Synthesis, characterization and DPPH scavenging activity of some benzimidazole derivatives

Article abstract of DOI:10.4314/bcse.v32i2.8

A base-catalyzed conversion of aldehydes to benzimidazoles has been achieved. The compounds have been characterized by IR, NMR, micoranalysis, and GC-MS. The reaction for the formation of benzimidazoles has been monitored with ¹H NMR and IR. Th

Full text of DOI:10.4314/bcse.v32i2.8

Bull. Chem. Soc. Ethiop. 2018, 32(2), 271-284.
2018 Chemical Society of Ethiopia and The Authors
DOI: https://dx.doi.org/10.4314/bcse.v32i2.8
ISSN 1011-3924
Printed in Ethiopia

SYNTHESIS, CHARACTERIZATION AND DPPH SCAVENGING ACTIVITY OF
SOME BENZIMIDAZOLE DERIVATIVES
1
*
2
1
1
3
Felix Odame , Jason Krause , Eric C. Hosten , Richard Betz , Kevin Lobb , Zenixole R.
1
2
Tshentu and Carminita L. Frost
1
Department of Chemistry, Nelson Mandela University, P.O. Box 77000, Port Elizabeth 6031,
South Africa
2
3
Department of Biochemistry and Microbiology, Nelson Mandela University, P.O. Box 77000,
Port Elizabeth 6031, South Africa
Department of Chemistry, Rhodes University, P.O. Box 94, Grahamstown 6140, South Africa
(
Received January 23, 2018; Revised June 14, 2018; Accepted June 20, 2018)
ABSTRACT. A base-catalyzed conversion of aldehydes to benzimidazoles has been achieved. The compounds
have been characterized by IR, NMR, micoranalysis, and GC-MS. The reaction for the formation of
1
benzimidazoles has been monitored with H NMR and IR. The crystal structures of two derivatives, 2-(2-
chlorophenyl)-1H-benzimidazole and 2-(1H-benzimidazol-2-yl)-4-nitrophenol, are presented. A study of the
DPPH scavenging activity of these compounds showed that 2-(1H-benzimidazol-2-yl)phenol (2), 2-p-tolyl-1H-
benzimidazole (3) and 2-(4-methoxyphenyl)-1H-benzimidazole (7) gave IC50 values 1974, 773 and 800 µM.
KEY WORDS: Benzimidazole, o-Phenylenediamine, Aldehydes, Base catalysis, DPPH scavenging activity
INTRODUCTION
Benzimidazoles have been used in the synthesis of a variety of compounds and materials [1-2].
The common route for the synthesis of benzimidazoles from diamines is through a condensation
reaction with carboxylic acids under acid catalysis at high temperatures [3]. Microwave
irradiation has also been used to synthesize benzimidazoles by the reaction of anthranilic acid
and o-phenylenediamine, however, via the base-catalyzed route (using potassium carbonate and
ethanol) [4].
The aldehyde route has also been explored to achieve benzimidazoles from diamines despite
the fact that monoimines or diimines should be the expected products. 2-Substituted
benzimidazoles have been synthesized in the presence of alumina–methanesulfonic acid (AMA)
by microwave irradiation [5]. Catalytic amounts of zinc acetate have been used to synthesize
benzimidazoles from aldehydes and o-phenylenediamine at room temperature [6]. Boron
trifluoride dietherate has also been used to catalyze the synthesis of benzimidazoles from o-
phenylenediamine and aldehydes under solvent free conditions [7]. 1,3-Dibromo-5,5-dimethyl-
hydanton (DBH)–catalyzed solvent-free synthesis of 2-arylbenzimidazoles has been achieved
via microwave irradiation of o-phenylendiamine and aryl aldehydes [8].
A wide variety of 2-substituted benzimidazoles and bis-benzimidazoles have been
synthesized in high yields by PEG-mediated catalyst-free synthesis under solvent-free
conditions [9]. The synthesis of benzimidazoles through the coupling of aldehydes with o-
phenylenediamine by using highly acidic nanoporous aluminosilicate with 3D structure and
cage-type pores as the catalyst has been achieved. The catalyst resulted in excellent yields in
short reaction times presumably due to its high acidity, large pore diameter, high surface area,
and cage-type 3D porous structure [10].
_
*
_________
Corresponding author. E-mail: felixessah15@gmail.com
This work is licensed under the Creative Commons Attribution 4.0 International License

272
Felix Odame et al.
