1Kinetics of the reactions of the OH radical with 2-methyl-1-propanol, 3-methyl-1-butanol and 3-methyl-2-butanol between 241 and 373 K
Type: Object |
Advantage: None |
Novelty: New |
ConceptID: Obj1
1
Kinetics of the reactions of the OH radical with 2-methyl-1-propanol, 3-methyl-1-butanol and 3-methyl-2-butanol between 241 and 373 K
Type: Object |
Advantage: None |
Novelty: New |
ConceptID: Obj1
2
Absolute rate constants have been measured for the gas-phase reactions of hydroxyl radicals with 2-methyl-1-propanol (k1), 3-methyl-1-butanol (k2) and 3-methyl-2-butanol (k3).
Type: Object |
Advantage: None |
Novelty: New |
ConceptID: Obj2
3
Experiments were carried out using two different techniques, the relative rate method and the pulsed laser photolysis-laser induced fluorescence technique.
Type: Method |
Advantage: None |
Novelty: Old |
ConceptID: Met1
4
The kinetic data were used to derive the following Arrhenius expressions in the temperature range 241–373 K(in units of cm3 molecule−1 s−1):k1 = (3.1±0.9) × 10−12exp[(352±82)/T]k2 = (2.8±0.9) × 10−12exp[(503±98)/T]k3 = (2.6±0.6) × 10−12exp[(456±65)/T]At 298 K, the reaction rate constants obtained by the two methods were in very good agreement.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res1
5
The results are presented, discussed and used to estimate the atmospheric lifetimes for the studied alcohols.
Type: Goal |
Advantage: None |
Novelty: None |
ConceptID: Goa1
Introduction
6
Alcohols are emitted into the atmosphere by a wide number of anthropogenic and natural processes.1
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac1
7
The main sources are due to their use as solvent in different industries.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac1
8
For example, 2-methyl-1-propanol (isobutanol) is commonly used in coatings, as an additive in gasoline, an intermediate for glycol ethers and agricultural chemicals, and in anti-corrosion and anti-wear additives in engine oils.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac1
9
As a chemical intermediate, 2-methyl-1-propanol is used to make esters and ethers.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac1
10
3-methyl-1-butanol (isopentanol) is used as a chemical intermediate and solvent, and in pharmaceutical products and medicines.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac1
11
The release of these oxygenated volatile organic compounds is likely to contribute to the formation of ozone and other components of photochemical smog such as aldehydes, ketones and PAN-type compounds in urban areas.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac1
12
To assess the impact of these chemical species on the environment, a detailed understanding of the kinetics and mechanisms of their atmospheric degradation is required.
Type: Motivation |
Advantage: None |
Novelty: None |
ConceptID: Mot1
13
The gas-phase atmospheric oxidation of alcohols is primary initiated by reaction with OH radicals.2,3
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac2
14
Several kinetic studies of the OH reaction with alcohols have been reported previously.2,3
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac2
15
However, only a limited number of studies have been conducted on the absolute measurements of the OH reaction rate constant with alcohols with more than four carbon atoms.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac2
16
In this article, we report the rate coefficient data for the reactions of OH radicals with 2-methyl-1-propanol (k1), 3-methyl-1-butanol (k2) and 3-methyl-2-butanol (k3): OH + (CH3)2CHCH2OH→products: k1OH + (CH3)2CHCH2CH2OH→products: k2OH + (CH3)2CHCH(OH)CH3→products: k3Experiments were carried out using two different techniques.
Type: Object |
Advantage: None |
Novelty: New |
ConceptID: Obj3
17
The pulsed laser photolysis–laser induced fluorescence (PLP-LIF) technique was used to determine the absolute rate coefficients for the above reactions in the temperature range 241–373 K at around 100 Torr while the relative method enabled us to conduct the measurements at room temperature and atmospheric pressure.
Type: Method |
Advantage: None |
Novelty: Old |
ConceptID: Met1
18
As far as we know, this work provides the first temperature dependence studies for the three reactions.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac3
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac4
20
The present work also aims at extending the kinetic data base to be used to develop structure-reactivity relationships for OH reaction with oxygenated VOCs.
Type: Goal |
Advantage: None |
Novelty: None |
ConceptID: Goa2
Experimental section
Relative measurements
21
The experimental set-up used to measure the relative rate constant consisted of a 120 L Teflon bag and lamps positioned in a wooden box with the internal faces covered with aluminium foil.
