Kinetic and equilibrium studies of σ-adduct formation and nucleophilic substitution in the reactions of 2-chloro-3,5-dinitropyridine and 2-ethoxy-3,5-dinitropyridine with p-substituted anilines in DMSO

Article abstract of DOI:10.1007/s00706-012-0860-z

Rate and equilibrium results for the reactions of 2-chloro-3,5- dinitropyridine and 2-ethoxy-3,5-dinitropyridine with a series of p-substituted anilines in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO) were studied in DMSO. The reactions yielded 2

Full text of DOI:10.1007/s00706-012-0860-z

Monatsh Chem (2013) 144:301–306
DOI 10.1007/s00706-012-0860-z
ORIGINAL PAPER
Kinetic and equilibrium studies of r-adduct formation
and nucleophilic substitution in the reactions of 2-chloro-
3,5-dinitropyridine and 2-ethoxy-3,5-dinitropyridine
with p-substituted anilines in DMSO
Basim H. Asghar
Received: 23 March 2012 / Accepted: 1 September 2012 / Published online: 18 October 2012
Ó Springer-Verlag 2012
Abstract Rate and equilibrium results for the reactions of
2-chloro-3,5-dinitropyridine and 2-ethoxy-3,5-dinitropyri-
dine with a series of p-substituted anilines in the presence of
1,4-diazabicyclo[2.2.2]octane (DABCO) were studied in
DMSO. The reactions yielded 2-anilino-3,5-dinitropyridine
derivatives, and no accumulation of intermediates could be
detected spectrophotometrically. The rates were compatible
with a two-step mechanism involving initial nucleophilic
attack followed by either base-catalysed or uncatalysed
conversion to the product. The base-catalysed pathway was
likely to involve rate-limiting proton transfer from the
zwitterionic intermediate to the base to yield the anionic
r-adduct. Plots of log K1kDABCO against pKa values gave
good straight lines at 25 °C, with slopes of 0.42 for
2-chloro- and 0.45 for 2-ethoxy-3,5-dinitropyridine. The
results were compared with those for the reactions of
2-phenoxy-3,5-dinitropyridine with substituted anilines.
area [5–9]. The presence of electron-withdrawing ring
substituents, such as the nitro group, strongly accelerates
substitution, and ring nitrogen atoms have also been found
to exert a powerful activating effect [7]. However, a ring
nitrogen ortho to the substitution position has generally
been found to be less effective than a corresponding nitro
group [8–12]. It was found that two or three nitro groups
exert a very powerful effect, such that 2,4,6-trinitrochlo-
robenzene and an acyl chloride were comparable in
hydrolytic reactivity, whereas chlorobenzene requires a
higher temperature for hydrolysis [13–18].
In organic bases such as pyridine and quinoline, the
nitrogen heteroatom is an activating group which is almost
as strong as a nitro group [19, 20]. Pyridine derivatives
undergo nucleophilic substitution much more easily than
the corresponding benzenes, especially at both the 2- and
4-positions, owing to the presence of the electronegative
nitrogen atom in the aromatic ring [15, 21–25]. This
increase in the susceptibility of pyridines to nucleophilic
attack is a further reflection of the electron-attracting
character of the ring nitrogen [23, 24].
