Aromatic nitration is important because it is a convenient way of adding a nitrogen sub stituent to the ring and because it stops cleanly after one nitro group has been added. Double nitration of benzene is possible but stronger conditions must be used—fuming nitric acid instead of normal concentrated nitric acid—and the mixture must be refluxed at around 100 °C.

The second nitro group is introduced meta to the first: evidently the nitro group is deactivating and meta-directing. The nitro group is conjugated with the π system of the benzene ring and is strongly electron withdrawing—and it withdraws electrons specifi cally from the ortho and para positions. We can use curly arrows to show this:

The nitro group withdraws electron density from the π system of the ring thereby making the ring less reactive towards an electrophile. Since more electron density is removed from the ortho and para positions, the least electron-deficient position is the meta position. Hence the nitro group is meta directing. In the nitration of benzene, it is much harder to nitrate a second time and, if we insist, the second nitro group goes in meta to the fi rst. Other reactions go the same way: bromination of nitrobenzene gives meta-bromonitro benzene in good yield. The combination of bromine and iron powder provides the necessary Lewis acid catalyst (FeBr₃) while the high temperature needed for this unfavourable reaction is easily achieved as the boiling point of nitrobenzene is over 200 °C.

In drawing the mechanism it is best to draw the intermediate and to emphasize that the positive charge must not be delocalized to the carbon atom bearing the nitro group.

Nitro is just one of a number of groups that are also deactivating towards electrophiles and meta directing because of electron withdrawal by conjugation. Others include carbonyl groups (aldehydes, ketones, esters, etc.), nitriles, and sulfonates. The 1H NMR shifts of rings carrying these substituents confirm that they remove electrons principally from the ortho and para positions.

Points to note:

 • Each of the compounds contains the unit Ph–X=Y, where Y is an electronegative element, usually oxygen.

 • In each compound, all the protons have larger chemical shifts than benzene because the electron density at carbon is less.

 • The protons in the meta position have the smallest shift and so the greatest electron density. Nitro is the most electron-withdrawing of these groups and some of the other compounds are nearly as reactive (in the meta position, of course) as benzene itself. It is easy, for example, to nitrate methyl benzoate and the m-nitro ester can then be hydrolysed to meta-nitrobenzoic acid very easily.

●Electron-withdrawing groups make aromatic rings more reluctant to undergo electrophilic substitution, but when they do react, they react in the meta position.

One group of substituents remains and they are slightly odd. They are ortho, para-directing but they are also deactivating. They are the halogens.

مواضيع ذات صلة

Halogens show evidence of both electron withdrawal and donation

Electron-withdrawing substituents give meta products

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