The kinetics and mechanism of the homogeneous, unimolecular gas-phase elimination of 2-(4-substituted-phenoxy)tetrahydro-2H-pyranes

Article abstract of DOI:10.1002/poc.3778

The gas-phase elimination kinetics of tetrahydropyranyl phenoxy ethers: 2-phenoxytetrahydro-2H-pyran, 2-(4-methoxyphenoxy)tetrahydro-2H-pyran, and 2-(4-tert-butylphenoxy)tetrahydro-2H-pyran were determined in a static system, with the vessels deactivated with allyl bromide, and in the presence of the free radical inhibitor toluene. The working temperature and pressure were 330 to 390°C and 25 to 89?Torr, respectively. The reactions yielded DHP and the corresponding 4-substituted phenol. The eliminations are homogeneous, unimolecular, and satisfy a first-order rate law. The Arrhenius equations for decompositions were found as follows: 2-phenoxytetrahydro-2H-pyran log k₁ (s^?1)?=?(14.18?±?0.21)???(211.6?±?0.4)?kJ?mol^?1 (2.303 RT)^?1 2-(4-methoxyphenoxy)tetrahydro-2H-pyran log k₁ (s^?1)?=?(14.11?±?0.18)???(203.6?±?0.3)?kJ?mol^?1 (2.303 RT)^?1 2-(4-tert-butylphenoxy)tetrahydro-2H-pyran log k₁ (s^?1)?=?(14.08?±?0.08)???(205.9?±?1.0)?kJ?mol^?1 (2.303 RT)^?1. The analysis of kinetic and thermodynamic parameters for thermal elimination of 2-(4-substituted-phenoxy)tetrahydro-2H-pyranes suggests that the reaction proceeds via 4-member cyclic transition state. The results obtained confirm a slight increase of rate constant with increasing electron donating ability groups in the phenoxy ring. The pyran hydrogen abstraction by the oxygen of the phenoxy group appears to be the determinant factor in the reaction rate.

Full text of DOI:10.1002/poc.3778

Received: 25 July 2017
DOI: 10.1002/poc.3778
Revised: 11 September 2017
Accepted: 12 September 2017
R E S E A R C H A R T I C L E
The kinetics and mechanism of the homogeneous,
unimolecular gas‐phase elimination of 2‐(4‐substituted‐
phenoxy)tetrahydro‐2H‐pyranes
Alvaro Alvarez‐Aular
| Loriett Cartaya | Alexis Maldonado | David Santiago Coll |
Gabriel Chuchani†
Centro de Química, Instituto Venezolano
de Investigaciones Científicas (IVIC),
Caracas, Venezuela
Abstract
The gas‐phase elimination kinetics of tetrahydropyranyl phenoxy ethers: 2‐
phenoxytetrahydro‐2H‐pyran, 2‐(4‐methoxyphenoxy)tetrahydro‐2H‐pyran, and
2‐(4‐tert‐butylphenoxy)tetrahydro‐2H‐pyran were determined in a static system,
with the vessels deactivated with allyl bromide, and in the presence of the free
radical inhibitor toluene. The working temperature and pressure were 330 to
390°C and 25 to 89 Torr, respectively. The reactions yielded DHP and the cor-
responding 4‐substituted phenol. The eliminations are homogeneous,
unimolecular, and satisfy a first‐order rate law. The Arrhenius equations for
decompositions were found as follows:
Correspondence
Alvaro Alvarez‐Aular, Centro de Química,
Instituto Venezolano de Investigaciones
Científicas (IVIC), Apartado, 21827
Caracas, Venezuela.
