Gas-phase pyrolytic reaction of 3-phenoxy and 3-phenylsulfanyl-1-propanol derivatives: Kinetic and mechanistic study

Article abstract of DOI:10.1002/kin.20276

3-Phenoxy-l-propanols 1a-c and 3-phenylsuIfanyl-1-propanols 2a-c containing primary, secondary, and tertiary alcohols were prepared and subjected to gas-phase pyrolysis in a static reaction system. Pyrolysis of 4-phenyl-1-butanol 3, 2-methyl-3-phenyl-1-propanol 4, and 2-methyl-3-phenylpropanoic acid 5 was also studied, and results were compared with those obtained for compounds 1-3. The pyrolytic reactions were homogeneous and followed a first-order rate equation. Analysis of the pyrolysate showed the products to be phenol (from la to 1c), thiophenol (from 2a to 2c), and toluene (from 3 to 5) and carbonyl compounds. The kinetic results and product analysis of each of the nine investigated compounds are rationalized in terms of a plausible transition state for the elimination pathway.

Full text of DOI:10.1002/kin.20276

Gas-Phase Pyrolytic
Reaction of 3-Phenoxy
and 3-Phenylsulfanyl-1-
propanol Derivatives:
Kinetic and Mechanistic
Study
H. H. DIB, M. R. IBRAHIM, N. A. AL-AWADI, Y. A. IBRAHIM, S. AL-AWADI
Chemistry Department, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait
Received 16 December 2006; revised 20 May 2007; accepted 19 June 2007
DOI 10.1002/kin.20276
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: 3-Phenoxy-1-propanols 1a–c and 3-phenylsulfanyl-1-propanols 2a–c containing
primary, secondary, and tertiary alcohols were prepared and subjected to gas-phase pyrolysis
in a static reaction system. Pyrolysis of 4-phenyl-1-butanol 3, 2-methyl-3-phenyl-1-propanol
4, and 2-methyl-3-phenylpropanoic acid 5 was also studied, and results were compared with
those obtained for compounds 1–3. The pyrolytic reactions were homogeneous and followed
a first-order rate equation. Analysis of the pyrolysate showed the products to be phenol (from
1a to 1c), thiophenol (from 2a to 2c), and toluene (from 3 to 5) and carbonyl compounds. The
kinetic results and product analysis of each of the nine investigated compounds are rationalized
in terms of a plausible transition state for the elimination pathway. ꢀ 2007 Wiley Periodicals,
C
Inc. Int J Chem Kinet 40: 51–58, 2008
lend support to a reaction pathway involving a five-
membered cyclic transition state previously suggested
by Chuchani et al. [2] for other different α-substituted
carboxylic acids (Scheme 1).
The order of reactivity of the aryl group is OPh >
SPh > NPh; this order corresponds to the trend
expected for the nature of incipient phenol, thiophenol,
and aniline [3].
A dramatic change in the reactivity is observed
when the acidity of the incipient hydrogen is re-
duced by replacing the carboxylic acid proton with
an alcohol hydroxyl hydrogen; a relative rate factor
INTRODUCTION
We have reported the kinetics and mechanism of
thermal gas-phase elimination of α-substituted car-
boxylic acids [1]. Product analysis and kinetic results
Correspondence to: N. A. Al-Awadi; e-mail: nouria@kuc01.
kuniv.edu.kw.
Contract grant sponsor: RA, University of Kuwait.
Contract grant sponsor: Analab/SAF.
Contract grant numbers: SC04/08, GS02/01, GS03/01.
2007 Wiley Periodicals, Inc.
c
ꢀ

52
DIB ET AL.
Scheme 1
Scheme 2
EXPERIMENTAL
General
1H and 13C NMR spectra were recorded on a Bruker
DPX 400 MHz superconducting NMR spectrometer.
