Quick answer: Hydrogenation is an addition reaction: hydrogen adds across the C=C double bonds of a vegetable oil in the presence of a nickel catalyst, turning the liquid oil into a solid fat (vanaspati). Chlorination of methane is a substitution reaction: in sunlight, chlorine replaces hydrogen atoms one by one, giving and . Unsaturated compounds give addition reactions; saturated compounds give substitution reactions.
Addition reactions
An addition reaction is one in which a molecule such as , or adds across a double or triple bond, forming a single product. Only unsaturated compounds (with C=C or C≡C bonds) can do this.
General example with hydrogen:
The double bond becomes a single bond, and the product is saturated.
Hydrogenation of vegetable oils
Vegetable oils such as groundnut, sunflower and mustard oil contain long carbon chains with C=C double bonds, so they are unsaturated.
When hydrogen gas is passed through warm oil in the presence of finely divided nickel as a catalyst, hydrogen adds to these double bonds:
Why does the liquid oil become solid?
- The double bonds put bends ("kinks") in the chains, so the molecules cannot pack closely. The oil stays liquid at room temperature.
- After hydrogenation, the chains are straighter and pack together more tightly, so the fat becomes solid or semi-solid.
- In industry the oil is usually only partly hydrogenated, to get the right texture.
Role of the catalyst: Nickel speeds up the reaction without being used up. Without a catalyst, hydrogen does not add to the oil at a useful rate.
Oils, fats and health
| Type | Main fatty acids | State at room temperature | Health note |
|---|---|---|---|
| Vegetable oils | Unsaturated | Liquid | NCERT advises choosing these for cooking |
| Animal fats (ghee, butter) | Mostly saturated | Solid | Said to be harmful for health if eaten in excess |
| Hydrogenated fats (vanaspati) | Saturated, and may contain trans fats | Solid | Diets high in saturated and trans fats are linked to raised LDL cholesterol and heart disease |
Exam answer: "Vegetable oils contain unsaturated fatty acids and are better for health. Animal fats contain saturated fatty acids, which are said to be harmful, so oils with unsaturated fatty acids should be chosen for cooking."
Test for unsaturation: bromine water (extension)
Syllabus note: The bromine water test is a standard laboratory test, but it is not described in the rationalised NCERT Class 10 chapter. It is useful background and is studied in later classes.
Bromine water is an orange-brown solution of bromine in water.
- Unsaturated compound (alkene): bromine adds across the double bond, so the colour disappears.
- Saturated compound (alkane): no addition takes place in the dark, so the orange-brown colour remains.
The product, 1,2-dibromoethane, is colourless.
Substitution reactions
Saturated hydrocarbons (alkanes) contain only single bonds and are fairly unreactive. In the presence of sunlight, however, chlorine can replace their hydrogen atoms one at a time. This is a substitution reaction.
Chlorination of methane
Step 1: One hydrogen is replaced, giving chloromethane:
This is the equation given in NCERT. With more chlorine, further hydrogens are replaced:
Step 2:
Step 3:
Step 4:
| Product | Name |
|---|---|
| Chloromethane | |
| Dichloromethane | |
| Trichloromethane (chloroform) | |
| Tetrachloromethane (carbon tetrachloride) |
At each step, hydrogen chloride gas () is formed as a by-product. In practice a mixture of these products is obtained.
Summary table: addition vs substitution
| Basis | Addition (hydrogenation) | Substitution (chlorination) |
|---|---|---|
| Type of hydrocarbon | Unsaturated (C=C or C≡C) | Saturated (only single bonds) |
| Condition | Nickel (or palladium) catalyst | Sunlight |
| What changes | Double bond becomes single bond | H atom replaced by Cl atom |
| Products | One product | Product plus HCl |
| Example | Oil + H₂ → vanaspati | CH₄ + Cl₂ → CH₃Cl + HCl |
Exam tips
- Always write the condition above the arrow: "Ni catalyst" for hydrogenation, "sunlight" for chlorination.
- When asked for one example of each reaction type, use hydrogenation of oils (addition) and chlorination of methane (substitution).
- Link the reaction to the structure: double bond → addition; only single bonds → substitution.
- For the health question, use NCERT's wording: unsaturated fatty acids in oils, saturated fatty acids in animal fats.
Common mistakes
- Saying alkanes give addition reactions. They have no double bond to add across.
- Calling the by-product "hydrochloric acid". It is hydrogen chloride gas; it forms hydrochloric acid only when dissolved in water.
- Forgetting the catalyst in the hydrogenation equation.
- Describing bromine water as colourless to begin with. It is orange-brown and becomes colourless with an alkene.
Frequently Asked Questions
What is hydrogenation of vegetable oil?
It is the addition of hydrogen to the double bonds of vegetable oil in the presence of a nickel catalyst. It converts liquid oil into solid vegetable ghee (vanaspati).
Which catalyst is used in the hydrogenation of oils?
Finely divided nickel is used. Palladium can also catalyse the reaction.
Why is hydrogenation an addition reaction?
Hydrogen adds across the C=C double bonds without any atom being removed, so two reactants combine into one product.
What is a substitution reaction? Give an example.
It is a reaction in which an atom or group replaces another atom in a molecule. For example, in sunlight chlorine replaces a hydrogen atom of methane: CH₄ + Cl₂ → CH₃Cl + HCl.
Why are vegetable oils considered healthier than animal fats?
Vegetable oils contain mainly unsaturated fatty acids, while animal fats contain mainly saturated fatty acids, which are linked to health problems such as heart disease when eaten in excess.
How can you test whether a compound is unsaturated?
Shake it with bromine water. An unsaturated compound removes the orange-brown colour; a saturated compound does not.