2-Methylnaphthalene, bromination

Let us now turn to compounds with more than one benzenoid ring. The first species are the isomeric a- and /J-naphthylamincs, 18a and 18b. The archival enthalpies of formation are found to be 157.6 6.9 and 133.8 5.1 kJmol-1. The 24 9 kJmol-1 difference of these two numbers is incompatible with the near-zero difference of the enthalpies of formation for the isomeric naphthols, methyl- and bromonaphthalenes32. Which or either naphthylamine has the correct enthalpy of formation The gas-phase enthalpies of formation of the naphthols differ from their single benzene ring analog, phenol, by 66 kJmol-1 in close agreement with the difference between the methylnaphthalenes and toluene, 63, and between the brominated and parent hydrocarbons, 69 6 and 68 2 kJmol-1 respectively. That is, it is plausibly asserted33 that the difference quantities 17 are nearly constant and equal. 

Is it possible that the unexpectedly slow oxidations of molecules such as mesitylene, durene, and the methylnaphthalenes which the author has described are caused by the depletion of the bromide portion of the catalyst In these cases, bromination of the activated nuclei could occur and the bromide would be lost permanently. 

A typical procedure is that described in Expt 6.119 for the synthesis of 1-naphthaldehyde. The synthesis of p-nitrobenzaldehyde provides an example in which the intermediate crystalline hexamine salt is isolated prior to hydrolysis. 2-Naphthaldehyde is prepared from the bromomethyl compound, the preparation of which illustrates the use of Af-bromosuccinimide for effecting benzylic bromination of 2-methylnaphthalene. 

A small amount of iodine and iron powder is added to a solution of 142 g. (1.0 mole) of 2-methylnaphthalene in 300 ml. of carbon tetrachloride. The mixture is cooled to 0°, and a solution of 160 g. (1.0 mole) of bromine in 300 ml. of carbon tetrachloride is added (8 hours) with stirring and exclusion of light, during which time the temperature is not allowed to rise above 5°. (Hood.) After standing overnight, the solution is washed with 10% sodiiun hydroxide solution and water followed by drying over calcium chloride. Distillation gives 186 g. (84%) of l-bromo-2-methylnaphthalene, b.p. 152-156°/14 mm. 

Methylnaphthalene can be converted into 2-methyl-6-acetylnaphthalene by acylation with BFs/acetic anhydride, with high selectivity and conversion rates. 2-Methyl-6-acetylnaphthalene, when oxidized at 130 °C and at a pressure of 6 to 8 bar using cobalt/bromine catalysts in acetic add produces naphthalene-2,6-di-carboxylic add. 

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