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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115

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