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18.2: Physical Properties of Carboxylic Acids

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  • Hydrogen Bonding

    Carboxylic acids show a high degree of association through hydrogen bonding. We have encountered such bonding previously with alcohols; however, acids form stronger hydrogen bonds than alcohols because their \(\ce{O-H}\) bonds are more strongly polarized as \(\ce{-} \overset{\delta \ominus}{\ce{O}} \ce{-} \overset{\delta \oplus}{\ce{H}}\). Furthermore, carboxylic acids are able to form hydrogen bonds to the negative oxygen of the carbonyl dipole rather than just to the oxygen of another hydroxyl group. Carboxylic acids in the solid and liquid states mostly exist as cyclic dimers, and these dimeric structures persist to some extent even in the vapor state and in dilute solution in hydrocarbon solvents:

    Figure 18-1, which is a plot of boiling points versus \(n\) (the total number of carbon atoms) for the homologous series \(\ce{CH_3(CH_2)}_{n-2} \ce{X}\), in which \(\ce{X}\) is \(\ce{-CO_2H}\), \(\ce{-CH_2OH}\), or \(\ce{-CH_2Cl}\).

    Table 18-1: Physical Properties of Representative Carboxylic Acids

    Figure 18-1: Boiling points of acids, \(\ce{CH_3(CH_2)}_{n-2} \ce{CO_2H}\); alcohols, \(\ce{CH_3(CH_2)}_{n-2} \ce{CH_2OH}\); and alkyl chlorides, \(\ce{CH_3(CH_2)}_{n-2} \ce{CH_2Cl}\)

    Hydrogen bonding also is responsible for the high water solubility of the simple carboxylic acids with less than five carbons; water molecules can solvate the carbonyl group through hydrogen bonds. Nonetheless, as the chain length of the hydrocarbon residue \(\ce{R}\) increases, the solubility decreases markedly, because the proportion of polar to nonpolar groups becomes smaller.

    Spectra of Carboxylic Acids

    The infrared spectra of carboxylic acids provide clear evidence for the hydrogen bonding discussed in the preceding section. This is illustrated in Figure 18-2, which shows the spectrum of ethanoic acid in carbon tetrachloride solution, together with those of ethanol and ethanal for comparison.

    Figure 18-2: Infrared spectra of ethanol, ethanoic acid, and ethanal as \(10\%\) solutions in carbon tetrachloride.

    The spectrum of ethanol has two absorption bands that are characteristic of the \(\ce{OH}\) bond; one is a sharp band at \(3640 \: \text{cm}^{-1}\), which corresponds to free or unassociated hydroxyl groups, and the other is a broad band centered on \(3350 \: \text{cm}^{-1}\) due to hydrogen-bonded groups. The spectrum of ethanoic acid shows no absorption from free hydroxyl groups but, like that of ethanol, has a very broad intense absorption ascribed to associated \(\ce{OH}\) groups. However, the frequency of absorption, \(3000 \: \text{cm}^{-1}\), is shifted appreciably from that of ethanol and reflects stronger hydrogen bonding than in ethanol. The absorption due to the carbonyl group of ethanoic acid \(\left( 1740 \: \text{cm}^{-1} \right)\) is broad, but is at about the same position as the carbonyl absorption in ethanal.

    The carboxyl function does absorb ultraviolet radiation, but the wavelengths at which this occurs are appreciably shorter than for carbonyl compounds such as aldehydes and ketones, and, in fact, are out of the range of most commercial ultraviolet spectrometers. Some idea of how the hydroxyl substituent modifies the absorption properties of the carbonyl group in carboxylic acids can be seen from Table 18-2, in which are listed the wavelengths of maximum light absorption \(\left( \lambda_\text{max} \right)\) and the extinction coefficients at maximum absorption \(\left( \epsilon_\text{max} \right)\) of several carboxylic acids, aldehydes, and ketones.

    Table 18-2: Wavelengths for Maximum Ultraviolet Absorption of Some Carboxylic Acids, Aldehydes, and Ketones \(\left( n \rightarrow \pi^* \right)\)

    Figure 18-3 by the spectra of phenylethanoic acid \(\left( \ce{C_6H_5CH_2CO_2H} \right)\) and phenylmethanol \(\left( \ce{C_6H_5CH_2OH} \right)\). The chemical shift of the carboxylic acid proton is here about \(9 \: \text{ppm}\) toward lower magnetic fields than that of the hydroxyl proton of the alcohol. This behavior parallels that of the enol hydrogens of 1,3-dicarbonyl compounds and is similarly related to hydrogen-bond formation (Section 17-1D).
    Figure 18-3: Proton nmr spectra of (a) phenylethanoic acid and (b) phenylmethanol in carbon tetrachloride solution at \(60 \: \text{MHz}\) relative to TMS.

    Contributors and Attributions

    John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format."