Neighbouring group participation in the gas-phase pyrolysis kinetics of 4-(N-methyl-N-phenylamino)-1-butyl acetate and 4-(N-phenylamino)-1-butyl acetate

Article abstract of DOI:10.1002/1099-1395(200005)13:5<266::AID-POC240>3.0.CO;2-Q

The pyrolysis kinetics of two phenylaminobutyl acetates were determined in a static system over the temperature range 359.7-399.6°C and the pressure range 23.8-95 Torr. The reactions, in vessels seasoned with allyl bromide and in the presence of the free radical inhibitor toluene, are homogeneous and unimolecular, and obey a first-order rate law. The overall rate coefficients are expressed by the following equations: for 4-(N-methyl-N-phenylamino)-1-butyl acetate, log[k₁ (s^-1)] = (13.92 ± 0.36) - (210.4 ± 4.5) kJ mol^-1 (2.303RT)^-1; and for 4-(N-phenylamino)-1-butyl acetate, log[k₁ (s^-1)] = (12.03 ± 0.43) - (188.3 ± 5.3) kJ mol^-1 (2.303RT)^-1. The decomposition of these substrates undergoes a parallel reaction. The predominant primary product, the corresponding heterocyclic product, appears to be the result of an anchimeric assistance of the amino substituent for a back-side displacement. This suggests that an incipient ion-pair type of mechanism may be operating during the process of elimination. The Arrhenius expressions for the parallel decomposition of each of the aminobutyl acetate substrates are presented and discussed. Copyright

Full text of DOI:10.1002/1099-1395(200005)13:5<266::AID-POC240>3.0.CO;2-Q

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2000; 13: 266–271
Neighbouring group participation in the gas-phase
pyrolysis kinetics of 4-(N-methyl-N-phenylamino)-1-
butyl acetate and 4-(N-phenylamino)-1-butyl acetate
Gabriel Chuchani,1* Nouria Al-Awadi,2 Rosa M. DomõÂnguez,1 Alexandra Rotinov,1 Armando Herize1
and Kamini Kaul2
1Centro de QuõÂmica, Instituto Venezolano de Investigaciones Cienti®cas (I VIC), Apartado 21827, Caracas 1020-A, Venezuela
2Department of Chemistry, University of Kuwait, P.O. Box 5969 Safat, 13060 Kuwait
Received 7 June 1999; revised 29 December 1999; accepted 4 January 2000
ABSTRACT: The pyrolysis kinetics of two phenylaminobutyl acetates were determined in a static system over the
temperature range 359.7–399.6°C and the pressure range 23.8–95 Torr. The reactions, in vessels seasoned with allyl
bromide and in the presence of the free radical inhibitor toluene, are homogeneous and unimolecular, and obey a first-
order rate law. The overall rate coefficients are expressed by the following equations: for 4-(N-methyl-N-
1
phenylamino)-1-butyl acetate, log[k1 (s 1)] = (13.92 Æ 0.36) (210.4 Æ 4.5) kJ mol (2.303RT) 1; and for 4-(N-
1
phenylamino)-1-butyl acetate, log[k1 (s 1)] = (12.03 Æ 0.43) (188.3 Æ 5.3) kJ mol (2.303RT) 1. The decom-
position of these substrates undergoes a parallel reaction. The predominant primary product, the corresponding
heterocyclic product, appears to be the result of an anchimeric assistance of the amino substituent for a back-side
displacement. This suggests that an incipient ion-pair type of mechanism may be operating during the process of
elimination. The Arrhenius expressions for the parallel decomposition of each of the aminobutyl acetate substrates are
presented and discussed. Copyright 2000 John Wiley & Sons, Ltd.
KEYWORDS: phenylaminobutyl acetates; gas-phase pyrolysis kinetics; neighbouring group participation
INTRODUCTION
In view of the scarce literature reports of true gas-
phase trans-eliminations of carboxylic esters, the present
work was aimed at examining the extent to which the
presence of a phenyl group at the nitrogen atom may
affect the participation of the amino group in the gas-
phase elimination reactions of Ph(CH3)NCH2CH2CH2
CH2OAc and PhNHCH2CH2CH2CH2OAc.
