Question

when compounds are nitrated, typicallya mixture of concentrated nitric and sulfuric acids are required. howerever, when phenol undergoes nitration only dilute nitric acid is required. why is this?

Answer 1

How does the -OH group affects the reactivity of Phenol?

Activation of the ring

The -OH group attached to the benzene ring in phenol has the effect of making the ring much more reactive than it would otherwise be.

For example, as you will find below, phenol will react with a solution of bromine in water (bromine water) in the cold and in the absence of any catalyst. It also reacts with dilute nitric acid, whereas benzene itself needs a nitrating mixture of concentrated nitric acid and concentrated sulphuric acid.

Inductive effect of the substituent plays important role in its reactivity. Most elements other than metals and carbon have a significantly greater electronegativity than hydrogen. Consequently, substituents in which nitrogen, oxygen and halogen atoms form sigma-bonds to the aromatic ring exert an inductive electron withdrawal, which deactivates the ring.

In summary, OH group present on ring activate ring and easily undergo nitration with dil. Acid while benzene ring or any deactivated compound require Conc.Acid.