Mass spectra were measured on VG Autospec-Q (high-
resolution, high-performance, tri-sector GC/MS/MS),
and with LCMS using Agilent 1100 series LC/MSD
with an API-ES/APCI ionization mode.
of 2 × 103 is obtained when the rate of pyrolysis of
2-phenoxypropanoic acid and 2-phenoxy-1-propanol
was compared [1].
Later, we studied the kinetic and mechanism of ther-
mal gas-phase elimination of β-substituted acids [4].
Product analysis and theoretical calculation suggested
a reaction mechanism compatible with a thermal retro-
Michael reaction pathway involving a four-membered
cyclic transition state (Scheme 2).
To investigate further in this elimination reaction,
in the present study we examined the result of the ki-
netic and product analysis of the thermal gas-phase
elimination reaction of 3-phenoxy-1-propanol 1a, 4-
phenoxy-2-butanol 1b, 4-phenoxy-2-methyl-2-butanol
1c, and their phenylsulfanyl derivatives 2a–c. The py-
rolysis of 4-phenyl-1-butanol 3, 2-methyl-3-phenyl-1-
propanol 4, and 2-methyl-3-phenylpropanoic acid 5
was also investigated (Scheme 3).
Synthesis
3-Phenoxy-1-propanol (1a). A solution of 3-
phenoxypropionic acid [5a] (1.66 g, 10 mmol) in dry
ether (20 mL) was added dropwise to a stirred slurry
of lithium aluminum hydride (0.50 g, 13 mmol) in
dry ether (100 mL) at rate sufficient to maintain re-
flux. After the addition was over, the reaction mix-
ture was heated under reflux for 4 h and then cooled
to room temperature. The excess hydride was decom-
posed by dropwise addition of water, and the resulting
white suspension was filtered off. The filtrate was dried
over anhydrous sodium sulfate, and the solvent was re-
1
moved in vacuo to afford 1.2 g (80%) of 1a [5b]. H
NMR (CDCl3): δ 2.01 (quint, 2H, J = 6.0, CH2), 3.04
(br, 1H, OH), 3.85 (t, 2H, J = 6.0, CH2), 4.11 (t,
2H, J = 6.0, CH2), 6.94 (d, 2H, J = 8.0), 6.98 (t, 1H,
J = 7.4), 7.32 (t, 2H, J = 7.8). MS: m/z = 152 (M+,
70%).
4-Phenoxy-2-butanol (1b). To a solution of 4-
phenoxybutan-2-one [6] (1.63 g, 10 mmol) in methanol
(20 mL), cooled at 0◦C, NaBH4 (0.50 g, 15 mmol) was
added portionwise with stirring. The reaction mixture
was then stirred at room temperature for 1 h, and the
solvent was then removed in vacuo. The product was
extracted with ether (3 × 50 mL), washed with water,
and dried over anhydrous sodium sulfate. The solvent
was then removed in vacuo to give colorless oily prod-
uct, yield 1.0 g (60%), which was purified by silica
gel column chromatography, using EtOAc-pet. ether
(60–80) as an eluent [Rf = 0.37 EtOAc-pet. ether 60–
1
Scheme 3 Substrates investigated in the study.
80 (1:4)]. H NMR (CDCl3): δ 1.29 (d, 3H, J 6.2,
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GAS-PHASE PYROLYSIS OF 3-PHENOXY AND 3-PHENYLSULFANYL-1-PROPANOL DERIVATIVES
53
CH3), 1.95 (q, 2H, J = 6.3, CH2), 2.24 (br, 1H, OH),
4.12 (m, 2H), 4.20 (m, 1H, CH OH), 6.93 (d, 2H,
J = 8.3), 6.98 (t, 1H, J = 7.2), 7.30 (t, 2H, J = 8.0).
13C NMR (CDCl3): δ 23.8 (CH3), 38.1 (CH2), 65.9
(CH2), 66.4 (C OH), 113.6 (2CH), 121.0 (CH), 129.6
(2CH), 158.8 (C). MS: m/z = 166 (M+, 90%), 107
(30%), 94 (100%).
methanol (20 mL), NaBH4 (0.50 g, 15 mmol) was
added portionwise at room temperature with stirring.