Email: aalvarez@ivic.gob.ve
2‐phenoxytetrahydro‐2H‐pyran
log k1 (s−1) = (14.18 0.21) − (211.6 0.4) kJ mol−1 (2.303 RT)−1
2‐(4‐methoxyphenoxy)tetrahydro‐2H‐pyran
log k1 (s−1) = (14.11 0.18) − (203.6 0.3) kJ mol−1 (2.303 RT)−1
2‐(4‐tert‐butylphenoxy)tetrahydro‐2H‐pyran
log k1 (s−1) = (14.08 0.08) − (205.9 1.0) kJ mol−1 (2.303 RT)−1
The analysis of kinetic and thermodynamic parameters for thermal elimination
of 2‐(4‐substituted‐phenoxy)tetrahydro‐2H‐pyranes suggests that the reaction
proceeds via 4‐member cyclic transition state. The results obtained confirm a
slight increase of rate constant with increasing electron donating ability groups
in the phenoxy ring. The pyran hydrogen abstraction by the oxygen of the
phenoxy group appears to be the determinant factor in the reaction rate.
1 | INTRODUCTION
examined in a static system and in the temperature range
of 389 to 530°C. The kinetics of thermal decomposition for
The gas‐phase thermal decomposition of ketal substrates
ethylal, dimethyl acetal, and diethyl acetal were reported.
The results for ethylal suggest that it proceeds via a radical
process. On the other hand, dimethyl and diethyl acetal
were considered to undergo rearrangement reactions.
Specifically, dimethyl acetal decomposed up to 50%
was first studied by Molera et al.[1] The reactions were
†Deceased April 30, 2017. To the memory of Dr. Gabriel Chuchani and
his nonstop 59 years of research in chemistry at IVIC.
J Phys Org Chem. 2017;e3778.
wileyonlinelibrary.com/journal/poc
Copyright © 2017 John Wiley & Sons, Ltd.
1 of 7
https://doi.org/10.1002/poc.3778

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ALVAREZ‐AULAR ET AL.
reaction, in parallel reactions, obeys a first‐order rate law
and is depicted in reaction 1.
SCHEME 1 Hα and Hβ of ketals
Rosas et al[7] examined the gas‐phase thermal
elimination of 2‐methoxytetrahydro‐2H‐pyran shown
in reaction 3. The elimination products are 3,4‐
dihydro‐2H‐pyran (DHP) and methanol. The tempera-
ture and pressure ranges were 400 to 450°C and 25
to 83 torr. The reaction proved to be homogeneous,
unimolecular and follows a first‐order rate law. The
rate constant is represented by the equation: log k1
(1)
Further studies by Molera et al,[2,3] the gas‐phase ther-
mal decompositions of ketals: methylal (472‐520°C)[2] and
propylene methylal (512−572°C)[3] have been reported.
The analyses of the products suggest that these reactions
proceed via radical mechanisms. In addition, a new exam-
ination of methylal pyrolysis in a flow system in the tem-
perature range of 600 to 732°C[4,5] has shown very
complex radical processes. The use of free radical inhibi-
tors has been unable to avoid those chain reactions.
The presence of a hydrogen atom attached at Cα or Cβ
in ketal structures (Scheme 1) and the leading role in the
transition state mechanism of gas‐phase elimination
kinetics for these compounds was studied.[6] This can be
explained based on the electronic effects of substituent Z
at β‐carbon that have a direct effect upon the ability of
the alkoxy group to abstract Cβ hydrogen, and it is depen-
dent of the polarization degree on the Cβ−H bond.
This study[6] examined the gas‐phase elimination
kinetics of 2,2‐diethoxyethyl amine and 2,2‐diethoxy‐N,
N‐diethyl ethylamine, in seasoned reaction vessels, in
the temperature range of 320 to 380°C and pressure range
of 40 to 150 torr. The results have determined the reac-
tions as homogeneous, unimolecular, and follow a first‐
order rate law. The elimination involves parallel processes
described in reaction 2. The first reaction produces etha-
nol and 2‐ethoxyetheneamine with a subsequent decom-
position to yield ethylene, CO, and the corresponding
amine. The second parallel step produced, in small
amounts, ethane and ethyl ester of an α‐amino acid.
According to this research, the hydrogen atom attached
to Cβ assists the elimination of ethanol.
(s−1) = (13.95
0.15) − (223.1
2.1) kJ mol−1
(2.303 RT)−1. The reaction occurs through a 4‐member cyclic
transition state. Thermodynamic parameters suggest that
the rate determining factor is the polarization of C−OCH3.