Evaluation of the antioxidant activities of natural substances has been of interest in recent
years. Antioxidants scavenge free radicals and reactive oxygen species and can be extremely
important in inhibiting oxidative mechanisms that lead to degenerative diseases [11]. Free
radicals have been implicated as playing a role in cardiovascular disease, cancer, Alzheimer’s
disease and Parkinson’s disease. The antioxidant capacity of compounds sources is usually
associated with their phenolic contents. Although plant polyphenols such as tannins and
flavonoids have problems of astringency and protein binding which have grouped them under
the category of anti-nutrients [12-13], they have been found useful as natural antioxidants in
scavenging deleterious free radicals released in the body by fat metabolism [14]. Some phenolic
compounds have also been tested for their antioxidant activity [15-17].
Herein, we report a base-catalyzed method for the synthesis of benzimidazoles from
aldehydes and o-phenylenediamine, the characterization of the compounds with IR, NMR,
microanalysis, GC-MS has been presented. The single crystal XRD of 2-(2-Chlorophenyl)-1H-
benzimidazole (4) and 2-(1H-benzimidazol-2-yl)–4-nitrophenol (8) have been discussed. The
DPPH scavenging activities of the compounds have also been discussed.
EXPERIMENTAL
Analytical grade reagents and solvents for synthesis and analysis which included; o-
phenylenediamine, dimethyl sulfoxide, m-toluoaldehyde and 5-nitrosalicyaldehyde were
obtained from Sigma Aldrich (USA) whilst tetrahydrofuran, salicyaldehyde, 2-bromo-
salicyaldehyde, 3-methoxybenzaldehyde, 4-methoxybenzaldehye, 4-methylbenzaldehyde, 2-
chlorobenzaldehyde, 4-nitrobenzaldehyde, toluene, ethyl acetate, ethanol, methanol and
triethylamine (99%) were obtained from Merck Chemicals (SA). The chemicals were used as
1
13
received (i.e. without further purification). H NMR and C NMR spectra were recorded on a
1
13
C
Bruker Avance AV 400 MHz spectrometer operating at 400 MHz for H and 100 MHz for
using DMSO-d as solvent and tetramethylsilane as internal standard. Chemical shifts are
6
expressed in ppm. FT–IR spectra were recorded on a Bruker Platinum ATR Spectrophotometer
Tensor 27. Elemental analyses were performed using a Vario Elementar Microcube ELIII.
Melting points were obtained using a Stuart Lasec SMP30 whilst the masses were determined
using an Agilent 7890A GC System connected to a 5975C VL-MSC with electron impact as the
ionization mode and detection by a triple-Axis detector. The GC was fitted with a 30 m x 0.25
mm x 0.25 µm DB-5 capillary column. Helium was used as carrier gas at a flow rate of 1.6
−
1
−1
mL.min with an average velocity of 30.2 cm s and a pressure of 63.7 kPa.
2-(3-Methoxyphenyl)-1H-benzimidazole (1)
o-Phenylenediamine (3.24 g, 0.030 mol) was mixed with 3-methoxybenzaldehyde (4.08 g, 0.030
o
mol) in 20 mL of triethylamine and heated at 120 C for 12 h. The reaction was followed by
TLC to completion. The solvent was removed using the high vacuum pump and to obtain brown
oil, which was washed with hot ethanol (150 mL) and the purified on a column using ethyl
acetate:methanol (1:1). The product recrystallized as a light brown solid from ethanol. Melting
o
−1
point = 203–204 C. Yield = (4.75 g) 70.62%. IR (ν , cm ) 3071 (N–H), 3002 (N–H), 2799
max
1
(
(
C–H), 2685, 1602 (C=N), 1491 (C–N), 1434 (C–N). 784. H NMR (ppm): 12.92 (s, 1H), 7.77
m, 2H), 7.68 (m, 1H), 7.54 (m, 1H), 7.48 (m, 1H), 7.22 (m, 2H), 7.07 (m, 1H), 3.89 (s, 3H).
13
C
NMR (ppm): 159.6, 151.1, 143.6, 134.9, 131.4, 130.1, 122.6, 121.7, 118.8, 118.7, 115.9, 111.3,
+
5
5.3. LRMS (m/z, M ): Found for C14
12 2
H N O = 224.10, Expected mass = 224.26. Anal. calcd.
for C14 O: C, 74.98; H, 5.39; N, 12.49. Found: C, 74.60; H, 5.41; N, 12.22.
12 2
H N
2-(1H-Benzimidazol-2-yl)phenol (2)
o-Phenylenediamine (3.24 g, 0.03 mol) was mixed with salicyaldehyde (3.66 g, 0.030 mol) in 20
o
mL of triethylamine and heated at 120 C for 12 h. The reaction was followed by TLC to
Bull. Chem. Soc. Ethiop. 2018, 32(2)

Synthesis, characterization and DPPH scavenging activity of benzimidazole derivatives 273
completion. The solvent was removed at the pump and to obtain a brown solid which was
washed with hot ethanol (150 mL) and purified on a column using ethyl acetate:methanol (1:1).