Keywords Rate and equilibrium ꢀ
2-Chloro-3,5-dinitropyridine ꢀ
2-Ethoxy-3,5-dinitropyridine ꢀ p-Substituted anilines
In agreement with the classic work of Orvik and Bunnett
[26] the results for ethyl 2,4,6-trinitrophenyl ether indi-
cated that substitution involves the specific base–general
acid catalysis mechanism (SB-GA) in which leaving group
expulsion is the overall rate-limiting step. However, the
phenoxy group is a considerably better leaving group than
the ethoxy group, and the observation of base catalysis in
reactions of phenyl aryl ethers was best explained in terms
of rate-limiting proton transfer from a zwitterionic inter-
mediate to the base [27, 28]. El-Zahraa et al. [29] have
previously examined the kinetics of the reaction of
2-chloro-3,5-dinitropyridine with meta- and para-substi-
tuted anilines in methanol at different temperatures to give
2-anilino-3,5-dinitropyridine derivatives. The kinetics of
Introduction
Rate studies for nucleophilic substitution reactions of aro-
matic substrates have been the subject of several excellent
reviews and books [1–4] and remain an active research
B. H. Asghar (&)
Department of Chemistry, Faculty of Applied Sciences,
Umm Al-Qura University, P.O. Box: 9569,
Makkah, Saudi Arabia
e-mail: basim15@yahoo.com
123

302
B. H. Asghar
Scheme 1
the displacement of chloride and phenoxide from the
strongly activated compounds 1-chloro-2,4-dinitrobenzene
and 1-phenoxy-2,4-dinitrobenzene by aliphatic amines
have also been investigated [30]. This study identified the
following major factors affecting values of k1, the rate
constant for nucleophilic attack: (a) steric effects at the
reaction centre; rate constant values decrease with steric
congestion at the reaction centre; steric effects increase in
the order Cl \ OPh; (b) ground-state stabilization,
involving resonance interactions between the phenoxy
group and the ring, may decrease reactivity.
Results and discussion
Spectroscopic characterisation of 2-anilino-3,5-
dinitropyridines
The reactions of 0.2 g 2-chloro-3,5-dinitropyridine (1a)
(1.3 9 10-3 mol) with 1 cm3 para-substituted anilines
2a–2d (0.01 mol) in the presence of 0.1 g DABCO (1 9
10-3 mol) dissolved in ethanol gave the corresponding
1
substitution products 5a–5d. H NMR spectra recorded in
DMSO-d6 indicated an anilinodechlorination process with
the formation of 2-anilino-3,5-dinitropyridine derivatives
(Table 1).
In continuation of our previous studies in the field of
nucleophilic aromatic substitution [19, 31–35], this work
involved the study of the kinetics of the reactions between
2-chloro-3,5-dinitropyridine (1a) or 2-ethoxy-3,5-dinitro-
pyridine (1b) with a series of substituted anilines (4-OMe,
4-Me, H and 4-Cl, 2a–2d), in the presence of 1,4-diaza-
bicyclo[2.2.2]octane (DABCO) in DMSO (Scheme 1).
The results were compared with those reported in earlier
studies for the reaction of 2-phenoxy-3,5-dinitropyridine
with anilines [33]. The pKa values for the substituted
anilinium ions in DMSO were available from previous
work using the proton-transfer equilibrium with 2,4-
dinitrophenol [36]. We investigated the heteroaromatic
reactivity and the effect of the aniline substituents on the
rate of the reaction.
Rate and equilibrium studies
Rate measurements of the reactions of 1a and 1b with
substituted anilines 2a–2d were generally made in the
presence of DABCO and DABCO hydrochloride,
0.01 mol dm-3 in DMSO at 25 °C. The products exhibited
absorption maxima at ca. 375 or 395 nm. The concentra-
tion of 1a or 1b was kept at 5 9 10-5 mol dm-3, and was
very much lower than that of the other components (one of
the substituted anilines 2a–2d and DABCO). Under these
conditions, accurate first-order rates were observed and the
variation in value of the rate constant with aniline and
123

Kinetic and equilibrium studies of r-adduct formation and nucleophilic substitution
303
1
Table 1 Properties and H NMR data for 2-substituted anilino-3,5-dinitropyridines 5a–5d
Comp.
