You saw that many ketones have a nucleophilic ‘alter ego’ known as an enol tautomer. Formation of the enol tautomer is catalysed by acid or by base, and because the ketone and enol are in equilibrium, enolization in the presence of D2O can lead to replace ment of the protons in the α positions of ketones by deuterium atoms. This is what happens to pentan-3-one in acidic D2O:

Because the enolization and deuteration process can be repeated, eventually all of the α-protons are replaced by deuterium. The way this ketone is deuterated provides evidence that its enol form exists, even though the keto/enol equilibrium greatly favours the ketone form at equilibrium. In this chapter we shall be discussing similar reactions of a compound that exists entirely in its enol form. That very stable enol is phenol and its stability is a consequence of the aromaticity of its benzene ring.

The proton NMR spectrum for phenol is shown below. Before reading any further cover up the rest of this page and make sure you can assign the spectrum.

The next spectrum is the proton NMR after shaking phenol with acidic D2O. Most of the peaks have almost disappeared because the H atoms have been replaced with D. Only one signal remains the same size, and even that is simplified because it has lost any coupling to adjacent protons it may have had previously.

The signal that remains is the 2H signal for the protons in the 3 and 5 positions of the aro matic ring, so the product must be the one shown in the margin. We can explain why by using the same mechanism we used with the ketone on the previous page. Phenol is deuterated in the same way as other enols, except that the final product remains in the very stable, aro matic, enol form rather than reverting to the keto form. The fi rst step (after initial replace ment of the OH with OD) is addition of D₃O+ to the enol.

Now this cation could lose the D from oxygen to leave a ketone (brown arrow below), or it could lose the proton from carbon to leave the phenol (orange arrows below). Alternatively, it could just lose the D and go back to the starting material, which is why there is an equilibrium arrow in the scheme above.

Our spectrum tells us that three ring protons are replaced by D—the ones at the 2, 4, and 6 positions. It’s not hard to see how the same process on the other side of the OH group replaces the proton at C-6. But how does the D at position 4 get there? The enol of phenol is conju gated, and we can push the curly arrows one stage further, like this:

The end product on treating phenol with D₃O⁺ has the protons in the 2, 4, and 6 positions (that is, the ortho and para positions) substituted by deuterium. D³O⁺ is an electrophile, and the overall process is called electrophilic substitution. It is a reaction characteristic of not only phenol but of other aromatic compounds. 

● When aromatic compounds react with electrophiles they generally do so by electrophilic aromatic substitution.

مواضيع ذات صلة

Nitration of benzene

Benzene and its reactions with electrophiles

Reactions of silyl enol ethers with halogen and sulfur electrophiles

Reactions of enol ethers Hydrolysis of enol ethers

Enol and enolate reactions at oxygen: preparation of enol ethers

Silyl enol ethers

Nitrosation of enols

The Robinson annelation

Intramolecular aldol reactions

How to control aldol reactions of aldehydes

How to control aldol reactions of esters

Specifi c enol equivalents from 1,3-dicarbonyl compounds