Few reactions involving neighboring group participation
in true gas-phase experiments have been reported.1–4 In
this respect, an unusual homogeneous, unimolecular back-
side displacement of an ester in the gas phase has been
described in the pyrolysis kinetics of 4-(N,N-dimethyl-
amino)-1-butyl acetate.5 This amino ester was found to
give N-methylpyrrolidine and methyl acetate. In this
respect, the result suggested that the (CH3)2N group
provided an anchimeric assistance in the elimination
reaction. The mechanism was explained in terms of a
modest intimate ion-pair type of intermediate, where the
acetoxy leaving group proceeds by an intramolecular
solvation or autosolvation to abstract a CH3 from (CH3)2N
to give the products as described in reaction (1).
RESULTS AND DISCUSSION
4-(N-Methyl-N-phenylamino)-1-butyl acetate
The pyrolysis products of 4-(N-methyl-N-phenylamino)-
1-butyl acetate described in reaction (2) require a twofold
*Correspondence to: G. Chuchani, Centro de Qu´ımica, Instituto
Venezolano de Investigaciones Cientificas (I VIC), Apartado 21827,
Caracas 1020-A, Venezuela.
Contract/grant sponsor: Consejo Nacional de Investigaciones Cientı´-
ficas
y Tecnolo´gicas (CONICIT); Contract/grant number: S1-
97000005.
Contract/grant sponsor: Kuwait University; Contract/grant number:
SC 088.
increase in the final pressure, Pf, i.e. Pf/P0 = 2.0. The
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 266–271

PYROLYSIS KINETICS OF PHENYLAMINOBUTYL ACETATES
Table 1. Stoichiometry of the reactions
267
4-(N-Methyl-N-phenylamino)-1-butyl acetate at 370.1°C:
Time (min)
Reaction (%) (pressure)
N-Phenylpyrrolidine (%) (GLC)
Acetic acid (%) (titration)
8
12
16
20
28.8
17.9
10.9
39.0
24.3
14.6
49.4
30.7
17.9
59.3
36.9
21.5
4-(N-Phenylamino)-1-butyl acetate at 389.5°C:
Time (min)
2
15.5
16.0
3
24.8
25.1
6
43.4
44.4
8
51.2
50.9
10
58.9
55.9
Reaction (%) (pressure)
Acetic acid (%) (titration)
Table 2. Homogeneity of the reactions
1 a
1 b
1 c
Compound
S/V (cm
)
104 k1 (s
)
104 k1 (s
)
4-(N-Methyl-N-phenylamino)-1-butyl acetate at 389.5 °C
1
6
1
6
21.33
20.75
19.70
22.64
20.85
21.00
15.33
15.10
4-(N-Phenylamino)-1-butyl acetate at 389.6 °C
aS = surface area; V = volume.
bClean Pyrex vessel.
cVessel seasoned with allyl bromide.
Table 3. Effect of the inhibitor toluene on ratesa
1
Substrate
P0 (Torr)
Pi (Torr)
Pi/P0
104 k1 (s
)
4-(N-Methyl-N-phenylamino)-1-butyl acetate at 370.1 °C
38.5
70
—
66
157.5
127
—
33.4
34.5
32.4
—
0.9
2.7
3.1
—
1.8
2.5
4.7
7.95
7.24
6.92
6.95
5.61
5.43
5.41
5.57
59
40.5
71
58.6
87.5
152.6
4-(N-Phenylamino)-1-butyl acetate at 370.1 °C
aP0 = pressure of the substrate; Pi = pressure of toluene inhibitor. Vessel seasoned with allyl bromide.
average experimental result for Pf/P0 at four different
temperatures and 10 half-lives is 2.09. An additional
confirmation of the stoichiometry of reaction (2), up to
60% decomposition, was obtained by comparing the
pressure measurements with the sum of the quantitative
gas chromatographic analysis of N-phenylpyrrolidine and
titration of acetic acid with 0.05 M NaOH solution
(Table 1).
