Kinetic study of the proton transfer reaction between 1-nitro-1-(4-nitrophenyl)alkanes and P1-t-Bu phosphazene base in THF solvent

Article abstract of DOI:10.1016/j.molstruc.2005.11.031

The rates of proton transfer reactions between C-acids of the series of nitroalkanes with increasing bulk of R=H, Me, Et, i-Pr substituents as 4-nitro-phenylnitromethane (1), 1-(4-nitrophenyl)-1-nitroethane (2), 1-(4-nitrophenyl)-1-nitropropane (3), 2-methyl-1-(4-nitrophenyl)-1-nitropropane (4) and the P₁-t-Bu phosphazene have been measured in THF at pseudo-first-order conditions. The product of the proton transfer reaction with P₁-t-Bu is associated into ion pair. The equilibrium constants decrease along with growing bulk of alkyl substituent in the reacting C-acid: >100 000, 1170, 590, and 11.8 for 1, 2, 3, 4, respectively. The second order rate constants (k_2H) for this very strong base and C-acids are rapidly declining: 9357, 2.31, 0.66, 0.09 dm³ mol^-1 s^-1 for 1, 2, 3, 4, respectively, and could not be accounted for the small values of the enthalpies of activation Δ H_H^≠ = 6.1, 18.0, 20.7 and 11.1 kJ mol^{- 1}. The appropriate values of the entropies of activation were all negative and relatively large Δ S_H^≠ = - 149.7, - 176.5, - 178.7, - 227.8 J mol^{- 1} deg^{- 1} indicating an importance of steric and ordering effects in forming the transition state. The primary deuterium kinetic isotope effects are large showing tendency of reverse relation towards steric hindrance of the reacting C-acids, k_H/k_D=15.8, 13.6, 13.2 for 1, 2, and 3, respectively. The explanation of the unusual slow proton transfer abstraction caused by this extremely strong base is undertaken.

Full text of DOI:10.1016/j.molstruc.2005.11.031

Journal of Molecular Structure 788 (2006) 152–158
www.elsevier.com/locate/molstruc
Kinetic study of the proton transfer reaction between 1-nitro-1-(4-nitro-
phenyl)alkanes and P1-t-Bu phosphazene base in THF solvent
˙
Zaneta Wisłocka, Iwona Nowak, Arnold Jarczewski
*
´
Faculty of Chemistry, A. Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland
Received 18 October 2005; accepted 14 November 2005
Available online 19 January 2006
Abstract
The rates of proton transfer reactions between C-acids of the series of nitroalkanes with increasing bulk of RZH, Me, Et, i-Pr substituents as
4-nitro-phenylnitromethane (1), 1-(4-nitrophenyl)-1-nitroethane (2), 1-(4-nitrophenyl)-1-nitropropane (3), 2-methyl-1-(4-nitrophenyl)-1-nitro-
propane (4) and the P1-t-Bu phosphazene have been measured in THF at pseudo-first-order conditions. The product of the proton transfer reaction
with P1-t-Bu is associated into ion pair. The equilibrium constants decrease along with growing bulk of alkyl substituent in the reacting C-acid:
O100 000, 1170, 590, and 11.8 for 1, 2, 3, 4, respectively. The second order rate constants (k2H) for this very strong base and C-acids are rapidly
declining: 9357, 2.31, 0.66, 0.09 dm3 molK1 sK1 for 1, 2, 3, 4, respectively, and could not be accounted for the small values of the enthalpies of
activation DHsH Z6:1; 18:0; 20:7 and 11:1 kJ molK1. The appropriate values of the entropies of activation were all negative and relatively large
DSsH ZK149:7;K176:5;K178:7;K227:8 J molK1 degK1 indicating an importance of steric and ordering effects in forming the transition state. The
primary deuterium kinetic isotope effects are large showing tendency of reverse relation towards steric hindrance of the reacting C-acids, kH/kDZ
15.8, 13.6, 13.2 for 1, 2, and 3, respectively. The explanation of the unusual slow proton transfer abstraction caused by this extremely strong base
is undertaken.
q 2005 Elsevier B.V. All rights reserved.
Keywords: Proton transfer; C-acids; Phosphazene; Rate constants; Kinetic isotope effects
1. Introduction
constants, always larger with the TBD than with MTBD base.
The ratios of kTBD/kMTBD varied from 50 to 160 in MeCN [4]
and 120 to 291 in THF [5] for 2, 3, and 4, respectively. Initially,
very different rate constants for these bases of almost equal
strength were discussed in terms of a little larger basicity of
TBD, steric hindrance brought to reaction center by the MTBD
molecule caused by the methyl substituent of the nitrogen atom
of the latter base, and finally the hypothetical additional
hydrogen bond formed in the transition state (TS) of the TBD
reaction [4,5]. For the reaction of TBD the remarkable blue
shift that was observed for the reaction product species
suggests double hydrogen bonding of the nitronate group by
protonated base TBDHC [4,5].