R
Yield/%
M.p./°C
Pyridyl protons
–NH
Aryl protons
0
H2 ,6
0
0
H3 ,5
0
H4
H6
1a
–
–
–
–
8.99 (d)
9.45 (d)
–
–
J4,6 = 2.2
9.31 (d)
J6,4 = 1.8
9.38 (d)
5aa
5bb
5cc
5d
4-OMe
4-Me
H
51
95
94
95
176
148
130
187
10.46 (s)
10.53 (s)
10.59 (s)
10.55 (s)
7.56 (d)
J = 8.8
7.56 (d)
J = 8.3
7.69 (d)
J = 7.8
7.49 (d)
J = 8.8
7.05 (d)
J = 8.8
7.33 (d)
J = 8.2
7.54 (t)
J = 7.3
7.49 (d)
J = 8.8
J4,6 = 2.2
9.34 (d)
J6,4 = 2.2
9.38 (d)
J4,6 = 2.3
9.37 (d)
J6,4 = 2.3
9.39 (d)
J4,6 = 2.3
9.36 (d)
J6,4 = 2.3
9.40 (d)
4-Cl
J4,6 = 2.4
J6,4 = 2.3
d/ppm, J/Hz
a
4-OCH3 protons appear at 3.92 ppm (s, 3H)
4-CH3 protons appear at 2.49 ppm (s, 3H)
H-40 proton appears at 7.38 ppm (t, 1H)
b
c
DABCO concentrations was examined. Measurements
were also made in the absence of DABCO, and plots of the
second-order rate constant, kobs/[aniline], versus aniline
concentration were linear with positive intercepts.
Table 2 Rate results for the reaction of 1a with 2a in DMSO at
25 °C
[2a]/mol dm-3
[DABCO]a/mol dm-3
k
obs/[2a]/mol-1 dm3 s-1
0.05
0.075
0.10
0.20
0.30
0.02
0.10
0.25
0.10
0.10
a
–
0.065
0.094
0.123
0.242
0.360
These results are interpreted in terms of the processes
shown in Scheme 1. It is known that phenoxide is a con-
siderably better leaving group than ethoxide, by a factor of
ca. 106 [27, 28, 37, 38]. Values of the rate constant [30] for
nucleophilic attack at the 1-position increase with increasing
ring activation, but may be reduced by steric repulsion at the
reaction centre, which increases in the order Cl \ OPh.
The failure to observe 4, the intermediate on the sub-
stitution pathway, may be attributed to its rapid
decomposition through loss of chloride. The assumption
that 3 may be treated as a steady-state intermediate leads to
the rate expression of Eq. (1), where kAn and kDABCO
represent the respective bases [28].
–
–
–
–
0.10
0.10
0.10
0.03
0.06
16.250
16.342
16.520
4.989
9.854
Solutions containing DABCO also contain DABCO hydrochloride,
0.01 mol dm-3. Measurements were made at 395 nm
k1½Anꢁðk2 þ kAn½Anꢁ þ kDABCO½DABCOꢁÞ
k
obs. Rate measurements at 375 nm showed a single first-
kobs
¼
ð1Þ
k
ꢂ1 þ k2 þ kAn½Anꢁ þ kDABCO½DABCOꢁ
order process for the reaction of 1b with 2a. Values of the
rate constant, kobs/[2a], for the reaction with 2a are given in
Table 3. Values calculated with K1kDABCO = 1.12 9
10-2 dm6 mol-2 s-1, K1kAn = 6.5 9 10-5 dm6 mol-2 s-1
and K1k2 = 3.3 9 10-5 dm3 mol-1 s-1 were in good agree-
The results, which provide evidence for base catalysis,
indicate that the condition
k
ꢃ k2 þ kAn½Anꢁ þ kDABCO½DABCOꢁ
applies, so that Eq. (1) reduces to Eq. (2):
ꢂ1
ment with values of kobs
.
