Kinetics of the reaction of N,N-dimethylaniline with 1-bromoalk-2-ynes

Article abstract of DOI:10.1134/S1070363217070076

Quaternization of N,N-dimethylaniline with propargyl bromide, 1-bromobut-2-yne, and 1-bromooct-2-yne were studied. It was shown that, with the lengthening chain of the substituent at the triple bond, the quaternization rate tends to increase.

Full text of DOI:10.1134/S1070363217070076

ISSN 1070-3632, Russian Journal of General Chemistry, 2017, Vol. 87, No. 7, pp. 1486–1489. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © V.P. Andreev, A.V. Ryzhakov, P.S. Sobolev, 2017, published in Zhurnal Obshchei Khimii, 2017, Vol. 87, No. 7, pp. 1093–1096.
Kinetics of the Reaction of N,N-Dimethylaniline
with 1-Bromoalk-2-ynes
V. P. Andreeva*, A. V. Ryzhakovb, and P. S. Soboleva
a Petrozavodsk State University, pr. Lenina 33, Petrozavodsk, 185910 Russia
*e-mail: andreevvp1@gmail.com
b Institute of Water Problems of the North, Karelian Research Center,
Russian Academy of Sciences, Petrozavodsk, Russia
Received March 16, 2017
Abstract―Quaternization of N,N-dimethylaniline with propargyl bromide, 1-bromobut-2-yne, and 1-bromooct-
2-yne were studied. It was shown that, with the lengthening chain of the substituent at the triple bond, the
quaternization rate tends to increase.
Keywords: nucleophilic substitution, quaternization of amines, reaction kinetics, carbocations
DOI: 10.1134/S1070363217070076
The introduction of a propargyl group into mole-
cules is the focus of a large number of articles and
patents describing the synthesis, transformations, and
biological properties of reaction products. At the same
time, the mechanism (in particular, the kinetics and
thermodynamics) of the nucleophilic substitution
involving propargyl halides has been studied much less
extensively than the allyl and benzyl analogs. For
example, even for primary propargyl halides (the
reactions with secondary derivatives are additionally
complicated by isomerization) there exist fairly
contradictory published data on the influence of the
structure of the reaction participants on the reaction
rate and mechanism [1, 2]. At present such reactions
are primarily described based on calculations [3–5].
conditions tends to increase as dictated by the change
in the +I effect [6]). The alkaline hydrolysis reactions
of monosubstituted triple-bond propargyl halides (see
the table, R = H) and their reactions with sodium
ethoxide follow the SN2 mechanism; otherwise the SN1
mechanism operates.
Previously, we studied the kinetics of quaterniza-
tion of saturated tertiary β-, γ-, and δ-alkynylamines
with alkynyl bromides in acetophenone and calculated
the activation parameters of those reactions [7]. We
found that chain lengthening from 4 to 8 carbon atoms
in the substituent at the triple bond in alkynyl bromides
affects negligibly the rate of reaction (1) with tertiary
β-, γ-, and δ-alkynylamines.
R12NR2 + BrCH2C≡CR3 → [R12NR2CH2C≡CR3]+ Br– , (1)
The electron-donor properties of alkyl groups (due
to +I effect) typically (with exception of the Baker-
Nathan effect) tend to increase as (H <) Me < Et < i-Pr <
t-Bu. Therefore, in SN1 reactions, the stability of the
carbocation formed from BrCH2C≡CR and the reaction
rate should exhibit the same trend. However, the rate
of the alkaline hydrolysis of RC≡CCMe2Cl varies
depending on the nature of R in a reverse order (see
table). The rate of solvolysis in aqueous ethanol upon
replacement of the hydrogen atom by the methyl group
(as in the reaction of RC≡CCH2Cl with potassium
iodide) initially increases and then begins to decrease
(the solvolysis rate for Me2RCCl under identical
R1 = CH3, C2H5, C4H9, C8H17; R2 = CH2C≡CC4H9, (CH2)2C≡CC4H9,
(CH2)3C≡CC4H9, C8H17; R3 = C4H9, C6H13, C8H17.
Kinetic studies showed that, in the reaction with
propargyl bromides, β-alkynylamines are more nucleo-
philic than stronger bases (γ- and δ-alkynylamines) due
to a greater accessibility of the reaction center, the
nitrogen atom (with four C–C≡C–C atoms lying in one
straight line). A special position of β-alkynylamines is
confirmed by thermodynamic parameters. Low Ea and
ΔH≠ values indicate weak steric hindrance for these
amines compared to γ- and δ-alkynylamines. More
negative activation entropies for tertiary β-alkynyl-
1486