The product recrystallized as a light brown solid from ethanol:THF (2:1). Melting point = 236–
o
−1
2
(
2
1
2
1
38 C. Yield = (4.62 g) 73.29%, IR (ν , cm ): 3238 (N–H), 3053 (N–H), 1630 (C=N), 1590
max
1
C=C), 1488 (C–N), 1454 (C–N). H NMR (ppm): 13.16 (br, 1H), 8.06 (d, J = Hz, 1H), 7.67 (m,
H), 7.39 (m, 1H), 7.29 (m, 2H), 7.04 (m, 2H). C NMR (ppm): 158.0, 151.6, 131.7, 126.2,
+
19.1, 117.2, 112.6.. LRMS (m/z, M ): Found for C13
10.23. Anal. calcd. for C13
3.24.
13
H
10
2
N O = 210.10, Expected mass =
H N O: C, 74.27; H, 4.79; N, 13.33. Found: C, 74.38; H, 4.64; N,
10 2
2-p-Tolyl-1H-benzimidazole (3)
o-Phenylenediamine (3.24 g, 0.03 mol) was mixed with 4-methylbenzaldehyde (3.60 g, 0.03
o
mol) in 20 mL of triethylamine and heated at 120 C for 12 h. The reaction was followed by
TLC to completion. The mother liquor was allowed to stand overnight. The product was filtered,
washed with hot ethanol (150 mL) and purified on a column with ethyl acetate:methanol (1:1).
The product recrystallized as a light brown solid from ethanol:THF (1:1). Melting point = 270–
o
−1
2
(
7
1
=
71 C. Yield = (4.72 g) 75.49%. IR (νmax., cm ): 3054 (N–H), 2915 (C–H), 1552 (C=C), 1500
1
C=C), 1475 (C–N), 1452 (C–N). H NMR (ppm): 12.86 (s, 1H), 8.11 (m, 2H), 7.57 (m, 1H),
13
.46 (m, 1H), 7.38 (m, 2H), 7.15 (m, 2H), 2.44 (s, 3H), C NMR (ppm): 151.37, 139.56,
+
29.31, 126.36, 125.96, 20.92. LRMS (m/z, M ): Found for C14
208.26. Anal. calcd. for C14 : C, 80.74; H, 5.81; N, 13.45. Found: C, 80.82; H, 5.68; N,
3.35.
12 2
H N = 208.10, Expected mass
12 2
H N
1
2-(2-Chlorophenyl)-1H-benzimidazole (4)
o-Phenylenediamine (3.24 g, 0.03 mol) was mixed with 2-chlorobenzaldehyde (4.22 g, 0.03
o
mol) in 20 mL of triethylamine and heated at 120 C for 12 h. The reaction was followed by
TLC to completion. The solvent was removed at the pump and to obtain a brown oil. The dry
product was washed with hot ethanol (150 mL) and the product recrystallized as a light brown
o
solid from ethanol:THF (1:1). Melting point = 227–228 C. Yield = (5.04 g) 73.64%. IR (νmax,
−
1
1
cm ): 3061 (N–H), 3001 (N–H), 1590 (C=C), 1441 (C–N). H NMR (ppm): 7.92 (d, J = Hz,
1
3
1
1
H), 7.64 (m, 3H), 7.52 (m, 2H), 7.24 (m, 2H). C NMR (ppm): 149.1, 132.0, 131.6, 131.2,
30.3, 129.9, 127.4, 122.2, 115.3. Found: C, 68.46; H, 3.89; N, 12.34. LRMS (m/z, M ): Found
+
for C13
H
9
ClN
2
O = 228.80, Expected mass = 228.68. Anal. calcd. for C11
ClN O : C, 68.28; H, 3.97; N, 12.25.
12 2 4 2
H N O S.H O: C,
4
6.15; H. Found. for C13
H
9
2
2-Phenyl-1H-benzimidazole (5)
o-Phenylenediamine (3.24 g, 0.03 mol) was mixed with benzaldehyde (3.18 g, 0.03 mol) in 20
o
mL of triethylamine and heated at 120 C for 12 h. The reaction was followed by TLC to
completion. The solvent was removed at the pump and to obtain brown oil. The dry product was
washed with hot ethanol (150 mL) and the product recrystallized as a light brown solid from
o
−1
ethanol:THF (1:1). Melting point = 291–292 C. Yield = (4.54 g) 78.00 %. IR (νmax, cm ): 3048
1
(
(
N–H), 1622 (C=N), 1591 (C=C), 1494 (C–N), 1462 (C–N). H NMR (ppm): 12.90 (s, 1H), 7.56
m, 2H), 7.48 (br, 1H), 7.35 (m, 4H), 7.01 (s, 2H). C NMR (ppm): 151.2, 130.0, 129.8, 128.9,
13
+
1
26.4. LRMS (m/z, M ): Found for C13
10 2
H N =194.10, Expected mass = 194.23. Anal. calcd. for
13 10 2
C H N
: C, 80.39; H, 5.19; N, 14.42. Found: C, 80.43; H, 5.25; N, 14.30.
