Step 1. Chlorination of ethane
#underbrace("CH"_3"CH"_3)_color(red)("ethane") + "Cl"_2 stackrelcolor(blue)(hν color(white)(m))(→) underbrace("CH"_3"CH"_2"Cl")_color(red)( "chloroethane") #
Step 2. Dehydrochlorination of chloroethane
# underbrace("CH"_3"CH"_2"Cl")_color(red)( "chloroethane") + "KOH" stackrelcolor(blue)("ethanol"//Δ color(white)(m))(→) underbrace("H"_2"C=CH"_2)_color(red)("ethene")#
Step 3. Addition of sulfur dichloride
#"H"_2"C=CH"_2 + underbrace("Cl-S-Cl")_color(red)("sulfur dichloride") + "H"_2"C=CH"_2 → underbrace("Cl-CH"_2"CH"_2"-S-CH"_2"CH"_2"-Cl")_color(red)("mustard gas")#