The pyrolytic elimination of reaction (2) is homo-
geneous since no significant variations in the rates are
observed in these experiments when using both clean
Pyrex and seasoned vessels with a surface-to-volume
ratio of 6.0 relative to that of the normal vessel, which is
equal to 1.0 (Table 2). The effect of a free radical
inhibitor is described in Table 3. The kinetic determina-
tions were carried out in the presence of at least 2 equiv.
of toluene in order to inhibit any possible free-radical
chain process of the substrate and/or products. No
induction period was observed. The rate coefficients are
reproducible with a relative standard deviation not
greater than Æ5% at a given temperature.
The overall rate coefficients determined in seasoned
vessels and in the presence of toluene were found to be
invariable with the initial pressure of the substrate (Table
4), and the first-order plots are satisfactorily linear up to
about 60% reaction. Beyond this percentage decomposi-
tion, the relative standard deviation is greater than Æ5%.
The variation of the overall rate coefficient with
temperature is given in Table 5, where rate coefficients
at the 90% confidence level obtained with a least-squares
procedure are given.
The partial rates for the formation of the products as
described in reaction (2) were determined, up to 60%
decomposition of the substrate, by the quantitative
chromatographic analyses of N-phenylpyrrolidine and
4-(N-methyl-N-phenylamino)-1-butene The variation of
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 266–271

268
G. CHUCHANI ET AL.
Table 4. Variation of rate coef®cients with initial pressure
4-(N-Methyl-N-phenylamino)-1-butyl acetate at 379.2°C:
P0 (Torr)
104 k1 (s
36
11.47
52.5
11.86
72.5
11.58
95.0
11.72
1
)
4-(N-phenylamino)-1-butyl acetate at 370.1°C:
P0 (Torr)
23.8
5.61
40.5
5.42
64.5
5.57
82.0
5.53
1
104 k1 (s
)
370.1
6.92
379.2
11.86
389.5
21.23
399.4
38.66
Temperature (°C)
359.9
3.16
370.1
5.57
380.8
9.55
389.5
15.33
399.6
27.29
1
104 k1 (s
)
Rate equation: log[k1 (s 1)] = (12.03 Æ 0.43) [(188.3 Æ 5.3) kJ mol 1](2.303RT) 1;r = 0.9997
aSeasoned vessel and in the presence of toluene inhibitor.
Table 6. Variation of rate coef®cients with temperature for formation of products from 4-(N-methyl-N-phenylamino)-l-butyl
acetate
1
104 k1 (s
)
Temperature (°C)
N-Phenylpyrrolidine
4-(N-Methyl-N-phenylamino)-l- butene
359.7
370.1
379.2
389.5
399.4
2.24
4.30
7.38
13.27
24.05
1.36
2.62
4.48
8.06
14.61
ꢀ
polarization Cꢀ‡
O
is rate determining. The incipient
…
the rate coefficients for the formation of these products
with temperature (Table 6) gives, by the least-squares
method and with 90% confidence coefficient, the
following Arrhenius expressions:
positive carbon atom is stabilized through anchimeric
for N-phenylpyrrolidine:
1
log‰k1ꢀs †Š ˆ ꢀ13:72 Æ 0:35† ‰ꢀ210:4
1
1
Æ 4:4† kJ mol Š ꢀ2:303RT†
r ˆ 0:9999
;
assistance by the methylphenylamino group to form an
intimate ion-pair intermediate. The acetoxy ion proceeds
through intramolecular solvation or autosolvation to
attack the CH3 of C6H5(CH3)N group and form methyl
acetate and N-phenylpyrrolidine. With regard to Path 2,
the mechanism may be explained as commonly described
for ester pyrolysis, involving a semi-polar six-membered
cyclic transition state reaction.
for 4-(N-methyl-N-phenylamino)-1-butene:
1
log‰k1ꢀs †Š ˆ ꢀ13:50 Æ 0:37† ‰ꢀ210:4
1
1
Æ 4:6†Š kJ mol Š ꢀ2:303RT†
;
r ˆ 0:9999
Based on the products formed and the kinetic
parameters, two different mechanisms are considered
(reaction (3)).