KINETICS OF THE REACTION OF N,N-DIMETHYLANILINE
1487
Kinetic data (k×104, L mol–1 h–1) for the alkaline hydrolysis
and solvolysis of RC≡CCMe2Cl at 25°C in 80% ethanol [1]
and for the reactions of RC≡CCH2Cl with potassium iodide
at 20°C in acetone and with sodium ethoxide at 50°C in
ethanol [2]
amines evidence a greater rigidity of the activated
complex in reactions with these compounds.
Here we studied the kinetics of reaction (2) of
quaternization of N,N-dimethylaniline 1 with propargyl
bromide 2a, 1-bromobut-2-yne 2b, and 1-bromooct-2-
yne 2c, differing in the carbon chain length.
Reaction with Reaction
Alkaline
R
Solvolysis potassium with sodium
PhN(CH3)2 + BrCH2C≡CR3
hydrolysis
iodide
0.279
0.419
–
ethoxide
1
2а–2c
b
7.1а
4.9
0.00237
5.13
–
→ [PhN(CH3)2CH2C≡CR3]+ Br–,
3а–3c
(2)
H
Me
Et
2.24
R3 = Н (а), СН3 (b), n-С5Н11 (c).
4.6
4.90
–
–
–
Salts 3a–3c are colorless crystalline substances that
are readily soluble in water. Compound 3a has been
described in the literature [8]; the composition of salts
3b, 3c was determined by elemental analysis for
nitrogen and ionic bromine, and their structure was
confirmed by 1H NMR spectroscopy.
i-Pr
3.45
3.42
–
t-Bu
2.77
2.80
–
a Second-order reaction. b Reaction of complicated second-order.
The rate of reaction (2) was followed by measuring
the decrease in optical density of amine 1 with time at
λ = 299 nm corresponding to the maximum of the
longwave absorption band n→π*. Bromo derivatives
2a–2c and salts 3a–3c do not absorb in this spectral
region.
The formation of alkynyl carbenium cations has
been proven by various methods; the degree of delo-
calization and, consequently, the structure and be-
havior of such cations depend on the nature of the
substituents and the reaction medium. The data on
solvolysis for the series of halogen derivatives A and
D under a wide variety of conditions show that the
reactions follow the SN1 mechanism, with halides A
reacting much faster than the corresponding isomers D
[10]. Factors favoring unimolecular process are
enhanced stability of the carbocation resulting from
halide dissociation and the use of protic solvents.
Reaction (2) is accelerated on going from propargyl
bromide to 1-bromooct-2-yne. The rate constants of
the second-order reaction in acetonitrile at 30°C for
halides 3a–3c with lengthening R3 substituent were
estimated at 0.00636±0.00038, 0.0185±0.0011, and
0.0256±0.0011 L mol–1 min–1, respectively.
Nucleophilic substitution reactions can follow the
uni- (with the formation of carbocations) or bimole-
cular mechanisms, with one of them dominating [9].
Attempts to experimentally detect alkynyl carbenium
ions were based on studying the kinetics of solvolytic
processes [10]. To resonance forms of propargyl
cations B belong allenyl ions C; these cations can be
recovered by solvolysis of both acetylenic (A) and
allenic (D) halides.
Reaction (2) seems to follow the bimolecular
mechanism to a larger extent. First, the reaction rate
constant calculated by the second-order reaction
equation is invariant for all halides 2a–2c. Second, the
addition of quaternary salt 3a to the reaction system in
some of the experiments with propargyl bromide does
not decelerate the reaction with amine 1 (k =
0.00652±0.00068 L mol–1 min–1). The unimolecular
reaction should be decelerated by the action of the
same ion.
+
C
R1R2C
_
R1R2C
C
CR3
CR3
_Х
On the one hand, the differences in the chain length
(volume) of substituents at the triple bond of com-
pounds 2a–2c, with their large separation from the
reaction center, should not significantly affect the
quaternization of amine 1. On the other hand, the alkyl
substituents differ in the magnitude of the inductive
effect. The reaction is accelerated with increasing +I
effect in bromides 2a–2c, so the transition state
Х
A
B
+
R1R2C
_
R1R2C
C
CR3
C
CR3
_Х
Х
D
C
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 87 No. 7 2017