The introduction of a new, strong sterically hindered base
tert-Butylimino-tris(dimethylamino)phosphorane, commonly
known as P1-t-Bu phosphazene [1], gives an
additional opportunity to study the factors influencing the
mechanism of proton transfer reactions in solution. In our
previous papers we already took an advantage using
commercially available two very strong bicyclic guanidines
C
TBD and MTBD of similar basicity (pKTBDH Z24:7 or 25:96;
C
pKMTBDH Z24:97 or 25:43) [2,3], differing by just one frontal
methyl group. The mechanistic study of the proton transfer
between C-acids of the series of nitroalkanes with increasing
bulk of RZH, Me, Et, i-Pr substituents as: 4-nitrophenylni-
tromethane (1), 1-(4-nitrophenyl)-1-nitroethane (2), 1-(4-nitro-
phenyl)-1-nitropropane (3), 2-methyl-1-(4-nitrophenyl)-1-
nitropropane (4) and the TBD and MTBD bases in acetonitrile
[4] and THF [5] solvents unexpectedly show very different rate
The equilibrium constant for the TBD reaction has been
found larger than 11000 MK1 and is considerably greater than
that of 44 MK1 for MTBD reaction. This shows that the
Brønsted relationship does not hold for the reactions of
nitroalkanes with bicyclic guanidines. Such breakdowns
frequently indicate the importance of steric hindrance but in
the case of TBD base it results from an additional hydrogen
bond, formed both in the TS and in the product [4,5].
*
Corresponding author. Tel.: C48 61 8291 479; fax: C48 61 8658 008.
E-mail address: arnold@amu.edu.pl (A. Jarczewski).
Generally, along with growing crowding in the vicinity
of reaction center due to steric hindrance brought by the bulk
0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2005.11.031

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Z. Wisłocka et al. / Journal of Molecular Structure 788 (2006) 152–158
153
of alkyl substituents in the C-acid: 2, 3, 4 and MTBD base,
significant reduction of the rate constants was observed [4]
which is consistent with earlier observations made by Bordwell
[6,7]. This points out that the steric factors play an important
role and cannot be neglected in considering the planarity of the
carbanion formed and, most of all, the different structure of the
transition state [5].
combined with large steric hindrance of very strong base tert-
Butylimino-tris(dimethylamino)phosphorane, named P1-t-Bu
phosphazene (pKaMeCNZ26:88 [1]) offers a new ability to study
the factors influencing mechanism of the proton transfer reaction.
The reaction carried out in low polarity solvent THF
proceeds the Scheme 1:
NO2
Usually the primary deuterium kinetic isotope effects KIE
increase with the steric hindrance [8], either by removal of the
solventmoleculesfromthevicinityofthereactionsiteorasteeper
energy barrier that favours proton tunneling [9]. However, just
reverse effectwehavealready found. One canstatesthatthesteric
hindrance in both the base and C-acid may also reduces the KIE.
These unusual values of KIE we found for the reaction of studied
series of C-acids with MTBD base where they are declining with
the increase of bulk of the R substituents [10] in both acetonitrile
[4] and THF [5] solvents. We, undoubtedly found, that in case of
the TBD and MTBD reactions with this series of C-acids the
reaction of MTBD proceeds with different structure of the
transitionstate. Actually, twofactorsinfluencingKIEvalueshave
been found. Firstly, the additional stabilization of the TS in the
case of TBD reaction by a hydrogen bond and secondly the
planarity ofthenitro grouptowards theprotontransferaxisgiving
the dihedral HCNO angle dependent on the bulk of the R
substituent [4]. The growing bulk of R substituent in the C-acid
causes a decrease in HCNO dihedral angle what is manifested in
the larger KIE values. Generally, for the reaction of nitroalkanes
with guanidine bases, the KIE values increase when the structure
of the transition state is less product-like. In conclusion, we say
that the KIE values are greatly influenced by the factor resulting
from the structure of the transition state complex which is formed
with the hydrogen bonds between a base and oxygen atoms of
nitrogroupsofthe reactingC-acid. This is evidentlyconfirmedby
the ab initio predictions and the energetics we estimated for the
structures of the complexes of 1-nitro-1-(4-nitrophenyl)ethane
and TBD and MTBD bases [4,11].
O2N
C
R
H + B
I.P.
CH3
CH3
CH3
C
N
P
Where: R= H, Me, Et, i-Pr; B =
I.P. = Ion-pair
N(CH3)2
(H3C)2N
N(CH3)2
Scheme 1. Reaction between 1-nitro-1-(4-nitrophenyl)alkanes and phospha-
zene P1-t-Bu.
The product of the proton transfer reactions (Scheme 1) in
THF of lmaxZ484, 496, 498, and 496 nm for 1–4, respectively,
appears to be an ion pair as indicated by a blue shift w20 nm
compared to that of free anions previously found in acetonitrile
solvent of lmaxZ508–515 nm [4,17,18]. Such a blue shift is an
indication of a strong stabilization by the interaction with
countercation [10]. The appropriate chemical shifts found in 1H
and 13C NMR spectra are given in Tables 1 and 2. The 1H NMR
spectrum of the reaction of 1 with P1-t-Bu, mixture 1:1 shows
the complete conversion of the carbon acid 1 into its anion
manifested by the shift of the signal of the acidic proton s, dZ
5.740 to 6.790. The intensity of the band found is equivalent to
the loss of the one Ca proton. There were insignificant shifts of
the bands attributed to ortho (2H2,6) and meta (2H3,5) hydrogen
atoms (Table 1).
