Monitoring of reduction reaction of 2-((phenylimino)methyl)phenol compound with sodium borohydride in solution by infrared spectroscopy

Article abstract of DOI:10.1080/00387010.2020.1792503

In this study, an imine compound 2-((phenylimino)methyl)phenol was synthesized by the reaction of salicylaldehyde and aniline for in situ monitoring of reduction with sodium borohydride. The reduction of C = N double bond to C–N in solution was continuous

Full text of DOI:10.1080/00387010.2020.1792503

Spectroscopy Letters
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Monitoring of reduction reaction of 2-
((phenylimino)methyl)phenol compound with
sodium borohydride in solution by infrared
spectroscopy
Onur Turhan & Hatice Yaşar
To cite this article: Onur Turhan & Hatice Yaşar (2020): Monitoring of reduction reaction of
2-((phenylimino)methyl)phenol compound with sodium borohydride in solution by infrared
spectroscopy, Spectroscopy Letters, DOI: 10.1080/00387010.2020.1792503
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SPECTROSCOPY LETTERS
https://doi.org/10.1080/00387010.2020.1792503
Monitoring of reduction reaction of 2-((phenylimino)methyl)phenol
compound with sodium borohydride in solution by infrared spectroscopy
Onur Turhan and Hatice Ya s¸ ar
Department of Chemistry, Faculty of Arts and Sciences, Balıkesir University, C¸ a gꢀ ı s¸ , Turkey
ABSTRACT
ARTICLE HISTORY
Received 24 May 2020
Accepted 1 July 2020
In this study, an imine compound 2-((phenylimino)methyl)phenol was synthesized by the
reaction of salicylaldehyde and aniline for in situ monitoring of reduction with sodium boro-
hydride. The reduction of C ¼ N double bond to C–N in solution was continuously moni-
tored by infrared spectroscopy using the background defining method in liquid cell at
ambient temperature. The structures of the purified 2-((phenylimino)methyl)phenol com-
pound and the resulting 2-((phenylamino)methyl)phenol compound were elucidated by
KEYWORDS
Background defining;
imines; infrared
spectroscopy; reaction
monitoring; reduction
1
13
Infrared Spectroscopy, H and C Nuclear Magnetic Resonance Spectroscopy.
Introduction
reaction of aniline with salicylaldehyde. The reac-
tion medium of the imine to the amine was
monitored in the FT-IR liquid cell by the back-
ground (bg) defining method.
Amines are important compounds used in
organic chemistry as reagents or intermediates
and in many industrial fields.
widely used as raw materials in polymer chemis-
try, solvents in organic chemistry, additives in
the plastic and textile industries. It is also fre-
[
1]
Amines are
There are methods in the literature for moni-
toring various organic reaction environ-
[
8–10]
ments.
In the method used in the study,
[
2]
[3]
unlike other FT-IR methods, the beginning of the
reaction was defined as the background (bg) and
then the reaction environment was monitored by
FT-IR. This method, which was previously used
quently used in the paint,
and food industry.
pharmaceutical
The amine derivatives
[
4]
obtained by the reaction of aldehydes and
ketones with ammonia, primary or secondary
amines in a reducing reagent medium are synthe-
sized for use in biological and chemical proc-
[
11]
by us to monitor some imination reactions,
hydrazone formation reactions and complex for-
[
5]
[12]
esses.
Different substituted amine derivatives
mations,
has some advantages.
[
6]
can be obtained by reducing the imines.
[
7]
Since amines are biologically important and
interesting compounds in addition to their use in
many fields, the reaction of reducing imines to
the amine examined in the study is also very
important. Therefore, the reduction reaction of 2-
Materials and methods
The chemicals used in the experiments were pur-
chased commercially from Riedel, Fluka, Merck
companies and used without further purification.
FT-IR measurements were obtained using Perkin-
(
(phenylamino) methyl) phenol, an imine com-
Elmer Brand Spectrum 65 model FT-IR CaF2
pound, with NaBH in methanol solvent was
4
liquid cell with cell thickness (light path length)
monitored by FT-IR.
The secondary amine compound (2-((phenyla-
mino)methyl)phenol) in this study was obtained
by reduction of the imine compound (2-((phenyl-
imino)methyl)phenol) which obtained from the
of 0.015 mm. FT-IR spectrum measurements were
recorded ten scans and averaged with a reso-
lution of 1 cm . NMR’s were obtained with
ꢀ1
Agilent Technologies brand 400 MHz NMR.
CONTACT Onur Turhan
oturhan@balikesir.edu.tr
Department of Chemistry, Faculty of Arts and Sciences, Balıkesir University, C¸ a gꢀ ı s¸ , Turkey.
ß 2020 Taylor & Francis Group, LLC