2-(4-Nitrophenyl-1H-benzimidazole (6)
o-Phenylenediamine (3.24 g, 0.03 mol) was mixed with 4-nitrobenzaldehyde(4.53 g, 0.03 mol)
o
in 20 mL of triethylamine and heated at 120 C for 12 h. The reaction was followed by TLC to
Bull. Chem. Soc. Ethiop. 2018, 32(2)

274
Felix Odame et al.
completion. The solvent was removed at the pump and to obtain brown oil. The dry product was
washed with hot ethanol (150 mL) and the product recrystallized as a light brown solid from
o
−1
ethanol:THF (1:1). Melting point = 320–321 C. Yield = (5.52 g) 76.94%. IR (νmax., cm ): 3035
1
(
(
N–H), 2952 (C–H), 1604 (C=N), 1512 (C=C), 1434 (C–N). H NMR (ppm): 8.40 (m, 4H), 7.64
m, 2H), 7.27 (m, 2H). C NMR (ppm): 149.0, 147.8, 135.9, 127.3, 124.2, 122.9. LRMS (m/z,
M ): Found for C H N O = 239.60, Expected mass = 239.23. Anal. calcd. for C H N O : C,
13
+
1
3
9
3
2
13
9
3
2
6
5.29; H, 3.79; N, 17.56. Found: C, 65.46; H, 3.85; N, 17.48.
2-(4-Methoxyphenyl)-1H-benzimidazole (7)
o-Phenylenediamine (3.24 g, 0.03 mol) was mixed with 4-methoxybenzaldehyde (4.08 g, 0.03
o
mol) in 20 mL of triethylamine and heated at 120 C for 12 h. The reaction was followed by
TLC to completion. The solvent was removed at the pump and to obtain brown oil. The dry
product was washed with hot ethanol (150 mL) and the product recrystallized as a light brown
o
solid from ethanol:THF (1:1). Melting point = 220–222 C. Yield = (5.26 g) 78.16%. IR (νmax.,
−
1
cm ): 3054 (N–H), 2953 (C–H), 2835 (C–H), 1611 (C=N), 1500 (C=C), 1476 (C–N), 1454 (C–
1
13
N). H NMR (ppm): 12.78 (br, 1H), 8.18 (m, 2H), 7.50 (m, 2H), 7.22 (m, 2H), 7.16 (m, 2H). C
+
NMR (ppm): 160.6, 151.3, 128.0, 122.7, 114.3, 67.0, 55.3, 25.1. LRMS (m/z, M ): Found for
C H N O =224.10, Expected mass = 224.26. Anal. calcd. for C H N O : C, 74.98; H, 5.59;
1
4
12
2
14 12
2
N, 12.49. Found: C, 74.72; H, 5.42; N, 12.63.
2-(1H-benzimidazol-2-yl)–4-nitrophenol (8)
o-Phenylenediamine (3.24 g, 0.03 mol) was mixed with 5-nitrosalicyaldehyde (5.01 g, 0.03 mol)
o
in 20 mL of triethylamine and heated at 120 C for 12 h. The reaction was followed by TLC to
completion. The solvent was removed at the pump to obtain a yellow solid which was washed
with hot ethanol (150 mL) and recrystallized as a yellow solid from DMSO:tolulene (1:1)
o
−1
.
Melting point = 335–336 C, Yield = (4.65 g) 60.69%. IR (ν , cm ): 3274 (N–H), 3097 (N–
max
1
H), 1628 (C=N), 1587 (C=C), 1407 (C–N). H NMR (ppm): 9.12 (s, 1H), 8.26 (d, J = Hz, 1H),
.73 (m, 2H), 7.35 (m, 2H) 7.20 (d, J = Hz, 1H). C NMR (ppm): 164.1, 149.7, 139.3, 136.4,
27.0, 123.5, 122.9, 118.3, 115.0, 112.6. LRMS (m/z, M ): Found for C13H N O = 255.80,
9 3 3
13
7
1
+
Expected mass = 255.23. Anal. calcd. for C H N O : C, 61.18; H, 3.55; N, 16.46. Found: C,
13
9
3
3
6
1.29; H, 3.64; N, 16.56.
