What are substitution reactions? Illustrate with an example.

Explanation

### Step-by-Step Solution:

1. **Definition of Substitution Reactions**:
A substitution reaction is a type of chemical reaction where one or more atoms or groups in a molecule are replaced by different atoms or groups. This reaction occurs without changing the rest of the molecule.

2. **Example of a Substitution Reaction**:
Let's take methane (CH₄) as an example of an organic compound. In this reaction, one of the hydrogen atoms in methane is replaced by a chlorine atom.

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