Using PCl5 as a chlorinating agent for alcohols

We use phosphorus pentachloride for chlorinating a primary alcohol. But we use Hydrochloric acid for chlorinating a tertiary alcohol. Why is that? ie. Why not use the acid for primary alcohols?

Oh and also, when brominating an alcohol, we use 50% concentrated acid. What is that supposed to mean? Is it dilute or concentrated?! or half half? and why do we use 50%? is to make sure we produce bromine ions instead of bromine gas?

The third question is why do we need separate methods of halogenating alcohols. Why can't it be like halogenating alkanes via an electrophilic substitution reaction?

Thx!

(edited 7 years ago)

Reply 1

charco

Study Forum Helper

Original post by Zuzy

We use phosphorus pentachloride for chlorinating a primary alcohol. But we use Hydrochloric acid for chlorinating a tertiary alcohol. Why is that? ie. Why not use the acid for primary alcohols?

It is more difficult to chlorinate 1º alcohols, so a more vigorous chlorinating agent is needed.

Oh and also, when brominating an alcohol, we use 50% concentrated acid. What is that supposed to mean? Is it dilute or concentrated?! or half half? and why do we use 50%? is to make sure we produce bromine ions instead of bromine gas?

50% means just what you say, 50% pure acid and 50% water.
100% concentrated acid would oxidise the bromide ions to bromine, which is not wanted.

The third question is why do we need separate methods of halogenating alcohols. Why can't it be like halogenating alkanes via an electrophilic substitution reaction?

Thx!

You cannot halogenate alkanes via electrophilic substitution.

Separate methods are used as the reactivity of the 1º, 2º, and 3º alcohol reactivity is not the same. The method used is the optimum (AFAWK) for that specific alcohol, although this is a generalisation.