2-(3-Methylphenyl)-1H-benzimidazole (9)
o-Phenylenediamine (3.24 g, 0.03 mol) was mixed with m-methylbenzaldehyde (3.60, 0.03 mol)
o
in 20 mL of triethylamine and heated at 120 C for 12 h. The reaction was followed by TLC to
completion. The solvent was removed at the pump and to obtain brown solid, which was washed
with ethanol and purified on a column using ethyl acetate:methanol (1:1). The product was
recrystallized from methanol:THF (1:1) and obtained as a white solid. Melting point = 213–215
o
–1
C, Yield = (4.87 g) 78.00%. IR (νmax, cm ): 3052 (N–H), 2986 (C–H), 2879 (C–H), 1661
1
(
C=N), 1590 (C=C), 1487 (C–N). H NMR (ppm): 12.88 (s, 1H), 8.04 (d, J = Hz, 1H). 7.99 (d, J
=
Hz, 1H), 7.66 (m, 1H), 7.54 (s, 1H), 7.41 (m, 1H), 7.31 (m, 1H), 7.20 (m, 2H), 2.41 (s, 3H).
1
3
C NMR (ppm): 151.5, 138.1, 130.4, 130.2, 128.77, 127.0, 123.6, 121.9, 115.0, 21.0. LRMS
+
(
m/z, M ): Found for C14
H
12
N
2
=194.10. Expected mass = 194.23. Anal. calcd for C23
H
24
N
2
O
3
:
C, 74.97; H, 6.71; N, 11.66. Found: C, 74.67; H, 6.42; N, 11.95.
X-ray crystallography
X-ray diffraction analyses of 4 and 8 were performed at 200 K using a Bruker Kappa Apex II
diffractometer with monochromated Mo Kα radiation (λ = 0.71073 Å). APEXII [18] was used
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Synthesis, characterization and DPPH scavenging activity of benzimidazole derivatives 275
for data collection and SAINT [18], for cell refinement and data reduction. The structures were
solved by direct methods using SHELXS–2013 [19], and refined by least-squares procedures
using SHELXL-2013 [20], with SHELXLE [20], as a graphical interface. All non-hydrogen
atoms were refined anisotropically. Carbon-bound H atoms were placed in calculated positions
(
C–H 0.95 Å for aromatic carbon atoms and C–H 0.99 Å for methylene groups) and were
included in the refinement in the riding model approximation, with Uiso (H) set to 1.2Ueq (C).
The H atoms of the methyl groups were allowed to rotate with a fixed angle around the C–C
bond to best fit the experimental electron density (HFIX 137 in the SHELX program suite [19],
with Uiso (H) set to 1.5Ueq (C). Nitrogen-bound H atoms were located on a difference Fourier
map and refined freely. Data were corrected for absorption effects using the numerical method
implemented in SADABS [19].
DPPH assay
1
,1-Diphenyl-2-picryl-hydrazyl (DPPH) and ascorbic acid were purchased from Sigma–Aldrich
(
South Africa). A stock solution of DPPH (0.2 mM) was prepared in methanol. The solvents and
-
6
other chemicals were of analytical grade. Compounds 1 to 9 (64 to 30x10 mM) were tested for
-
8
DPPH scavenging activity. Ascorbic acid (500 to 23x10 μM) was used as a positive control.
Briefly, 100 μL of each compound was added to a 96 well microtitre plate followed by addition
of 100 μL DPPH (0.2 mM). Microtitre plates were wrapped in aluminium foil and kept in the
dark at room temperature for 30 min. Spectrophotometric measurements were done at 517 nm
using a BioTek Epoch 2 microtitre plate reader. The data was expressed as mean ± SD. The
experiment was carried out in triplicate. An average was obtained from the triplicate values for
sample and blank, the blank was subtracted from all samples. The % DPPH scavenging = ((Abs
DPPH-Abs Sample)/(Abs DPPH))*100. The result was then plotted against the log of
concentration.
RESULTS AND DISCUSSION
General studies
The disappearance of the carbonyl of the aldehydes and ketones were confirmed by both the IR
1
3
1
and the C NMR spectra for all the compounds. The H NMR spectrum of compound 1 gave a
singlet peak at 12.92 ppm for the proton of the benzimidazole and another singlet at 3.89 ppm
1
3
for a methyl group. The C NMR and DEPT spectra confirmed the formation of a
benzimidazole by the carbon at the 2-position appearing at 159.6 ppm as well as the presence of
1
the methyl group at 55.3 ppm. A broad signal at 13.16 ppm in the H NMR spectrum of
1
3
compound 2 confirmed the hydroxyl proton. The signal at 158.0 ppm in the C NMR spectrum
confirmed the formation of benzimidazole of compound 2.