2
O. TURHAN AND H. YAŞAR
Figure 1. The infrared spectra of (a) chloroform in background mode, (b) benzaldehyde dissolved in chloroform in background
mode (c) obtained by reading the benzaldehyde solution after recording the b spectrum as background in the range of
ꢀ1
1
250–4000 cm . Infrared vibrations of all components in benzaldehyde solution (benzaldehyde and chloroform) were ignored by
the device in spectrum (c). (Chloroform allows working in this wave numbers range due to its infrared vibrations). These three
spectra explain the basis of the background defining method in the study.
Background defining method
components (solvent, solute) in the solution are
ignored by the FT-IR, the spectrum is obtained
as a straight line. “Defining the background at
any time in infrared measurements allows the
vibration of all the components in the environ-
ment to be ignored by the device” is the basis of
the bg definition method.
Receiving an IR spectra in solutions is a slightly
different than obtaining an IR spectrum of solids.
Because the substance to be measured is dis-
solved in organic solvents, the solvent also
absorbs the IR rays. Therefore, before the IR
measurement the IR spectrum of the solvent has
to be taken and recorded in the background
mode to eliminate the vibrations come from the
solvent. Then, the scanned spectrum of the solu-
tion is shown only the substance like to
be measured.
The idea of background defining is, if the
infrared spectra of a pure solvents can be defined
as background (bg), the solutions can also be
recorded as background. In Fig. 1a chloroform
recorded as bg mode. In Fig. 1b is the solution of
benzaldehyde in chloroform in background
mode. When the spectrum is examined, the
Organic solvents are compounds with certain
organic functional groups therefore there are IR
regions where they give very strong absorbance.
If the sample to be measured gives absorbance at
the same wavelength ranges as the solvent, it is
not possible to measure these vibrations. Even if
background identification is done, the IR spec-
trum of the solution can be measured in the
wavelength range (range in which there is no
severe absorbance) allowed by the solvent. In
order to perform IR analysis in solution medium,
the wavelength range allowed by the solvent must
be determined in advance. The best working
range for methyl alcohol used in this study was
ꢀ1
vibrations seen at 3019 and 2400 cm belong to
ꢀ1
ꢀ1
chloroform, while the vibration at 2360 cm is
caused by CO in the air. The vibration band at
determined as 1500–2000 cm .
2
The background defining method has been
previously used to monitor some organic reac-
tions in the liquid cell with FT-IR. The basis of
this method is to ignore vibrations caused by all
components in the environment at the beginning
ꢀ1
1
702 cm belongs to the benzaldehyde molecule.
After recording the spectrum (b) as background,
a new scan was performed and the spectrum (c)
is obtained. Since vibrations from air and all