(3)
SCHEME 2 Structure of cyclic ketals
SCHEME 3 Selected signals from substrate and product to
estimate the degree of reaction through integration ratios
TABLE 1 Stoichiometry of the reactionsa
% DHP
Temp Time % DHP
(°C)
(min) (Pressure) NMR)b
(1H‐
Substrate
2‐Phenoxytetrahydro‐2H‐ 371.6
pyran
4
7
14
23.1
34.6
57.7
63.4
23.1
33.8
57.1
62.4
22
2‐(4‐Methoxyphenoxy)
tetrahydro‐2H‐pyran
350.7
360.0
5
33.3
56.7
60.0
73.3
34.1
55.9
57.8
72.1
11
13
19
2‐(4‐Tert‐butylphenoxy)
tetrahydro‐2H‐pyran
9
45.9
64.9
81.1
89.2
45.1
64.3
80.6
88.9
15
23
32
(2)
There is a limited number of previous works about the
gas‐phase thermal elimination of ketals containing a ring
in its structure (Scheme 2).
aSeasoned vessel and in the presence of toluene inhibitor.
bIn CDCl3.

ALVAREZ‐AULAR ET AL.
3 of 7
The aim of the present work on the thermal elimina-
tion kinetics of 2‐phenoxytetrahydro‐2H‐pyran, 2‐(4‐
methoxyphenoxy)tetrahydro‐2H‐pyran, and 2‐(4‐tert‐
butylphenoxy)tetrahydro‐2H‐pyran is to obtain more
information about the decomposition reaction of ketals
with 1 of the oxygen atom belonging to a cyclic system
as a heterocyclic compound. Moreover, the substituent
effect in position 4 of the phenoxy ring and their impact
on the rate coefficients for these reactions was studied.
2 | RESULTS AND DISCUSSION
The gas‐phase thermal elimination of tetrahydropyranyl
phenoxy ethers was carried out in a static system seasoned
with allyl bromide and inhibited with toluene. The
decomposition is depicted in reaction 4.
(4)
TABLE 2 Ratio of final (Pf) to initial pressure (P0)a
The extent of decomposition obtained via pressure
measurements was compared with the integration
analyses of H‐NMR signals taken from these reactions
T (° P0
Pf
Pf/
Substrate
C) (torr) (torr) P0 Average
1
2‐Phenoxytetrahydro‐
2H‐pyran
348.3 35
360.8 41
371.6 38
378.8 30
388.0 37
69
78
75
58
72
1.97 1.95 0.03
1.90
1.97
1.93
1.95
at different reaction times (Scheme 3).
The degree of formation of DHP as a product of
thermal decomposition, in a given time of reaction for
all substrates, is defined by Equation 1.
2‐(4‐Methoxyphenoxy) 329.7 44
tetrahydro‐2H‐pyran 339.1 48
350.7 30
87
95
59
61
91
1.98 1.97 0.01
1.98
1.97
1.97
1.98
I
HB
%
¼
×100
(1)
DHP
360.1 31
368.9 46
I
þ I
HA
HB
2‐(4‐Tert‐butylphenoxy) 340.0 42
tetrahydro‐2H‐pyran 350.1 33
360.0 42
83
65
83
91
80
1.98 1.98 0.01
where IH and IH are the integration values of the
A
B
1.97
1.98
1.98
2.00
selected hydrogen signals of substrate and DHP, respec-
tively. These integration values are taken from spectra at
certain evolution time of the thermal elimination.
The experimental stoichiometry for the elimination of
tetrahydropyranyl ethers was examined by comparing the
370.5 46
380.3 40
aSeasoned vessel and in the presence of toluene inhibitor,
TABLE 3 Homogeneity of the elimination reactionsa
Substrate
Temp (°C)
S/V (cm−1
)
104 k (s−1 b
)
104 k (s−1 c
)
2‐Phenoxytetrahydro‐2H‐pyran
371.6
1
4.4
23.01 1.72
31.52 3.83
12.08 0.33
12.14 0.12
2‐(4‐Methoxyphenoxy)tetrahydro‐2H‐pyran
2‐(4‐tert‐butylpenoxy)tetrahydro‐2H‐pyran
350.7
360.0
1
4.4
31.44 2.26
43.59 7.96
11.98 0.14
12.01 0.17
1
4.4
31.16 7.15
48.47 11.37
12.52 0.23
12.20 0.08
aPresence of toluene inhibitor.
bClean Pyrex vessel.
cDeactivated Pyrex vessel with allyl bromide.