The reaction mixture was then stirred at room tempera-
ture for 1 h, and the solvent was then removed in vacuo.
The product was extracted with ether (3 × 50 mL),
washed with water, and dried over anhydrous sodium
sulfate. The solvent was then removed in vacuo,
and the remaining colorless oily product, yield 1.4 g
(60%), was purified by silica gel column chromatog-
raphy using EtOAc–hexane as an eluent, (Rf = 0.24
EtOAc:hexane 1:4). 1H NMR (CDCl3): δ 1.20 (d, 3H,
J = 6.2, CH3), 1.76 (m, 2H, CH2), 2.63 (s, 1H, OH),
3.04 (m, 2H, CH2), 3.94 (sextet, 1H, J = 6.2, CH–O),
7.19 (t, 1H, J = 7.2), 7.30 (t, 2H, J = 7.5), 7.35 (d,
2H, J = 7.7). 13C NMR (CDCl3) δ: 24.1 (CH3), 30.6
(CH2), 38.7 (CH2), 67.4 (C OH) 126.5 (CH), 129.6
(4CH), 137.0 (C). MS: m/z = 182 (M+, 90%), 123
(50%), 110 (100%).
4-Phenoxy-2-methyl-2-butanol (1c). To a solution of
CH3MgI (freshly prepared from 1.0 g Mg and 2.5 mL
CH3I in 40 mL dry ether) was added methyl 3-
phenoxypropionate [4] (1.8 g, 10 mmol) in dry ether
(20 mL) portionwise at room temperature with stirring
under nitrogen atmosphere. The reaction mixture was
heated under reflux for 24 h. After cooling at room
temperature, the mixture was quenched with saturated
aqueous ammonium chloride solution and the ethereal
layer was separated and the aqueous layer was fur-
ther extracted with ether (3 × 50 mL). The combined
ethereal extracted was dried over anhydrous sodium
sulfate. The solvent was then removed in vacuo to give
colorless oil, yield 1.2 g (66%), which was purified by
silica gel column chromatography using CHCl3-pet.
ether (60–80) as an eluent [Rf = 0.7 CHCl3:pet. ether
2-Methyl-4-phenylsulfanyl-2-butanol (2c). To a solu-
tion of thioanisole (1.2 mL, 10 mmol) in dry THF (40
mL) at −78◦C and n-BuLi in hexanes (5.6 mL, 2.0 M),
2,2-dimethyloxirane (1.2 mL, 14 mmol) was added
dropwise over several minutes. The reaction mixture
was then allowed to slowly warm to room tempera-
ture and stirred over night at room temperature before
quenching with 10% HCl. The organic layer was ex-
tracted with ether, dried over anhydrous sodium sul-
fate, and the solvent was removed in vacuo to give
the crude product as yellow oil, yield 1.6 g (79%)
[7,10a,b], which was purified by silica gel column
chromatography using EtOAc–hexane as an eluent sol-
vent (Rf = 0.3 EtOAc:hexane 1:9). 1H NMR (CDCl3):
δ 1.28 (s, 6H, 2CH3), 1.59 (br, 1H, OH), 1.83 (m,
2H, CH2), 3.05 (m, 2H, CH2), 7.20 (t, 1H, J = 7.2),
7.30 (t, 2H, J = 7.6), 7.36 (d, 2H, J = 7.9). 13C NMR
(CDCl3): δ 28.5 (CH2), 29.3 (2CH3), 42.7 (CH2), 70.8
(C-OH), 125.9 (CH), 128.8 (2CH), 128.9 (2CH), 136.4
(C). MS: m/z = 196 (M+, 80%), 109 (100%).