Type: Method |
Advantage: None |
Novelty: New |
ConceptID: Met2
22
Measured amounts of reagents were flushed from calibrated bulbs into the Teflon bag through a stream of ultra pure air.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp1
23
The photoreactor was then filled to its full capacity at atmospheric pressure with ultra-pure air.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp1
24
OH radicals were produced by photolysing H2O2 using lamps emitting at 254 nm (Sylvania G30W) and illuminating the hole gas sample.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp1
25
A gas chromatograph equipped with flame ionisation detector (GC-FID, CP-3800 or Star 3600 CX, Varian) was used for the quantitative analysis of the reactants.
Type: Method |
Advantage: None |
Novelty: Old |
ConceptID: Met3
26
Chromatographic separation was achieved by using a DB-1 capillary column (J&W Scientific, 30 m, 0.25 nm id, 5 μm film).
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp2
27
The column was operated at the temperature of 318 K, and helium was used as the carrier gas.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp2
Absolute measurements
28
The pulsed laser photolysis–laser induced fluorescence (PLP-LIF) technique and the methodology used have been described in detail previously [e.g. in ref. 7], only a brief description is given here.
Type: Method |
Advantage: None |
Novelty: Old |
ConceptID: Met4
29
Hydroxyl radicals were produced by photolysis of H2O2 at λ = 248 nm (KrF excimer laser) and their concentration was monitored at various reaction times ranging from about 10 μs to 10 ms by pulsed laser induced fluorescence.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp3
30
A Nd:YAG-pumped frequency-doubled dye laser was used to excite the OH radicals at λ = 282 nm.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp3
31
Fluorescence from the OH radicals was detected by a photomultiplier, fitted with a 309 nm narrow bandpass filter.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp3
32
The output pulse from the photomultiplier was integrated for a preset period by a gated charge integrator.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp3
33
Typically the fluorescence signal from 10 different delay times and 100 probe laser shots were averaged to generate OH concentration-time profiles over at least three lifetimes.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp3
34
H2O2 was introduced into the reaction cell by passing a small flow of helium through a glass bubbler containing H2O2 solution.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp3
35
Alcohols were premixed with helium in a 10 L glass bulb to form 0.2–0.6% mixture at a total pressure of about 850 Torr.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp4
36
The gas mixture, the photolytic precursor (H2O2), and the bath gas (helium) flowed through the cell with a linear velocity ranging from 3 to 10 cm s−1.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp4
37
The gas mixture illuminated was then renewed every two laser shots.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp4
38
The concentrations of the reactants were calculated from their mass flow rates, the temperature, and the pressure in the reaction cell.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod1
39
All flow rates were measured with mass flow meters calibrated by measuring the rate of pressure increase in a known volume.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp5
40
The pressure in the cell was measured with a capacitance manometer connected at the entrance of the cell.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp5
41
The helium carrier gas (UHP certified to >99.9995% (Alphagas)) was used without purification.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp5
42
The 50 wt % H2O2 solution, from Prolabo, was concentrated by bubbling helium through the solution to remove water for several days prior to use and constantly during the experiment.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp5
43
2-methyl-1-propanol (≥99.8 %), 3-methyl-1-butanol (≥99.5) and 3-methyl-2-butanol (≥99.5%) were from Fluka, they were further purified by repeated freeze, pump, and thaw cycles and fractional distillation before use.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp5
Results and discussion
Relative rate measurements
44
Relative rate constants were measured by comparing the OH reaction rate with alcohols to that with reference compounds:OH + alcohol → products: kalcoholOH + reference → products: krefAssuming that the compounds are only consumed by reaction with OH, it can be shown that:ln([alcohol]0/[alcohol]t) = (kalcohol/kref)ln([ref]0/[ref]t)where the subscripts 0 and t indicate concentrations before irradiation and at time t, respectively.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod2
45
The rate constant for the reaction of OH with the three alcohols were measured at (298 ± 3) K and 760 Torr, relatively to the rate constant of OH with 1-butanol and 1,3-dioxolane.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp6
46
The concentrations of alcohols and references were in the range 30–100 ppm.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp6
47
The compounds showed a good stability when they were mixed in the chamber with H2O2 and kept in the dark for about 1 h.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs1
48
Moreover, in absence of H2O2, the photolysis of the mixtures (alcohols and references in air) for more than one hour did not show any decrease in the concentrations.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs2
49
Fig. 1 shows an example of the obtained relative loss of alcohols versus that of the reference compounds in the presence of OH.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs3
50
In Table 1, the obtained rate constant ratios are given along with the derived rate constants for the OH reactions with the three alcohols.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs4
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod3
52
While averaging the values from different experiments, we obtained:k1 = (0.92 ± 0.15) × 10−11 cm3 molecule−1 s−1.k2 = (1.4 ± 0.2) × 10−11 cm3 molecule−1 s−1.k3 = (1.25 ± 0.2) × 10−11 cm3 molecule−1 s−1.The quoted errors represent twice the standard deviation, to which we have added an estimated error of 10% on the reference rate constants.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res2
Absolute measurements
53
Kinetic experiments have been performed under pseudo-first-order conditions with the concentration of alcohols in large excess over that of OH concentration ([alcohol]0 > 100[OH]0).