TABLE 4 Effect of inhibitor on ratesa
Substrate
T (°C)
Parametersb,c
Values
2‐Phenoxytetrahydro‐2H‐pyran
371.6
Pinh/P0
0
6.79
0.5
10.78
0.8
12.08
1.5
11.91
2.4
12.07
104 k (s−1
)
)
)
2‐(4‐Methoxyphenoxy)tetrahydro‐2H‐pyran
2‐(4‐Tert‐butylphenoxy)tetrahydro‐2H‐pyran
350.7
Pinh/P0
104 k (s−1
0
0.8
14.05
1.0
11.89
2.1
11.98
2.7
11.93
20.15
360.0
Pinh/P0
104 k (s−1
0
6.58
1.6
12.42
2.1
12.25
2.3
12.16
4.1
11.93
aSeasoned vessel.
bP0 = pressure of the substrate.
cPinh = pressure of the inhibitor toluene.

4 of 7
ALVAREZ‐AULAR ET AL.
TABLE 5 Invariability of rate coefficient with initial pressure of
the substratea
amount of elimination of the substrates. It was calculated
not only from pressure measurements but also from H‐
1
NMR data analyses of substrates and DHP (Table 1).
Additional evidences of stoichiometry of reaction 4
were obtained by carrying out the elimination process
until no further pressure increase was observed. The
mean values on ratio of final pressure (Pf), to the initial
pressure (P0), were found for each substrate at 5 different
temperatures and 10 half‐lives (Table 2). The theoretical
stoichiometry must to satisfy Pf = 2 P0.
The homogeneity of these decomposition reactions, in
the presence of the radical inhibitor toluene, was examined
by making several runs in a reaction vessel with surface‐to‐
volume ratio of about 4.4 relative to that of the normal
vessel, which is equal to 1. The packed and unpacked clean
Pyrex vessel had a significant effect on the rates. Neverthe-
less, when the packed and unpacked vessels are seasoned
with allyl bromide, no significant effect on the rates
coefficients of these substrate eliminations was observed
(Table 3). Bearley et al[8] show that the wall of the reaction
chamber had some influence on the determination of rate
on the thermal elimination of tert‐butyl chloride, but
the experiments became reproducible when the surface
of the vessel was covered with a polymer layer formed by
thermal decomposition of allyl bromide.[9]
Temp P0
(°C)
Substrate
(Torr) 104 k (s−1
)
2‐Phenoxytetrahydro‐2H‐pyran
371.6 25
12.07 0.45
33
34
42
52
11.91 0.42
12.08 0.49
11.88 0.48
11.91 0.39
2‐(4‐Methoxyphenoxy)tetrahydro‐ 350.7 20
11.93 0.18
11.87 0.26
11.89 0.38
11.93 0.20
11.90 0.21
2H‐pyran
27
30
54
59
2‐(4‐Tert‐butylphenoxy)
tetrahydro‐2H‐pyran
360.0 21
12.12 0.27
12.09 0.17
12.08 0.12
12.23 0.16
12.26 0.32
39
42
46
53
aSeasoned vessel and in the presence of toluene inhibitor.
Kinetic and thermodynamic parameters (Table 7)
were determined by using the Arrhenius relationship
k = Ae−Eact/RT, Eyring equation k = kBT/h e−ΔG≠/RT
,
ΔS≠ = [lnA − ln(enkBT/h)]R. Activation enthalpy is
expressed as ΔH≠ = Eact − nRT and ΔG≠ = ΔH≠ − TΔS≠.
The analysis of the data in Table 7 explains the
elimination rates of 2‐phenoxytetrahydro‐2H‐pyran,
The kinetic runs for the thermal elimination of these
substrates were carried out at different ratios of substrate
pressures (P0), to the radical chain suppressor pressures
(Pinh). The effect of different proportions of toluene in
the process is shown in Table 4.