4-(N-Phenylamino)-l-butyl acetate
Path 1 is analogous to the reaction described for 4-
(N,N-dimethylamino)-l-butyl acetate,1 where the bond
One of the products obtained from the gas-phase
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 266–271

PYROLYSIS KINETICS OF PHENYLAMINOBUTYL ACETATES
269
Table 7. Variation of rate coef®cients with temperature for formation of products from 4-(N-phenylamino)-1-butyl acetate
1
104 k1 (s
)
Temperature(°C)
N-Phenylpyrrolidine
4-(N-Phenylamino)-1- butene
Aniline
359.9
370.1
380.8
389.5
399.6
2.13
3.75
6.42
10.31
17.75
0.66
1.16
1.99
3.20
5.51
0.38
0.66
1.13
1.82
3.14
pyrolysis of 4-(N-phenylamino)-l-butyl acetate described
in reaction (4), butadiene (reaction (4), path 3),
polymerizes to some extent. However, verification of
stoichiometry (4) was possible, when obtaining, up to
60% decomposition, good agreement between pressure
measurements and the quantitative titration analysis of
acetic acid (Table 1) with 0.05 M NaOH solution.
these products with temperature are given in Table 7
(90% confidence coefficient from the least-squares
procedure) and their corresponding Arrhenius equations
are as follows: for N-phenylpyrrolidine:
1
log‰k1ꢀs †Š ˆꢀ11:84 Æ 0:44† ‰ꢀ188:1
1
1
Æ 5:5† kJ mol Š ꢀ2:303RT†
r ˆ 0:9997
;
for 4-(N-phenylamino)-1-butene:
1
log‰k1ꢀs †Š ˆꢀ11:36 Æ 0:44† ‰ꢀ188:3
1
1
Æ 5:5† kJ mol Š ꢀ2:303RT†
r ˆ 0:9997
;
;
for aniline:
The homogeneity of reaction (4) was examined by
using a vessel with a surface-to-volume ratio six times
greater than that of the unpacked vessel. The clean
packed and unpacked Pyrex vessels showed a significant
effect on the rates (Table 2). However, the packed and
unpacked vessels seasoned with allyl bromide no effect
on the rate coefficients (Table 2). The addition of
different proportions of the free radical suppressor
toluene (Table 3) also showed no effect on rates, even
though the pyrolysis experiments were carried out in the
presence of at least an equal amount of toluene to prevent
any possible radical chain reaction. Moreover, no
induction period was detected.
1
log‰k1ꢀs †Š ˆꢀ11:05 Æ 0:51† ‰ꢀ187:5
1
1
Æ 6:3† kJ mol Š ꢀ2:303RT†
r ˆ 0:9996
The products obtained from this substrate [reaction
(4)], together with the kinetic parameter determinations,
suggested three different mechanisms [reaction (5)].
The overall rate coefficients for elimination showed no
significant variation with change of initial pressure
(Table 4), and the first-order plots of log (2P0 Pt) vs
time t are linear up to 55% decomposition. Further
decomposition leads to a relative standard deviation
greater than Æ5%. The temperature dependence of the
overall rate coefficients is described in Table 5, which
gives rate coefficients at the 90% confidence level from a
least-squares method.
The partial rates of product formation depicted in
reaction (4) were determined, up to 55% decomposition,
by the quantitative chromatographic analyses of N-
phenylpyrrolidine, 4-(phenylamino)-l-butene and aniline.
