Chapter 8 FTIR and FTR Investigations on Beneoic acid methyl ester
Chapter 8
F T I R and FTR Investigations on Beneoic acid methyl ester
CHAPTER - 8
INTRODUCTION
Benzoic acid is an organic acid formed by attachment of a carboxyl group
to a benzene ring, giving the formula C,H,COOH. It is also called carboxy
benzene, benzenecarboxylic acid and phenyl formic acid. It occurs in nature in
free and combined forms.
It has white scales or needle crystals with odor of benzoin or
benzaldehyde. It has a melting point of 121.25"C and boiling point of 249.2OC.
It particularly sublimes at 100°C freely, volatile in steam and flash point of
121.1°C. It is soluble in alcohol, ether, chloroform, benzene, carbondisulfide,
carbon tetrachloride and turpentine. It is slightly soluble in water and also
combustible. It is derived from (a) Decarboxylation of phthalic anhydride in the
presence of catalysis, (b) Chlorination of toluene to yield bemotrichloride which
is hydrolyzed to benzoic acid, (c) Oxidation of toluene and (d) from benzoin
resin. The ultra-pure benzoic acid is prepared for use as titrimetric and
calorimetric standard [I]. It is moderately toxic by ingestion and has restricted
use in foods upto 0.1%.
It is used in preserving foods, fats, fruit juices and alkaloidal solution. It is
also used in the manufacture of benzoates and benzoyl compound dyes. It is used
as a mordant in calicoprinting and for curing tobacco. It is used as standard in
volumetric and calorimetric analysis and intermediate in organic synthesis.
It is used as an- agents for superficial fungus infations of skin.
Together with salicylic acid in ointments, it is used for the tnatment of ring
worm in dogs and other species.
Benzoic acid methylester, derivative of benzoic acid is an essence or oil of
Niobe with molecular formula C,H,COOCH,. It is a colorless, oily bansparent
liquid with a pleasant, odor. It has boiling point 198.6"C and flash point 82.7OC.
It is insoluble in water, miscible with alcohol, ether and methanal. It is stable
against oxidation and easily saponified by a smng base.
Benzoic acid methyl ester is obtained (a) by heating methyl alcohol and
benzoic acid in presence of sulfiuic acid and (b) passing dry hydrogen chloride
through a solution of benzoic acid in methanal. It occurs naturally in oils of dove,
ylang ylang and tuberose. It is used as a perfiune and dye carrier. It is also used as
solvent for cellulose esters, ethers, resins, rubber and flavoring.
The relaxation times and the activation energies of some substituted
benzoic acids and their esters have been determined in the 3-cm microwave
region at 20°C, in dilute solutions of benzene [2 ] . Korobkov and Zharikov
reported low-frequency Raman spectra of benzoic acid and some of its
derivatives [3]. Abdullin and Furer studied the calculation of band intensities in
IR spectra and conformation of aromatic esters 141.
The FTIR and laser Raman spectra of para chlom benzoic acid have been
recorded in the regions 200-4000 cm-' and 30-4000 cm-'. Vibrational spectra and
normal coodinate calculations of para chloro benzoic acid was reported by
Mohan [5].
Sanchez de la Blanca and others [6] reported the vibrational analysis of
in- and Raman spectra in solid phase of some 0-substituted benzoic acid
derivatives. Yesook Kim and Katsunosuke Machida [7] reported vibrational
spectra, normal vibrations and m M intensities of six isotopic benzoic
acids.
However, there is no report is available in the literature about the
vibrational spectra and analysis of benzoic acid methyl ester. Hence, the present
investigation has been undertaken to record and study the FTIR and FT b n a n
spectra of this compound affesh and also to perform normal coordinate analysis
to check the validity of the assignment.
8.1 EXPERIMENTAL DETAILS
The FTIR spectrum of benzoic acid methyl ester is recorded on Brucker
IFS 66V FTIR spectrometer in the region 4000-200 cm-'. The FT Raman
spectrum of the same compound is also recorded on the same instrument with
FRA 106 Raman module equipped with Nd : YAG laser source operating at
1.06 pm line with a scanning speed of 30 cm-I min-' of spectral width 20 cm-I.
The frequencies for all sharp bands were accurate to i 1 cm.'. The structure of the
compound is shown in Fig.8.1. The recorded spectrum of benzoic acid methyl
ester is shown in Fig.8.2.
8.2 THEORETICAL CONSIDERATIONS
The molecular symmetry of a molecule helps to determine and classify the
actual number of fUndamental vibrations of the system. The observed spectrum is
<-y< OCH,
Fig.8.1 Structure of BENZOIC ACID METHYL ESTER
explained on the basis of C, point group symmetry. The 48 optically active
fundamental vibrations are distributed as r,,, = 35a' (in-plane) + 13a"
(out-of-plane).
All the modes are active in both Raman and Infrared Assignments have
been made on the basis of relative intensities, magnitudes of the muencies and
polarisation of the Raman l ies. The vibrational assignments are discussed in
terms of the potential energy distribution which was obtained from the evaluated
constants.
8.3 NORMAL CO-ORDINATE ANALYSIS
The normal coordinate calculations have been performed to obtain
vibrational fitquencies and the potential energy distribution for the various
modes. In the normal coordinate analysis, the potential energy distribution plays
an important role for the characterisation of the relative contributions from each
internal coordinates to the total potential energy associated with particular normal
coordinate of the molecule.