It is known that reaction at unsubstituted ring positions
to give anionic adducts may precede the attack at the
2-position [27, 28, 37, 38]. Previous work at lower base
concentrations and in different solvents did not find evi-
dence for base catalysis [26–28, 39]. However, the present
work showed that in DMSO, such catalysis occurs, and the
values of K1kAn and K1kDABCO for reactions of 1a and
1b with anilines can be calculated. Base catalysis indicates
kobs ¼ K1½Anꢁðk2 þ kAn½Anꢁ þ kDABCO½DABCOꢁÞ
ð2Þ
Rate measurements at 395 nm showed a single first-order
process for the reaction of 1a with 2a. Values of the rate
constant, kobs/[2a], for the reaction with 2a are given in
Table 2. Values calculated with K1kDABCO = 162.18 dm6
mol-2 s-1, K1kAn = 1.18 dm6 mol-2 s-1 and K1k2 = 5.5 9
10-3 dm3 mol-1 s-1 were in good agreement with values of
123

304
B. H. Asghar
The highest value for 4-OMe indicates that the higher
basicity of 4-OMe is reflected in higher values of K1k2,
K1kAn and K1kDABCO. Values of K1kDABCO, K1k2 and K1kAn
decrease strongly as the substituent R is made more elec-
tron withdrawing.
Table 3 Rate results for the reaction of 1b with 2a in DMSO at
25 °C
[2a]/mol dm-3
[DABCO]a/mol dm-3
k
obs/[2a]/mol-1 dm3 s-1
0.05
0.075
0.10
0.20
0.30
0.02
0.2
–
0.000036
0.000037
0.000039
0.000046
0.000053
0.0015
–
The proton transfer process, kDABCO, will be strongly
favoured thermodynamically. This is because protons are
transferred to a strong base (DABCO). In the absence of
steric effects, values would be close to the diffusion limit.
The reaction centre in 3 is quite hindered; hence, values
will be lower than the diffusion limit. However, since the
steric effects will be the same for 2a–2d, the value of
kDABCO will be the same for each aniline. Therefore,
changes in K1kDABCO in 2a–2d reflect changes in values of
K1, the equilibrium constant for the formation of the
–
–
–
0.10
0.10
0.10
0.03
0.06
0.0016
0.3
0.0017
0.10
0.10
a
0.00037
0.00071
Solutions containing DABCO also contain DABCO hydrochloride,
0.01 mol dm-3. Measurements were made at 375 nm
zwitterions, rather than changes in kDABCO
.
It is customary to correlate the effect of the para-sub-
stituent in the 2-anilino-3,5-dinitropyridines by the
Hammett relationship [41]. Hammett constants r are
applied when there is no direct mesomeric interaction
between the substituent and the reaction centre. Thus, a
linear plot of values of log K1kDABCO for the reactions of
1a and 1b versus Hammett r values gives a slope, q, of
-2.2 for 1a in Fig. 1, and -1.9 for 1b (not shown). Since
values of kDABCO are expected to be independent of the
nature of the remote substituent, this q value reflects
the substituent effect on values of K1. The negative value
obtained is consistent with the increase in the positive
charge on nitrogen associated with the formation of 3.
The Brønsted relationship is generally applicable to
combinations of nucleophiles and electrophiles. It has been
successfully applied to SNAr reactions of nitro-activated
aromatic compounds with amines [42]. The Brønsted plot
of log K1kDABCO shown in Fig. 2 for the reaction of 1a with
anilines versus the pKa values of the corresponding ani-
linium ions provides further evidence for the steric
requirement of the anilines. Thus, the slope of the plot,
Table 4 Summary of results for the reactions of anilines 2a–2d with
1a in DMSO at 25 °C
Aniline K1kDABCO
/
K1kAn
/
K1k2/
pKa
dm6 mol-2 s-1 dm6 mol-2 s-1 dm3 mol-1 s-1 (DMSO)a
4-OMe 162.18
1.18
0.70
0.30
0.10
5.5 9 10-3
3.3 9 10-3
1.4 9 10-3
4.5 9 10-4
5.08
4.48
3.82
2.86
4-Me
H
77.62
39.81
14.12
4-Cl
a
Results from Ref. [36], pKa values listed correspond to the conju-
gate acids of the anilines
Table 5 Summary of results for the reactions of anilines 2a–2d with
1b in DMSO at 25 °C
Aniline K1kDABCO
/
K1kAn
/
K1k2/
pKa
dm6 mol-2 s-1 dm6 mol-2 s-1 dm3 mol-1 s-1 (DMSO)a
4-OMe 1.12 9 10-2
6.5 9 10-5
4.2 9 10-5
2.0 9 10-5
7.6 9 10-6
3.3 9 10-5
2.1 9 10-5
1.0 9 10-5
3.8 9 10-6
5.08
4.48
3.82
2.86
4-Me
H
5.88 9 10-3
3.16 9 10-3
1.09 9 10-3
4-Cl
bnuc, has a value of 0.42 for 1a (and 0.45 for 1b, not
shown), and this low value is compatible with a steric
a
Results from Ref. [36], pKa values listed corresponding to the
conjugate acids of the anilines
that proton transfer from the zwitterion to the base is rate
limiting.