TABLE 6 The dependence of rate coefficients with temperaturesa
The absence of toluene increases the rate coefficient
for 2‐(4‐methoxyphenoxy)tetrahydro‐2H‐pyran. The
probable cause for this behavior is the formation of
collateral gaseous products by complex radical processes.
However, the lack of toluene in the kinetic runs, for 2‐
phenoxytetrahydro‐2H‐pyran and 2‐(4‐tert‐butylphenoxy)
tetrahydro‐2H‐pyran, generates lower rate constants; this
may be due to the trend of products to polymerize.
Therefore, thermal decomposition of all substrates had
to be carried out in the presence of at least twice the
amount of inhibitor with respect to the substrate. Under
this condition, rate constants in all reactions remain with
no significant variation.
The rate constants were calculated according to
k1 = (2.303/t) log [P0/(2P0 − Pt)]. In all cases, constants
were found to be invariable with changes of the initial
pressure, P0 of the substrates (Table 5). Plots of log
(2P0 − Pt) against time t exhibit a linear behavior up to
80% of decomposition.
Temp (°
C)
Substrate
104 k (s−1
)
2‐Phenoxytetrahydro‐2H‐pyran
348.3
360.8
371.6
378.8
388.0
2.69 0.03
6.63 0.18
12.08 0.49
18.45 0.46
31.72 0.45
log k1 (s−1) = (14.18 0.2) − (211.6 0.4) kJ mol−1 (2.303 RT)−1
,
r = 0.9997
2‐(4‐Methoxyphenoxy)tetrahydro‐2H‐
pyran
329.7
339.1
350.7
360.1
368.9
3.14 0.09
5.78 0.02
11.98 0.44
21.35 0.79
37.82 0.24
log k1 (s−1) = (14.11 0.18) − (203.6 0.3) kJ mol−1 (2.303 RT)−1
r = 0.9998
,
2‐(4‐Tert‐butylphenoxy)tetrahydro‐2H‐
pyran
340.0
350.1
360.0
370.2
380.2
3.48 0.05
6.64 0.04
12.52 0.03
23.38 0.21
41.37 0.15
The dependence of the first‐order rate constant with
temperature leads Arrhenius equations (Table 6 and
Figure 1). All of these were determined by using the
least‐squares procedure and 90% confidence coefficient.
log k1 (s−1) = (14.08 0.08) − (205.9 1.0) kJ mol−1 (2.303 RT)−1
r = 0.9999
,
aSeasoned vessel and in the presence of toluene inhibitor.

ALVAREZ‐AULAR ET AL.
5 of 7
values for thermal eliminations of all tetrahydropyranyl
phenoxy ethers are smaller than reported previously by
Rosas7 (223.1 kJ mol−1) for the decomposition of 2‐
methoxy‐tetrahydro‐2H‐pyran, which have no phenyl
resonance structures.
The effect of the substituent on position 4 in the
phenoxy ring was also examined. A slight increase in
the rate coefficients with electronic donor groups can be
noticed with respect to 2‐phenoxytetrahydro‐2H‐pyran.
Although polarization Cαδ+....δ−OR exists as predicted by
Rosas,[7] it does not seem to be the determinant process
for the decomposition of the phenoxy‐pyran ketals.
Electron donor groups increase negative charge upon
the oxygen atom of the phenoxy group. They also increase
the double‐bond character between this oxygen and
the carbon atom belonging to phenyl ring, conferring it
carbonylic character.
Similarly, previous works by Rotinov and
coworkers[12] in the thermal decomposition of primary,
secondary, and tertiary β‐hydroxy‐ketones had shown
the importance of the carbonyl nucleophilic ability, to
facilitate these reactions. In most cases, substrates
undergo a retro‐aldolic‐type elimination. The acid nature
of the hydroxyl hydrogen is not determinant on the rate
increase but important in assistance during elimination.
