Carbon-hydrogen bonds acidity

Convince yourself of this fact by writing an equation using the structural formulas 1 and 3. In contrast, bromoethane can be obtained from structure 2 only through a complicated rearrangement. Two carbon-oxygen and one carbon-hydrogen bond would have to be broken. Experience shows that such complicated reshufflings of atoms rarely occur. Therefore, the reaction between ethanol and hydrobromic acid, HBr, to form bromoethane provides more evidence that ethanol has structure 1. 

Hydrocarbons undergo related reaction.s in the super-acid media, such as fluorosuiphuric acid and antimony pentachloride. It has been suggested that the initial one-electron processes during the electrochemical oxidation of alkanes in fluorosuiphuric acid involve a protonated carbon-hydrogen bond with formation of a carbon radical and release of two protons [15]. 

Oxidation of methylpyridines in 60-80 % sulphuric acid at a lead dioxide anode leads to the pyridinecarboxylic acid [213]. The sulphuric acid concentration is critical and little of the product is formed in dilute sulphuric acid [214]. In these reactions, electron loss from the n-system is driven by concerted cleavage of a carbon-hydrogen bond in the methyl substituent. This leaves a pyridylmethyl radical, which is then further oxidised to the acid, fhe procedure is run on a technical scale in a divided cell to give the pyridinecarboxylic acid in 80 % yields [215]. Oxida-tionof quinoline under the same conditions leads to pyridine-2,3-dicarboxylic acid [214, 216]. 3-HaIoquino ines afford the 5-halopyridine-2,3-dicarboxylic acid [217]. Quinoxaline is converted to pyrazine-2,3-dicarboxylic acid by oxidation at a copper anode in aqueous sodium hydroxide containing potassium permanganate [218]. 

In any reaction where the cleavage of a carbon-hydrogen bond is important, the introduction of a metal ion into the molecule in the proper position will facilitate reaction. For example, in the elimination of the elements of a phosphoric acid monoester from the molecule below, the electrostatic attraction of the cupric ion facilitates removal of the proton on the o -carbon atom with subsequent elimination of the phosphoryl residue (8). 

Another reaction in which the cleavage of a carbon-hydrogen bond is important is the bromination of ketones. In the bromination of ethyl acetoacetate and 2-carboethoxycyclopentanone, it was shown that multivalent cations are catalysts. In the latter reaction, cupric, nickelous, lanthanum, zinc, plumbous, manganous, cadmium, magnesium, and calcium ions were effective (45). One can interpret the effect of the metal ion in terms of its catalysis of the proton transfer from the ester to a base, whether the reaction is carried out in dilute hydrochloric acid solution (acid-catalyzed bromination) or in acetate buffer (base-catalyzed bromination). 

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