Chemistry

Aromatic reactions - nucleophilic aromatic substitution Ar-SN

Aromatic reactions - nucleophilic aromatic substitution Ar-SN



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Nucleophilic substitution on aromatics (Ar-SN)

In addressing the electrophilic substitution of aromatics, it has been shown that electron donating substituents greatly facilitate attack by electrophiles. Conversely, the question arises whether sufficient electron-withdrawing substituents can also lead to the aromatic system becoming amenable to nucleophilic attack.

In fact, it can be observed that halobenzenes bearing nitro groups in the ortho and para positions are derived from nucleophiles such asOC.H3 be attacked in a substitution reaction.

The comparison of the electrostatic potential surfaces shows the stronger positive polarization of the chlorine-substituted carbon atom due to the electron-withdrawing effect of the nitro groups, which makes nucleophilic attack easier. (blue = more positive potential, red = more negative potential)

The more electron withdrawing substituents are present, the easier the reaction becomes. First and foremost they are ortho- andparaPositions relevant, while substituents inmeta-Position only make a small contribution. These observations can be explained in terms of a mechanism that is similar to electrophilic aromatic substitution and, so to speak, its nucleophilic counterpart.

Here, too, there is an addition-elimination mechanism, in which, however, in the first, rate-limiting step, the aromatic system is abolished by the attack of a nucleophile Nu. In the resulting carbanion, the negative charge across the five sp2-hybridized carbon atoms distributed. In the final step, the rearomatization takes place by splitting off the leaving group X. The likewise conceivable elimination of the former nucleophile Nu, which would lead to the regression of the educt, is generally of minor importance, since the nucleophiles usually represent poor leaving groups.

Further delocalization through conjugation with acceptor substituents is possible if these are in ortho- andpara-Position to the sp3The stabilization through electron-withdrawing inductive effects also has an effect in the same direction, which is most strongly in the direct vicinity of the center with the highest negative charge density, i.e. inparaPosition and only slightly less in ortho-Position.

Both the nucleophile Nu and the leaving group X show little leaving group activity, and the aromatic has three nitro groups inortho- and paraPosition, the intermediate carbanionic complexes become so long-lived that they can be isolated. Stable carbanions of this structure type are called Meisenheimer complexes, and the non-isolable intermediates of nucleophilic aromatic substitution are consequently referred to as Meisenheimer complex-analogous intermediates.

The reactivity of nucleophilic anions can be significantly increased by carrying out the reaction in a polar aprotic solvent. As a result, the anions are not stabilized by hydrogen bridging. Under these conditions, for example, non-activated halobenzenes can also be used for nucleophilic aromatic substitution.