The carbonyl nucleophilic ability enhances the abstrac-
tion of such hydrogen. Consequently, the ability of the
oxygen atom in the phenoxy group, to abstract the
dihydropyran hydrogen, appears to be the determinant
process on the reaction rate of the gas‐phase elimination
of tetrahydropyranyl phenoxy ethers in study (Reaction
6). Therefore, electron‐donor groups (R+) enhance
double‐bond and nucleophylic character on oxygen atom
FIGURE 1 Arrhenius plots for elimination rates of the following:
─♦─, 2‐phenoxytetrahydro‐2H‐pyran; ─▴─, 2‐(4‐tert‐
butylphenoxy)tetrahydro‐2H‐pyran; and ─
methoxyphenoxy)tetrahydro‐2H‐pyran
─, 2‐(4‐
•
2‐(4‐methoxyphenoxy)tetrahydro‐2H‐pyran, and 2‐(4‐tert‐
butylphenoxy)tetrahydro‐2H‐pyran. The results given in
Table 8, suggest a polar concerted 4‐member cyclic transi-
tion state type (Reaction 5). This conclusion is consistent
with the small positive entropy values, and the log
A > 13.4 for all substrates, in agreement to Benson[10,11]
proposal for the geometry of this transition states.
(5)
The presence of the phenoxy group as a substituent on
Cα of the ketal system (Reaction 5) stabilizes the polariza-
tion of the Cα―OPh bond, in the sense Cαδ+....δ−OPh by
resonance. This suggestion justifies the fact that the Eact
TABLE 7 Kinetic and thermodynamic parameters at 360°C
Substrate
Eact (kJ/mol)
log A
ΔH≠ (kJ/mol)
ΔS≠ (J/mol K)
ΔG≠ (kJ/mol)
2‐Phenoxytetrahydro‐2H‐pyran
211.6 0.4
14.18 0.21
205.8
11.77
198.4
2‐(4‐Methoxyphenoxy)tetrahydro‐2H‐pyran
2‐(4‐Tert‐butylphenoxy)tetrahydro‐2H‐pyran
203.6 0.3
205.9 1.0
14.11 0.18
14.08 0.08
198.4
200.7
11.80
9.9
191.0
194.7
TABLE 8 Reaction of phenol tetrahydropyranylation
Phenol
Phenol Mass (mol)
Product Mass
Yield (%) (Time)
Appearance
5.59 g (59.44 mmol)
10.35 g
98 (60 min)
Pale yellow liquid
5.11 g (41.19 mmol)
7.22 g (48.10 mmol)
8.27 g
97 (40 min)
Ambar liquid
Ambar liquid
10.96 g
97 (60 min)

6 of 7
ALVAREZ‐AULAR ET AL.
to abstract the hydrogen atom belonging to pyran ring. It
is proposed as the rate determining step.
10% NaOH solution was added (approx twice the volume
of the initial mixture), and the reaction mixture was
stirred for 30 minutes. Aqueous layer was discarded, fresh
NaOH solution was added again, and the mixture was
stirred for additional 30 minutes. The organic layer
was extracted and dried with Na2SO4. After filtration,
the solvent was evaporated from organic layer under
vacuum at room temperature. All products were obtained
in good yields (Table 8).
(6)
3 | CONCLUSIONS
4.2 | Spectral data
The products of the thermal elimination of 2‐
4.2.1 | 2‐Phenoxytetrahydro‐2H‐pyran
1H‐NMR δ ppm: 1.50 to 2.02 (m, 6H), 3.60 (m, 1H), 3.92
phenoxytetrahydro‐2H‐pyran,
tetrahydro‐2H‐pyran, and
2‐(4‐methoxyphenoxy)
2‐(4‐tert‐butylphenoxy)
tetrahydro‐2H‐pyran are DHP and the corresponding 4‐
substituted phenol. The reactions are unimolecular,
homogeneous, and follow first‐order reaction kinetics.
The observed rate constants are represented by the follow-
ing equations: for 2‐phenoxytetrahydro‐2H‐pyran: log k1
(m, 1H), 5.42 (t, 1H), 6.95 to 7.30 (m, 5H).
MS m/z: 178 (M+), 94 (PhOH+), 84 (DHP+).
