Solvation models in the reaction between phenacyl bromide and 2-mercaptobenzimidazole

Article abstract of DOI:10.1080/17415991003664755

The reaction between phenacyl bromide and 2-mercaptobenzimidazole has been studied in 12 different protic and aprotic solvents. The kinetic study shows that the reaction is second-order with first-order dependence each on (phenacyl bromide) and (2-mercapt

Full text of DOI:10.1080/17415991003664755

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Solvation models in the reaction
between phenacyl bromide and 2-
mercaptobenzimidazole
Brahmanapally Kavitha a & Prerepa Manikyamba a
a Department of Chemistry, Kakatiya University, Warangal, 506
009, AP, India
Published online: 07 Apr 2010.
To cite this article: Brahmanapally Kavitha & Prerepa Manikyamba (2010) Solvation models in the
reaction between phenacyl bromide and 2-mercaptobenzimidazole, Journal of Sulfur Chemistry,
31:2, 103-108, DOI: 10.1080/17415991003664755
To link to this article: http://dx.doi.org/10.1080/17415991003664755
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Journal of Sulfur Chemistry
Vol. 31, No. 2, April 2010, 103–108
Solvation models in the reaction between phenacyl bromide and
2-mercaptobenzimidazole
Brahmanapally Kavitha and Prerepa Manikyamba*
Department of Chemistry, Kakatiya University, Warangal-506 009, AP, India
(Received 2 May 2009; final version received 31 January 2010)
The reaction between phenacyl bromide and 2-mercaptobenzimidazole has been studied in 12 different
protic and aprotic solvents. The kinetic study shows that the reaction is second-order with first-order
dependence each on (phenacyl bromide) and (2-mercaptobenzimidazole). Analysis of rate data shows that
there is no direct correlation between the rate constant and dielectric constant of the solvent. Correlation of
rate data with different solvent parameters like hydrogen bond acceptor basicity, polarizability and solvent
electrophilicity, using linear multiple regression analysis shows that the reaction is influenced by these
properties of the solvent. From the regression coefficients, information on the mode of solvation of the
reactants and the transition state is obtained. The reaction has also been studied at different temperatures
and the thermodynamic parameters ꢀH#, ꢀS# and ꢀG# are evaluated.
Keywords: solvent effects; linear solvation energy relationship (LSER); phenacyl bromide;
2-mercaptobenzimidazole; solvation parameters
1. Introduction
A solvent influences the rate of a reaction by solvating the reactants and the transition state. A
substance is solvated due to two types of interactions with the solvent, namely specific and non-
specific interactions (1).All solvents interact with the substances non-specifically. The intensity of
these non-specific interactions is measured in terms of polarity of the solvent (Y) and polarizability
(P) of the solvent. Specific solvation of the substance primarily occurs when the solvent interacts
with a charged atom or group in a molecule. The intensity of these interactions is measured
in terms of electrophilicity (E), nucleophilicity (B), hydrogen bond donor (HBD) ability (α),
hydrogen bond acceptor ability (β), etc. So, the general term ‘polarity’means the solvating ability
of the solvent due to either all or some of the above properties of the solvent. Therefore, the
effect of solvent on reaction rate is to be expressed not by a single parameter equation, but by a
multiparameter equation (2), i.e.
log k = log ko + aA + bB + cC + · · ·
(1)
*Corresponding author. Email: mani_prerepa@yahoo.co.in
ISSN 1741-5993 print/ISSN 1741-6000 online
© 2010 Taylor & Francis
DOI: 10.1080/17415991003664755
http://www.informaworld.com

