Kinetics and Mechanisms of Dehydration of Secondary Alcohols

Subscriber access provided by UNIV OF NEWCASTLE

Article

Kinetics and Mechanisms of Dehydration of Secondary Alcohols Under Hydrothermal Conditions Christiana Bockisch, Edward D. Lorance, Hilairy E. Hartnett, Everett L Shock, and Ian Robert Gould ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.8b00030 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Earth and Space Chemistry

Kinetics and Mechanisms of Dehydration of Secondary Alcohols Under Hydrothermal Conditions

Christiana Bockisch,# Edward D. Lorance,‡ Hilairy E. Hartnett,*,#,¶ Everett L. Shock,*,#,¶ and Ian R. Gould,*, # # School of Molecular Sciences, Arizona State University, Tempe, AZ 85287 ‡ Department of Chemistry, Vanguard University, Costa Mesa, CA 92926 ¶ School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287 Keywords: Geochemistry, hydrothermal, organic chemistry, mechanism, elimination, kinetics, deoxygenation ABSTRACT Alcohol dehydration by elimination of water is central to a series of functional group interconversions that have been proposed as a reaction pathway that connects hydrocarbons and carboxylic acids under geochemically relevant hydrothermal conditions, such as in sedimentary basins. Hydrothermal dehydration of alcohols is an example of an organic reaction that is quite different from the corresponding chemistry under ambient laboratory conditions. In hydrothermal dehydration, water acts as the solvent and provides the catalyst, and no additional reagents are required. This stands in contrast to the same reaction at ambient conditions, where concentrated strong acids are required. Hydrothermal dehydration is thus of potential interest in the context of green chemistry. We have investigated the mechanism of hydrothermal alcohol dehydration for a series of secondary alcohols using studies of kinetics and stereoelectronic effects, in order to establish reaction mechanisms. The E1 elimination mechanism dominates over the corresponding E2 mechanism, with the E2 mechanism being competitive with E1 only

1

ACS Paragon Plus Environment

ACS Earth and Space Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

for the most favorable stereoelectronically restricted alcohols included in the present study. These results are relevant to understanding the kinetics and product distributions of alcohol dehydration reactions in natural geologic systems, and can guide the development of organic chemical reactions that mimic geologic organic reactions under laboratory green chemistry conditions.

2


Page 2 of 42

Page 3 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


• INTRODUCTION Organic reactions commonly occur under hydrothermal conditions in a wide variety of geochemically relevant processes, from petroleum generation to the deposition of minerals, and as part of the deep carbon cycle.1-5 A wide range of hydrothermal organic chemical reaction types have been surveyed.6-9 Of particular interest are a series of reversible functional group interconversions described by Seewald that connect simple alkanes to carboxylic acids, Scheme 1.2 Although product distributions of many hydrothermal reactions have been investigated, kinetic and mechanistic studies of organic transformations under these conditions have received much less attention. Understanding mechanisms and kinetics is important in order to build theoretical geochemical models of the deep subsurface.10,11 Dehydration of alcohols to form alkenes is unusual in that it is relatively well understood in both thermodynamic and mechanistic terms at both hydrothyermal and ambient conditions.1216

At ambient conditions, addition of water to an alkene to form an alcohol, the reverse of

dehydration, is favorable due to formation of a stronger σ-bond at the expense of a weaker πbond, although Brønsted or Lewis acid catalysis is required.15,16 Under these conditions, the favorable enthalpic contribution to the free energy of formation of the alcohol is larger than the unfavorable entropic contribution associated with the conversion of two molecules (alkene and water) into one molecule (alcohol), and equilibrium favors the alcohol rather than the alkene and water.12,17 Under hydrothermal conditions at elevated temperatures, however, entropic effects become more important, and the equilibrium shifts so that it lies on the side of alkene and water. Propanol is a simple representative example. As shown in Figure 1, the equilibrium constant for

3



R

R' alkane

-H2

R

R' +H2

alkene

OH

+H2O -H2O

Page 4 of 42

R

R'

O

-H2 +H2

alcohol

R

R' ketone +3 H2O

-2 CO2 R' + R

O 3 H2 +

alkanes

+ R'

R

HO

OH

O

carboxylic acids

Scheme 1. Functional group interconversions that connect alkanes and carboxylic acids under hydrothermal conditions.2 Alcohol dehydration and the reverse reaction, alkene hydration, and indicated by the box.

