Objectives After completing this section, you should be able to identify the two characteristic infrared absorptions displayed by all carboxylic acids. state the approximate 1 H NMR absorption of a carboxylic acid proton. use infrared and NMR spectroscopy data to assist in the identification of an unknown carboxylic acid, with or without the assistance of a table of characteristic absorptions. IR The carboxyl group is associated with two characteristic infrared stretching absorptions which change markedly with hydrogen bonding. The spectrum of a CCl 4 solution of propionic acid (propanoic acid), shown below, is illustrative. Carboxylic acids exist predominantly as hydrogen bonded dimers in condensed phases. The O-H stretching absorption for such dimers is very strong and broad, extending from 2500 to 3300 cm -1 . This absorption overlaps the sharper C-H stretching peaks, which may be seen extending beyond the O-H envelope at 2990, 2950 and 2870 cm -1 . The smaller peaks protruding near 2655 and 2560 are characteristic of the dimer. In ether solvents a sharper hydrogen bonded monomer absorption near 3500 cm -1 is observed, due to competition of the ether oxygen as a hydrogen bond acceptor. The carbonyl stretching frequency of the dimer is found near 1710 cm -1 , but is increased by 25 cm -1 or more in the monomeric state. Other characteristic stretching and bending absorptions are marked in the spectrum. 0 1 2 The Infrared Spectrum of Propanoic Acid The Infrared Spectrum of Propanoic Acid The Infrared Spectrum of Propanoic Acid Nitriles show a distinctive absorption for the C-N triple bond which appear near 2250 cm -1 The Infrared Spectrum of Benzonitrile 1 H Nuclear Magnetic Spectroscopy The acidic O-H protons of carboxylic acids are highly deshielded due to the electronegativity of oxygen and anisotropy from the C=O carbonyl bond. They tend to be among the least shielded protons appearing far downfield in the 10–12 ppm region which is considered distinctive for carboxylic acids. Due to hydrogen bonding the proton of a carboxylic acid often appears as a broad singlet and adding D 2 O causes the signal to disappear due to hydrogen-deuterium exchange. Protons on carbons adjacent to a carboxylic acid absorb in the 2-3 ppm region. Some deshielding occurs due to the fact that the carbonyl oxygen is pulling electron density away from the carbonyl carbon which inductively pulls electron density away from the adjacent carbon. The Typical 1 H Peaks for a Carboxylic Acid The 1 H Peaks for Butanoic Acid A 1 H NMR Spectra for Butanoic Acid