104 B. Kavitha and P. Manikyamba
where k is the rate constant of the reaction in any solvent and ko is the rate constant in a
reference solvent which is inert. The coefficients a, b, c are the susceptibilities of k to the respec-
tive solvent parameters A, B, C, etc. These coefficients indicate the relative solvation of the
reactants over the transition state and the mode of solvation. With this end in view, the reac-
tion between phenacyl bromide and 2-mercaptobenzimidazole has been studied in 12 protic
and aprotic solvents and the results are presented. The role of solvent effects in the rates of
reactions has been studied extensively (3–12) for nucleophilic substitution at sp3 and sp2 car-
bon using a variety of nucleophiles. Similar studies using phenacyl halide are few (13, 14).
However, the substituent effect and nucleophile effect involving phenacyl bromide as substrate,
and the oxidation of phenacyl bromide by V5+, Mn3+ and Ce4+ are reported in the literature
(15–17).
2. Results and discussion
The second-order rate constants are determined in 12 different protic and aprotic solvents in
the 303–318 K temperature range. These are presented in Table 1. Correlation of these log kII
values with different solvent parameters, namely polarity (Y) (18), polarizability (P) (18), solvent
electrophilicity (E) (18), solvent nucleophilicity (B) (18), HBD acidity (α) (19), hydrogen bond
acceptor basicity (β) (19) and specific polarizability (π∗) (19), did not give satisfactory results,
indicating that the rate of reaction is not influenced by any single property of a solvent. Out of
these, a better correlation is observed with polarity – polarizability parameter with the following
relation:
log kII = −1.50 + 0.76π∗ r = 0.81
SE = (0.14)(0.18) (0.14)
(2)
The values in the parentheses are standard errors of the coefficients. A glance at these values
indicates that these values are highly dependent on the nature of the solvent. However, there
appears to be no correlation between log kII and the dielectric constant of the solvent. Addition of
the second parameter to the single parametric equation improved the strength of the correlation
Table 1. Second-order rate constants at different temperatures and thermodynamic parameters at 303 K. [Phenacyl
bromide] = [2-mercaptobenzimidazole] = 2.00 × 10−2 mol dm−3
.
kII × 102 dm3 mol−1 s−1 at T/K
Ea
(kJ mol−1
ꢀH#
(kJ mol−1
ꢀS#
(Jk−1 mol−1
ꢀG#
(kJ mol−1
δꢀG=
(kJ mol−1
)
Solvent
303 308 313
318
)
)
)
)
Methanol
Ethanol
10.96 23.99 36.31
7.94 15.85 22.92
7.58 29.08 46.77
7.08 13.80 19.06
6.61 31.62 60.23
11.22 33.88 53.70
8.91 20.28 42.62
26.92 41.69 53.70
22.39 36.31 47.86
53.71
34.66
67.61
27.54
93.33
83.18
63.43
67.61
61.66
81.88
95.67
111.49
69.41
110.05
102.36
104.92
47.69
79.36
93.15
108.97
66.89
107.53
99.84
102.40
45.17
49.90
−1.55
41.29
93.13
79.83
80.64
80.75
80.93
81.10
79.77
80.35
77.57
78.04
0
0.81
0.92
1.10
n-Propanol
i-Propanol
n-Butanol
2-Butanol
i-Butanol
Benzyl alcohol
N,N-dimethyl
formamide
Formamide
−46.35
87.22
1.27
66.24
72.78
−0.06
0.52
−106.93
−2.26
−1.79
52.42
−92.85
20.89 33.88 45.71
64.57
51.29
61.65
38.13
55.97
24.7
35.61
53.45
22.18
−140.60
−83.24
78.21
78.67
76.65
−1.62
−1.16
−3.19
Dimethyl sulfoxide 17.38 20.62 28.48
2-Ethoxy ethanola 38.91 41.69 50.12
−179.76
Note: aEliminated from statistical analysis as complete set of solvent parameters data is not available.

Journal of Sulfur Chemistry 105
slightly, and some of these biparametric relationships are
log kII = −1.28 + 0.62 π∗ − 0.19 α R = 0.84
SE = (0.24)(0.22)(0.17) (0.13)
log kII = −0.80 + 0.64 π∗ − 0.77β R = 0.96
SE = (0.16)(0.099)(0.16) (0.071)
(3)
(4)
Although there is an increase in the value of the multiple regression coefficient R, it is not a
satisfactory correlation. So the analysis is extended using three parameters:
log kII = −0.66 − 0.14 α − 0.74 β + 0.53 π∗ R = 0.97
SE = (0.16)(0.079)(0.14)(0.11) (0.063)
(5)
t-stat = (−4.07)(−1.79)(−5.32)(4.99) F = 37.95
The statistical analysis is not extended further using four solvent parameters as there are no
sufficient data points to use linear four parametric equations. The validity of this LSER obtained
is tested by conducting the following tests:
(1) F-test: The Fcal is very high (37.95) compared with the table value (Ftable7.59) at 1% level
of significance (20). This suggests that this LSER is not a chance correlation.
(2) t-test: The significance of individual parameters is tested by comparing the tcal with ttable. This
comparison indicates that the parameters π∗ and β are significant at 99% confidence level
and α is significant at 90% confidence level (20).
(3) Correlation of log kcal with log kobs (21): This correlation indicates that there is excellent
correlation between these two values with a correlation coefficient (r) of 0.97 and a slope
of 1.00.
From the above observations the following conclusions can be drawn:
(1) The rate of the reaction is influenced by HBD acidity i.e., α of the solvent. As there are polar
centers in the reactants, the transition state solvent can easily form hydrogen bonds. The
negative sign of this coefficient in the above equation suggests that the reactants are more
solvated than the transition state due to hydrogen bond donation. The rate decreases due to
decrease in the HBD ability of the solvent.
(2) The reaction is facilitated in solvents having higher hydrogen bond acceptor ability measured
by their β values. The negative sign and the magnitude of the coefficient of β in Equation (9)
suggest that HBA solvents more strongly solvate the reactants than the transition state does
and such solvents do not assist the formation of the transition state.
(3) The rate of the reaction is influenced by π*, which is a measure of the ability of the solvent
to stabilize a charge or a dipole by virtue of its polarity – polarizability. The positive sign
of this coefficient in the above equation suggests that the transition state is more extensively
solvated by such solvents than the reactants. Hence, the rate of this reaction increases with
the increase in the π* value of the solvent.