1
60–80 (1:9)] [7]. H NMR (CDCl3): δ 1.34 (s, 6H,
2CH3), 2.03 (t, 2H, J = 6.2), 2.10 (br, 1H, OH), 4.21
(t, 2H, J = 6.2), 6.94 (d, 2H, J = 8.2), 6.99 (t, 1H,
J = 7.3), 7.29 (t, 2H, J = 7.9). 13C NMR (CDCl3): δ
30.2 (2CH3), 42.2 (CH2), 65.6 (CH2), 71.0 (C OH)
115.1 (2CH), 121.6 (CH), 130.1 (2CH), 159.1 (C O).
MS: m/z = 180 (M+, 30%), 94 (100%).
3-Phenylsulfanyl-1-propanol (2a). A solution of 3-
phenylsulfanylpropanoic acid [8a] (3.64 g, 0.02 mol)
in dry ether (20 mL) was added dropwise to a stirred
slurry of lithium aluminum hydride (0.76 g, 20 mmol)
in dry ether (100 mL) at rate sufficient to maintain
reflux. After the addition was complete, the reaction
was refluxed for 4 h and then cooled to room tempera-
ture. The excess hydride was decomposed by dropwise
addition of water, and the resulting white suspension
was filtered off. The filtrate was dried over anhydrous
sodium sulfate, and the solvent was removed in vacuo
to afford yellow oil; yield 2.5 g (71%), which was puri-
fied by vacuum distillation (lit. [8b] bp 105◦C, 0.1 mm).
1H NMR (CDCl3): δ 1.90 (quint, 2H, J = 6.4, CH2),
2.10 (br, 1H, OH), 3.05 (t, 2H, J = 7.0, CH2), 3.78 (t,
2H, J = 6.0, CH2), 7.20 (t, 1H, J = 7.2), 7.30 (t, 2H,
J = 7.6), 7.38 (d, 2H, J = 7.8). 13C NMR (CDCl3):
δ 30.8, 32.2, 61.9, 126.6, 129.5, 129.8, 136.8. MS:
m/z = 168 (M+, 80%).
4-Phenyl-1-butanol (3). This compound was pur-
chased from Merck (Schuchardt OHG, Germany).
2-Methyl-3-phenyl-1-propanol (4). To a suspension
of lithium aluminum hydride (0.20 g, 5 mmol) in dry
THF (20 mL), 2-methyl-3-phenylpropionic acid [11]
(0.33 g, 2 mmol) in THF (10 mL) was added dropwise
with stirring at rate sufficient to keep the solution re-
fluxing. After complete addition, the reaction mixture
was heated under reflux for 3 h. The mixture was cooled
toroomtemperature, hydrolyzedwithwater(5mL)and
extracted with ethyl acetate (3 × 20 mL), dried with an-
hydrous sodium sulfate, and the solvent was evaporated
in vacuo to give 2.5 g (83%) as pure product [12]. 1H
4-Phenylsulfanyl-2-butanol (2b). To a solution of 4-
phenylsulfanyl-2-butanone [9] (1.82 g, 10 mmol) in
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54
DIB ET AL.
NMR (CDCl3): δ 0.92 (d, 3H, J = 6.6, CH3), 1.98 (m,
1H, CH), 2.41 (dd, 1H, J = 13.3, 8.0), 2.78 (dd, 1H,
J = 13.3, 6.0), 3.24 (br, 1H, OH), 3.49 (m, 1H), 3.56
(m, 1H), 7.20 (d, 2H, J = 7.3) 7.23 (t, 1H, J = 7.6).
7.61 (t, 2H, J = 7.5). MS: m/z = 150 (M+, 80%), 92
(90%).
to characterize the consistituents of the pyrolysate. Per-
cent yields were determined from 1H NMR.
Kinetics and Data Analysis
Stock solution (7 mL) was prepared by dissolving 5–
10 mg of the substrate in acetonitrile to give a concen-
tration of 1 × 103 to 2 × 103 ppm. An internal standard
was then added, and the amount of which was adjusted
to give the desired peak area ratio of substrate to stan-
dard (2.5:1). The solvent and standard were selected
to be stable under the conditions of pyrolysis, and be-
cause they do not interact or react with either substrate
or product. The internal standards used in the present
study were chlorobenzene, 1,3-dichlorobenzene, and
1,2,4-trichlorobenzene. Each mixture was filtered be-
fore use to ensure a homogeneous solution.
