Question

Formation of Peroxides.

Answer 1

On standing in contact with air, ethers are converted into unstable peroxides (R₂O → O) which are highly explosive even in low concentrations. Hence ether is always purified before distillation. Purification (removal of peroxide) can be done by washing ether with a solution of ferrous salt (which reduces peroxide to alcohols) or by distillation with conc. H₂SO₄ (which oxidises peroxides).

The presence of peroxides in ether is indicated by formation of red colour when ether is shaken with an aqueous solution of ferrous ammonium sulphate and potassium thiocyanate. The peroxide oxidises Fe²⁺ to Fe³⁺ which reacts with thiocyanate ion to give red colour of ferric thiocyanate.

\(Peroxide +Fe^{2+} \longrightarrow Fe^{3+}\)

\(\xrightarrow{CNS^-}\underset {Red}{Fe(CNS)_3}\)

However, the formation of peroxide is prevented by adding a little Cu₂O to it.

Answer 2

Over time and in the presence of oxygen, chemicals will typically form peroxides due to autoxidation. Autoxidation is the slow reaction between chemicals and oxygen. Peroxide forming chemicals usually have a listed manufacturer date.