Question

Write reactions for:
(i) Pyrolysis of n-hexane
(ii) Wurtz synthesis

Answer 1

The heating of hydrocarbons at high temperatures is known as pyrolysis. N-hexane contains six-carbon. The pyrolysis products contain carbon less than six. Wurtz reaction is used to combine two same or different types of alkyl halide. The reagent used in the Wurtz reaction is sodium in presence of dry ether.

Complete answer
(i) Pyrolysis of n-hexane
Pyrolysis is also known as cracking. By pyrolysis, the higher hydrocarbons decompose into lower hydrocarbons (alkane, alkenes) and water.
The pyrolysis of alkane follows a free radical mechanism. The formula of n-hexane is ${{\text{C}}_{\text{6}}}{{\text{H}}_{{\text{14}}}}$. The pyrolysis of n-hexane takes place at $773$K.

The pyrolysis of n-hexane is shown as follows:
${{\text{C}}_{\text{6}}}{{\text{H}}_{{\text{14}}}}\, \to {{\text{C}}_{\text{6}}}{{\text{H}}_{{\text{12}}}}\, + \,{{\text{H}}_2}$
${{\text{C}}_{\text{6}}}{{\text{H}}_{{\text{14}}}}\, \to {{\text{C}}_4}{{\text{H}}_8}\, + \,{{\text{C}}_2}{{\text{H}}_6}$
${{\text{C}}_{\text{6}}}{{\text{H}}_{{\text{14}}}}\, \to {{\text{C}}_3}{{\text{H}}_6}\, + \,{{\text{C}}_2}{{\text{H}}_{4\,}}\, + \,{\text{C}}{{\text{H}}_{\text{4}}}$

The pyrolysis of n-hexane gives n-hexene, hydrogen, butene, ethane, propene, ethane and methane.
Therefore, the reaction for pyrolysis of n-hexane are,
${{\text{C}}_{\text{6}}}{{\text{H}}_{{\text{14}}}}\, \to {{\text{C}}_{\text{6}}}{{\text{H}}_{{\text{12}}}}\, + \,{{\text{H}}_2}$
${{\text{C}}_{\text{6}}}{{\text{H}}_{{\text{14}}}}\, \to {{\text{C}}_4}{{\text{H}}_8}\, + \,{{\text{C}}_2}{{\text{H}}_6}$
${{\text{C}}_{\text{6}}}{{\text{H}}_{{\text{14}}}}\, \to {{\text{C}}_3}{{\text{H}}_6}\, + \,{{\text{C}}_2}{{\text{H}}_{4\,}}\, + \,{\text{C}}{{\text{H}}_{\text{4}}}$

(ii) Wurtz synthesis
In presence of ${\text{Na/ether}}$ , two alkyl halide join, the reaction is known as Wurtz fitting reaction.
${\text{2}}\,{\text{RX}}\,{\text{ + }}\,\,{\text{Na}}\mathop \to \limits^{{\text{dry ether}}} \,R - {\text{R}}\,\,{\text{ + }}\,{\text{2}}\,{\text{NaX}}\,$
The RX reacts with sodium in presence of dry ether to give R-Na which generates ${{\text{R}}^ - }$ nucleophile. The nucleophile attacks on the second RX to give RR.

Note: The total number of carbon atoms and hydrogen atoms remains the same on both sides of the pyrolysis reaction. The carbon number in each product of pyrolysis will be less than the carbon number of the reactant. Pyrolysis gives lower hydrocarbon from higher hydrocarbons. Wurtz reaction gives higher hydrocarbons from lower hydrocarbons. If a single type of alkane is used as a reactant then, the product formed by the Wurtz reaction will have double carbon number than the carbon present in the reactant. Wurtz reaction is also known as Wurtz fitting reaction.