Effects of halogens on the aromatic system of benzoic acid

Journal of Molecular Structure, 293 (1993) 89-92

89

Elsevier Science Publishers B.V., Amsterdam

Effects

of halogens on the aromatic system of henzoic acid

W. Lewandowski’*, H. Baranskab, P. Mokibrodaa and B. Dasiewicza *Institute of General Chemistry, Warsaw Agricultural University, SGGW, Rakowiecka 26/30,02-528 Warsaw, Poland bLaboratory of Applied Spectroscopy, Industrial Chemistry Research Institute, Rydygiera 8, 01-793 Warsaw, Poland

In the IR and Rsman spectra of phalogenbenzoic acid we have stated systematic changes in number, frequency and intensity of some bands of the aromatic system in the series F, Cl, Br, I. The most essential differences occur within the ranges of y(CH) 29742950, P(CH) 1180-1160, y(CH) 825-802 and y(CH1768-753 cm-‘. These regularities depend on the ionic potential of halogens.

1. INTRODUCTION

3. RESULTS AND DISCUSSION

The effect of some metals on the aromatic system of benzoic [1,21, salicylic [33 and nicotinic [41 acids has been investigated by the methods of molecular spectroscopy. Benxoic, salicylic and nicotinic acids are regarded as model systems representing a wide group of aromatic ligands which are incorporated into enzymes. The aim of the present paper has been to study (by IR and Raman spectroscopy) the effect of fluorine, chlorine, bromine, and iodine on the aromatic system of benzoic acid.

3.1. Vibrational assignments

2. EXPERIMENTAL

Raman spectra of solid samples in the range 4000-10 cm-l were recorded with a Coderg Model PHO spectrometer with SpectraPhysics argon ion laser (excitation lines 514.5 and 488 nm). IR spectra in the range 4060400 cm-l were recorded with a Carl Zeiss IR-75 spectrophotometer using pressed potassium bromide pellets. Ionic potentials were calculated using ionic radio given by Pauling [51. ‘To whom correspondence should be addressed 0022-2860/93/$06.00

Table 1 contains assignment of IR and Raman bands of the aromatic system for p-halogenbenzoic acids. The assignments were made on the basis of literature data [6,73. Normal vibrations of the aromatic ring were given by Versanyi 161.The symbol 8 denotes deformation in-plane vibrations of functional groups, whereas y denotes out-of-plane vibrations, and a&CC) and a(cC) denote deformation in-plane and out-of-plane vibrations of the ring, respectively. The results in Table 1 indicate that average differences between the band frequencies of the same vibrations in the IR and Raman spectra not exceed more than few cm-l. 32. E&ct

of halogens on the aromatic system of benzoic acid Analysis of the number, frequency and

intensity of bands of the aromatic ring in the IR and Raman spectra of benzoates [1,2], salicylates C33and nicotinates [43 of selected thirty metals showed that alkaline and some heavy metals [such as Ag, Hg(II), Cd and Pb(III1 perturb the aromatic system of these acids.by deforming the uniform distribution of

0 1993 Elsevier Science Publishers B.V. All rights reserved.

Table 1 Band fkquency of the aromatic system in IR and Raman spectra of phalogenbenzoic

acidsa

Frequency (cm-l)

normal vibration of the

p-tluorobenzoic acid

p-chlorobenzoic acid

p-bromobenzoic acid

p-iodobenzoic acid

IR

R

IR

R

IR

R

IR

R

-

3091 s

-

3094m

-

3092vw

-

-

V(CH),

20a

1602 sh

1607 s -

1598 sh

1595 vs -

-

1606m -

-

-

v(CzC),

8a

v(C-_C),

8b

-

1574 sh -

-

v(CzC),

19a

1573 sh 1504 sh 1426 m

1508 w -

1486m

1422 vw

1433 m

1428 vw

-

1428 w

-

v(CzC),.

19b

1320 m -

1321 s

1312 vw -

1319 w -

-

v(C-_C),

14

-

JXCH)

13

1178 w

1175 m -

1180 w

1177 m -

PO3 $@H) P(CH)

9a

1319 sh

1230 sh

1231 w

1233 sh

1160 m

1176 vw

1107 sh

1158 m -

1110 sh

1176 s -

1094 sh

-

1092 m

1094 vs

1015 w

-

1015 w

1015 m -

972 vw

1233 sh 1110 w 1012 vw

-

18a

fiCH)

17b

811 w

802 m -

$H)

1Oa

yiCH)

6a

672 w

act)

4

623 w -

c&CC)

6b

825 m

820 w

821 m

816 w

768 sh

767 w

760 w

762 m

681 vw -

698 m

757 w -

809 w -

as

p(CH) y(CH)

850 w

851 w

538 w

-

-

543 vw

1007 vw 973 w

852 w

629 vs -

1

973 w

-

633 9

-

18b

-

854 m -

685W

1108 w -

1010 vw -

682 w -

ring

1489 sh

1329 w

-

aromatiC

1569 sh -

1572 sh

1327 sh

968vv.I

Assignment

542 w

60QW

624 s -

= strong; m = medium; w = weak, sh = shoulder; v = very

753 w 548 w

17a

16b

161

91

k 765

F

2

2‘760 5 $755 L= 4.0 5.0 6.0 7.0 8.0 9.0 Ionic potential

Figure 1. Dependence of the frequencies of y(CH) bands No 6a of phalogenbenxoic acids in the IR spectra on the metal ionic potentials.a

1

_640( 7

E 635 z I

4.0 5.0 6.0 7.0 8.0 9.0 tonic potential

Figure 3. Dependence of the frequencies of r(CH) bands No 10a of p-bslogenbenzoic acids in the Raman spectra on the metal ionic potentials.a @The straight lines (Fig. 1, 2 and 3) are obtained for linear regression by using the last square method. These lines are given by following equations: y=4.8!5 x+609.40 with correlation coefficient p = 0.985 (Fig. 11,y = 2.85 x+609.60, p=O.94 (Fig. 2) and y=5.93x+ 77745, p = 0.86 (Fig. 3).

