Substituted benzenes, carbon atom reactivity with

A very reactive nitrogen atom is required to convert benzenes or naphthalenes into pyridines, and there are a number of such reactions which involve nitrenes or nitrenoid species. A number of substituted benzenes have been treated with sulfonyl diazide or carbonyl diazide and moderate yields of pyridines recorded (27CB1717). Thus p-xylene gives 2,5-dimethylpyridine there is no indication of the fate of the carbon atom which is lost. More controlled reaction is possible in intramolecular insertions. The examples in which o-nitrotoluene is converted into a derivative (759) of 2-acetylpyridine, and where 2,3-diazidonaphthalenes give 3-cyanoisoquinolines (760) are quoted in a review (81 AHC(28)231>. 

Naphthalene intermediates [61] are always built up by substitution reactions starting from the cheap and plentiful hydrocarbon using, in the main, only seven basic reactions. Most of these reactions are generally familiar from benzene chemistry but with some modification, since naphthalene has two different possible positions of substitution. These positions are often designated a and [3, the four a-positions being ortho and the four P-positions meta to the nearest carbon atom of the central bond. A further modifying influence is the lower level of aromaticity of naphthalene compared with benzene, leading to increased reactivity. 

How is the course of halogen substitution in the benzene nucleus to be explained It is not at all probable that direct replacement of hydrogen occurs, such as we must assume in the formation of benzyl chloride and in the reaction between methane and chlorine, since the hydrogen attached to the doubly bound carbon atom of olefines exhibits no special reactivity. However, various facts which will be considered later (p. 164) indicate that benzene reacts with halogen in fundamentally the same way as does ethylene. The behaviour of ethylene towards bromine is the subject of the next preparation. 

It is agreed that attack of an electrophilic reagent is directed towards the most reactive centers of a core with high electron density. These centers are carbon atoms in the ortho- and /j ra-positions of the benzene substituted if a substituting group is an electron donor, and in the meta-positions if a substituting group is an electron acceptor. 

These inductive and resonance effects oppose each other. The carbon-halogen bond (shown at left) is strongly polarized, with the carbon atom at the positive end of the dipole. This polarization draws electron density away from the benzene ring, making it less reactive toward electrophilic substitution. 

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