The presence of chloride or ethoxy groups rather than a
phenoxy group is not expected to drastically affect the
value of K1 for zwitterion formation [29, 40]. The k2 step is
likely to involve intramolecular proton transfer from
nitrogen to oxygen coupled with carbon–oxygen cleavage.
Leaving group expulsion is part of the rate-limiting step
here.
Values obtained for K1kDABCO, K1k2 and K1kAn are
summarised in Tables 4 and 5, together with pKa values for
the conjugate acids of the anilines, measured in DMSO.
Fig. 1 Plot of log K1kDABCO for the reaction of 1a with anilines
2a–2d against the Hammett constant
123

Kinetic and equilibrium studies of r-adduct formation and nucleophilic substitution
305
that steric effects will be dominant in determining values of
kDABCO
.
The ratio of the K1kAn values for 1a/1b is ca. 16,000,
and is higher than the corresponding ratio of K1kDABCO
values. This is likely a result of the reduction in the value
of kAn in the reaction of 1b, because the proton transfer
process involved (Scheme 2) is less thermodynami-
cally favourable than in the corresponding reaction
involving 1a.
In 1a, 2-phenoxy-3,5-dinitropyridine, and 1b, the rate
constants for nucleophilic attack at the 2-position increased
with increasing ring activation, which correlates with the
electronegativities of the respective substituents. Table 6
compares the product K1kDABCO for the reactions of the
anilines 2a–2d with 1a, 1b, and 2-phenoxy-3,5-dinitro-
pyridine [33]. The product had a higher value for the
reaction of 1a than for 1b or the phenoxy compound (1a/1b
was ca. 15,000; 1a/2-phenoxy-3,5-dinitropyridine was ca.
1,300). As previously discussed [37], the dominant factor
was the higher value of kDABCO associated with the lower
steric hindrance to proton transfer when the reaction
occurred at an unsubstituted ring position. The value of K1
must also be considerably higher for 1a than for 1b.
Despite the fact that the bulky phenoxy group is a good
leaving group and also benefits from greater steric strain
relief in the transition state compared to the chloride, the
large difference in Brønsted acidities for the conjugate
acids of these groups would appear to dominate their
leaving group abilities in this reaction, as reflected by the
rate constants. The order of reactivities was found to be
1a [ 2-phenoxy-3,5-dinitropyridine [ 1b, which illus-
trates the difficulty in quantifying these factors. It should
be noted that because proton transfer is rate determining in
all these reactions, it is not possible to obtain values for the
rate constants k1 and k-1 relating to formation of the
zwitterionic intermediates.
Fig. 2 Brønsted plot of log K1kDABCO versus pKa values of the
anilines 2a–2d has a slope, b, of 0.42 for 1a
effect involving a large separation of reactants in the
transition state. As the transition state of a nucleophile
implies (partial) donation of an electron pair from the
nucleophile to the reaction centre, the Brønsted coefficient,
bnuc, is related to the extent of bond formation between the
nucleophile and the reaction centre. However, the moderate
values of q and bnuc for the title reaction indicate that the
bond-making process lags behind the p-bond breaking of
the pyridine ring in the transition state to give the zwit-
terionic intermediate 3.
