Spectroscopic Analysis of Carboxylic Acids

PROBLEM 19.12 Show the bonding changes that occur, and write the structure of the intermediate formed in the thermal decarboxylation of 

SAMPLE SOLUTION (a) By analogy to the thermal decarboxylation of malonic acid, we represent the corresponding reaction of benzoylacetic acid as 

Acetophenone is the isolated product it is formed from its enol by proton-transfers. 

The thermal decarboxylation of -keto acids is the last step in a ketone synthesis known as the acetoacetic ester synthesis. The acetoacetic ester synthesis is discussed in Section 21.6. 

NMR Like other carbonyl groups, the carbon of the —CO2H group of a carboxylic acid is strongly deshielded (8 160-185 ppm), but not as much as that of an aldehyde or ketone (190-215 ppm). 

Infrared The most characteristic peaks in the infrared spectra of carboxylic acids are those of the hydroxyl and carbonyl gronps. As shown in the infrared spectrum of 

UV-VIS In the absence of any additional chromophores, carboxylic acids absorb at a wavelength (210 nm) that is not very useful for diagnostic purposes. 

Mass Spectrometry Aside from a peak for the molecular ion, which is normally easy to pick out, aliphatic carboxylic acids undergo a variety of fragmentation processes. The dominant fragmentation in aromatic acids corresponds to loss of OH, then 

Section 18.2 Like the carbonyl group of aldehydes and ketones, the carbon of a C=0 unit in a carboxylic acid is sp -hybridized. Compared with the carbonyl group of an aldehyde or ketone, the C=0 unit of a carboxylic acid receives an extra degree of stabilization from its attached OH group. 

Section 18.3 Hydrogen bonding in carboxylic acids raises their melting points and boiling points above those of corr5)arably constituted alkanes, alcohols, aldehydes, and 

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