2-Methyl-3-phenylpropionic Acid (5). A mixture of
diethyl 2-benzyl-2-methylmalonate [11a] (5.0 g, 19
mmol), KOH (5g, 89 mmol) in water (20 mL) was
heated under reflux for 20 h. The reaction mixture
was then diluted with water (20 mL), and the result-
ing ethanol was removed in vacuo. The residue was
cooled in ice, and conc. H2SO4 (5 mL) was added.
The reaction mixture was heated under reflux for 3 h,
cooled to room temperature, and extracted with ether
(3 × 50 mL). The ethereal extract was then dried over
anhydrous sodium sulfate, and the solvent was then re-
moved in vacuo to give the crude product as colorless
oil, yield 2.4 (77%), which was purified by column
chromatography using ether–pet. ether 60–80 as an
eluent, (Rf = 0.8 ether:pet. ether 60–80 1:1) [11b]. 1H
NMR (CDCl3): δ 1.22 (d, 3H, J = 6.9,CH3), 2.72 (dd,
1H, J = 8.0, 13.3, CH), 2.80 (sext, 1H, J = 6.9 CH),
3.12 (dd, 1H, J = 6.0, 13.3,CH), 7.23 (d, 2H, J = 8.0),
7.27 (t, 1H, J = 7.0), 7.34 (t, 2H, J = 7.3),10.5 (br, 1H,
OH).13C NMR (CDCl3): δ 16.4 (CH3), 39.2 (CH2),
41.2 (CH), 126.4 (CH), 128.4 (2CH), 129.0 (2CH),
138.9 (C), 182.3 (CO). MS: m/z = 164 (M+, 50%), 92
(90%).
The ratio of the amount of substrate with respect to
the internal standard was calculated from the ratio of
the substrate peak area to the peak area of the internal
standard. The kinetic rate was obtained by tracing the
rate of disappearance of the substrate with respect to the
internal standard as follows: An aliquot part (0.2 mL)
of each solution, containing the substrate and the in-
ternal standard, was pipetted into the pyrolysis tube,
which was then cooled in liquid nitrogen and sealed
at reduced pressure (0.28 mbar). The tube was then
placed in the pyrolyzer for 6 min under nonthermal
conditions (ambient temperature). A sample was then
analyzed using Water’s HPLC consisting of Water’s
HPLC pump (model 515) and Water’s UV/vis detec-
tor (model 2487). HPLC columns used were Supelco
LC-8, LC-18, and ABZ+ (25-cm length, 4.6-mm in-
ternal diameter, and 54-µm pore size). The wavelength
of the detector λ = 250–260 nm was used to calculate
the standardization value (A0). Several HPLC mea-
surements were obtained with an accuracy of ≥2%.
The temperature of the pyrolysis block was then raised
until ca. 10% pyrolysis of the substrate was deemed
to occur over 900 s interval; the HPLC measurements
were carried out as above. This process was repeated
after each ca. 10◦C rise in the reaction temperature un-
til >95% reaction was achieved. The relative ratios of
the sample and the internal standard (A) at each reac-
tion temperature was calculated for a minimum of two
kinetic runs, made at each of these temperatures that
were in agreement within 2% to ensure reproducible
values of A [13,14]. The rate coefficients at T = 600 K
were calculated using the kinetic relation log k = log
A − Ea/2.303 RT, where log A and Ea are the Arrhe-
nius parameters obtained for the substrates. The limits
of error in Table I represents the correction of statisti-
cal values. A typical Arrhenius plot for the pyrolysis
of compound 1a is shown in Fig. 1.