The 13C NMR spectrum for the same reaction products shows
a significant shift for acidic a-carbon atom equal to 31.664, and
much smaller for the 2C2,6 carbon atoms of the anion and only
insignificant changes for other carbon atoms (Table 1). The shift
of the signals towards the lower field suggests the strong olefinic
character of the exocyclic carbon atom. Hence the dominant
structure of the anion in its resonance hybrid has the negative
charge localized on the oxygen atoms of the exocyclic nitro
group of the sp2 hybridization of the Ca atom.
The further study of the mechanism of proton transfer
may utilize the very strong phosphazene P1-t-Bu base
ðpKaMeCN Z26.88 [1]Þ which is the weakest in the phospha-
zene’s family [1,3,12], however, stronger than TBD and
MTBD by one order of pKa unit (pKa TBD
MeCN
[2,3] (pK
MeCN Z25:96 or 24:7)
Z25:43 or 24:97) [2,3] and even three pKa
1
The H NMR spectrum of the reaction of 4 with P1-t-Bu,
a MTBD
MeCN Z23:9) [13].
mixture 1:1 shows the signals derived from both the C-acid and
its anion (Table 2). Taking into account the integrated signals
of the aromatic protons, one can assume w30% of the
conversion of the C-acid 4 to its anionic form. The same
conclusions can be drawn by analyzing the signals of 13C
corresponding to the P1-t-Bu phosphazene base and the cation
of 4 (Table 2). This amount of conversion is not so well
observable in the equilibrium experiments carried out by
spectrophotometry, where a similar degree of conversion has
been noted only for the larger than 1:1 concentration of the
base. The shifts assigned to the ortho and para protons 2H2,6
and 2H3,5 shows merely insignificant shift as was already
observed for the reaction of 1 (Table 2).
units stronger compared to DBU (pKa DBU
Moreover, phosphazene P1-t-Bu is much less nucleophilic
in comparison with DBU and not sensitive towards hydrolysis
revealing particularly useful properties for deprotonation [14].
The use of this neutral, strong and most of all highly hindered
nitrogen base seems to be promising in our study of the factors
influencing the structure of the transition state and mechanisms
of the proton transfer reactions in solutions [4,15].
2. Results and discussion
1-Nitro-1-(4-nitrophenyl)alkanes are moderately strong acids
(pKaZ20.39, 23.24, 23.57 and 25.73 for 1, 2, 3, and 4,
respectively) [16] with gradually increasing bulk of alkyl R
substituents: H (1), Me (2), Et (3), i-Pr (4). These properties
Generally the spectrum of 13C NMR of the 1:1 mixture of 1
and P1-t-Bu phosphazene base is not clear perhaps due to low

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Z. Wisłocka et al. / Journal of Molecular Structure 788 (2006) 152–158
154
Table 1
Chemical shifts (d) found for the reaction of 4-nitrophenylnitromethane 1 and P1-t-Bu phosphazene in THF
1H (d-ppm rel. to TMS)
1
1 anion
2H2,6 DdZK0.052
2H3,5 DdZ0.368
1Ha, DdZK1.050
P1-t-Bu
P1-t-BuHC
2H2,6
2H3,5
1Ha
m, dZ7.739
m, dZ8.285
s, dZ5.740
m, dZ7.791
m, dZ7.917
s, dZ6.790
9H, C–CH3,
d, dZ1.146
m, dZ2.582
9H, C–CH3, DdZK0.200
18H, N–CH3, DdZK0.184
d, dZ1.346
m, dZ2.766
18H, N–CH3,
13C (d-ppm rel. to TMS)
1Ca
dZ78.707
dZ132.370
dZ124.541
dZ138.154
dZ149.704
1Ca
s, 110.371
s, 122.400
s, 124.354
s, 141.509
s, 145.061
3C (C–CH3)
6C (N–CH3)
1C (C–CH3)
d, 36.197
d, 38.128
d, 49.732
3C (C–CH3) DdZ4.634
6C (N–CH3) DdZ0.251
1C (C–CH3) DdZK3.109
s, 31.563
d, 37.877
s, 52.841
2C2,6
2C3,5
1C1
2C2,6
2C3,5
1C1
1C4
1C4
concentration of the anion. Nevertheless the signals assigned to
free P1-t-Bu phosphazene base as well as its cation in the amount
adequate to converted amount of C-acid 4 are visible (Table 2).
The conductometric titration of the C-acid 1 with P1-t-Bu
phosphazene base in THF solvent shows rather low, however,
distinct changes of the conductance (Fig. 1). The molar
reactions KZkf/kb are large 315, 273 and 9.5 MK1 for 2, 3,
4, respectively. The appropriate values of the equilibrium
constants derived from equilibrium using Benesi and
Hildebrand [20] formalism are in good agreement
(Table 4), indicating that the ion-pair products are not
involved in any side reactions.
conductivity at the stoichiometric point is 0.72 UK1 cm2 molK1
.
