C2H6 + Cl2 → C2H5Cl + HCl is an example of which reaction type?

• A. Addition
• B. Substitution
• C. Combustion
• D. Fermentation

Answer: (B)

The key idea here is how a hydrogen on an alkane is replaced by a halogen. In this reaction, chlorine gas reacts with ethane to give chloroethane and hydrogen chloride, which means one hydrogen atom in the alkane is swapped for a chlorine atom. That describes substitution, specifically halogen substitution of an alkane. Alkanes are saturated and have only single bonds, so they don’t undergo addition across a double or triple bond. The common mechanism in the presence of light or heat is a radical chain process: Cl2 splits to form Cl radicals, a Cl radical abstracts a hydrogen from ethane to produce HCl and an ethyl radical, and that radical then reacts with Cl2 to form chloroethane plus another Cl radical, continuing the cycle. This differs from combustion, which would involve reacting with oxygen to make CO2 and H2O, and from fermentation, which is a biological process producing alcohols or acids.