Structural and Physical Properties of Carboxylic Acids

What is the structure of a typical carboxylic acid What are the characteristic physical properties of carboxylic acids This section answers these questions, beginning with the structure of formic acid. We shall see that carboxylic acids exist mainly as hydrogen-bonded dimers. 

The carboxy group is polar and forms hydrogen-bonded dimers 

Carboxylic acids, especially those possessing relatively low molecular weights and correspondingly high volatility, exhibit strong odors. For example, the presence of butanoic acid contributes to the characteristic flavor of many cheeses, and (f )-3-methyl-2-hexenoic add is one of the compounds responsible for the smell of human sweat. 

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Q Draw and name carboxylic acids and dicarboxylic acids, and use spectral information to determine their structures. Q Describe the trends in the acidity and physical properties of carboxylic acids, and explain how their acidity varies with their substituents. 

Structure and Nomenclature of Acid Derivatives 982 21-3 Physical Properties of Carboxylic Acid Derivatives 988 21-4 Spectroscopy of Carboxylic Acid Derivatives 991 21-5 Interconversion of Acid Derivatives by Nucleophilic Acyl Substitution 997... 

Write structures and describe the physical properties of carboxylic acids. 

Lutz, R., Aserin, A., Wicker, L., Garti, N. (2009). Structure and physical properties of pectins with block-wise distribution of carboxylic acid groups. Food Hydrocolloids, 23, 786-794. 

Structure and Properties of Carboxylic Acids 755 Table 20.2 Physical Parameters for Acetic Acid... 

Trimerization to isocyanurates (Scheme 4.14) is commonly used as a method for modifying the physical properties of both raw materials and polymeric products. For example, trimerization of aliphatic isocyanates is used to increase monomer functionality and reduce volatility (Section 4.2.2). This is especially important in raw materials for coatings applications where higher functionality is needed for crosslinking and decreased volatility is essential to reduce VOCs. Another application is rigid isocyanurate foams for insulation and structural support (Section 4.1.1) where trimerization is utilized to increase thermal stability and reduce combustibility and smoke formation. Effective trimer catalysts include potassium salts of carboxylic acids and quaternary ammonium salts for aliphatic isocyanates and Mannich bases for aromatic isocyanates. 

The primary structure of a polypeptide is its sequence of amino acids. It is customary to write primary structures of polypeptides using the three-letter abbreviation for each amino acid. By convention, the structure is written so that the amino acid on the left bears the terminal amino group of the polypeptide and the amino acid on the right bears the terminal carboxyl group. Figure 13-35 shows the two dipeptides that can be made from glycine and serine. Although they contain the same amino acids, they are different molecules whose chemical and physical properties differ. Example shows how to draw the primary stmcture of a peptide. 

As part of a more extensive study of cocrystals formed by isonicotinamide with carboxylic acids, 1 1 products containing the dicarboxylic fumaric or succinic acids [59]. In the structures of these particular cocrystals, the typical discrete dimeric synthon was not observed, but instead effectively infinite assemblies of one-dimensional chains were found instead. In a subsequent work, cocrystals of isonicotinamide containing mixed fumaric/succinic acids were prepared using both solid-state grinding and solution crystallization [60]. A full physical characterization of the products demonstrated that the products consisted of a single cocrystal phase, and were not simple physical mixtures of two cocrystal components. Such solid solutions were proposed as yet another method whereby one might obtain even finer control over the physical properties of cocrystal systems proposed as drug substances. 

Structure of the Carbonyl Group 817 18-3 Nomenclature of Ketones and Aldehydes 818 18-4 Physical Properties of Ketones and Aldehydes 820 18-5 Spectroscopy of Ketones and Aldehydes 822 18-6 Industrial Importance of Ketones and Aldehydes 828 18-7 Review of Syntheses of Ketones and Aldehydes 829 18-8 Synthesis of Ketones from Carboxylic Acids 833 18-9 Synthesis of Ketones and Aldehydes from Nitriles 833... 

Because the carboxylic acid functional group is structurally related to both ketones and alcohols, we might expect to see some familiar properties. Like ketones, the carboxyl carbon has sp hybridization, and carboxylic acid group.s are therefore p anar with C-C=0 and 0=C-0 bond angles of approximately 120°. The physical parameters of acetic acid are given in Table 20.2. 

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