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19-1 Naming the Carboxylic Acids Chemical Abstracts retains the common names for the two simplest carboxylic acids, formic acid and acetic acid. The IUPAC system derives the name of carboxylic acids by replacing the ending –e in the parent alkane by the ending –oic acid. The alkanoic acid stem is numbered by assigning 1 to the carbonyl carbon and labeling any substituents along the longest chain incorporating the CO2H group accordingly. The carbonyl group and the functional groups of its derivatives take precedence in naming over any other groups discussed so far: When other functional groups are present, the main chain is chosen to include other functional groups as much as possible. Saturated cyclic acids are named as cycloalkanecarboxylic acids. Aromatic acids are named benzoic acids. Dicarboxylic acids are referred to as dioic acids. 19-2 Structural and Physical Properties of Carboxylic Acids Formic acid is planar. The molecular structure of formic acid is roughly planar, which is characteristic of carboxylic acids in general. The carboxy group is polar and forms hydrogen-bonded dimers. The carboxy function is strongly polar and forms hydrogen bonds to other polarized molecules such as water, alcohols and other carboxylic acids. Carboxylic acids up to butanoic acid are completely soluble in water. As neat liquids, and even in fairly dilute solutions, carboxylic acids form hydrogenbonded dimers (6–8 kcal mol-1). Carboxylic acids have relatively high melting and boiling points due to hydrogen bonding in both the solid and liquid states. 19-3 NMR and IR Spectroscopy of Carboxylic Acids The carboxy hydrogen and carbon are deshielded. Hydrogens on a carbon next to a carbonyl group are slightly deshielded. The effect diminishes rapidly with increasing distance from the carbonyl. The hydroxyl proton resonates at very low field ( = 10-13 ppm). Its chemical shift varies strongly with concentration, solvent and temperature because of its involvement in hydrogen bonding. The 13C NMR chemical shifts of carboxylic acids are similar to those of aldehydes and ketones. The amount of deshielding is smaller because of the presence of the extra OH group. The smaller deshielding can be attributed to the extra resonance form present in carboxylic acids: The carboxy group shows two important IR bands. Stretching frequencies for both the carbonyl group and the hydroxy substituent are seen in the IR spectra of carboxy groups. The O-H bond exhibits a very broad band at 2500–3300 cm-1, lower than for alcohols because of strong hydrogen bonding. Mass spectra of carboxylic acids show three modes of fragmentation. Fragmentation of carboxylic acids occurs in several ways, which results in a fairly weak molecular ion peak.