Values of K1k2 show similar changes with the nature of
the substituent as seen for the values of K1kDABCO. This
implies that values of k2 vary only slightly with the nature
of the substituent R.
Values of K1kAn decrease much more dramatically on
going from 2a to 2d. This indicates that the kAn process is
far more favourable with 4-methoxyaniline than with
4-chloroaniline. A possible explanation is that the proton-
transfer equilibrium in Scheme 2 is not strongly thermo-
dynamically favoured. Hence, as the basicity of the aniline
decreases, the rate constant for the process is reduced.
The results in Tables 4 and 5 allow comparison of the
reactions of 1a and 1b with the anilines 2a–2d. They show
that values of both K1kDABCO and K1kAn were considerably
higher for 1a than for 1b. Both electronic (leaving group)
and steric factors will be important in determining the
relative reactivities. Proton transfer from the zwitterions to
DABCO will be strongly favoured thermodynamically, so
Table 6 Comparison of the reactions of 2-phenoxy-3,5-dinitropyri-
dine, 1a, and 1b with anilines 2a–2d in DMSO
Anilines K1kDa ABCO
dm6 mol-2 s-1
/
K1kbDABCO
/
K1kcDABCO
/
dm6 mol-2 s-1
dm6 mol-2 s-1
4-MeO 0.120
162.18
77.620
39.810
14.120
1.12 9 10-2
5.88 9 10-3
3.16 9 10-3
1.09 9 10-3
4-Me
H
0.052
0.028
0.012
4-Cl
a
Results from Ref. [33]: reaction of 2-phenoxy-3,5-dinitropyridine
with anilines
b
Results from the present work: reaction of 2-chloro-3,5-dinitro-
pyridine (1a) with anilines
c
Results from the present work: reaction of 2-ethoxy-3,5-dinitro-
pyridine (1b) with anilines
Scheme 2
123

306
B. H. Asghar
5. Asghar BHM, Crampton MR (2007) J Phys Org Chem 20:702
6. Asghar BHM, Crampton MR (2007) Org Biomol Chem 5:1646
7. Asghar BHM, Crampton MR, Isanbor C (2008) Can J Chem
86:225
8. Asghar BHM (2010) J Saudi Chem Soc 14:261
9. Crampton MR, Emokpae TA, Howard JAK, Isanbor C, Mondal R
(2003) Org Biomol Chem 1:1004
10. Emokpae TA, Uwakwe PU, Hirst J (1993) J Chem Soc Perkin
Trans 2:125
11. Emokpae TA, Uwakwe PU, Hirst J (1991) J Chem Soc Perkin
Trans 2:509
12. Emokpae TA, Hirst J, Uwakwe PU (1990) J Chem Soc Perkin
Trans 2:2191
13. Loudon JD, Smith DM (1964) J Chem Soc 2806
14. Etsinger RIL, Ramsay OB, McCian JH (1965) J Am Chem Soc
87:2945
15. Bunnett JF, Zahler RE (1951) Chem Rev 49:273
16. Ross SD (1963) Prog Phys Org Chem 1:31
17. Beckwith AL, Leahy GD, Miller J (1952) J Chem Soc 3552
18. Bunnett JF, Nudelman NS (1969) J Org Chem 34:2038
19. Hamed EA (1997) Int J Chem Kinet 29:599
20. Chapman NB, Russell-Hill DQ (1956) J Chem Soc 1563
21. Pietra FQ (1969) Rev Chem Soc 23:504
22. Eggiman W, Schmid P, Zollinger H (1975) Helv Chim Acta
58:257
23. Brenelli ECS, Moran PJS (1989) J Chem Soc Perkin Trans
2:1219
24. Brenelli ECS, Moran PJS, Rodrigue JAR (1989) J Chem Res
(S) 389
25. Chupakhin ON, Charushin VN (1988) Tetrahedron 44:1
26. Orvik JA, Bunnett JF (1970) J Am Chem Soc 92:2417
