Carboxylic acids spectroscopic features

In this chapter, we first introduce the system of naming carboxylic acids and then list some of their physical and spectroscopic characteristics. We then examine their acidity and basicity, two properties that are strongly influenced by the interaction between the electron-withdrawing carbonyl group and the hydroxy function. Methods for the preparation of the carboxy group are considered next, followed by a survey of its reactivity. Reactions of carboxylic acids will feature a new two-step substitution pathway, addition-elimination, for the replacement of the hydroxy group by other nucleophiles, such as halide, alkoxide, and amide. The chemistry of the carboxylic acid derivatives, which result from these transformations, is the subject of Chapter 20. 

The establishment of the main features of the reactions of carboxylate ions with 3-unsubstituted isoxazolium salts is fully described in the classic paper by Woodward and Olofson.165 A generalized reaction scheme is shown as Scheme 10. Deprotonation at the 3-position is rapidly followed (or more probably accompanied) by ring opening to give the ketoketenimine (54). Ketoketenimines have been detected spectroscopically and are sufficiently stable for isolation in some cases.166,167 Reaction of carboxylic acid... 

P. anthracophilus found on a Eucalyptus species were established at the same time. The work on eburicoic acid was carried out by Robertson in Liverpool. The elucidation of the structure of polyporenic acid A (5.161) benefited from experience gained in studies on the structure of lanosterol and on the penta-cyclic triterpenes. The identification of some structural features also showed the developing role of UV and IR spectroscopic methods in the 1950s. However, the purification of these compounds was impeded by the difficulty of separating the 8(9)-enes from the 7(8),9(ll)-dienes that often accompanied them. Initial studies on polyporenic acid A revealed the presence of a carboxylic acid, two hydroxyl groups and two double bonds, each of differing reactivity. One double bond was a terminal methylene since formaldehyde was obtained on ozonolysis whilst the other was more hindered. Typical of the tetracyclic triterpenes. 

The most characteristic feature in the spectroscopic analysis of ethers is the strong band of C-0 stretching. It occurs in the 1060-1300 cm range. In this region, many bands are present, but this alone does not establish the presence of an ether. For note, alcohols, carboxylic acids, and esters also show C-0 stretching. They can be eliminated as possibilities, however, by the fact alcohols would also show the strong and broad 0-H stretch in the 3200-3600 cm , car-... 

Structures and properties of other strongly associated liquids like hydrogen fluoride or carboxylic acids share many features with liquid water. Therefore, a separate discussion of other pure liquids is not needed here. Spectroscopic techniques can also be applied to solutions of hydrogen-bonded species in solvents that do not form hydrogen bonds. This method is particularly usefiil for investigations of solutions, with two solutes forming a strong... 

Infrared (IR) spectroscopy is probably the quickest and cheapest of the spectroscopic techniques in determining the functional groups of the sample. The samples can be soUds, liquids, or gases and can be measured in solution or as neat liquids mulled with KBr or mineral oil. Comparison of IR spectra of substances of known structure has led to many correlations between wavelength (or frequency) of IR absorption and features of molecular structure. Certain structural features can easily be established. For example, in an organic compound that contains only C, H and O, the oxygen can only be present as C=0,0—H, or C—O—C or a combination of these, such as the ester or carboxylic acid group. 

The structure (121) of thomasic acid was established by extensive spectroscopic and chemical degradation studies (116,119,120) and presents several interesting features. It is one of the few lignans occurring naturally in racemic form and in possessing a free carboxyl group. In addition, the trans vicinal substituents (at C-1,2) adopt a diaxial conformation. The synthesis of this product (Scheme 25) was... 

Again by analogy with peptide hydrolysis, metalloenzymes catalyzing ester hydrolysis may take advantage of additional chemical features provided by amino-acid residues present in the active-site cavity. This situation occurs with car-boxypeptidase, " which shows esterase activity in vitro. Although the rate-limiting steps for carboxylic esters and peptides may differ, several features, such as the pH dependences of cat and and the presence of two spectroscopically observable intermediates, point to substantially similar mechanisms. On the other hand, carboxylic ester hydrolysis catalyzed by carbonic anhydrase seems to rely on fewer additional features of the active-site cavity, perhaps only on the presence of a metal-coordinated hydroxide that can perform the nucleophilic attack on the carbonyl carbon atom." ... 

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