineering Progress 2012, 9, 041. (18) Levin, M.; Gonzales, N.; Zimmerman, L.; Yang, J. Kinetics of Acid-Catalyzed Cleavage of Cumene Hydroperoxide. J. Hazard. Mater. 2006, 130, 88.

References (1) Ogunniyi, D. S. Peroxide Vulcanisation of Rubber. Prog. Rubber Plast. Technol. 1999, 15, 95. (2) Di Somma, I.; Marotta, R.; Andreozzi, R.; Caprio, V. Dicumyl Peroxide Thermal Decomposition in Cumene: Development of a Kinetic Model. Ind. Eng. Chem. Res. 2011, 51, 7493. (3) Naskar, K.; Kokot, D.; Noordermeer, J. Influence of Various Stabilizers on Ageing of Dicumyl Peroxide-cured Polypropylene/ethylenepropylene-diene Thermoplastic Vulcanizates. Polym. Degrad. Stab. 2004, 85, 831. (4) Dluzneski, P. R. Peroxide Vulcanization of Elastomers. Rubber Chem. Technol. 2001, 74, 451. (5) Andrigo, P.; Caimi, A.; Cavalieri d'Oro, P.; Fait, A.; Roberti, L.; Tampieri, M.; Tartari, V. Phenol-Acetone Process: Cumene Oxidation Kinetics and Industrial Plant Simulation. Chem. Eng. Sci. 1992, 47, 2511. (6) Knifton, J. F.; Sanderson, J. R. Method for Production of Phenol/Acetone from Cumene Hydroperoxide. Pat. No. US4898995 A . Google Patents: 1990. (7) Seubold Jr, F. H.; Vaughan, W. E. Acid-Catalyzed Decomposition of Cumene Hydroperoxide. J. Am. Chem. Soc. 1953, 75, 3790. (8) Dialkyl Peroxides. Arkema, Inc., 2007. Industry Publication: https://intranet.ssp.ulaval.ca/cgpc/fsss/fichiers/LUPER OX%20101XL45_dialkyl-peroxides%20tech.pdf (9) Applications: Free Radical Initiators. Sigma-Aldrich, 2016. Industry Publication: http://www.sigmaaldrich.com/content/dam/sigmaaldrich/docs/Aldrich/General_Information/thermal_initi ators.pdf (10) The concentration of DBSA was below the experimentally determined CMC range in Figure S2 (0.0025 to 0.006 M). It should be emphasized that the CMC of DBSA in dodecane was

15



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16


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O S

S

OH

Ph

O

O

Ph

H

O

H Ph

O

OH


Mechanism of Acid-Catalyzed Decomposition of Dicumyl Peroxide in

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