Hydrogenation of aromatic carboxylic acids

Direct hydrogenation of aromatic carboxylic acids to the corresponding aldehydes has been industrialized by use of modified ZrC>2 catalyst by Mitsubishi Chemical Co.[16]. Although the reaction mechanisms are not clear at present, the hydrogenation and dehydration abilities of Zr02, which are associated with basic properties, seem to be important for promoting the reaction. By modification of Zr02 with metal ions such as Cr and Mn, the activity is increased, crystallization is suppressed and the coke formation is avoided. 

Direct hydrogenation of aromatic carboxylic acids to the corresponding aldehydes using modified Zr02 (associated with basic properties) has been commercialized by Mitsubishi Chemical Company (Maki et al., 1993), and is an important example of the industrial application of solid base catalysts. 

The reactivities differ for ortho, meta, and para positions of toluene. In anthranilic acid (10), positions 3 and 5 in the molecule possess 49.8 and 27.4% of the total activity thus, tritium incorporation tends to be highest at positions of highest electron density.35 In isopropanol,35 the hydroxyl group incorporates approximately three times as much isotope as any other hydrogen position, while tritiation of the secondary C-H is slightly more efficient than the primary C-H. In the aliphatic esters of aromatic carboxylic acids, the aryl hydrogens tritiate ten times faster than alkyl hydrogens.10,36... 

Methyl groups by reduction of aromatic carboxylic acids with trichbrosilane-tri-n-propylamine.2 In a hood well vented to permit open atmospheric transfer of trichloro-silane and to remove hydrogen chloride off gas produced, a 300-ml. three-necked. 

Cr -modified Zr09 has been used for the hydrogenation of a variety of aromatic carboxylic acids the results are given in Table 3. Alkyl- and phenoxy-substituted benzoic acids are hydrogenated to the corresponding aldehydes with selectivity up to 95%. Terephthalaldehyde and 4-carbomethoxybenzaldehyde are formed by hydrogenation of dimethyl terephthalate. 

Imidoyl chlorides of aromatic carboxylic acids ( ). An equimolar mixture of the N-monosubstituted amide and phosphorus pentachloride is heated for 15-180 min at 60-140 C. After completion of the reaction, which is indicated by the cessation of hydrogen chloride evolution, the phosphoryl chloride is removed under reduced pressure, and the remaining imidoyl chloride is distilled under vacuum. The reported yield ranges from 41-96%, but generally yields of 80-90% are obtained. 

Rare earth oxides have been studied to a lesser extent than alkaline earth oxides. However, they show characteristic selectivity in the dehydration of alcohols. Secondary alcohols form 1-olefins, whereas the same reaction over an acid catalyst produces the thermodynamically more stable 2-olefin (312). An example of an industrially important rare earth oxide catalyst is Zr02. It has several applications, including the reduction of aromatic carboxylic acids with hydrogen to aldehydes (314), the dehydration of 1-cyclohexyl ethanol to vinyl cyclohexane (315), and the production of diisobutyl ketone from isobutyraldehyde (316). The extensive use of Zr02 is mainly due to its resistance to poisoning by H2O and CO2, and its inherent catalytic activity is a result of its bifunctional acid-base properties. It contains both weakly acidic and basic sites, neither of which is susceptible to poisoning. The acid-base functionality of Zr02 is displayed in the reaction of alkylamine to nitrile (278) (Fig. 33). To form nitriles from both secondary and tertiary amines, both acid and base sites are required. 

Yokoyama T, Setoyama T, Fujita N, Nakajima M, Maki T, Fuji K. Novel direct hydrogenation process of aromatic carboxylic acids to the corresponding aldehydes with zirconia catalyst. Appl Catal A. 1992 88 149-61. 

Sulfur Tetrafluoride and Aromatic Carboxylic Acids. Ben2otrifluorides also are prepared from aromatic carboxyhc acids and their derivatives with sulfur tetrafluoride (SF (106,107). Hydrogen fluoride is frequently used as a catalyst. Two equivalents of sulfur tetrafluoride are required ... 

Oxidation, of acids to peroxy acids by hydrogen peroxide, 43, 96 of alkylarenes to aromatic carboxylic acids, 43, 80... 

Thus, the observed aromatic carboxylic acids at chain ends [11, 25] would be accounted for by the hydrogen abstraction by the carboxyl radical (Scheme 18.1, path A). This, of course, generates another radical species, R, (not shown) capable of carrying on further degradative reactions. 

Alkene loss via McLafferty rearrangement at the alkoxy group of aliphatic and aromatic carboxylic acid esters competes with yet another reaction path, where two hydrogens instead of one as in the normal McLafferty product are transferred to the charge site. This second pathway leading to alkenyl loss has early been noticed [94] and became known as McLafferty rearrangement with double hydrogen transfer (r2H) ... 

Simple aliphatic carboxylic acids having upto four carbon atoms are miscible in water due to the formation of hydrogen bonds with water. The solubility decreases with increasing number of carbon atoms. Higher carboxylic acids are practically insoluble in water due to the increased hydrophobic interaction of hydrocarbon part. Benzoic acid, the simplest aromatic carboxylic acid is nearly insoluble in cold water. Carboxylic acids are also soluble in less polar organic solvents like benzene, ether, alcohol, chloroform, etc. 

According to the figure below, reacting 2,6-dimethylanilme with the acid chloride of pyridine-carboxylic acid first gives the 2,6-xylidide of a-picoUnic acid (2.2.4). Then the aromatic pyridine ring is reduced to piperidine by hydrogen in the presence of a platinum on carbon catalyst. 

The mechanochemical formation of hydrogen-bonded co-crystals between 4-amino-W-(4,6-dimethylpyrimidin-2-yl)benzenesulfonamide and aromatic carboxylic acids was investigated by Caira et al. [65]. 

Aromatic carboxylic acids are, in general, less reactive towards sulfur tetrafluoride than aliphatic acids. The reactivity strongly depends on the nature of the aromatic ring substituents. Benzoic, 4-methylbenzoic and particularly 4-methoxybenzoic acid (la-c) give poor yields of the respective (trifluoromethyl)benzenes 2a-c, but the yields of 2 increase with increasing electron-withdrawing power of the substituents.122 Yields of (trifluoromethyl)benzenes substantially increase, even under milder conditions, in the presence of an excess of anhydrous hydrogen fluoride.123... 

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