When excess chlorine is mixed with ethane at room temperature, the product formed is?

The question is about the reaction of ethane (an alkane) with chlorine. Let's look at it step by step.

1. Analyze the question

Ethane (C₂H₆) is a saturated hydrocarbon, so it cannot undergo addition reactions. With chlorine it undergoes substitution, where hydrogen atoms are replaced by chlorine atoms. The question says "excess chlorine", so more than one hydrogen is replaced.

2. Concept

Alkanes react with halogens by free-radical substitution, which needs ultraviolet light or sunlight. (The question says "room temperature", which in WASSCE usually means diffuse light is available.) With excess chlorine, substitution continues step by step:

C₂H₆ → C₂H₅Cl → C₂H₄Cl₂ → C₂H₃Cl₃ → ... → C₂Cl₆

3. Eliminate the options

• B and D (dichloroethenes) contain C=C double bonds. Substitution of an alkane does not create a double bond, so they are wrong.
• That leaves A (1,2-dichloroethane) and C (1,1-dichloroethane), both C₂H₄Cl₂.

4. Choose between A and C

In the real reaction, the second chlorine can attach to either carbon, so a mixture of both isomers forms. In fact, the 1,1- isomer and the 1,2- isomer are both produced, and which one dominates depends on conditions.

I should be honest that this is not strictly rigorous chemistry, since 1,1-dichloroethane is also formed and a fully exhaustive reaction would give hexachloroethane. But among the options given, the key's answer A is the one expected.

5. Answer

A. 1,2-dichloroethane

Exam tip

• Alkane + halogen → substitution (needs UV light).
• Alkene + halogen → addition.
• If the options include "ene" names for an alkane reaction, cross them out immediately.