27. Chamberlin R, Crampton MR (1995) J Chem Soc Perkin Trans
2:1831
28. Crampton MR, Robotham IA (1998) Can J Chem 76:627
29. El-Zahraa F, El Hegazy M, Abdel Fattah SZ, Hamed EA, Sharaf
SM (2000) J Phys Org Chem 13:549
30. Crampton MR, Emokpae TA, Isanbor C, Batsanov AS, Howard
JAK, Mondal R (2006) Eur J Org Chem 1222
31. Hamed EA, El-Bardan AA, Saad EF, Gohar GM, Hassan GM
(1997) J Chem Soc Perkin Trans 2:2415
32. El-Bardan AA (1999) J Phys Org Chem 12:347
33. Asghar BH (2008) Int J Chem Kinet 41:198
34. Asghar BH (2010) E J Chem 7:253
Conclusions
Activation parameters and q values were determined for
the nucleophilic aromatic substitution reactions of various
anilines with 2-substituted 3,5-dinitropyridines in the
presence of DABCO in DMSO. The kinetic data suggested
that the SNAr reaction occurred through a two-step mech-
anism involving initial nucleophilic attack followed by
either base-catalysed or uncatalysed conversion (direct
elimination of the leaving group) to the product. The base-
catalysed pathway was likely to involve rate-limiting pro-
ton transfer from the zwitterionic intermediate to DABCO
to yield the anionic r-adduct. The trends in reactivity and
leaving group ability were consistent with established
substituent effects.
Experimental
2-Chloro-3,5-dinitropyridine (1a) was obtained from
Aldrich and purified by crystallization twice from metha-
nol/light petroleum as yellow needles. 2-Ethoxy-3,5-
dinitropyridine (1b) was prepared by the reaction of 1a
with one equivalent sodium ethoxide in ethanol [43].
4-Methoxyaniline (2a), 4-methylaniline (2b), aniline (2c)
and 4-chloroaniline (2d) were supplied by Aldrich and
were purified by crystallization or vacuum distillation.
DABCO and DMSO were the purest commercially avail-
able grades. Amine salts were prepared in solution by the
accurate neutralization of amines with concentrated
hydrochloric acid.
1H NMR spectra were recorded in DMSO-d6 at 24 °C
using a Bruker Avance-400 instrument. UV–Vis spectra
and rate measurements were conducted with a Shimadzu
UV-2101 PC. All measurements were made at 25 °C. First-
order rate constants, precise to 3 %, were evaluated using
standard methods.
35. Asghar BH, Fathalla FM, Hamed EA (2009) Int J Chem Kinet
41:777
36. Asghar BHM, Crampton MR (2005) Org Biomol Chem 3:3971
37. Crampton MR, Robotham AI (1997) J Chem Res (S) 22
38. Bernasconi CF, Muller MC (1978) J Am Chem Soc 100:5530
39. Hirst J, Hussain G, Onyido I (1986) J Chem Soc Perkin Trans
2:397
References
40. Chamberlin R, Crampton MR, Knight RL (1993) J Chem Res
(S) 444
41. Hammett LP (1954) Physical organic chemistry. McGraw-Hill,
New York
42. Dixon JE, Bruice TC (1974) J Am Chem Soc 94:2052
43. Barycki J, Plazek E (1963) Roczniki Chem 37:1443
1. Miller J (1969) Aromatic nucleophilic substitution. Elsevier, New
York, p 18
2. Strauss M (1970) Chem Rev 70:667
3. Terrier F (1991) Nucleophilic aromatic displacement. VCH, New
York
4. Terrier F (1982) Chem Rev 82:77
123