State that in a substitution reaction one atom or group of atoms is replaced by another atom or group of atoms

Objective

In a substitution reaction one atom or group of atoms is replaced by another atom or group of atoms.

Analogy

Think of a sentence: “The cat sat on the mat.” If you replace cat with dog, the sentence still makes sense but the subject has changed. In a substitution reaction, the “subject” (an atom or group) in the alkane is swapped out for a new one, but the rest of the molecule stays the same.

Example

Methane reacts with chlorine under UV light to give chloromethane and hydrogen chloride:

\$\mathrm{CH4 + Cl2 \xrightarrow{hv} CH_3Cl + HCl}\$

Here, one hydrogen atom in the methane is replaced by a chlorine atom.

General Reaction Pattern

Exam Tips

• Look for keywords like “substitution” or “replace” in the question.
• Remember the carbon skeleton of the alkane never changes.
• Check the reaction conditions: UV light often drives halogenation.
• Write the balanced equation and highlight the atom or group being swapped.
• Use the general pattern R–H + X → R–X to quickly sketch the product.