Explanation: The structure of methanamine is CH₃NH₂ and the structure of ethanamine is CH₃CH₂NH₂. To convert methanamine into ethanamine (the number of carbon atoms are increasing from one carbon atom two carbon atoms) so we need ethanenitrile which we can get from ethyl chloride. We can obtain ethyl chloride from alcohol which can be obtained from diazonium salt (R--N₂⁺Cl^-)

Step 1: Convert methyl amine to methanol

Methyl amine (CH₃NH₂) is first treated with HNO₂ and HCl, which gives a fresh diazonium salt (R--N₂⁺Cl^-) and then diazonium salt undergoes hydrolysis to form methanol.

Step 2: Convert methanol to methyl chloride

Methanol is treated with PCl₅ or thionyl chloride, gives ethanenitrile (CH₃Cl) by the replacement of OH atom by Cl atom.

Step 3: Convert methyl chloride to ethanenitrile

Methyl chloride (CH₃Cl) is treated with ethanolic NaCN or KCN, gives ethanenitrile (CH₃CN)

Step 4: Convert ethanenitrile to ethanamine

Ethanenitrile (CH₃CN) undergoes a reduction in the presence of sodium and ethyl alcohol to form ethanamine (CH₃CH₂NH₂ – final product) which has one carbon atom more than the ethanenitrile.