Question

Acetylene and hydrogen cyanide is passed through a red hot tube 2:1 to form _________.

Answer 1

Acetylene ( ${\mathrm{C}}_2{\mathrm{H}}_2$) on passing through a red-hot iron tube at $873\mathrm{K}$ undergoes cyclic polymerization. Three molecules of acetylene undergo polymerization to produce benzene ${\mathrm{C}}_6{\mathrm{H}}_6$.

${\left({\mathrm{C}}_2{\mathrm{H}}_6\right)}_3\left(\mathrm{aq}\right)\stackrel{}{\stackrel{\mathrm{red}\mathrm{hot}\mathrm{tube}}{\to }{\mathrm{C}}_6{\mathrm{H}}_6\left(\mathrm{aq}\right)}$

This is Diels Alder reaction: The Diels-Alder reaction is a conjugate addition reaction of a conjugated diene to an alkene or alkyne (the dienophile) to produce cyclohexane.

Diels Alder reaction

• The Diels-Alder reaction is an important organic chemical reaction where the reactants include a substituted alkene and a conjugated diene.
• This substituted alkene is commonly referred to as a dienophile.
• This reaction gives rise to a substituted derivative of cyclohexene.
• The Diels Alder reaction is a very good example of pericyclic reactions which proceed via concerted mechanisms.

Thus, Acetylene and hydrogen cyanide is passed through a red hot tube 2:1 to form benzene.