Product Analysis
The reactor used for kinetic and product analysis is a
Chemical Data System (CDS) custom-made pyrolyzer
consisting of an insulated aluminum alloy block fit-
ted with a platinum resistance thermocouple connected
to a Comark microprocessor thermometer for reac-
tor temperature readout, accurate to <0.5◦C. The al-
loy was chosen for its high-thermal conductivity and
low-temperature gradient, and may be heated for up
to 530◦C. The temperature of the reactor was con-
trolled by means of a Eurotherm 093 precision temper-
ature regulator to provide 0.1◦C incremental change.
The reaction tubes were Pyrex, 8-cm length for ki-
netic runs and 12 cm for product analysis, having
internal and outside diameters of 1.5 and 1.7 cm,
respectively.
A quantity of 0.2 g of the substrate was introduced
in the reaction tube, cooled in liquid nitrogen, sealed
under vacuum (0.06 mbar), and put in the pyrolyzer
for 900 s at a temperature comparable to that used for
complete pyrolysis in the kinetic studies. The content
of the tubes was then analyzed by 1H NMR and LCMS
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55
Table I Rate Coefficients, k/(s−1), and Arrhenius Parameters of Compounds 1–5
Compound
T (K)
104 k (s−1
)
log A (s−1
)
E (k J mol−1
)
K
(s−1
)
600 K
a
1a
681.85
704.15
714.15
724.45
734.65
744.15
607.45
626.05
644.65
653.25
700.55
719.25
734.05
509.25
519.15
539.25
559.25
579.05
691.75
709.95
718.85
728.15
737.55
746.95
632.45
644.85
658.15
684.15
710.55
538.85
564.75
578.15
591.15
604.15
614.15
731.65
744.45
757.75
770.45
783.65
712.25
722.15
734.65
745.85
624.40
634.10
639.00
644.00
649.00
653.60
658.20
1.615
4.931
7.530
12.61
19.63
29.96
0.738
1.079
2.011
4.088
8.749
11.35
19.85
1.042
1.959
8.855
15.64
40.34
2.658
6.164
9.393
14.76
22.34
33.93
2.264
3.563
6.305
15.43
32.37
2.297
7.219
10.09
17.87
25.89
39.87
4.161
6.764
10.47
16.02
26.06
3.424
4.942
8.887
14.98
8.430
16.06
19.09
23.45
26.98
36.47
49.27
11.33 0.12
197.41 1.72
1.39 × 10−6
1b
4.065 0.31
95.55 4.02
5.52 × 10−5
1c
2a
9.127 0.94
11.44 0.143
127.33 9.76
199.10 1.97
1.10 × 10−2
1.31× 10−6
2b
2c
6.984 0.26
6.215 0.27
128.62 3.25
101.58 3.06
6.14 × 10−5
2.36 × 10−3
3
8.48 0.196
166.21 2.85
1.04 × 10−6
4
5
10.09 0.45
12.44 0.39
185.10 6.24
173.63 4.79
9.63 × 10−7
2.10 × 10−3
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DIB ET AL.
Figure 1 Arrhenius plot for the pyrolysis of 2-phenoxy-1-ethanol 1a.
namely (3-phenoxy) primary, secondary, and tertiary
alcohols, reflects the influence of the methyl group
on the reactivity of alcohols 1a–c. Tertiary alcohol is
more reactive than its primary and secondary coun-
terparts by a factor of 8 × 103 and 39, respectively.
This appreciable rate enhancement 3◦ > 2◦ > 1◦ is
systematic and consistent with the stabilizing electron-
donating effects of the methyl groups on the stability
of the transition state by stabilizing the partial charge
development at the carbon atom.
RESULTS AND DISCUSSION
Synthesis
The required substrates 1, 2, 4, and 5 were prepared and
fully characterized by NMR and MS, as described in
the Experimental section. Compound 3 was purchased
from Merck.