4


Page 5 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.5

OH

Keq

100

150 200 250 Temperature, °C

+ H2O

1.0 0.5 Log Keq 0.0 -0.5 -1.0 -1.5 0

50

300

350

Figure 1. Equilibrium constants (Log Keq) for dehydration of 1-propanol as a function of temperature at the saturated water vapor pressure. Above ca. 155°C Log Keq is positive and the equilibrium favors alkene and water rather than the alcohol. The values for Log Keq were calculated with the revised Helgeson-Kirkham-Flowers equation of state,18 using data and parameters from Refs. 17 and 19.

5



dehydration of this alcohol increases with temperature until the corresponding alkene and water are favored at temperatures above ca. 155°C.17-19 The result is that in the temperature range of ca. 200 - 350°C (and at the relevant associated confining pressures), alcohols can undergo quite rapid dehydration to form alkenes, even though they are in the presence of a high concentration of water as the solvent.13,14,17,20 The mechanism of hydrothermal alcohol dehydration has been the subject of several previous studies.13,21-31 Previous work suggests that in hot pressurized water the mechanism proceeds via conventional, homogeneous, Brønsted acid-catalyzed elimination.21-31 Autoionization of water under hydrothermal conditions is characterized by a larger Kw compared to ambient conditions.32-34 The hydronium ions formed by autoionization can protonate the alcohol oxygen to form an oxonium cation that can then undergo elimination with water as the leaving group, Scheme 2.28-34 Two different mechanisms are normally considered for elimination of alcohols, E1 and E2, Scheme 2.35,36 Both include protonation of the hydroxyl group of the alcohol as the first step. In the E1 mechanism, the protonated oxygen leaves as a water molecule to generate a cationic intermediate that subsequently undergoes deprotonation to form the alkene. In the E2 mechanism, loss of water and deprotonation occur simultaneously, and no intermediates are involved. Primary alcohols almost certainly dehydrate via an E2 mechanism, since the primary carbocation intermediate that would be formed via E1 elimination is too unstable.22-24 For tertiary alcohols, formation of a tertiary carbocation intermediate is facilitated as a consequence of the increased stability of the more substituted cation, and dehydration by an E1 mechanism seems to dominate in these cases.25,26 For secondary alcohols the mechanism is much less clear and it is possible that E1 and E2 mechanisms are competitive, Scheme 2.13,27,28

6


Page 6 of 42

Page 7 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


– H2O (E2) OH

OH2 + H3O+

– H2O

H2O

(E1)

(E1) H2O (E1)

+ H3O+

+ H 3 O+

Scheme 2. Comparison of the E1 and E2 mechanisms for hydrothermal dehydration of 4heptanol (as a representative secondary alcohol). The E1 mechanism (grey box) involves a carbocation intermediate that can undergo rearrangement, whereas the E2 mechanism is concerted. Not all of the possible isomeric alkene products are included in this Scheme. The results described in this work suggest that for this particular alcohol, E1 is the major elimination mechanism, see text.