SPECTROSCOPY LETTERS
3
Figure 3. Reduction reaction of 2-((phenylamino)methyl)phe-
nol. Reduction of the imine compound monitored by using
background defining method in solution.
Figure 2. Imine formation reaction of salicylaldehyde and anil-
ine in ethanol. The synthesis of imine compound which is
used for reduction reaction.
Reduction of 2-((phenylimino)methyl)phenol
of the reaction by the FT-IR device. Reaction
medium FT-IR measurements taken over time
result only from changes in the medium. As a
result, the received spectrum includes increasing
and decreasing inputs in the environment.
The method, while the starting point of the
reaction (mixture of input materials and solvent)
is defined as background, all vibrations in the
environment ignored by FT-IR. Obtaining a spec-
trum at any time of the reaction medium shows
the vibrations of the input substances above the
transmittance, while the vibrations of the product
formed below the transmittance line.
In this study, the reduction reaction of the
imine compound, obtained from the reaction of
salicylaldehyde and aniline, with NaBH4 in
MeOH was investigated by FT-IR using the back-
ground defining method.
For this purpose, firstly the imine product is
synthesized by the reaction of salicylaldehyde
with aniline (Fig. 2), then this imine compound
is converted to amine by reduction with NaBH4.
This reduction reaction is also monitored by
means of background defining method by FT-IR
in solution.
The reduction reaction of the imine compound
pmp (Fig. 3) was monitored in solution media by
FT-IR in the liquid cell.
0.01 mol (1.97 g) of the synthesized pmp com-
pound was taken and its solution in methanol
was prepared in a 50 mL flask.
The reduction reaction was monitored in solu-
tion using the bg defining method. For this pur-
pose, the first used solvent (MeOH) is defined as
background and the range of the wavenumbers
ꢀ1
(2000–1500 cm ) is set as a default. The pmp
solution prepared in methanol was taken into the
liquid cell and the IR spectrum was scanned and
then recorded against the methanol background.
Thus, only vibrations of bmp were obtained. For
further measurements the same bmp solution
(reaction starting point) is recorded as bg. By this
way, all vibrations originating from the pmp in
solvent were reset by the device. Then, while
mixing the pmp solution in a flask at ambient
temperature, NaBH was added to the medium as
4
a certain amount of solid in order for the reduc-
tion reaction to take place. Since bg was defined
at the beginning of the reaction, only changes in
the reduction reaction medium were observed
with FT-IR measurements.
Synthesis of 2-((phenylimino)methyl)phenol (pmp)
In order to follow the reduction reaction step
1
1
5
0 mmol (0.912 ml) aniline was added to the
0 mmol (1.043 ml) salicylaldehyde solution in
0 mL of ethyl alcohol. 1–2 drops of glacial acetic
by step, 0.0005 mol (0.0189 g) NaBH was added
4
into the pmp solution, which continued to mix,
and after the gas discharge process was com-
pleted, some reaction mixture was transferred to
the FT-IR liquid cell and the IR spectrum of the
medium was recorded. The addition of NaBH4
repeatedly into the solution continued until the
peak intensities in the IR spectra did not
observed. The change in peak intensities is ended
on the eighth addition.
acid were added to the reaction mixture as a
catalyst. The reaction mixture was stirred at
ꢁ
70 C under reflux for 24 hours. A yellowish
crude product is obtained by evaporating the
solvent and recrystallized from ethanol. The
structure of the product was determined by FT-
1
13
IR, H-NMR and C-NMR.