Kinetic Analysis
The kinetic data on the gas-phase elimination of com-
pounds 1–5 are summarized in Table I. The reactions
followed first-order kinetics. Each rate constant at a
given temperature represents an average of at least two
measurements in agreement within 2% rate spread.
The Arrhenius plots were strictly linear over >95%
reaction.
3-Phenylsulfanyl-1-propanol (2a), 4-Phenylsulfanyl-
2-butanol (2b), and 2-Methyl-4-phenylsulfanyl-2-
butanol (2c). The products of the gas-phase elimi-
nation reaction of 2a–c were identified as thiophe-
nol together with CH2O from 2a, CH3CHO from 2b,
and (CH3)2CO from 2c. This indicates similar ther-
mal behavior with their 3-phenoxy counterparts 1a–c
(Scheme 5).
The reactivity order of this series 2a–c parallels
that of 1a–c. The most reactive substituted alcohol
is the tertiary. It is 2 × 103 and 38 times more reactive
than its primary and secondary analogues, respectively.
This lend support to the electron-donating effect of the
methyl group in stabilizing the TS and, hence, enhanc-
ing the reaction rate.
Reaction Products and Mechanism
To ensure complete substrate pyrolysis, reactions for
product analysis were carried out at temperatures ex-
ceeding those required for 98% reaction as per kinetic
measurements. Absence of substrates from product
mixtures was further confirmed by inspection of data
on reaction products; the pyrolysates were analyzed us-
ing LCMS and 1H NMR. Typical representative results
are shown in Table II.
4-Phenyl-1-butanol (3). The pyrolysate from this
compound consists of toluene and CH2O, suggest-
ing a similar mechanistic pathway to its oxygen and
thio analogues 1 and 2. The rate constant at 600 K is
3-Phenoxy-1-propanol (1a), 4-Phenoxy-2-butanol
(1b), and 2-Methyl-4-phenoxy-2-butanol (1c). The
pyrolysates from 1a–c were ascertained to be phenol
together with CH2O from 1a, CH3CHO from 1b, and
(CH3)2CO from 1c. These results could be fully ex-
plained on the basis of a reaction mechanism with a
cyclic six-membered TS (Scheme 4).
1.04 × 10−6 s−1
.
To facilitate comparison, the rate constants, k/(s−1),
of gas-phase elimination reaction at 600 K of com-
pounds 1–3 are compiled in Scheme 6.
The analysis of the rate data shows that the influence
of the aryl substituents ( OPh, SPh, and CH2Ph) on
the reactivity of the present alcohol is not significant in
magnitude. The hydroxyl (OH) function of the alcohols
Analysis of the kinetic results reported in Scheme 4,
using the proposed reaction mechanism for 1a–c,
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Table II Pyrolysis Products of Compounds 1–5 and Percentage Yield
Number
Compound
Product (% Yield)
CH2O (22)
1a
PhOH (77)
PhOH (73)
CH2 CH2
CH2 CH2
1b
CH3CHO (29)
1c
PhOH (81)
(CH3)2CO (17)
CH2 CH2
2a
2b
PhSH (82)
PhSH (87)
CH2O (27)
CH2 CH2
CH2 CH2
CH3CHO (16)
2c
PhSH (91)
(CH3)2CO (22)
CH2 CH2
3
4
PhCH3 (90)
PhCH3 (88)
CH2O (23)
CH2 CH2
CO
CH3CHO (13)
5
PhCH3 (79)
CH3CHO (13)
CO
1–3 is equivalent; hence, the influence of this function
on the molecular reactivity of alcohols 1–3 should be
equivalent. This lend support to the importance of the
OH bond breaking (Scheme 6) to the overall reactivity
of the alcohols under study; the effect of this bond on
molecular reactivity provides an explanation for the
comparable rate constant of the gas-phase elimination
reaction of alcohols 1–3.