7



Understanding the mechanism and kinetics of dehydration is important in order to build models for organic reaction pathways under geologically relevant conditions. As discussed in detail below, the E1 mechanism is associated with a negative entropy of activation, whereas the E2 mechanism is associated with a positive entropy of activation. This suggests that E2 elimination may become less favorable compared to other possible competing reactions at higher temperatures whereas the E1 mechanism may become more competitive. Hydrothermal dehydration of alcohols is also of interest as an alternative to conventional dehydration of alcohols at ambient conditions. Alcohol dehydrations performed at ambient conditions require concentrated acids and the use of forcing conditions, such as physical separation of reactants and products via distillation.35,36 Hydrothermal dehydration, however, proceeds with high chemical yield, without the need for external reagents or catalysts, and product isolation is exceedingly simple because the non-polar organic products simply float on top of the water after quenching to ambient temperature. The lack of reagents, the use of water as the solvent, and the simple product isolation procedures mean that this reaction conforms to many of the standard principles of green chemistry.37 Hydrothermal reactions of organic chemicals in general are of increasing interest for green chemistry applications,38-42 and also in the broader context geomimicry, the geochemical analogue of biomimicry.43 Here we describe a detailed study of the mechanism of dehydration of a series of secondary cyclic and acylic alcohols. Conformationally locked cyclohexanols are used so that stereoelectronic effects can be used to probe the elimination mechanism, together with a detailed analysis of the minor products at early reaction times. The results of these studies suggest that a Brønsted-acid-catalyzed E1 mechanism dominates over the corresponding E2 mechanism for most secondary alcohols under hydrothermal conditions.

8


Page 8 of 42

Page 9 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


• EXPERIMENTAL SECTION Materials. Cyclohexanol and 5-nonanol were purchased from Fisher Scientific. The cisand trans-4-t-butylcyclohexanols and 4-octanol were purchased from TCI America. The remaining alcohols were purchased from Sigma Aldrich. All chemicals were assessed for purity by GC, then used as purchased without further purification. Methods. Reactions were performed in fused-silica tubes (6mm OD, 2mm ID), with starting alcohol concentrations of 0.2 molal in 18.2 MΩ⋅cm (Barnstead Nanopure™) water. Prior to sealing, the headspaces of the tubes were purged with ultrapure argon for 10 minutes, submerged to the inner liquid level in liquid nitrogen, and evacuated to ~30 mTorr using a rotary vane pump. The tubes containing the frozen samples under vacuum were then sealed with a hydrogen-oxygen torch. The sealed tubes were 11-12 cm long and the 200 µL sample volumes occupied roughly 6 cm of the tube length at room temperature (RT), the remainder of the volume being the evacuated headspace. The sample volume increased to occupy roughly 7.8 cm of the tube length at the experimental temperature volume (i.e. to roughly 61% of the volume), the rem ainder being headspace. At 250°C the water vapor pressure in the tube was determined to be ca. 40 bar.34 Under the experimental conditions the reactions are assumed to proceed in the water phase because they are catalyzed by solution-phase hydronium ion,21-31 and because experiments in which the solution volume to headspace ratio was varied between 50% liquid, 30% liquid, and 25% liquid, resulted in no appreciable change in the reaction kinetics. For reaction times one hour or less, the sealed fused-silica tubes were placed in a large brass block heater that was pre-heated to 250°C using cartridge heaters. A thermocouple inside the block next to the sample tube monitored the reaction temperature. For reaction times longer than an hour, the tubes were placed in vented metal pipes capped at each end that acted as shields

9



in case of tube failure, then placed in a gas chromatography oven set at 250˚C. At the end of the reaction time, tubes were quenched by plunging into room temperature water. Kinetic studies were performed by allowing different samples to react for different lengths of time. The reaction mixtures were extracted with 1 mL of dichloromethane (Fisher) and the dichloromethane solutions were analyzed directly using gas chromatography with flame ionization detection (Bruker Scion 456 with an Equity-5 column [5% diphenyl/95% dimethyl siloxane]) or GC-mass spectrometry (Thermo Electron Trace 1300-ISQ with a TG-SQC column). GC calibration curves were determined for all reactants and products for which authentic samples were available. The calibration curves corrected for any differences in detection and extraction efficiencies. Although the calibration curves do not allow the relative importances of extraction and detection efficiencies to be separated, they allow direct determination of solution concentrations from the integrated areas of the relevant GC peaks. Decane was used as an internal gas chromatography standard to calculate reaction conversion, chemical yields and mass balances. Reaction kinetics were determined from time course experiments by determining the % of the starting alcohol converted at different reaction times, and, when the mass balances were close to 100% (see below), as the percentage of the alcohol that had converted to a particular product at the same reaction times. Assignment of gas chromatography peaks was performed using authentic samples of the starting alcohols and product alkenes. Isomeric alkene products were assumed to have the same response factors, as were alcohol isomer products of the starting alcohols. The mass balances for the reactions of the three cyclohexanols were 95 - 100%. The corresponding values for the linear alcohols were 70 100%, with most being greater than 85%. Mass balances of less than 100% are almost certainly due to mechanical loss of volatile alkene products in the isolation steps, since the mass balances