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp7
54
Typically, the initial OH concentration, [OH]0, was around 2×1011 molecule cm−3.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp7
55
The rate of disappearance of the OH radical followed a simple exponential rate law:[OH]t = [OH]0e−k′t where k′ = ki [alcohol] + k′0ki represents the rate constants for the reaction of OH with the three alcohols.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod4
56
The decay rate k′ is the first-order OH disappearance rate in the presence of the alcohol and k′0 is the first-order rate constant for OH removal in absence of alcohol (attributed to the diffusion of OH radicals out of the detection zone and to their reaction with H2O2).
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod4
57
Typically, alcohol and H2O2 concentrations were in the ranges (1.6–92)×1013 and (1–10)×1013 molecule cm−3, respectively. k′0 and k′ were in the ranges 60–140 s−1 and 200–13000 s−1, respectively.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp8
58
Experiments were conducted in the temperature range 241–373 K and a total pressure of (106 ± 5) Torr of helium.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp8
59
In all conditions, the OH decays were found to be exponential over at least three lifetimes.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs5
60
The high [alcohol]/[OH]0 ratios and low OH concentrations made negligible contribution from secondary reactions involving the products of reactions to the measured rate constants.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res3
61
The contribution of the reaction of OH with photofragments of alcohol was negligible since alcohols are not photolysed at 248 nm,10 the wavelength used to generate OH radicals.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac5
62
As expected, variation in the photolysis fluence (3–26 mJ cm−2) had no effect on the determined rate constants.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs6
63
The three alcohols were purified to better than 99.5% and hence loss of OH radicals by reaction with impurities in the gas mixtures is expected to be insignificant.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con1
64
Fig. 2 shows examples of the plots of k′ versus the alcohol concentrations obtained at 298 K.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs7
65
The obtained values of k1–k3 and the experimental conditions are listed in table 2.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs8
66
The room temperature rate constant taken as the average of all values obtained at (298 ± 2) K are:k1 = (1.0 ± 0.1)×10−11 cm3 molecule−1 s−1.k2 = (1.5 ± 0.1)×10−11 cm3 molecule−1 s−1.k3 = (1.2 ± 0.1)×10−11 cm3 molecule−1 s−1.The quoted errors for k1–k3 include 2σ from the least-squares analysis and the systematic error (5%, due to uncertainties in measured concentrations).
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res4
67
These values are in good agreement with those obtained by the relative method.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con2
68
The measured values of k1–k3 shown in Table 2 are plotted in the Arrhenius form in Fig. 3.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs9
69
An un-weighted least squares analysis of the ln kivs. 1/T plot yields the following expressions for the temperature dependence of k1–k3 in the temperature range 241–373 K (in units of cm3 molecule−1 s−1 ):k1 = (3.1±0.9) × 10−12exp[(352±82)/T]k2 = (2.8±0.9) × 10−12exp[(503±98)/T]k3 = (2.6±0.6) × 10−12exp[(456±65)/T]Uncertainties are 2AσlnA and 2σE/R, for A and E/R, respectively.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res5
70
The rate constants obtained in this work are compared with the previously reported data in Table 3.
