Carboxylic acids with aromatic rings

The direct electrochemical reduction of carbon dioxide requires very negative potentials, more negative than —2V vs. SCE. Redox catalysis, which implies the intermediacy of C02 (E° = —2.2 V vs. SCE), is accordingly rather inefficient.3 With aromatic anion radicals, catalysis is hampered in most cases by a two-electron carboxylation of the aromatic ring. Spectacular chemical catalysis is obtained with electrochemically generated iron(0) porphyrins, but the help of a synergistic effect of Bronsted and Lewis acids is required.4... 

Double bonds conjugated with aromatic rings and with carbonyl, carboxyl, nitrile and other functions are readily reduced by catalytic hydrogenation and by metals. These reductions are discussed in the appropriate sections aromatics, unsaturated aldehydes and ketones, unsaturated acids, their derivatives, etc. 

Partial reduction of the aromatic ring is especially easy in anthracene-9-carboxylic acid which was reduced to 9,10-dihydroanthracene-9-carboxylic acid with 2.5% sodium amalgam in aqueous sodium carbonate at 10° in 80% yield [987]. Aromatic carboxylic acids with hydroxyl groups in the ortho positions suffer ring cleavage during reductions with sodium in alcohols and are converted to dicarboxylic acids after fission of the intermediate j8-keto acids. 

Carboxylic acid fluorides, intermediates in the pathway to trifluoromethyl derivatives, are readily formed in reactions of all types of carboxylic acids with sulfur tetrafluoride derivatives (see Section 8.2.4.). Little attention has been paid to fluorination of other carboxylic acid halides. Aroyl chlorides are converted into aroyl fluorides by treatment with sulfur tetrafluoride, e.g. formation of l,41 or to (trifluoromethyl)arenes, e.g. 2 and 3. in the presence of hydrogen fluoride.45 Chlorination of the aromatic ring occurs in some cases.41... 

Ring synthesis by y- closure is a somewhat more versatile entry into this class of compound, though even then it is restricted to suitably activated precursors such as (142) (equation 101) (72T5197). Thiochromones and thioxanthones are probably most frequently prepared by this mode of closure, usually via an intramolecular Friedel-Crafts condensation of a carboxylic acid with an aromatic ring (equation 102) (71GEP2006196). 

Sodium hypochlorite is used for the epoxidation of double bonds [659, 691] for the oxidation of primary alcohols to aldehydes [692], of secondary alcohols to ketones [693], and of primary amines to carbonyl compounds [692] for the conversion of benzylic halides into acids or ketones [690] for the oxidation of aromatic rings to quinones [694] and of sulfides to sulfones [695] and, especially, for the degradation of methyl ketones to carboxylic acids with one less carbon atom [655, 696, 697, 695, 699] and of a-amino acids to aldehydes with one less carbon [700]. Sodium hypochlorite is also used for the reoxidation of low-valence ruthenium compounds to ruthenium tetroxide in oxidations by ruthenium trichloride [701]. 

Conjugation with aromatic rings and donble bonds, and interactions with halogens and cyclic structures, all affect the band positions, as in the mid-infrared. Also, these second overtones are sometimes split into two as there may be two or more small bands in the vicinity of the calculated second overtone. The second overtone of the C=0 of a carboxylic acid appears at about 5260 cm" (1900 nm) and is particnlarly clear in a spectrum of perfluorocaproic acid taken in solution, as this acid has no CH absorptions. It can be seen in Figure 7.2, a comparison between octanoic and octadecanoic acids in carbon tetrachloride. With the shorter-chain acid, the acid carbonyl peak is more prominent. 

Fig. 14.15. Lattice energies of computational crystal structures for corannulene carboxylic acid. Each point in the graph corresponds to a separately optimized crystal structure. Volumes in and energies in kJ mol . Structures in the upper cluster with energies above—165 kJ mol are closely packed with aromatic ring stacking but are without hydrogen bonds.

Metal organic frameworks (MOFs) are crystalline porous materials whose structure is constituted by metal ions or clusters of metal ions held in places by coordination with bipodal or multipodal rigid organic linkers [1 ]. Typical organic compounds employed for the synthesis of MOFs are aromatic polycarboxylates that coordinate by electrostatic and coordinative bonds with metal ions or metal clusters. The aromatic ring provides conformational rigidity of the linker, making possible the directionality of the interaction of the carboxylic acids with the metallic nodes. There have been reported MOFs for virtually all the transition metals and also for alkali earth and other nontransition metals [5-7]. 

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