Since we presume that hydrogen bonds between the carbon
acid and P1-t-Bu base with the hydrogen that was initially
attached to a-carbon atom of the C-acid, lags behind the proton
transfer from this carbon, then the pKa values of the C-acids and
P1-t-Bu base, even though not measured in THF solvent, may in
fact reflect the reactivity better than the equilibrium constants
for their reactions in THF. For our purpose it is useful to accept
pKa values found in acetonitrile [1,16] as a measure of ‘inherent’
basicity providing the appropriate equilibrium constants
This is a direct and convincing indication of the ion-pair
product taking into account that values of conductance
obtained for the system of reaction made of C-acid 1 and
MTBD base in acetonitrile solvent was larger by two orders of
magnitude 80.15 UK1 cm2 molK1 [19].
The plots of kobs vs base concentration show a measurable
intercepts for the reactions of 2, 3, and 4 while for the
reaction of 1 the intercept is zero within two standard
deviations. Then the equilibrium constants calculated as the
ratio of the rate constants for the forward and reverse
K
258R1.6!105 MK1, K258Z1170, 590 and 12 MK1 for the
reaction of phosphazene P1-t-Bu with carbon acids 1, 2, 3, and 4,
Table 2
Chemical shifts (d) found for the reaction of 2-methyl-1-(4-nitrophenyl)-1-nitropropane 4 and P1-t-Bu phosphazene in THF
4
4, anion
P1-t-Bu
P1-t-BuHC
1H (d-ppm rel. to TMS)
2H2,6
m, dZ7.822
2H2,6 DdZK0.096
m, dZ7.918
9H, C–CH3
d, dZ1.146
9H, C–CH3
d, dZ1.178
DdZK0.032
18H, N–CH3
2H3,5
m, dZ8.280
s, dZ5.429
m, dZ2.556–2.824
d, dZ0.757
2H3,5 DdZ0.204
1Ha
m, dZ8.076
not present
m, dZ3.356
d, dZ0.754
d, dZ1.104
18H, N–CH3
m, dZ2.582
m, not legible
1Ha
1H, CH
3H, CH3
3H, CH3
1H, CH
3H, CH3
3H, CH3
d, dZ1.105
13C (d-ppm rel. to TMS)
1Ca
dZ97.502
1Ca
d, dZnot
observed
3C (C–CH3)
6C (N–CH3),
1C (C–CH3),
d, dZ36.197
d, dZ38.128
d, dZ49.732
3C (C–CH3)
DdZ5.246
6C (N–CH3)
DdZ0.127
1C (C–CH3)
s, dZ30.951
d, dZ38.001
2C2,6
2C3,5
1C1
dZ130.251
dZ124.507
dZ141.470
dZ149.629
2C2,6 DdZ3.809
2C3,5 DdZ1.721
1C1
s, dZ126.442
s, dZ122.786
s, dZnot
observed
s, dZnot
observed
s, dZnot
observed
dZ36.012
1C4
1C4
1C (C)
dZ33.672
dZ18.402
dZ19.858
1C (C) DdZK2.340
1C (CH3) DdZK0.698 dZ19.100
1C (CH3) dZ19.528
1C (CH3)
1C (CH3)

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Z. Wisłocka et al. / Journal of Molecular Structure 788 (2006) 152–158
155
Table 3
4
3.5
3
Observed and second order rate constants for proton/deuteron transfer reactions
between 1-nitro-1-(4-nitrophenyl)alkanes O2NC6H4C(L)NO2R; RZH, Me, Et,
i-Pr; LZH, D* and P1-t-Bu phosphazene in THF
T (8C)
P1-t-Bu !
103 (M)
kobs!102
(sK1
kf
kH/kD
2.5
2
)
(dm3 molK1 K1
)
s
1-Nitro-1-(4-nitrophenyl)methane (1), (LZH)
1.5
1
10
15
25
35
45
0.5–2.5
0.5–2.5
0.5–2.5
0.5–2.5
0.5–2.5
369–1818
400–1925
429–2290
507–2527
564–2694
7336G111
7718G115
9357G118
10218G238
10718G113
0.5
0
0
0.005
0.01
0.015
0.02
0.025
1-Nitro-1-(4-nitrophenyl)methane (1), (LZD)
10
15
25
35
45
0.5–2.5
0.5–2.5
0.5–2.5
0.5–2.5
0.5–2.5
23–106
23–123
27–150
38–186
51–251
401G23
474G36
592G32
723G24
995G18
18.3G1.1
16.3G1.2
15.8G0.8
14.1G0.5
10.8G0.2
B [mol–1 dm3]
Fig. 1. Conductometric titration of 4-nitrophenylnitromethane 1 with P1-t-Bu
phosphazene in THF solvent.