10


Page 10 of 42

Page 11 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


were larger for the larger molecules, in particular those with t-butyl substituents, that formed less volatile alkenes. Percent recovery of the alcohol reactants under the experimental conditions was close to 100% based on repeated test extractions and repeated determinations of calibration curves. Thus, small differences in mass balance from 100% almost certainly do not influence determination of the kinetics of reaction of the alcohols. Kinetic fitting of the time-course data was performed using the COPASI software package, version 4.15 (Build 95),44 by numerical simulation of the data sets with minimization of errors using the Levenberg-Marquardt algorithm. For the t-butylcyclohexanols, the time course data for the starting alcohols and all of the products were fitted simultaneously, so that the rate constants obtained by minimization of errors represented the best global fits to all of the reactant and product data associated with each specific reaction.

• RESULTS AND DISCUSSION Cyclohexanol. Hydrothermal dehydration of cyclohexanol has been studied previously by Akiya and Savage.28 In agreement with this previous work, hydrothermal dehydration of cyclohexanol at 250°C and 40 bar forms cyclohexene as essentially the only product. 1Methylcyclopentene is detected as a very minor product, at less than 0.1% of the cyclohexene at all times. Studies as a function of time show that, as described previously,28 the kinetics of dehydration are pseudo-first order (Figure 2). Several different kinds of rate constants are discussed in this work (see Scheme 3). The pseudo-first order rate constant for disappearance of

11



Page 12 of 42

80

mole 60 fraction (%)

OH -H2O

40

250°C 40 bar

20

0

0

10

20

30

40

50

time, hrs Figure 2. Cyclohexanol conversion (mol %) as a function of time, for hydrothermal dehydration at 250°C and 40 bar. The solid curve is a first-order fit to the data with an observed rate constant, kobs, of 0.196 ± 0.038 hr-1 for 88% alcohol conversion at infinite time.

12


Page 13 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


OH k –H2O A

+ H 2O

k +H2O k obs = k –H2O + k +H2O [H2O] OH

K eq =

k +H2O [H2O]

OH2 k +H+

B

k –H2O

kE

k –H+

+ H 2O

(E1 or E2)

+ k –H2O = k +H k E k –H+

Scheme 3. Kinetics of hydrothermal dehydration of cyclohexanol in water showing (A) interpretation of the observed rate constant for loss of alcohol (and formation of alkene products), kobs, in terms of the rate constants for the alcohol dehydration (k-H2O) and alkene hydration (k+H2O) steps; and (B) interpretation of k-H2O in terms of a rapid pre-equilibrium to form an oxonium ion that eliminates (kE) via either an E1 or E2 mechanism. For the tbutylcyclohexanols studied here, kE can be further decomposed into more elementary steps, (see for example, Scheme 5).

13



the alcohol and formation of the alkene is directly related to the experimental observables of reactant and product concentrations as a function of time; thus, we indicate this rate constant as kobs. Under the current experimental conditions of 250°C and 40 bar, kobs for cyclohexanol is 0.20 hr-1 (Table 1). After 20 hours under these conditions, this reaction reaches an apparent equilibrium consisting of 12% starting alcohol and 88% cyclohexene (Figure 2). Observation of an equilibrium allows kobs to be further decomposed into the rate constants for dehydration (k– H2O)

and hydration (k+H2O), by combining kobs and the equilibrium constant Keq, Scheme 3. The

rate constant for dehydration of cyclohexanol obtained this way, k-H2O, is 0.17 hr-1(Table 1). This rate constant also includes any other reaction that forms a product; however, the yield of the only other detectable product (1-methylcyclopentene,

Kinetics and Mechanisms of Dehydration of Secondary Alcohols

Recommend Documents