Substitution reaction
One atom takes the place of another.
A reluctant molecule
Saturated hydrocarbons such as methane are fairly unreactive. But in sunlight, chlorine reacts with them in a very fast reaction.
What do you think chlorine does to the methane molecule?
What this lesson covers
The idea
Saturated hydrocarbons are fairly unreactive, but in sunlight chlorine rapidly replaces their hydrogen atoms one by one; a reaction in which one type of atom or group takes the place of another is a substitution reaction.
A reluctant molecule
Saturated hydrocarbons such as methane are fairly unreactive. But in sunlight, chlorine reacts with them in a very fast reaction.
What do you think chlorine does to the methane molecule?
- It takes the place of a hydrogen atom
- It is added to the carbon-carbon double bond
- It breaks the molecule into carbon and hydrogen
Chlorine and methane in sunlight
Press Next step. A teacher does this in the lab.
Replacing, not adding
In this reaction chlorine does not join a double bond. It takes the place of a hydrogen atom, so we call it substitution.
Saturated hydrocarbons are fairly unreactive, but in sunlight chlorine rapidly replaces their hydrogen atoms one by one. A reaction in which one type of atom or group takes the place of another is a substitution reaction.
Notes
Saturated hydrocarbons are fairly unreactive, but in sunlight chlorine rapidly replaces their hydrogen atoms one by one. A reaction in which one type of atom or group takes the place of another is a substitution reaction.
Check yourself
What is a substitution reaction?
What is needed for chlorine to react quickly with a saturated hydrocarbon?
In CH₄ + Cl₂ giving CH₃Cl + HCl, what does chlorine do to methane?
How many hydrogen atoms does CH₃Cl have?
Answer: 3
One of the 4 hydrogen atoms of methane is replaced by chlorine, so 3 remain.
- One in which one type of atom or group takes the place of another — correct. Yes!
- One in which an unsaturated hydrocarbon adds hydrogen. That is an addition reaction.
- One in which a substance burns in oxygen. That is an oxidation reaction. In a substitution reaction an atom or group takes the place of another.
- A nickel catalyst. Nickel is the catalyst for adding hydrogen to unsaturated compounds. Chlorine reacts with saturated hydrocarbons in sunlight.
- Sunlight — correct. Yes!
- Concentrated sulphuric acid. Concentrated sulphuric acid is a dehydrating agent. Chlorine reacts with saturated hydrocarbons in sunlight.
- It adds to a double bond. Methane has no double bond. Chlorine replaces a hydrogen atom.
- It removes a carbon atom. The carbon atom stays. Chlorine replaces a hydrogen atom.
- It replaces a hydrogen atom — correct. Yes!