Characterization of naphthalene dicarboxylic acids. 2. Infrared absorption spectra

Characterization of naphthalene dicarboxylic acids. 2. Infrared absorption spectra Clive Davies Department of Chemical Engineering, Glamorgan Polytechnic, Llantwit Road, Treforest, Pontypridd, G/am., UK (Received 12 October 1973)

The infrared absorption spectra of the ten isomeric naphthalene dicarboxylic acids in the solid state, in potassium bromide discs, have been examined in the range 4000-250 cm-‘. The spectra are sufficiently different as to allow characterization of these isomers.

In a previous communication’ the importance of naphthalene carboxylic acids in the study of the chemical constitution of coal was stressed. The ultraviolet absorption spectra of the ten dicarboxylic acids reported’, although useful, do not possess sharp peaks to allow easy characterization of every acid. The infrared spectra can fingerprint these compounds and in conjunction with ultraviolet absorption spectra facilitate characterization.

EXPERIMENTAL The infrared (i.r.) absorption spectra were measured with a Perk&Elmer Spectrophotometer model 457. The range examined was 4000-250 crnl and all spectra were recorded with the scan speed at medium (14 min). All acids were examined in the solid form using potassium bromide discs. The preparation and characterization of the naphthalene dicarboxylic acids has already been reported’.

RESULTS AND DISCUSSION All the acids examined show broad absorption bands in the 345 O-2600 cm- 1 range. Absorption in the 30002400 cm-l region may be ascribed to hydrogen bonding arising from the carbonyl groups. Naphthalene itself also absorbs in this region and Friedman’ has shown that the dimethyl naphthalenes, which absorb strongly at just below 3000 cm-l, show marked variation in intensity of the bands with the position on the naphthalene nucleus. Each acid shows a strong broad band near 1700 cm-l; this may be attributed to stretching of aromatic carbonyl. Naphthalene- 1,8-dicarboxylic acid shows two very strong peaks at 1770 and 1740 cm-‘l which clearly characterize this particular isomer. These two peaks are also found in naphthalic anhydride (1,s) and are probably associated with the special structure of the peri isomer. The i.r. patterns show as a whole a great deal of fine structure in the range 1700-400 cm-l. This is in agreement with the observations of Gonzalez-Sanchez3 on the i.r. spectra of the benzene carboxylic acids. The spectra for this region are reproduced in Figure I. Tables of absorption wavelengths and intensities are not included since the relative intensity of peaks is best shown on the absorp tion spectra. These data are important in characterizing the isomers. Most of the acids are clearly distinguishable from one another. The two isomers with most similar

spectra are the 1,3 and 1,4 isomers. Even in these cases the spectra are sufficiently different to allow characterization. The U.V. spectra of these two acids are also almost indistinguishable’. Hawkins, Ward and Whiffen have tabulated characteristic frequencies of substituted naphthalenes for 170 compounds; there were no dicarboxylic acids in their collection. The results of the present investigation are only in general agreement with these correlations; many of the peaks indicated by Hawkins et L.# are absent. The last observation is in general agreement with van Gemert’ who investigated the spectra of the dihydroxynaphthalenes. The simplest absorption pattern was observed with the 1,5 and 2,6 isomers respectively. This is expected because these isomers possess the greatest degree of symmetry. Czuchajowski and Lawson6 investigated the i.r. spectra of some polycondensates derived from hydroxynaphthaquinones and their relation to the 1600 cm-l band shown by coals. They found that it was possible to produce mixtures with absorptions in the region 165&1580 cm-l and showing some similarity with the characteristic coal band. Although they did not suggest that low-rank coals necessarily contain naphthalene-type nuclei the importance of the naphthalene nuclei is evident; for example, formaldehyde with 5-hydroxy-1 ,Cnaphthaquinone yielded a product with a spectrum in the 1600 cm-l region very similar to that of a lignite from Thailand. Naphthalene carboxylic acids could be obtained by the oxidation of the polycondensates investigated. The naphthalene dicarboxylic acids examined in the present investigation also show absorption peaks in the 1650-1580 cm-l region.

ACKNOWLEDGEMENTS Grateful acknowledgement is made to the Department Chemistry for use of the spectrophotometer.

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REFERENCES 1 2 3 4

Davies, C. Fuel, Land. 1973, 52, 270 Friedman, H. M. Spectrochim. Acta 1966,22, 1465 Gonzalez-Sanchez, F. Spectrochim. Acta 1958, 12, 17 Hawkins, J. G., Ward, E. R. and Whiffen, D. H. Spectrochim. Acta 1957, 10, 105 5 Van Gcmert, J. T. Aust. J. Chem. 1968, 21, 2203 6 Czuchajowski, L. and Lawson, G. J. Fuel, Lond. 1963,42,131

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I. R. absorption spectra of naphthalene dicarboxylic acids: C. Davies Wavelrrgth Irml

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Figure 1 Infrared spectra in KBr solid naphthalene dicarboxylic acids (a) naphthalene-1 ,Zdicarboxylic acid (b) naphthalene-13dicarboxylic acid (c) naphthalene-1 ,4-dicarboxylic acid (d) naphthalene-15dicarboxylic acid (e) naphthalene-16dicarboxylic acid

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C. Davies: 1.R. absorption spectra of naph thalene dicarbox ylic acids

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