106 B. Kavitha and P. Manikyamba
Considering these points, the mode of solvation of the reactants and the transition state can be
represented as follows.
Solvation due
to polarity
HBD solvation
HBD solvation
HBA solvation
H
H
C
C
Br
H
S
H
–
N
+
H
O
C
C
C
Br
H
N
N
H
O
C
S
+
N
H
O
C
H
N
H
S
C
C
HBr
+
N
H
The normalization of the coefficients used in the above LSER indicates that the contributions of
the solvent parameters α, β and π∗ towards overall solvation of the reactants and the transition
state are 8, 55 and 37%, respectively. The solvation of the reactants by specific hydrogen bond
acceptor interaction exceeds that of the transition state. The dipolar interactions between transition
state and solvent is more than the reactant – solvent interactions. The second-order rate constants
are evaluated in all the solvents at different temperatures ranging from 303 to 318 K.
The thermodynamic parameters ꢀH#, ꢀS# and ꢀG# computed are given in Table 1. A perusal
of this data shows that ꢀH# and ꢀS# are also dependent on the nature of the solvent. The reaction
is entropy-controlled, indicating that the extent of solvation of the reactants is strongly dependent
on the nature of the solvent. The present reaction involves a neutral nucleophile and a negatively
charged leaving group Br−. Therefore, the charges on the two ends of the transition state, and
hence their solvation energies, are different. A change from a protic to dipolar aprotic solvent
results in a large difference in the relative solvation energies of the two ends of the transition state.
This difference in solvation energies is large enough to cause a shift in the electron density in the
−
−
− −
area between S and Br atoms along Br C S bond axis in the transition state, resulting in a
change in the structure of the transition state, as the solvent is changed.
The inherent limitation in the applicability of multiparameter correlation for any reaction is that
the mechanism of the process under examination should not change with change in the solvent.
The free energy of activation ꢀG# is nearly constant (79.38 2.25 kJ mol−1), suggesting the
operation of a common mechanism in all the solvents. Thus it can be concluded that in the
present system, the transition state and the reactants are solvated by specific solvent – solute
interactions.
The differential free energy values computed by taking methanol as the reference solvent are
presented in Table 1. These values suggest that the transition state is more stabilized in 2-butanol,
benzyl alcohol, formamide, N,N-dimethyl formamide, dimethyl sulfoxide and 2-ethoxy ethanol
when the reaction system is changed from methanol. This data also suggests that the differential
solvation is maximum in the case of benzyl alcohol and 2-ethoxy ethanol. This suggests the role
of intramolecular hydrogen bonding in stabilizing the transition state. In the rest of the solvents,
it is less stabilized compared with methanol.

