What evidence is there for intermediates like the ones we have been using in this section? When reactions like this last example are carried out, a purple colour often appears in the reaction mixture and then fades away. In some cases the colour is persistent and thought to be due to the intermediate. Here is an example with RO− attacking a nitrated aniline. This intermediate is persistent because neither potential leaving group (NR₂ or OR) is very good.

What is the nature of this intermediate? Well, in essence it is an anion delocalized over five sp² hybridized carbons of a six-membered ring (the sixth, the point at which the nucleophile attacked, is sp³ hybridized). It’s possible to make a simple homologue of such a species by deprotonating cyclohexadiene. Delocalizing the anion generates the three structures below.

You’ve seen before that ¹³C NMR spectra are revealing when it comes to distribution of charge, and the details of the ¹³C NMR spectrum of this anion are shown below, along with those of benzene itself and also of the cation generated by protonating benzene.

These results are very striking. The shifts of the meta carbons in both ions are very slightly different from those of benzene itself (about 130 ppm). But the ortho and para carbons in the anion have gone upfi eld to much smaller shifts, indicating greater electron density. By contrast, ortho and para carbons in the cation have gone downfi eld to much larger shifts. The differences are very great—about 100 ppm between the cation and the anion! It is very clear from these spectra that the ionic charge is delocalized almost exclusively to the ortho and para carbons in both cases. The alternative structures in the margin show this delocalization. This means that stabilizing groups, such as nitro or carbonyl in the case of the anion, can only have an effect if they are on carbons ortho or para to the position being attacked by the nucleophile. A good illustration of this is the selective displacement of one chlorine atom out of these two. The chlorine ortho to the nitro group is lost; the one meta is retained.

The mechanism works well if the nucleophile (the anion derived from the thiol) attacks the carbon bearing the chlorine ortho to the nitro group as the negative charge can then be pushed into the nitro group. Satisfy yourself that you cannot do this if you attack the other chlorine position. This is a very practical reaction and is used in the manufacture of a tranquillizing drug.

مواضيع ذات صلة

How to reduce carboxylic acids to alcohols

How to reduce amides to amines

The benzyne mechanism

Other nucleophiles

The SN1 mechanism for nucleophilic aromatic substitution: diazonium compounds

The activating anion-stabilizing substituent

The leaving group and the mechanism

The addition–elimination mechanism

Nucleophilic aromatic substitution

Nucleophilic epoxidation

Conjugate substitution reactions

Unsaturated nitriles and nitro compounds