On the other hand, the TS proposed involves C X
bond breaking, one would expect that compound 3
(X = CH2) would be less reactive than compound 1
(X = O) and 2 (X = S). Although it is not significant, it
is of interest here to note that the results of the theoret-
ical ab initio calculations show gas-phase thermody-
namic stabilities of the arene fragments to be in order
∼
of PhNH2 > PhCH3
PhOH > PhSH [4], so the
=
thermodynamic stability of PhCH3 in the gas-phase
elimination reaction compensates the lower polarity of
the (C X) bond in compound 3. It is worth mentioning
that the rate constant of the gas-phase elimination reac-
tion of 3-anilino-1-propanol at 600 K is 5.95 × 10−5
.
This is 42- to 45-times faster than the rate constant
of the gas-phase elimination of phenol and toluene
from compounds 1 and 3, respectively. This confirms
further the importance of thermodynamic stability of
arene produced from the reaction rate.
Scheme 4 Gas-phase elimination of 1a–c and the rate
constant at 600 K.
Scheme 5
International Journal of Chemical Kinetics DOI 10.1002/kin

58
DIB ET AL.
2-phenoxy-1-propanol. 2-Phenoxy-1-propanol and 2-
N-phenylamino-1-propanol were compared with the
reactivities of 2-phenoxypropanoic acid and 2-N-
phenylaminopropanoic acid, respectively [1].
BIBLIOGRAPHY
1. Al-Awadi, N. A.; Kaul, K.; El-Dusouqui, O. M. E. J
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1995, 27, 849.
3. Al-Awadi, S.; Abdallah, M.; Hasan, M.; Al-Awadi, N.
A. Tetrahedron 2004, 60, 3045–3049.
Scheme 6 Rate constants at 600 K for compounds 1–3 and
3-N-phenyl aminopropanol.
4. Al-Awadi, S. A.; Abdallah, M. R.; Dib, H. H.; Ibrahim,
M. R.; Al-Awadi, N. A.; El-Dusouqui, O. M. E. Tetra-
hedron 2005, 61, 5769–5777.
5. (a) Lai, S. M. F.; Orchison, J. J. A.; Whiting, D. A.
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Chem Soc 2003, 125(29), 8696–8697; (b) Tanikaga,
R.; Yamada, S.; Nishikawa, T.; Matsui, A. Tetrahedron,
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Scheme 7
7. Waugh, K. M.; Berlin, K. D.; Ford, W. T.; Holt, E. M.;
Carrol, J. P.; Schomber, P. R.; Thompson, M. D.; Schiff,
L. J. J Med Chem 1985, 28, 116–124.
Scheme 8
8. (a) Ahn, Y.; Cohen, T. J Org Chem 1994, 59, 3142–
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Tetrahedron, 1998, 54(31), 8933–8940.
2-Methyl-3-phenyl-1-propanol (4). Product analysis
revealed formation of PhCH3 and CH3CHO from com-
pound 4, at a rate constant of 9.63 × 10−7 at 600 K.
Identity of the products suggests the mechanistic path-
way that is shown in Scheme 7.
To prove this mechanism, we have prepared and
pyrolyzed 2-methyl-3-phenylpropanoic acid 5, and the
pyrolysates were ascertained to consist of toluene
PhCH3 and CH3CHO. This suggests a mechanis-
tic pathway similar to 2-methyl-3-phenylpropanol 4
(Scheme 8) with a rate constant of 9.12 × 10−4 at
600 K. This high reactivity of 3-phenylpropanoic acid
5 of 947 relative to 2-methyl-3-phenylpropyl alcohol
4 is attributed to the increase in the acidity of the in-
cipient hydrogen by replacing an alcohol hydroxyl hy-
drogen by a carboxylic acid proton; such an increase
in the rate of reaction was observed when the reac-
tivity of 2-phenoxypropanoic acid was compared with
10. (a) Jensen, A. W.; Manczuk, J.; Nelson, D.; Caswell, O.
J Heterocyclic Chem 2000, 37, 1527–1531; (b) Foubelo,
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International Journal of Chemical Kinetics DOI 10.1002/kin