The normal coordinate analysis is necessary for complete assignment of
the vibrational frequencies of larger polyatomic molecules and for a quantitative
description of the vibrations. The values of bond-length and bond-angles have
been taken from d i e d molecules and Sutton table (8). lnternal co-ordinates for
the out-of-plane torsional vibrations are defined as recommended by IUPAC. The
simple valence force field has been adopted for both in-plane and out-of-plane
vibrations. The normal dinate ate calculations have been performed using the
Program of Fuhrer et al., (9) with modifications. The initial set of force constants
have been ~ I I I derivatives of allied related molecules.
8.4 POTENTIAL ENERGY DISTRIBUTION
To check whether the chosen set of assignments contribute maximum to
the potential energy associated with normal coordinates of the molecules, the
potential energy distribution (PED) has been calculated using the relation
FIIL2,L PED = -
h,
where F, are the force constants defined by damped least square technique, L,,
the nomalised amplitude of the associated element (i,k) and 1, the eigen value
corresponding to the vibrational frequency of the element k. The PED
contribution corresponding to each of the observed frequencies over 10% are
alone listed in the present work.
8.5 RESULTS AND DISCUSSION
The experimentally observed frequencies in IR and Raman spectra with
their relative intensities and the calculated 'equencies along with the PED of
various modes of vibration of benzoic acid methyl ester are presented in table 8.1.
The assignment of frequencies is made as follows.
Stretching
Methyl modes
Very strong Raman band at 3075 cms', weak i n M band at 3063 cm-'
have been assigned to CH asymmetric stretching mode in CH,. The weak
i f i red band at 3025 cm-I is assigned to C-H symmetric stretching mode.
From PED, it is clear that they are pun modes. They agree quite well with the
calculated wave numbers 3071,3058 and 3019 cm".
The CH, rocking vibrations of the benzoic acid methyl ester normally
occw in the mnge 1200-1450 cm-I [lo]. The medium IR band at 1456 cm.' and
strong IR at 1438 cm-' have been assigned to CH, deformation and CH3 rocking
which agree with the calculated frequencies at 1450 cm-' and 1428 cm". The
PED calculation shows that the CH3 deformation and CH, rocking are always of
mixed modes.
C-H stretching
The substituted benzene gives rise to C-H stretching. Usually the bands
around 3000 cm-' are assigned to C-H st~tching vibration [I 11. They are not
affected by the nature of the substituents. The observed frequencies at 2850,
2907,2950,3000 and 3013 cm-' in ow case have been assigned to C-H stretching
modes which agree with the calculated frequencies. These modes appear to be
pure modes except at calculated frequency at 2940 cm-' where there is mixing of
the C-C stretching vibration (22%).
C=C Stretching
C=C vibrations are more interesting if the double bond is in conjugation
with the ring. The actual positions are determined not so much by the nature of
substituents but by the form of substituents around the ring [12, 131. The three
C=C stretching modes of benzene are assigned to 1600, 1644 and 1688 cm.' of
obser~ed fkpencies which agree with the calculated frequencies at 1592, 163 1
and 1680 cm-I. The pED indicates that C=C stretching vibration is of pure mode.
C - C Stretching
The modes corresponding to C-C stxtching in benzene are assigned to the
bands at 1184, 1168, 11 12 and 1075 cm.' in benzoic acid methyl ester which are
pure modes.
In plane and out of plane bendings
The aromatic structure shows the presence of C-H in plane bending in the
region 900 - 1100 cm-' and C-H out-of-plane bending in the region
800-980 cm-I which permits ready identification for this structure. In this region
the bands are not appreciably affected by the nature of the substituents.
The frequencies at 1500, 1388, 13 19, 1281 and 1 195 cm" are assigned to
C-H in plane bending and are in favourable agreement with values given in the
literatures [14,15]. The ffequencies 988,969,925,838 and 825 cm-' are assigned
to C-H out-of-plane bendings and these assignments are in good agreement with
the literature value [16].
Methyl Ester - CO.OCH,Vibrations
The bands due to the esters are C=O, C-0,O-C and C-OCH, stretchings.
The strong absorption band due to the C=O stretching vibration is observed in the
region 1850-1550 an-' [15]. The C = 0 stretching vibration band of Benzoic acid
methyl ester is assigned to 1725 cm-' which agrees with the calculated value
1719 an-'. The pED reveals that 74% of C = 0 stretching vibration is combined
with 20% of CC stretching vibration.
The bands due to the ester C-0 and 0-C stretching vibrations are strong,
partly due to an interaction with C - C vibration and occur normally in the range
of 1450 - 1 100 cur1. The infr-arad bands at 1588 cm are 1563 cm" have been
assigned to C-0 and 0-C stretching vibrations and they quite agree with the
calculated wave numbem 1580 and 1558 cm-I.
The strong Raman band at 1025 cm" has been assigned to C-OCH,
stretching which agrees with the calculated wave number 10 19 cm" .
The nature of other bands for in plane bending vibrations and out of plane
bending vibrations can be seen h m table 8.1. The potential energy distribution is
evaluated in the present work indicates the contribution of an individual force
constant to the vibrational energy of normal modes. It also clearly indicates that
there is m i x i i of internal displacement coordinates.
Conclusion
A complete vibrational spectra and analysis is reported in the present
work for the first time for benzoic acid methyl ester. The close agreement
between the observed and calculated frequencies confirm the validity of the
present assignment.
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