1
A singlet peak at 12.92 ppm in the H NMR spectrum of compound 3 confirmed the proton
of the benzimidazole. Another singlet signal at 2.44 ppm for three protons confirmed the
1
3
presence of the methyl group. The signal at 151.37 ppm in the C NMR spectrum confirmed the
1
3
formation of the benzimidazole in compound 3. The signal at 149.1 ppm in the C NMR
spectrum of compound 4 was attributed to the benzimidazole carbon.
1
The proton of the benzimidazole occurred as a singlet signal at 12.90 ppm in the H NMR
1
3
spectrum of compound 5, the signal at 151.2 ppm in the C NMR spectrum also confirmed the
1
3
formation of benzimidazole. In the C NMR spectrum of compound 6 the signal at 149.0 ppm
1
is attributable to the benzimidazole carbon. In the H NMR spectrum of compound 7, the
1
3
benzimidazole proton occurred as a broad signal at 12.78 ppm whilst the C NMR spectrum
confirmed the benzimidazole carbon at 160.6 ppm.
1
The benzimidazole proton occurred as a singlet at 9.12 ppm in the H NMR spectrum of
1
3
C
compound 8 and the occurrence of a benzimidazole carbon was noticed at 151.5 ppm in the
Bull. Chem. Soc. Ethiop. 2018, 32(2)

276
Felix Odame et al.
1
NMR spectrum. The H NMR spectrum of compound 9 gave a signal at 12.88 ppm attributable
1
3
to the benzimidazole proton whilst the signal at 151.5 ppm in the C NMR spectrum confirmed
the formation of the benzimidazole. Table 1 gives the scope and yields of the different
derivatives of the synthesized benzimidazoles. The base, triethylamine, was used as a solvent.
Table 1. Scope and yields of substituted benzimidazoles
Reaction
Time (h)
Entry
Aldehyde
Product
%Yields
OCH
3
H
N
1
3-OCH
3
6
C
6
H
4
CHO
12
71
N
HO
H
N
2
3
4
5
2-OHC
H
4
CHO
12
12
12
12
73
75
74
78
N
H
N
4-CH
3
C
6
H
4
CHO
CH
3
N
Cl
H
N
6 4
2-ClC H CHO
N
H
N
6 5
C H CHO
N
H
N
O
O
6
7
4-NO
2
C
6
H
4
CHO
N
12
12
77
78
N
H
N
4-OCH
3
C
6
H
4
CHO
OCH3
N
HO
H
N
2
-OH-5-
8
9
12
12
61
78
NO C H CHO
2
6 3
N
N
O
O
CH
3
H
N
3 6 4
3-CH C H CHO
N
Bull. Chem. Soc. Ethiop. 2018, 32(2)

Synthesis, characterization and DPPH scavenging activity of benzimidazole derivatives 277
R
2
R
1
R
2
R
1
H
H
2
2
N
N
O
H
N
Et N
3
R
3
R
3
N
H
R
R
R
1
2
3
=H, OH, Cl,
= H, OCH ,NO
= H, OCH ,NO
3
2
, CH
, CH
3
3
3
2
Scheme 1. Synthesis scheme of benzimidazoles from aldehydes and o-phenylenediamine
Scheme 1 gives the synthesis route for the base-catalyzed formation of benzimidazoles from
aldehydes and o-phenylenediamine. The reaction is proposed to proceed by the abstraction of a
proton from an amine of o-phenylenediamine by triethylamine (I), which allows for the attack
of the carbonyl of the aldehyde (II) by the amine forming a hydroxylamine (III) (Scheme 2).
The loss of water from (III) gives an imine (IV) which loses a proton to yield (V), and a further
proton loss and rearrangement leads to the formation of (VI). The proton loss stages are
proposed to be activated by triethylamine, of which the last step is facilitated by the presence of
oxygen hence the base-catalysis.
O
H
H
N
CH
3
OH
CH
3
NH
NH
H
H
2
NH
2
1
II
III
-
H O
2
CH
3
CH
3
N
N
H
N
H
V
NH
H
IV
H
O2
CH
3
N
N
VI
H
Scheme 2. Proposed mechanism for the synthesis of compound 2 from an aldehyde.
1
The H NMR of the reaction for the first hour was done at ten minutes intervals. The
formation of the aldehyde (IV in Scheme 2) is evidenced by the peak at 9.96 ppm (Figure 1).
Bull. Chem. Soc. Ethiop. 2018, 32(2)

278
Felix Odame et al.