Type: Object |
Advantage: None |
Novelty: None |
ConceptID: Obj4
71
For isobutanol, the measured value of k1 at room temperature is in good agreement with that obtained by Wu et al. using the relative rate method.4
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs10
72
These authors measured k1 relative to the rate constant of OH reactions with propane and cyclohexane and obtained (9.08±0.35) × 10−12 and (9.94±0.47) × 10−12 cm3 molecule−1 s−1, respectively.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac6
73
Wu et al. measured also the rate constant for the reaction of OH with 3-methyl-1-butanol using the same references, they obtained (13.8±0.5) × 10−12 and (13.7±1.1) × 10−12 cm3 molecule−1 s−1 with propane and cyclohexane, respectively.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac6
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs11
75
For 3-methyl-2-butanol, our value of k3 at room temperature agrees with that reported earlier by Wallington et al. obtained using the flash photolysis–resonance fluorescence technique (k3 = (12.4±0.7) × 10−12 cm3 molecule−1 s−1).6
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs12
76
This work provides the first temperature dependence parameters for the OH reaction with 2-methyl-1-propanol, 3-methyl 1-butanol and 3-methyl 2-butanol.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac7
77
The results show a negative temperature dependence for k1, k2 and k3 similarly to other alcohols for which temperature dependence data are available such as n-propanol, i-propanol and n-butanol.8
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res6
78
The present rate constant values k1–k3 can be compared with those calculated from the structure–activity relationship (SAR) of Atkinson based on group reactivity using the revised substituent factors F(–OH) = 2.9 and F(–CH2OH) = F(CHOH) = F(COH) = 2..611,12
Type: Object |
Advantage: None |
Novelty: New |
ConceptID: Obj5
79
To calculate k1–k3 we used also the following parameters as defined by Kwok and Atkinson: F(–CH3) = 1, F(–CH2–) = F(CH–) = F(C) = 1.23, kprim = 0.136, ksec = 0.934, ktert = 1.94 and kOH = 0.14 (k are in units of 10−12 cm3 molecule−1 s−1) where F(–CH3), F(–CH2–), F(CH–), and F(C) are the substituent factors and kprim, ksec, ktert and kOH represent the rate constants for the H-atom abstraction from –CH3, –CH2–, CH–, and –OH, respectively.11
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod5
80
The calculated k1–k3 values compared to the experimental ones (in brackets) are k1 = 0.89 (0.96) ; k2 = 0.92 (1.45) and k3 = 1.28 (1.23) (units of 10−11 cm3 molecule−1 s−1).
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs13
81
While the experimental values of k1 and k3 are in good agreement with the calculated ones, that of k2 is a factor of 1.6 higher than that calculated using the SAR.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs14
82
It has to be noticed that this kind of discrepancy between the experimental and calculated rate constants values for the reaction of OH with hydroxyl-containing organic compounds has already been mentioned earlier.12,13
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac8
83
The measured k2 value supports the statement of Bethel et al. indicating that accurate estimation of OH radical rate constants for all hydroxyl-containing compounds (to within better than a factor of two) does not appear possible using the general approach of the SAR developed by Atkinson.12
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res7
84
One of the possible reasons for this discrepancy is that the SAR of Atkinson considers only effects of the OH substituent on H-atom abstraction at the α- and the β- positions for hydroxyl-containing compounds while the available data shows that the oxygenated functional groups have long-range effects with respect to H-atom abstraction at sites remote from the substituent groups.3
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con3
85
This long-range effect indicates an alternative pathway to the direct concerted hydrogen abstraction process such as the formation of a hydrogen-bonded complex in which, first, a hydrogen bond is formed between the H atom of the OH radical and the O atom of the oxygenated compound then a second hydrogen bond is formed between the O atom of the OH radical and a H atom in the hydrocarbon chain, resulting in intermolecular H-atom transfer via a cyclic transition state.3,14
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac9
86
In all cases, the resulting reaction is an abstraction of a hydrogen atom from the oxygenated substrate.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac9
87
At room temperature, reaction of OH radicals with alcohols proceeds mainly by an H-atom abstraction process from the C–H bonds of the CH3–, –CH2- and CH– groups with a minor contribution of the H-atom abstraction from the -OH group.2,15–20
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res8
88
Product distribution studies on the reaction of OH radicals with long chain linear alcohols in the presence of NOx in air have shown that H-atom abstraction from different positions (α, β, or γ) to the –OH group makes a significant contribution to the total reaction.2,11–16
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac10
89
Under atmospheric conditions, the resulting hydroxy alkyl radicals react with oxygen.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac10
90
The α-hydroxy alkyl radicals formed through H-atom abstraction from the CHx group bonded to OH react solely with O2 to lead to carbonyls:2,20RR′CHOH + OH → RR′C˙OH + H2ORR′C˙OH + O2 → RR′C(O) + HO2The other hydroxy alkyl radicals (β, γ, etc.) react with O2 and NO to lead to different alkoxy hydroxy radicals which then react with O2 or decompose.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac10
91
In addition, recent mechanistic studies have suggested that isomerization of this type of intermediate oxy radicals may also be important in the oxidation of long chain alcohols, through a six-membered transition state, leading to the formation of hydroxyketones and hydroxyaldehydes.16–19
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac11
92
In the gas phase, the primary degradation step of alcohols in the troposphere is reaction with OH radicals since other processes such as reactions with O3 and NO3, and photolysis are of minor importance.2,3
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac12
93
The measured OH reaction rate constants (k) along with a typical OH concentration of [OH] = 2 × 106 molecule cm−3,21 can be used to estimate the tropospheric lifetimes (τ = 1/k[OH]) of the studied alcohols.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac13
94
The derived lifetimes are around 10 h for the three alcohols which indicate that these compounds are quickly removed from the atmosphere and give rise to other stable products that have longer lifetimes such as acetone.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res9