THE 1ST FRIEDEL-CRAFTS REACTION OF NITROBENZENE

It is a widely accepted generalization in chemistry that low selectivity in a reaction implies high reactivity. We were therefore intrigued by the apparent inertness of benzenes with meta-directing substituents, especially the nitro group, toward Friedel—Crafts alkylations, in which the attacking electrophile is R+.1-3 Friedel-Crafts al-kylations exhibit the lowest selectivity among electrophilic aromatic substitutions,4 implying an extremely early transition state, and reflecting the power of the alkyl cation as an electrophile. Why then, we wondered, is it possible to, say, nitrate a similarly substituted compound with the weaker electrophile N02 +, but not to alkylate it? The mystery is further complicated in light of σ+ values, which provide a relative measure of the susceptibility of the substrate to electrophilic attack. The higher the σ+ value, obviously, the lower its reactivity with respect to an attacking electrophile. Yet, some of the polyhalobenzenes, such as pentachlorobenzene, have been alkylated through the Friedel—Crafts reaction, in spite of the fact that their σ+values are higher than those of benzenes bearing meta-directing substituents, such as nitrobenzene.5 The apparent deactivation toward Friedel—Crafts alkylations observed in benzenes with meta-directing substituents therefore stands in direct conflict with expectations based on both the selectivity/reactivity relationship, and σ+values. By all indications, these compounds should be inherently reactive toward Friedel—Crafts alkylations. © 1990, American Chemical Society. All rights reserved.