Spectroscopy and Mass Spectrometry of Carboxylic Acids

In Summary The carboxy function is planar and contains a polarizable carbonyl group. Carboxylic acids exist as hydrogen-bonded dimers and exhibit unusually high melting and boiling points. 

The polarizable double bond and the hydroxy group also shape the spectra of carboxylic acids. In this section, we show how both NMR and IR methods are used to characterize the carboxy group. We also illustrate how carboxylic acids fragment in the mass spectrometer. 

Human bitter taste is mediated by so-called G-protein-coupled receptors, a family of proteins that sense odors, hormones, and neurotransmitters outside the cell to elicit corresponding cellular responses. The simple carboxylic acid pictured below (as the carboxylate anion) has the right three-dimensional shape of its hydrophobic chain and hydrogen-bonding carboxy group to maximize docking to the protein s receptor site, thus blocking this function. It may become useful as an additive to vitamins and other supplements to reduce their often bitter aftertastes. 

The NMR chemical shifts of carboxylic acids also are similar to those of the aldehydes and ketones, with moderately deshielded carbons next to the carbonyl group and the typically low-field carbonyl absorptions. However, the amount of deshielding is smaller because the positive polarization of the carboxy carbon is somewhat attenuated by the presence of the extra OH group. 

The contribution of the second resonance form, though minor, explains the strong deshielding of the carbonyl and adjacent carbons 

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Spectroscopy and Mass Spectrometry of Carboxylic Acids CHAPTER 19... 

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