The variations of the rate coefficients for the formation of
The formation of N-phenylpyrrolidine may be explaned
as before, that is, through neighboring group participation
of the phenylamino group as described in path 1, reaction
(5). An intimate ion-pair type of mechanism appears to be
the process for this decomposition. The mechanism for
the production of phenylamino-1-butene may be ex-
plained in the usual manner as for the pyrolytic
elimination of carboxylic esters, which is through a six-
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 266–271

270
G. CHUCHANI ET AL.
Table 8. Comparative kinetic parameters for neighbouring group participation at 380.0°C
1
1
1
Substrate
104 k1 (s
)
Ea (kJmol
)
logA (s
Reference
(CH3)2NCH2CH2CH2CH2OAc
Ph(CH3)NCH2CH2CH2CH2OAc
PhNHCH2CH2CH2CH2OAc
678.19
7.76
6.22
163.5 Æ 4.8
210.4 Æ 4.4
188.1 Æ 5.5
11.91 Æ 0.43
13.72 Æ 0.35
11.84 Æ 0.44
1
This work
This work
membered cyclic transition state as described in path 2.
The phenylamino-1-butene appears to undergo a partial
consecutive decomposition (path 3) to give aniline and
1,3-butadiene. The mechanism of this process may be
rationalized in terms of a discrete four-membered cyclic
transition state as shown in reaction (5) path 3.
with 10% hydrochloric acid. The toluene layer was
separated and the aqueous layer treated by slow addition
of sodium hydrogencarbonate. The aqueous solution was
extracted several times with diethyl ether, dried over
sodium sulfate and evaporated in vacuo. The product was
distilled several times (b.p. 140°C at 0.75 Torr, yield
72%) and the fraction with 99.9% purity as determined
by GLC was used. 1H NMR(CDCl3) ꢀ 1.48–1.78 (m, 4H,
2CH2), 2.02 (s, 3H, CH3), 2.91 (s, 3H, N-CH3), 3.09–3.43
(t, 2H, CH2), 3.97–4.12 (t, 2H, CH2), 6.58–6.75 (m, 3H,
The comparative influence of the N atom for
neighbouring group participation may be appreciated in
Table 8. Apparently, these results suggest that the lower
the nucleophilicity of the amino substituent the less is its
anchimeric assistance in the stabilization of the polarized
Ca—O bond in the transition state. This argument may be
justified when the CH3 group of the (CH3)2N substituent
is replaced by a phenyl group. The electron delocaliza-
tion of the nitrogen lone pair in the aromatic ring reduces
its availability for anchimeric assistance. Consequently, a
decrease in the rate of elimination is obtained. This
consideration is consistent with the order of rate
coefficients [(CH3)2N > C6H5(CH3)N ꢁ C6H5NH ] in
Table 8 from their anchimeric at the 4-position for
elimination. This sequence appears to be contrary to that
expected on the basis of steric acceleration, a factor
previously considered and discarded in previous work.5
‡
ArH), 7.09–7.36 (m, 2H, ArH). MS, m/z 221 (M ), 206
‡
[(C6H5)CH3N(CH2)4OCO ],
162
[(C6H5)CH3N
‡
‡
‡
(CH2)4 ], 120 ((C6H5)CH3NCH2 106 (C6H5NCH3 ),
91 (C6H5N ), 77 (C6H5 ), 43 (CH3CO ).
‡
‡
‡
The quantitative analyses and identifications of sub-
strates and products were carried out by GLC MS (Saturn
2000, Varian). Capillary column: DB-5MS, 30 m Â
0.250 mm i.d., 0.25 mm. The internal standard used for
quantitative GC analyses was pyrrolidine.
Kinetics. The aminobutyl acetates were pyrolyzed in a
static reaction apparatus as previously described,6,7 but
with some modifications and additions of modern
electronic and electrical devices. The reaction vessel
was seasoned with allyl bromide, and the decomposition
process was carried out in the presence of toluene as a
radical chain inhibitor. The amount of substrate em-
ployed for each pyrolytic run was around 0.05–0.1 ml.