4
O. TURHAN AND H. YAŞAR
Figure 4. Infrared spectra of (a) salicylaldehyde, (b) aniline and (c) 2-((fenilimino)methyl)phenol compound. ATR-Infrared spectra of
ꢀ
1
the synthesized imine compound and the input materials aniline and salicylaldehyde are given in the range of 600–4000 cm for
ꢀ
1
comparison. The infrared spectra are represented on the % transmittance axis (arbitrary unit). The carbonyl vibration at 1661 cm
ꢀ1
in the infrared spectrum (a) of salicylaldehyde was converted to C ¼ N vibration at 1614 cm in the spectrum (c) of the imine
ꢀ
1
compound. Primary amine vibration at 3431 and 3352cm in the infrared spectrum (b) of aniline was lost in the product spec-
trum (c).
ꢀ
1
ꢀ1
After the reaction is completed, the solvent
was evaporated and crude product 2-((phenyla-
mino)methyl)phenol (pamp) was crystallized
from the mixture of MeOH and EtOH (1/2-v/v).
The solution of the purified pamp compound in
MeOH was prepared and IR spectrum was taken
3034 cm . The band in 1618 cm is caused by
the bending vibration of the –NH group.
2
In Fig. 4c, the spectrum of the imine com-
pound pmp obtained is given. In this spectrum,
it is seen that the carbonyl vibration that may
be caused by salicylaldehyde and the vibrations
belonging to the –NH group of the aniline dis-
appear and the vibration at 1614 cm
ꢀ1
in the range of 2000–1500 cm and recorded for
comparison with in situ IR spectrums.
2
ꢀ1
of the
imine structure of the C ¼ N double
bond occurs.
Results and discussion
The NMR data: 1H NMR (CDCl , d, ppm)
3
Structural analysis of 2-((phenylimino)methyl)
phenol (pmp)
6
4
.95 (t, 1H), 7.05 (d, 1H), 7.29 (m, 3H), 7.41 (m,
H), 8.63 (s, 1H, HC ¼ N)), 13.07 (b, 1H, OH).
1
Fig. 4a When the FT-IR spectrum of salicylalde-
hyde is examined, the band seen in
When the H-NMR results are interpreted, the
hydrogen in the imine structure signals at
8.63 ppm, the total of nine hydrogen signals in
the aromatic rings in the structure of the product
and the hydrogen in the phenolic –OH group.
ꢀ1
2845–2750 cm
originates from the symmetric
and asymmetric stretching vibrations of the alde-
hyde hydrogen and the band seen at 1661 cm
ꢀ
1
1
3
is due to the stretching vibration of the
C¼O group.
C NMR (CDCl , d, ppm) 117.3, 119.0, 119.2,
3
121.2, 126.8, 129.4, 132.2, 133.1, 148.5, 161.1
(Aromatic carbon atoms) 162.7 (C¼N). The pres-
ence of imine carbon at 162.7 ppm in 13 C NMR
results and 10 different C atom signals in the
aromatic region supports the structure of
the molecule.
Two peaks seen in the IR spectrum of aniline
ꢀ1
in Fig. 4b at 3431 and 3352 cm results from
symmetrical and asymmetrical tensile vibrations
of the primary amine (–NH ). Aniline aromatic
2
ꢀ1
CH stress bands seen at 3070 cm
and