respectively. The equilibrium constant for the reaction of C-acid
1, is only roughly estimated using Benesi–Hildebrand formal-
ism, for the increments of absorbance of the product, as a
functionof the base concentration were small and charged with a
large error. Taking into account very large steric hindrance
attributed to the phosphazene P1-t-Bu, these values seem to
harmonize with the values of equilibrium constants already
found for DBU base using Benesi–Hildebrand method KTHFZ
2060, 560, and 3.7 MK1 for 2, 3, and 4, respectively [10]. The
largest equilibrium constant for the reaction of 4 has been found
for TBD base KTHFZ11000 MK1 while with MTBD was only
44 MK1, however, this last was 10 times larger than that with
DBU [10] base KTHFZ3.7 MK1 and P1-t-Bu KTHFZ11.8 MK1
1-Nitro-1-(4-nitrophenyl)ethane (2), (LZH)
10
15
25
35
5–25
5–25
5–25
5–25
1.0–4.5
1.3–5.1
1.9–6.5
2.7–9.5
1.74G0.03
1.90G0.01
2.31G0.05
3.59G0.06
1-Nitro-1-(4-nitrophenyl)ethane (2), (LZD)
10
15
25
35
5–25
5–25
5–25
5–25
0.05–0.21
0.07–0.27
0.11–0.45
0.18–0.70
0.08G0.004
0.10G0.002
0.17G0.001
0.26G0.01
20.8G1.1
18.6G0.4
13.6G0.4
13.8G0.3
1-Nitro-1-(4-nitrophenyl)propane (3), (LZH)
10
15
25
35
5–25
5–25
5–25
5–25
0.24–1.10
0.37–1.27
0.57–1.88
0.96–2.81
0.43G0.02
0.45G0.01
0.66G0.01
0.92G0.03
1-Nitro-1-(4-nitrophenyl)propane (3), (LZD)
¨
(Table 4). These show that the Bronsted relationship does not
10
15
25
35
5–25
5–25
5–25
5–25
0.01–0.06
0.02–0.08
0.03–0.14
0.06–0.26
0.02G0.001
0.03G0.001
0.09G0.002
0.10G0.001
19.7G0.9
15.3G0.7
13.2G0.6
9.6G0.1
hold for the reactions of nitroalkanes with bicyclic guanidines/
amidines [10]and P1-t-Bu bases. This effect couldbe a result of a
steric crowding in the vicinity of the reaction center, however,
the alterations are extremely dramatic, then we say that this is an
indication of the different structure of the transition state TS and
consequently, the mechanism of reaction for each base (DBU
[10], TBD [5], MTBD [5], and finally P1-t-Bu) in THF solvent.
This is undoubtedly visible for the present kinetic
experiments. The rate constants for the reaction of 1-nitro-
1-(4-nitrophenyl)alkanes 1, 2, 3, 4 are collected in Table 3.
The fastest reaction in the group takes place for the least
hindered C-acid 1 k2258Z9357 dm3 molK1 sK1. The replace-
ment of the hydrogen atom on the a carbon atom by the
Me group (C-acid 2), causes the dramatic decrease of the
second-order rate constants by more than three orders of
magnitude (Table 3).
The ratio of k21=k22 is large equal to 4050.7. The gradual
increase of the bulk of R substituents on the a carbon atom
of the C-acid results in a further decrease of the rate constants,
however the reduction is not so large. The ratio of the rate
constants are only k22=k23 Z3:5 and k23=k24 Z7:7. These changes
are the indication of the steric factor, which plays in this system
of reaction a dominant role. However, it is clearly seen that the
replacement of the Me by the Et and i-Pr caused a decrease of
the rate constant only by a factor of 3.5 and 7.7. Then the most
dramatic change is observed while comparing the proton
transfer abstraction with P1-t-Bu phosphazene from not
2-Methyl-1-(4-nitrophenyl)propane (4), (LZH)
15
20
25
35
19.6–187.1
19.6–187.1
19.6–187.1
19.6–187.1
0.55–1.81
0.83–2.17
1.08–2.52
2.06–3.88
0.075G0.001
0.08G0.001
0.09G0.001
0.11G0.001
*Concentrations of C-acids 2.5!10K5 mol dmK3
.
substituted 1, and Me substituted 2 C-acids. Consequently,
taking into consideration this way of changes we claim that the
steric inhibition reached a particular level for the reaction of 2,
as a further increase of the bulk of alkyl substituents effected
moderate changes of the rate constants. This trend is reflected
in the values of the activation parameters collected in Table 5.
The smallest value of the enthalpy of activation has been
found for the reaction of 1, DHsH Z6:1 kJ molK1, while
Table 4
Equilibrium constants for proton transfer reactions between 1-nitro-1-(4-nitro-
phenyl)alkanes O2NC6H4CHNO2R and P1-t-Bu phosphazene in THF at 258
R
lmax (nm)
kf/kb
Kr (molK1 dm3) e (cmK1 molK1 dm3)
H
484
496
498
496
kby0
O100 000
1170G150
590G60
16200G90
13200G80
28500G280
19900G200
Me
Et
315G36
273G23
9.5G0.2
i-Pr
11.8G0.1

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Z. Wisłocka et al. / Journal of Molecular Structure 788 (2006) 152–158
156
Table 5
Activation parameters for proton/deuteron transfer reactions between 1-nitro-1-(4-nitrophenyl)alkanes O2NC6H4C(L)NO2R and P1-t-Bu phosphazene in THF
DHsH
DSsH
DHsD
DSsD
DGsH
DGsD
DHsD KDHsH
R
ðkJ molK1
6.1G1.1
18.0G3.6
20.7G2.5
11.1G1.5
Þ
ðJ molK1 degK1
K149.7G3.5
Þ
ðkJ molK1
16.0G1.1
31.4G1.0
39.9G1.7
Þ
ðJ molK1 degK1
K138.0G3.6
K154.3G3.5
K135.4G5.8
Þ
ðkJ molK1
50.7G1.5
70.6G5.14
74.0G3.5
80.0G2.1
Þ
ðkJ molK1
57.2G1.5
77.4G1.5
80.3G2.4
Þ
ðkJ molK1
Þ
9.9G1.1
13.4G3.7
19.2G3.0
H
Me
Et
K176.5G12.3
K178.7G8.4
K227.8G5.1
i-Pr
the reaction with more hindered C-acids 2, 3 and 4 have much
larger enthalpies of activation reaching
DHsH Z18:0; 20:7 and 11:1 kJ molK1,
respectively. The
enthalpy of activation for the reaction of 1 is unusually small
DHsH Z6:1 kJ molK1, characteristic for diffusion controlled
reactions. Unexpectedly the rate of proton abstraction for this
reaction found is relatively slow k2HZ9357 dm3 molK1 sK1
(Table 3), allowing for stopped flow measurements.