Journal of Sulfur Chemistry 107
3. Experimental
The solvents methanol, ethanol, n-propanol, i-propanol, n-butanol, i-butanol, benzyl alcohol,
dimethyl sulfoxide, formamide, N,N-dimethyl formamide and 2-ethoxy ethanol were all of
analytical reagent (AR) grade and were used after purification by methods in the literature.
Phenacyl bromide (Merck) and the nucleophile 2-mercaptobenzimidazole (Aldrich) were used as
such.
The solutions of the reactants of required concentrations were prepared by dissolving a known
volume/weight of phenacyl bromide/nucleophile in a known volume of the solvent. The reaction
was initiated by mixing the thermally equilibrated solutions of the substrate and the nucleophile at
the required temperature. The course of the reaction was followed by measuring the conductance
of the reaction mixture at different time intervals using a conductivity bridge (CENTURY make).
Measurement of rates at different concentrations of the substrate and the nucleophile showed that
the reaction is overall second-order, with first-order dependence each on the (substrate) and the
(nucleophile). The second-order rate constants were calculated using the relation (22)
1
Ct
kII =
(6)
at C∞ − Ct
where Ct and C are the conductances of the reaction mixture at t and infinite time intervals, and
∞
a is the initial concentration of the reactants. The rate constants thus determined were found to
be reproducible within 5% error. A PCL personal computer was used to carry out the multiple
regression analysis. F-test (23) and student t-test (23) were used to know the validity of the
bi-and tri-parameter equations obtained. The product separated from methanol had a melting
point of 120 2◦C. The IR spectral data (KBr) of this compound shows absorption bands around
2690 cm−1 and 1400 cm−1 indicating the presence of an S–CH2 group and another band around
700 cm−1 indicating the thioether link, C–S–C. These values agree with the literature values
(24) of 2700–2600, 1420 and 720 cm−1. These data suggest the formation of S-phenacyl 2-
mercaptobenzimidazole. One of the products formed in this reaction is HBr, and therefore the
conductance of the reaction mixture increases. Thus by noting the conductance at different time
intervals, the rate constant of the reaction is calculated.
References
(1) Koppel, I.A.; Palm, V.A. Advances in Linear Free Energy Relationship; Chapman, N.V., Shorter, J., Eds.; Plenum
Press: New York, 1972; p 222.
(2) Reichardt, C. Ang. Chem. 1979, 18, 98–110.
(3) Ohkata, K.; Nagai, T.; Tomaru, A.; Hanafusa, T. J. Chem. Soc. Perkin Trans 2 1986, 43–48.
(4) Martinez, R.D.; Mancini, P.M.E.; Voltero, L.R.; Nudleman, N.S. J. Chem. Soc. Perkin Trans 2 1986, 1427–1431.
(5) Manikyamba, P.; Sundaram, E.V. Int J. Chem. Kinet. 1990, 22, 1153–1165.
(6) Manikyamba, P. Indian J. Chem. 1992, 31A, 959–962.
(7) Manikyamba, P. Indian J. Chem. 1996, 35A, 334–336.
(8) Gholami, M.R.; Habibi Yangjeh, A. Int J. Chem. Kinet. 2001, 33, 118–123.
(9) Habibi Yangjeh, A.; Gholami, M.R. Indian J. Chem. 2003, 42B, 895–899.
(10) Ranga Reddy, S.; Manikyamba, P. Indian J. Chem. 2004, 43A, 1092–1094.
(11) Kalyani, P.; Manikyamba, P. Indian J. Chem. 2004, 43A, 76–78.
(12) Ranga Reddy, S.; Manikyamba, P. Indian J. Chem. 2006, 45A, 1844–1847.
(13) John Winston, S.; Jayaprakash Rao, P.; Sethuram, B.; Navneeth Rao, T. Indian J. Chem. 1996, 35A, 979–982.
(14) Shanmuga Sundaram, A.; Raj Kumar, M. Indian J. Chem. 1986, 25A, 71–73.
(15) Harikishan, B.; Sundaram, E.V. J. Indian Chem. Soc. 1989, 66, 363–364.
(16) Harikishan, B.; Sundaram, E.V. Indian J. Chem. 1989, 28A, 793–795.
(17) Harikishan, B.; Sundaram, E.V. J. Indian Chem. Soc. 1980, 57, 1079–1082.
(18) Aslam, M.H.; Collier, G.; Shorter, J. J. Chem. Soc. Perkin Trans 2 1981, 1572–1576.
(19) Taft, R.W.; Abboud, J.L.M.; Kamlet, M.J.; Abraham, M.H. J. Org. Chem. 1983, 48, 2877–2887.
(20) Weast, R.C.; Selby, S.M. Handbook of Chemistry and Physics, The Chemical Rubber Company, Linde Company:
New York, 1967 A; p 165.

108 B. Kavitha and P. Manikyamba
(21) International Group for Correlation Analysis in Organic Chemistry (Announcement), Quant. Struct. Act Relat,
1985, 4, 29.
(22) Frost, A.A.; Pearson, R.G. Kinetics and Mechanism; Wiley Eastern: New Delhi, 1970; p 37.
(23) Shorter, J. Correlation Analysis of Organic Reactivity; John Wiley & Sons, Research Studies Press: NewYork, 1982;
p 107.
(24) Silverstein, R.M.; Bassler, G.C.; Moill, T.C. Spectrometric Identification of Organic compounds, 4th ed.; John Wiley
& Sons: New York, 1981; p 190.