The temperature was controlled by a resistance thermo-
meter controller, Shinko DIC- PS 25RT, and an OMEGA
solid-state relay, Model SSR240AC45, maintained with-
in Æ0.2°C and measured with a calibrated platinum–
platinum–13% rhodium thermocouple. No temperature
gradient was found along the reaction vessel. The overall
rate coefficients for 4-(N-methyl-N-phenylamino)-1-bu-
tyl acetate were followed manometrically and the partial
rates for the formation of N-phenylpyrrolidine and 4-N-
methyl-N-phenylamino-1-butene were obtained by gas
chromatographic analyses. The overall rate coefficients
for 4-(phenylamino)- l-butyl acetate were determined by
pressure increases and the partial rates for the formation
of N-phenylpyrrolidine, 4-(phenylamino)-1-butene and
aniline by gas chromatographic analyses. The substrates
were injected with a syringe through a silicone-rubber
septum directly into the reaction vessel.
EXPERIMENTAL
4-(Phenylamino)-1-butyl acetate. A solution of 4-
bromobutyl acetate (19.5 g., 0.05 mol) and aniline (4.65
g., 0.05 mol) in 20 ml of toluene was refluxed for 9–12 h.
The reaction mixture was acidified with 10% hydro-
chloric acid. The toluene layer was separated and the
aqueous layer basified by slow addition of solid sodium
hydrogencarbonate. Several extracts with diethyl ether
and dried over sodium sulfate were concentrated in
vacuo. The product was distilled several times to 98.9%
purity by GLC. B.p. 120°C at 0.75 Torr, yield 56–65 %.
1H NMR(CDCl3), ꢀ 1.54–1.78 (m, 4H, 2CH2), 2.02 (s,
3H, COCH3), 3.17–3.38 (t, 2H, CH2) 3.96–4.16 (t, 2H,
CH2), 5.53 (bs, 1H, NH), 6.56–6.76 (m, 3H, Ar H), 7.05–
‡
7.37 (m, 2H, Ar H). MS, m/z 207 (M ), 148
‡
‡
(C6H5NHCH2CH2CH2CH2 ), 106 (C6H5NHCH2 ), 91
‡
‡
‡
(C6H5N ), 77 (C6H5 ), 43 (CH3CO ).
4-(N-Methyl-N-Phenylamino)-1-butyl acetate. A solu-
tion of 4-bromobutyl acetate (9.75 g., 0.05 mol) and N-
methylaniline (5.36 g., 0.05 mol) in 15 ml of toluene was
refluxed for 10–12 h. The reaction mixture was acidified
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 266–271

PYROLYSIS KINETICS OF PHENYLAMINOBUTYL ACETATES
271
Acknowledgements
REFERENCES
1. Chuchani G, Dominguez RM, Rotinov A, Martin I, Alvarez GJ, J.
Phys. Chem. 1990; 94: 3341.
2. Martin I, Chuchani G, Dominguez RM, Rotinov A, J. Phys. Org.
Chem. 1992; 5: 725.
3. Chuchani G, Martin I, Dominguez RM, Rotinov A, Pekerar S,
Gomez M, J. Phys. Org. Chem. 1993; 6: 85.
4. Chuchani G, In Supplement D2, The Chemistry of Halides, Pseudo-
halides and Azides, Patai S, Rappoport Z, (eds). Wiley: Chichester,
1995; Chap. 19, 1069.
We are grateful to CONICIT (Consejo Nacional de
Investigaciones Cientificas y Technologicas) of Vene-
zuela for financial support. Project No. SI-97000005, and
the support of Kuwait University received through
research grant SC 088 and the facilities of SAF are
gratefully acknowledged
5. Chuchani G, Rotinov A, Dominguez RM, Gonzalez N, J. Org.
Chem. 1984; 49: 4157.
6. Maccoll A, J. Chem. Soc. 1955; 965.
7. Swinbourne ES, Aust. J. Chem. 11: 314 (1958).
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 266–271