The comparison of the second-order rate constants for the
reactions carried out with DBU, MTBD, TBD and P1-t-Bu is
puzzling as the fastest reaction takes place for TBD base
while for the strongest P1-t-Bu base is the slowest in the
group. The exceptional acceleration of the proton transfer
reaction we already found for the reaction of TBD while for
MTBD, of nearly equal pKa, was two orders of magnitude
slower (Table 5). This effect was attributed to the stronger
stabilization of the TS by an additional hydrogen bond
present in the TBD complex. The MTBD and DBU bases are
not capable to form such cyclic structures of the TS so the
influence of the net steric hindrance brought by these bases is
visible. The reaction with MTBD is slower than with DBU,
which is a weaker base but with less steric crowding in the
vicinity of reaction center [5,10]. Then the unusual
deceleration of the proton transfer induced by very strong
P1-t-Bu phosphazene reflects very large sterical restraints
which suddenly grow after the hydrogen has been replaced
by methyl group in the reacting C-acid (Tables 3 and 5).
The values of the primary deuterium kinetic isotope k2H/k2D
are large ranging from 15 to 13 at 25 8C for this series of
C-acids 1–4 (Table 3). These values are a little larger than those
already found for the reaction with DBU [10], MTBD and TBD
[5] (Table 6). The k2H/k2D are indistinctly larger for the least
hindered C-acid 1, while for more hindered substrates 2, 3, and
4 are smaller, at least at 25 8C (Tables 3 and 6). This decrease
of the KIE values, together with the increasing steric hindrance
brought by reacting C-acids was already found for the system
of reactions between MTBD base and sterically hindered
1-nitro-1-(4-nitrophenyl)alkanes [5], and (2,6-dialkyl-4-nitro-
phenyl)phenylcyano-methanes [15]. However, this trend is
weaker for the present systems of reactions, although the
temperature dependence of k2H/k2D is much visible since the
25 8C increase of temperature caused significant reduction of
the KIE values, larger for more sterically hindered C-acids.
The large, negative values of the entropy of activation
DSsH ZK149:7,K176.5,K178.7,K227.8 J molK1 degK1, res-
pectively indicate considerable ordering of the transition state
(Table 5). However, a visible difference between DSsH values
for the least hindered C-acid 1 and two other carbon acids 2 and
3, for which they are almost equal within the range of standard
deviation while the reaction of the C-acid 4 has much more
negative value (Table 5). Then the large differences in the
enthalpy of activation between the reaction of 1 and the
phosphazene base and other C-acids (2 and 3), of DDHsH Z
12 and 14:6 kJ molK1 and only DDHsH Z5 kJ molK1 for the
reaction of (4) respectively are compensated by the negative
entropies of activation in the way that the appropriate values of
free enthalpy of activation are similar for 2,
3
DGsH y70 kJ molK1, much smaller for the reaction of 1 DGsH y
50 kJ molK1 and definitely larger for the reaction of 4
DGsH Z80 kJ molK1. Then the differentiation in the rate
constants k2258 Z9357 dm3 molK1 sK1 and k2258Z2:31, 0.66 and
0.09 dm3 molK1 sK1 for 1, 2, 3 and 4, respectively (Table 3) is
certainly attributed to the increase in the energy of the
transition state for more hindered carbon acid and the solvation
effects, particularly meaningful for the reaction of 4.
As can be seen (Table 3), the reaction of 1 with P1-t-Bu is
very fast k2258 Z9357 dm3 molK1 sK1, however, still measurable
by the stopped flow method, while the reactions with much
weaker bases DBU, TBD and MTBD were unexpectedly faster
and out of range of this method (Table 6) [5,10].