The aldehydic proton begins to disappear after 40 min, which is consistent with the formation of
V in the mechanism in Scheme 2. Also the protons at 7.80 and 7.43 ppm also disappear from the
spectrum within the first hour. Upon attachment of the carbon atom from the aldehyde the four
protons from the o-phenylene diamine molecule become non-equivalent this makes the protons
appear as four signals in the spectrum. The signal at 7.80 ppm merges with that at 7.82 ppm
which is a signal for two protons with integration for two protons, whilst the signal at 7.43 ppm
also merges with signal at 7.33 ppm which also gives integration for two protons. There is no
direct experimental evidence for the oxidation step it is postulated to happen via oxidation in
literature.
1
Figure 1. The H NMR spectrum of the progress of the reaction of o-phenylene diamine with 3-
methylbenzaldehyde was again measured at 1 hour intervals but no significant changes
were observed.
The IR spectrum (Figure 2) measured at 10 min internals for the first hour confirms
formation of a hydroxyl group as shown in III in the mechanism in Scheme 2. The hydroxyl
group disappears in the course of the reaction which is consistent with the loss of hydroxyl
group as water in IV of Scheme 2. The increase in the intensity of the signals at 1456 and 1492
-
1
cm in the IR spectrum confirms the formation of benzimidazole.
Characterization of crystal structures
Compound 4 recrystallized as a light brown solid from ethanol:THF(1:1), whilst compound 8
recrystallized as a yellow solid from DMSO:tolulene (1:1). The computed and crystallographic
data and selected bond lengths and bond angles for all the crystal structures are provided in
Tables 2 and 3. The ORTEP diagrams for compounds 4 and 8 at 50% ellipsoid are presented in
Figures 3 and 4. Compound 4 crystallized in the orthorhombic space group Pbca, while
compound 8 crystallized in the triclinic space group P-1.
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Synthesis, characterization and DPPH scavenging activity of benzimidazole derivatives 279
Figure 2. IR spectrum of the progress of reaction, measurement done at 10 min intervals.
Table 2. Crystallographic data and structure refinement summary for compounds 4 and 8
Property
Formula
CCDC Number
Formula Weight
Crystal System
Space group
a(Å)
Compound 4
ClN
1062232
228.67
Orthorhombic
Pbca
7.0310(4)
Compound 8
C H N O
13 9 3 3
C
13
H
9
2
1062215
255.23
Triclinic
P-1
6.4211(5)
8.2150(6)
10.7976(9)
100.794(3)
99.810(3)
94.002(3)
548.26(7)
2
b(Å)
c (Å)
α(°)
β(°)
9.9397(7)
32.1093(18)
90
90
90
γ(°)
V [Å ]
Z
^3
2244.0(2)
8
3
D(calc) [g/cm^ ]
Mu(MoKa) [ /mm ]
F(000)
1.354
0.311
944
1.546
0.114
264
Crystal Size [mm]
Temperature (K)
Radiation [Å]
θ Min-Max [°]
Dataset
0.11 x 0.25 x 0.41
0.04 x 0.16 x 0.31
200
200
0.71073
2.5, 28.3
-9: 8 ; -11: 13 ; -42: 42
0.71073
2.0, 28.3
8: 8 ; -10: 10 ; -14: 14
9939, 2718, 0.023
2052
Tot., Uniq. Data, R(int)
Observed Data [I > 2.0 sigma(I)]
Nref
19671, 2793, 0.023
2321
2793
2718
Npar
149
177
R,
wR2
S
0.0523
0.1293
1.08
0.0478,
0.1324
1.11
Max. and Av. Shift/Error
Min. Residual. Dens.[e/Å^3]
Max. Residual Dens. [e/Å^3]
0.00, 0.00
-0.37
0.31
0.00, 0.00
-0.27
0.29
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Felix Odame et al.
The bond distances of N1–C1, and N2–C1, which were 1.323(2), and 1.355(2) Å,
respectively, were consistent with the C–N bond of the amide of compound 4 whilst N2–C22
bond length was 1.376(2). The bond angles of C1-N1-C21 and C1-N2-C22 are 105.1(1) and
o
1
07.1(1) , respectively around the nitrogen atoms.
Figure 3. An ORTEP diagram of compound 4 at 50% ellipsoid.
The bond distances of O1–C22 and O2–N3 which were 1.339(2) and 1.236(2) Å,
respectively, in compound 8 are slightly longer than the average bond length of C=O bonds on
the CCDC database which is 1.228 Å [21]. The torsion angles of C22–N2–C3–O2 and C21–
o
o
N1–C2–O2 in compound I were 5.3(3) and 180.0(3) respectively.
Figure 4. An ORTEP diagram of compound 8.