1488
ANDREEV et al.
stabilization is more significant than the decrease in
the positive charge in the alkynyl bromide. The
formation of a new bond in SN2 reactions generally
precedes the rupture of the old one (anion-like
transition state) with displacement of the leaving group
[11]. The reason is that the reaction is accelerated by
electron-withdrawing groups.
N,N-dimethylaniline but was not recrystallized. Yield
1.38 g (89%), mp 109–110°C. H NMR spectrum, δ,
1
ppm: 0.77–1.24 m (9H, CH2CH2CH2CH3), 1.99 t (2H,
≡CCH2), 4.03 s [6H, (CH3)2N], 5.39 s (2H, CH2C≡C),
7.46 t (1H, Hp, J = 7.1 Hz), 7.56 t (2H, Hm, J =
7.6 Hz), 7.99 d (2H, Ho, J = 8.3 Hz). Found, %: Br
25.35; N 4.33. C16H24BrN. Calculated, %: Br 25.81; N
4.52.
Electron-donor groups, on the contrary, promote
the reaction (2). Therefore, it can be assumed that the
elimination of the bromine anion proceeds faster than
the formation of the new N–C bond. This is consistent
with a lower nucleophilicity (basicity) of anilines
compared to the amines we considered in [7]. In turn,
low nucleophilicity of dimethylaniline is responsible
for the enhanced selectivity of the reactions with
alkynyl halides 2a–2c differing in the chain length.
Kinetic studies were carried out in freshly distilled
acetonitrile (“for UV spectroscopy” grade) at 30°C.
Initial concentrations: 0.1 (1), 0.165 (2a, 2b), 0.087 M
(2c). To measure the amine 1 concentration over time,
aliquots of the reaction mixture were diluted with
acetonitrile to optical densities Do of 0.5–0.6. The
measurements were carried out at 30–60 min intervals
6–8 times as the reaction progressed (to the extent of ≥
50%). The rate constants (k) of the second-order
reaction of amine 1 with bromides 2a–2c were
calculated by Eq. (3).
EXPERIMENTAL
1
The H NMR spectra were recorded on a Bruker
WM 400 instrument (solvent CDCl3). The electron
absorption spectra were measured on an SF-2000
spectrophotometer (cell thickness 1 cm).
2.303
k = ———– log ———– .
(b – a)t b(a – x)
a(b – x)
(3)
Here, t is the time after the start of the reaction, min; a,
initial concentration of N,N-dimethylaniline 1 (0.1 M);
b, initial concentration of alkynyl bromide, 0.165 (2a,
2b), 0.087 M (2c); and x, concentration of the reaction
product formed at time t, M.
Alkynyl bromides 2a–2c were synthesized from the
corresponding acetylenic alcohols and phosphorus
tribromide by the procedure from [12]. N,N-dimethyl-
aniline and bromides 2a–2c were freshly distilled
before use.
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