Table 6
Comparison of second-order rate constants and primary deuterium kinetic isotope effects kH/kD for reactions of 1-nitro-1-(4-nitrophenyl)alkanes O2NC6H4
C(L)NO2R; RZH(1), Me(2), Et(3), i-Pr(4); LZH, D with DBU, MTBD, TBD and P1-t-Bu in THF at 25 8C
R
Base
DBU k2
(dm3molK1 K1
kH/kD
MTBD k2
(dm3molK1 K1
kH/kD
TBD k2
(dm3molK1 K1
kH/kD
P1-t-Bu k2
(dm3molK1 K1
)
kH/kD
s
)
s
)
s
)
s
pKaZ23.9
pKaZ25–25.4
pKaZ24.7–26
pKaZ26.9
H
–
–
–
–
–
–
9357
2.31
0.66
0.09
15.8
13.6
13.2
Me
Et
800
219
7.75
11.4
12.1
9.7
161
46
10.9
10.1
8.5
19000
6800
880
9.7
12.1
14.8
i-Pr
3.05

˙
Z. Wisłocka et al. / Journal of Molecular Structure 788 (2006) 152–158
157
These reduction covered 6.3, 7.0, and 10.1 KIE units for 1, 2,
and 3, respectively (Fig. 2).
(Table 5) indicating predominant influence of steric and
solvation effects visible for sudden decrease of the rate
constants found for the reaction of 2 and 3.
Our general conclusion is that the effect of steric hindrance
in the vicinity of reaction center brought by both reacting
C-acid and a base may conceal other important effects
including Brønsted relationship obviously valid for most
systems of proton transfer reactions [5,8,23–25].
Such a distinctive temperature dependence we already
observed in our earlier paper [15] and was confirmed by Leffek
et al. [21]. However, this is not expected to be extremely large
effect and would be offset by some steric inhibition of C–H and
C–D vibration frequencies in the initial state [22].
The sudden decrease of the rate constants kf found for the
reactions of more hindered C-acids 2 and 3 (Table 3) is
insignificantly reflected in the KIE values as they remain
constant within experimental error.
3. Experimental
Spectroscopic grade THF (Aldrich) was dried with CaH2
and fractionally distilled in the apparatus filled with argon. The
middle fraction of THF (66G058) was collected and kept under
argon to prevent moisture and oxygen. The purity of THF was
controlled by UV spectra to meet standards [26]. The influence
of extraneous water on the rate constants of the proton transfer
reaction between phosphazene P1-t-Bu and the most hindered
C-acid 4 has been tested by measuring the kinetics using
commercial, anhydrous, packed under nitrogen THF by
Aldrich (water content!0.003%).
In the extreme case of a complete loss of the C–H stretching
frequency and two C–H bending frequencies during the
activation process, the maximum isotope effect on the
activation parameters is 8.8 kJ molK1 for DHsD KDHsH and
5.9 J molK1 KK1 for DSsD KDSsH. Since our results are in
excess of these maximum values DHsD KDHsH Z9:9, 13.4, and
19.2 kJ molK1 (Table 5), it is clear that there is a significant
contribution from tunneling effect. One can assume that most
proton transfer goes via tunnel effect as we accepted the
maximum energy loss for three frequency modes instead of
stretching vibrations, commonly used for estimation of the
maximum value of the enthalpy differences. In such a case the
maximum value of the enthalpy difference would be
5.4 kJ molK1 only, so the above values exceed much the
predicted value for the complete loss of C–H stretching
frequency during the activation process. The largest contribution
from the tunneling could be attributed for the reaction of 1,
k2H/k2DZ15.8 with DHsH Z6:1 kJ molK1 while for the
reactions of more hindered C-acids k2H/k2D remain unchanged
13.6 and 13.2 at 25 8C respectively confirming our suggestion
that in our system of reactions the steric inhibition reached a
particular level after the hydrogen atom in the carbon acid 1 has
been replaced by methyl or more bulky substituents. The most
distinct alteration, however is manifested for the entropy
changes DSsD KDSsH Z11:7, 22.2, and 43.3 J molK1 KK1
Previously prepared samples of C-acids: 4-nitrophenylni-
tromethane (1) [27], 1-(4-nitrophenyl)-1-nitroethane (2), 1-(4-
nitrophenyl)-1-nitropropane (3), 2-methyl-1-(4-nitrophenyl)-
1-nitropropane (4) were used [10]. The melting points and
spectra after recrystallization from ethanol were in accord-
ance with the literature. The deuterated analogues of 1, 2, 3,
and 4 were prepared by the modified exchange method in
which the neutralization of the anion of the appropriate
C-acid was effected using acetate buffer instead of concen-
trated DCl in D2O [28], which eliminates the noxious Nef’s
reaction of the nitronic acid formed. The degree of
deuteration of the a-carbon was better than 98% as showed
1
by H NMR spectra (disappearence of dH signals at 5.74, 1
(1 H, s), 5.74, 2 (1 H, q, J 6.95 Hz, CHNO2), 5.49, 3 (1 H, m,
CHNO2), 5.43, 4 (1 H, d, J 10.94 Hz, CHNO2).
Phosphazene P1-t-Bu distributed by Fluka was used without
any further purification. The stock solutions of commercial
phosphazene P1-t-Bu and C-acids (1!10K2 M) were freshly
prepared before each kinetic or equilibrium experiment and
kept under argon, to protect the system against atmospheric
CO2, O2, and moisture. The purity of the base was satisfactory
40
(3)
20
0
1
as tested by H NMR and titration methods [2]. The working
40
(2)
solutions of the C-acids and the base were made up by dilution
of stock solutions. All operations with the solutions of the
carbon acids and the base were accomplished under a positive
pressure of argon.