4.0 5.0 6.0 7.0 8.0 9.0 Ionic potentiol

Figure 2. Dependence of the frequencies of &CC) bands No 6b of p-halogenbenzoic acids in the Raman spectra on the metal ionic potentials.a

x electron charges. On the other hand some transition metals [e.g. CrGID, Fe(IID, YUII), Ln(III)] and aluminium stabilize these systems. The absence of bands of the aromatic system and/ or the decrease in intensity or their shill towards lower wavenumbers (caused by weakening of the bonds) is an evidence of perturbation of the aromatic system [8,91. Disturbance in aromatic system involve polarization of bonds in the ring. On the other hand, a rise in the stabilization of the complex as compared with the ligand involves a rise in the delocalization energy by a more uniform distribution of the electronic charge. The parameter determining the degree of the perturbance of the aromatic system of ligands

investigated is the ionic potential of the metals (measured by the ratio of the charge to the radius). With decrease in the ionic potential of metals the perturbance of the ring increases. The metals with low ionic potential such as Ag, W, Hg(II>, K and Na strongly perturb the aromatic system of ligands. Transition metals [Cr(III), Ln(III), Yl and Al with large potential stabilize this system. It has been established that the influence of metals on the aromatic system depends on the hind of structure of the carboxylic anion 111. In the spectra of p-halogenbenzoic acids in the series F + I the number, frequency and intensity of some of IR and Raman bands in the aromatic system undergo essential changes (Table 1). They are bands with numbers: 20a (R), 8a (IR and R), 19a (IR and R), 14 (RI, 13 (IR and R), 1 (IR), 4 (IR), 6a flR and R), 9a UR and R> and 10a (IR and R). On the other hand, bands numbered: 8b (RX), 14 (IR),18b (IR), 17a (IR) and 17b (IR) undergo only slight changes in dependence on

92

the halogen introduced into the ring. For bands numbered: 18a (IR), 17b (IR), 1Oa (RI, 6a (IR) and 6b (R) we observed a decrease in frequency in the series F + I. The number of aromatic system bands is decreased in the direction from F to I for normaI vibrations numbered: 20a (R), 8a (IR and R), 19a (IR and R), 14 (RI, 13 (IR and R), 1 (IR), 4 (IR) and 6a (RI. In pfluorobenzoic acid there appear 15 active vibrations in IR., and 10 in Raman spectra; on the other hand in spectra of p-iodobenzoic acid there are 11 active vibrations in IR and 4 in Raman spectra. After substituting fluorine by iodine several main bands of aromatic systems disappear. The bands numbered 20a (R) * 3090 cm-l, 8a (IR and R) = 1600 cm-l, 19a (IR and R) = 1500 cm-l, 6a (R) = 770 cm-l and 4 CR) *I 680 cm-l are present in pfluombenzoic acid and absent in p-iodobenzoic acid spectra. A decrease in the number, frequency and intensity of bands of the aromatic ring indicates a rise in the degree of perturbance of the aromatic system [8,91. The data of vibrational spectroscopy indicate that iodine (from among halogens) perturbs the aromatic system of benzoic acid in the highest degree. However, bands numbered: 9a (IR and R) and 17a (IR) are exceptions to the dependence8 observed. In tbis cases in the series F + I we obeerved an increase in frequency. This indicates that in addition to force constants undergoing weakening under the influence of substituting fluorine by iodine there exist some bonds undergoing strengthening. The largest differences in the number, frequency and intensity of bands of phalogenbenzoic acids in the series F -_) I have been established within $CH) in the range 825-753 cm-l, which characterize the kind of substitution of the aromatic system in benzoic acid. On the other hand, in case of metal benzoates essential differences occur within the range 1610-1370 cm-l of bands of aromatic vibrations v(C--“C),r conjugated with vibrations of carboxylic anion v, and v,(CCC-1. A question arises: why iodine (among halogens) perturbs in the highest degree the aromatic system of benzoic acid? Which

parameter decides on the degree of electronic charge perturbation? As it is well known in the series F, Cl, Br, I the ionic potential of halogens and polarity of C-X bonds are decreased, while the reactivity and polarizabihty of C-X bonds are increased. Presented above results indicate that the decrease in frequency of some bands from F to I is correlated linearly with the decrease in ionic potentials of halogens in the series F -+ I. Some examples of the dependence are shown in Figures 1, 2 and 3. However, some deflections from linear dependence8 are observed for fluorine. In each of the figures fluorine is situated below the straight line. This indicates that the ionic potential is not the only factor influencing the perturbance degree of aromatic systems.

ACKNOWLEiDGEMENT The authors are grat.efuI to Piotr Bara&& for statistical analysis of the results.

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