So far, all of the substituted benzene rings we have considered have carried substituents capable of donating electron density to the ring: despite being electronegative atoms, oxygen and nitrogen have lone pairs which conjugate with the ring’s π system; a similar but weaker effect results from σ conjugation from a methyl group. Two consequences arise from these substitu ents: the ring becomes more reactive than benzene, and substitution takes place in the ortho and para positions. So, what happens with groups which pull electron density away from the ring? Such a group is the trimethylammonium substituent: the nitrogen is electronegative but unlike in aniline this electronegativity is not offset by donation of a lone pair—the nitrogen is tetrahedral and no longer has one to donate. Nitration of the phenyltrimethyl ammonium ion yields mainly the meta product. And it does so slowly too—this nitration proceeds about 107 times more slowly than that of benzene.

The same thing happens with the CF₃ group. The three very electronegative fluorine atoms polarize the C–F bonds so much that the Ar–C bond is polarized too. Nitration of trifluoro methylbenzene gives a nearly quantitative yield of meta nitro compound.

Draw the mechanism for this reaction and you see the reason for the switch to meta selectivity.

The intermediate cation is again delocalized over three carbons, but importantly none of these carbons is the one next to the CF₃ group.

If, on the other hand, the electrophile were to attack the ortho or para position (the hypo thetical reaction para to CF₃ is shown below) then the carbon next to CF₃ would have to carry a positive charge, which would be destabilized by the electron withdrawal, making this a high-energy intermediate.

Think of it this way: the electron-deficient ring would really rather not react with an electrophile (hence the slower rate) but if it has to (because the electrophile is so reactive) then it takes the least bad course of keeping the positive charge away from the electron-withdrawing groups—and that means meta substitution.

مواضيع ذات صلة

Halogens show evidence of both electron withdrawal and donation

Some substituents withdraw electrons by conjugation

The sulfonation of toluene

Selectivity between ortho and para positions

Making aromatic amines less reactive

A nitrogen lone pair activates even more strongly

Oxygen substituents activate a benzene ring

Reaction conditions

Conjugated alkenes can be electrophilic

Alkenes conjugated with carbonyl groups

Two reasons to avoid a Friedel–Crafts alkylation

Two or more substituents may cooperate or compete