20
0
Electronic absorption spectra were completed on a Hewlett–
Packard 8452A diode array spectrophotometer using a
thermostat cell holder. All spectra were recorded at 25 8C.
The kinetic measurements were performed using a stopped flow
spectrophotometer made by Applied Photophysics and a Hewlett–
Packard 8452A diode array depending on the rate of reaction. The
values of kobs were measured using 2.5!10K5 mol dmK3
concentration of C-acid and a varying, five concentration of
phosphazene P1-t-Bu base, always in a large excess over carbon
acid to ensure pseudo-first-order kinetics (Table 3). From the
40
(1)
20
0
0
5
10 15 20 25 30 35 40 45 50
T [Co]
Fig. 2. Temperature dependence of KIE for the proton transfer reactions from
1-nitro-1-(4-nitrophenyl)methane 1, 1-nitro-1-(4-nitrophenyl)ethane 2, 1-nitro-
1-(4-nitrophenyl)propane 3 to P1-t-Bu phosphazene in THF

˙
Z. Wisłocka et al. / Journal of Molecular Structure 788 (2006) 152–158
158
[2] G. Schroeder, B. Leska, A. Jarczewski, B. Nowak-Wydra, B. Brzezinski,
J. Mol. Struct. 344 (1995) 77.
kinetic curves the pseudo-first-order rate constants kobs measured
by ‘stopped flow’ method have been achieved by an ‘end point
method’, but for slower reaction of deuteron transfer, measured by
means of ‘diode array’ spectrophotometer were calculated by
Guggenheim method.
[3] R. Schwesinger, Chimia 39 (1985) 269.
[4] I. Grzeskowiak, W. Galezowski, A. Jarczewski, Can. J. Chem. 79 (2001)
1128.
[5] W. Galezowski, I. Grzeskowiak, A. Jarczewski, J. Chem. Soc., Perkin
Trans. 2 (1998) 1607.
The observed rate constants for proton transfer reaction
were a function of base concentration according to the
following equation:
[6] F.G. Bordwell, H.E. Fried, Tetrahedron Lett. (1977) 1121.
[7] F.G. Bordwell, J.E. Bartmess, J.A. Hautala, J. Org. Chem. 43 (1978) 3107.
[8] E.S. Lewis, L.H. Funderburk, J. Am. Chem. Soc. 89 (1967) 2322.
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[11] A. Jarczewski, J. Koput, I. Nowak, Crystal structure and infrared spectra
of dicesium trans-tetraaquadichlorochromium (III) chloride, J. Mol.
Struct. 708 (2004) 105–111.
kobs Z kfHcB CkbH
(1)
where kfH and kbH are the rate constants for the forward and
reverse reactions respectively and cB is the concentration
of the base. The second-order rate constants are the
calculated slopes of kobs versus base concentration. The
intercepts for proton transfer reaction are the rate constants
of the backward reaction kbH. For reactions with deutero
analogues of C-acids the initial part of the curves was
ignored to minimize the possible error caused by
contamination of C-D-acid with not deuterated C–H acid
in accordance with Bergson [29] and Melander and
Saunderss [22] who suggested that this procedure reduces
the probability of results being distorted. The remaining
part of Guggenheim plot was a good straight line. Then
Eq. (1) was applied in order to determine second order rate
constants kf.
[12] R. Schwesinger, Nachr. Chem. Tech. Lab. 38 (1990) 1214.
[13] K.T. Leffek, P. Pruszynski, K. Thanapaalasingham, Can. J. Chem. 67
(1989) 590.
[14] R. Schwesinger, J. Willaredt, H. Schlemper, M. Keller, D. Schmitt,
H. Fritz, Chem. Ber. 127 (1994) 2435.
[15] K. Minksztym, A. Jarczewski, J. Mol. Struct. 631 (2003) 203.
[16] M. Stanczyk-Dunaj, W. Galezowski, A. Jarczewski, Can. J. Chem. 80
(2002) 1259.
[17] W. Galezowski, A. Jarczewski, J. Chem. Soc., Perkin Trans. 2 (1989) 1647.
[18] W. Galezowski, M. Stanczyk, I. Grzeskowiak, A. Jarczewski, J. Chem.
Soc., Perkin Trans. 2 (1996) 2647.
´
[19] I. Nowak, PhD Thesis, University of A. Mickiewicz, Poznan, Poland,
2001, pp. 121.
[20] H.A. Benesi, J.H. Hildebrand, J. Am. Chem. Soc. 71 (1949) 2703.
[21] K.T. Leffek, P. Pruszynski, Can. J. Chem. 66 (1988) 1454.
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Wiley, New York, 1980, pp. 287–291 (Chapter X).
Acknowledgements
[23] R.P. Bell, The Proton in Chemistry, second ed., Chapman & Hall,
London, 1973 (Chapters 7, 10, 12).
The authors are grateful for financial support from
Polish State Committee for the grant No. 3 T09A 035 29,
and for the funds from Faculty of Chemistry, A.
Mickiewicz University.
[24] F. Terrier, L. Xiao, P.G. Farrell, D. Moscowitz, J. Chem. Soc., Perkin
Trans. 2 (1992) 1259 (and refs therein).
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1980, pp. 98.
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