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Synthesis, characterization and DPPH scavenging activity of benzimidazole derivatives 281
Table 2. Selected bond lengths (Å) and bond angles (°) of compounds 4 and 8
Bond lengths
Compound 4
Cl1–C12
C21–C22
N1–C1
C23–C24
N2–C1
N2–C22
C25–C26
C12–C13
C14–C15
Compound 8
O1–C22
N3–C25
O2–N3
O3–N3
N1–C1
C21–C22
N1–C11
N2–C1
1.733(2)
1.401(2)
1.323(2)
1.379(3)
1.355(2)
1.376(2)
1.374(3)
1.389(4)
1.386(4)
1.339(2)
1.451(2)
1.224(2)
1.236(2)
1.327(2)
1.414(3)
1.390(2)
1.359(2)
1.381(2)
N2–C12
Bond angles
Compound 4
Compound 8
C1-N1-C21
N2-C22-C23
C1-N2-C22
N1-C1-N2
N1-C1-C11
N2-C1-C11
N1-C21-C22
N1-C21-C26
N2-C22-C21
105.1(1)
132.1(2)
107.1(1)
112.8(1)
122.8(2)
124.2(2)
109.5(1)
130.3(2)
105.5(1)
C1-N1-C11
C1-N2-C12
O2-N3-C25
O3-N3-C25
O2-N3-O3
N1-C1-C21
N1-C1-N2
105.3(2)
107.2(1)
119.3(1)
117.8(1)
122.9(1)
122.8(2)
112.5(2)
109.4(2)
124.6(2)
N1-C11-C12
N2-C1-C21
DPPH scavenging activity
Antioxidant activity is a commonly studied area of research [22, 23]. DPPH is a stable free
radical that can accept an electron or hydrogen radical to become a stable diamagnetic molecule.
Due to its odd electron, the methanolic solution of DPPH shows a strong absorption band at 517
nm. DPPH radical reacts with various electron donating molecules (reducing agents or
antioxidants). When electrons become paired off, bleaching of DPPH solution is the outcome,
this results in the formation of the colourless 2,2'-diphenyl-1-picryl hydrazine. Reduction of the
DPPH radicals can be estimated quantitatively by the decrease in absorbance 517 nm [24].
Scheme 3 gives the formation of the DPPH radical. The activity of the compounds being tested
is determined by their ability to easily contribute a proton to the DPPH radical. The data was
expressed as mean ± SD. The data is given in Table 2. Plots of percentage scavenging activity
against log of concentrations of compounds 1-9 and ascorbic acid are shown in Figure 5.
NO
2
NO
2
RH
N
N
2
O N
O
2
N
N
H
N
NO
2
NO
2
Scheme 3. The formation of the DPPH radical.
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282
Felix Odame et al.
Figure 5. Plots of percentage scavenging activity against log of concentrations of compounds 1-
and ascorbic acid.
9
Bull. Chem. Soc. Ethiop. 2018, 32(2)

Synthesis, characterization and DPPH scavenging activity of benzimidazole derivatives 283
Table 2. DPPH scavenging activity of compounds 1-9.
Compound
IC-50 (μM)
NC
1
2
1974
3
773
4
18843
0.0828
0.005078
800
5
6
7
8
7601
9
8909
Ascorbic acid
2.37
In these set of compounds their scavenging activity is greatly influenced by the ease of loss
of a proton. Of all the compounds tested only compounds 2, 3 and 7 exhibited DPPH activity. In
the case of compounds 3 and 7 their activity could be due to their symmetrical nature allowing
them to easily occupy the binding site of the DPPH. The very low activities of these compounds
could be because they do not have readily available proton for scavenging but might interact via
association with the radical to form a stable complex. Compound 2 on the other hand could
donate the proton on the hydroxyl group in it DPPH scavenging activity.
CONCLUSION
A base-catalyzed synthesis method for the formation of benzimidazoles from aldehydes and
diaminobenzene has been developed, and the derivatives that represent the scope of the reaction
are presented and have been fully characterized using IR, NMR, microanalysis and GC-MS. The
single crystal XRD of 2-(2-chlorophenyl)-1H-benzimidazole (4) and 2-(1H-benzimidazol-2-yl)–
4-nitrophenol (8) have been discussed. The DPPH scavenging activities of the compounds have
also been presented.
ACKNOWLEDGEMENTS
We thank MRC for the research funding (MRC-SIR). F. Odame thanks the Nelson Mandela
Metropolitan University for the awarding him a bursary.
Supplementary material. Supplementary data associated with this article can be found in the
online version. CCDC numbers 1062232 and 1062215 contain the crystal structures associated
with this article.
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