4.2.2 | 2‐(4‐Methoxyphenoxy)tetrahydro‐
2H‐pyran
1H‐NMR δ ppm: 1.60 to 1.97 (m, 6H), 3.57 (m, 1H), 3.73 (s,
3H), 3.92 (m, 1H), 5.28 (t, 1H), 6.81 (m, 2H), 6.91 (m, 2H).
MS m/z: 208 (M+), 124 (H3CO―PhOH+), 84 (DHP+).
(s−1) = (14.18
RT)−1
log k1 (s−1) = (14.11
(2.303 RT)−1, and for 2‐(4‐tert‐butylphenoxy)tetrahydro‐
2H‐pyran: log k1 (s−1
(14.08 0.08) −
0.21) − (211.6
for 2‐(4‐methoxyphenoxy)tetrahydro‐2H‐pyran:
0.18) − (203.6
0.3) kJ mol−1
0.4) kJ mol−1 (2.303
,
)
=
(205.9 1.0) kJ mol−1 (2.303 RT)−1. The reaction mecha-
nisms pass through a polar concerted 4‐member cyclic
transition state. The phenoxy group as a substituent in
the tetrahydropyranyl phenoxy ethers stabilizes the polar-
ization of the transition state by resonance respect to anal-
ogous aliphatic substrates. Electron donor groups on
position 4 in the phenoxy ring increase the negative polar-
ization on the oxygen and the double‐bond character
between this oxygen and the carbon atom belonging to
phenyl ring, enhancing the hydrogen abstraction from
dihydropyran ring, which is proposed as determining
reaction process.
4.2.3 | 2‐(4‐Tert‐butylphenoxy)tetrahydro‐
2H‐pyran
1H‐NMR δ ppm: 1.32 (s, 9H), 1.88 (m, 3H), 2.03 (m, 3H),
3.61 (m, 1H), 3.95 (m, 1H), 5.41 (t, 1H), 7.02 (m, 2H),
7.33 (m, 2H).
MS m/z: 234 (M+), 150 (t‐Bu―PhOH+), 135
([CH3]2CH―PhOH+), 119 ([CH≡C―PhOH+), 85
(DHP―H+).
4.3 | Kinetics
4 | EXPERIMENTAL
The kinetics determinations were performed in a static
system as reported in previous works.[15,16] The rate coef-
ficients were calculated from pressure increase measured
manometrically. The temperature was controlled by a
resistance thermometer controller and an Omega Model
SSR280A45 solid‐state relay maintained within 0.2°C
and measured with a calibrated platinum‐platinum 13%
rhodium thermocouple with an Omega DP41‐TC/DP41‐
RTD high‐performance digital temperature indicator. No
temperature gradient was detected along the reaction ves-
sel. The substrates were injected directly into the reaction
vessel with a syringe through a silicone rubber septum.
The amount of substrates used for each run was ≈0.05
to 0.15 mL.
4.1 | Synthesis of tetrahydropyranyl
ethers
All reagents were acquired commercially (Aldrich). A
modified procedure from Anselmi[13] and Bognini[14]
was employed. A round bottom flask containing the
corresponding phenol (Table 8) and DHP in equimolar
amounts dissolved in dichloromethane was placed in an
ice bath. A catalytic amount of p‐toluenesulfonic acid
was added into the solution under vigorous stirring. After
5 minutes, the ice bath was removed. The reaction
mixtures were allowed to react, under stirring, for the
indicated time at room temperature (Table 8). Then, a

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The products of decomposition were evacuated from
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solution during 30 minutes under stirring. Organic layer
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Alvaro Alvarez‐Aular
1726
http://orcid.org/0000-0003-4042-
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How to cite this article: Alvarez‐Aular A,
Cartaya L, Maldonado A, Coll DS, Chuchani G. The
kinetics and mechanism of the homogeneous,
unimolecular gas‐phase elimination of 2‐(4‐
substituted‐phenoxy)tetrahydro‐2H‐pyranes. J Phys
Org Chem. 2017;e3778. https://doi.org/10.1002/
poc.3778
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