SPECTROSCOPY LETTERS
5
Figure 5. Infrared spectra of (a) 2-((phenylimino)methyl)phenol (imine) and (b) 2-((phenylamino)methyl)phenol (reduced product).
ꢀ
1
ATR-Infrared spectra of the synthesized imine compound and reduced product (amine) are given in the range of 600–4000 cm
for comparison. The infrared spectra are represented on the % transmittance axis (arbitrary unit). The vibration of C ¼ N at
ꢀ
1
ꢀ1
1
614 cm in the spectrum of the imine compound disappeared after reduction. Secondary amine vibration is seen at 3261 cm
in the spectrum of the reduced product.
Structural analysis of 2-((phenylamino)methyl)
phenol (pamp)
amine group at 48.7 ppm also support the
molecular structure.
When the IR spectrum in Fig. 5a is examined,
the vibration of the C¼N double bond in pmp is
Investigation of pamp compound formation
reaction with FT-IR simultaneously
ꢀ1
seen in 1614 cm . In the spectrum of the pamp
ꢀ1
(
b) N–H vibration is seen at 1594 cm . The
In Fig. 6, spectrum (a) shows the IR spectra of
methanol bg pmp (c) product (pamp). The spec-
trum (b) each IR spectrum illustrates a scan of
the reaction media.
Focusing on Fig. 6b, it is seen that there are
bands below and above the transmittance line
since the starting point of the reaction is defined
as background. While the vibrations above the
transmittance line belong to the input substance
product has a single –N–H tensile vibration at
ꢀ1
3
261 cm seen in Fig. 5b. These IR data show
that pmp converts from imine to the
amine pamp.
1
The NMR data of pamp: H NMR (CDCl , d,
3
ppm) 4.42 (s, 2H), 5.30 (b, 1H, OH), 6.86 (d,
2H), 6.93 (m, 3H), 7.18 (d, 1H), 7.26 (m, 3H).
The hydrogen of the –CH – adjacent to the
2
–NH– group appears at 4.42 ppm, and the signal
(
pmp), the bands bellow of the transmittance line
in the aromatic 9 hydrogen. In addition, the fact
that the hydrogen in the phenolic –OH group
signals at 5.30 ppm supports the structure of
the compound.
are the vibrations of the product (pamp). The
vibration of the C¼N double bond observed in
ꢀ1
the spectrum (b) at 1614 cm increases on the
13
transmittance line with the addition of NaBH4.
C NMR (CDCl , d, ppm) 48.7 (–CH –),
3
2
1
1
15.9, 116.6, 120.0, 120.8, 122.9, 128.7, 129.2,
This indicates that the pmp is decreasing in the
1
3
29.4, 147.2, 156.7 (Aromatic carbon atoms).
C
reaction medium. The increase of a vibration
ꢀ1
NMR results, the aliphatic –CH – carbon and 10
different carbon atoms adjacent to the secondary
band at 1604 cm below the transmittance line
indicates that the amount of the pamp is
2

6
O. TURHAN AND H. YAŞAR
Figure 6. The infrared spectra of (a) 2-((phenylimino)methyl)phenol, (b) reaction media, (c) pure 2-((phenylamino)methyl)phenol.
ꢀ
1
Infrared spectra are given in the range of 1500–2000 cm to allow methanol to work. Since the reaction beginning solution
methanol and imine) is defined as background, vibrations have occurred below and above the transmittance line with the add-
(
ition of sodium borohydride. Vibrations below the transmittance line belong to the product (amine) and vibrations above the
transmittance line belong to the input substance (imine).
increasing by the addition of NaBH . The reason
monitoring method over other conventional
reduction reactions can be listed as follows:
4
why the vibration below the transmittance line of
the product does not increase as much as at
ꢀ1
ꢀ1
ꢂ Before the reduction process, the imine solution
of the is defined as bg and so the vibrations of
any components in the environment are elimi-
nated by resetting the background.
1
594 cm is that the peak at 1589 cm overlaps
by the input material decreases at the same time
and we see the difference between these
two vibrations.
ꢂ While the vibration bands originating from the
imine compound (reactand) rise above the trans-
mittance line, the vibration bands from the
amine (product) appear below the transmittance
line. On this way, it is easier to understand
increasing and decreasing component in the
reaction media.
ꢂ The products formed during the reaction can be
observed in the reaction medium without
purification.
ꢂ The intermediate compounds formed during the
reaction can also proven bye the peaks not
belonging to the products during the reaction.
ꢂ Since the peak heights in the IR spectrum are
proportional to the amount of substance in the
sample, it is possible to obtain information
about the amount of the input materials and
products in the environment by compearing the
peak heights.
Conclusions
The basis of the BG method is (blind reading)
process used to ignore any interference before
the IR spectrum of the media scanned. In the
study, bg identification process was made for the
input materials and the vibrations originating
from all the substances in the environment were
reset by the device and only the changes in the
reaction medium were examined.
Reduction reactions of imine compounds to
amine are available in the literature.
background defining method previously used in
the monitoring of imine formation reactions,
complex formation reactions and hydrazone for-
[
13]
However,
[
14]
[
15]
mation reactions
applying the method on the reduction reaction.
The advantages of this in situ reaction
this study is the first time

SPECTROSCOPY LETTERS
7
Acknowledgments
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Frequency Comb Spectroscopy. Journal of Molecular
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helpful discussion.
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