6.1.1 (i,j) Electrophilic Substitution of Phenols.

Syllabus

(i) the electrophilic substitution reactions of Phenol:

(i) with Bromine to form 2,4,6-tribromophenol

(ii) with dilute Nitric acid to form 2-nitrophenol

{Note that nitration with phenol does not require concentrated HNO₃ or the presence of a concentrated H₂SO₄ catalyst.}

(j) the relative ease of electrophilic substitution of Phenol compared with benzene, in terms of electron pair donation to the pi-system from an Oxygen p-orbital in Phenol

{Illustrated by reactions with bromine and with Nitric Acid.}

{Explanation is only in terms of susceptibility of ring to 'attack' and not in terms of stability of intermediate.}

What does this mean?

Reactivity of Phenols and Benzene rings.

One reason that Alkenes easily undergo electrophilic addition with bromine is because the electron density in the double bond is high (4 electrons) and thus polarises the Bromine molecule.

This doesn't happen to the same extent with Benzene as the electron density between each Carbon atom is closer to 3 electrons than it is to 4, since there are 9 bonding pairs (18 electrons) shared between 6 bonds

Image result for electron density benzene

But with phenols the lone pairs in the Oxygen atom can delocalise into the ring, activating it - making the electron density higher and polarising the Bromine more.

This makes phenol much more attractive to electrophiles than benzene would be.

So, it is much easier to carry out substitution reactions with a Phenol than it is with Benzene (no halogen carrier needed), but the energetic stability of the ring means that substitution is still much easier than addition (which would destroy the ring).

Reaction of Phenols and Bromine

You should know that Phenols will decolourise Bromine like Alkenes and will form a white ppt at the same time - making the reaction with Bromine a useful test.

So, you should be able to write the equation above and name the product.

You should also be able to explain that Phenol can dissolve in water due to Hydrogen Bonding.

Most of the molecule is hydrophobic but the Hydrogen bonding is strong enough to overcome this problem.

The 2,4,6-tribromophenol won't dissolve because the extra three Bromine atoms mean that so much of the molecule is now hydrophobic that the (still equally strong) Hydrogen Bonding is no longer enough to overcome it.

You're not expected to know the mechanism but, if you treat the ring as three double bonds, you could easily suggest the below.

You could do the same with the broken ring model that we used with the Nitration of Benzene.

Nitration of Phenol

Again, nitration of Phenol is much more easily accomplished than with Benzene - no concentrated Sulphuric acid required to make NO₂⁺ ions (Nitronium).

So dilute Nitric Acid can be used and the reaction can be done at Room Temperature.

A mixture of mono-nitrated Phenols will form, with the 2-Nitro form being the more common form than the 4-Nitro isomer.