Hydrocarbons are organic compounds made up of only carbon (C) and hydrogen (H) atoms. Due to the catenation property of carbon, a large number of hydrocarbons exist.
Aliphatic hydrocarbons are classified based on bond type and saturation:
- Alkanes (Paraffins) – Saturated hydrocarbons with only single bonds.
- Alkenes – Unsaturated hydrocarbons with a double bond.
- Alkynes – Unsaturated hydrocarbons with a triple bond.
So:
- If all carbon valencies are fully satisfied → Saturated (Alkanes)
- If carbon valencies are not fully used and contain double/triple bonds → Unsaturated (Alkenes & Alkynes)
💡 Tip for Students:
- Saturated → Maximum hydrogen atoms attached
- Unsaturated → Fewer hydrogen atoms
📥 Download Class 12 Chemistry Unit 8 – Aliphatic Hydrocarbons Notes
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❓FAQ – Aliphatic Hydrocarbons (Class 12 Chemistry Unit 8)
Q1: What are aliphatic hydrocarbons?
A: Aliphatic hydrocarbons are organic compounds made up of carbon and hydrogen only. They can be saturated (alkanes) or unsaturated (alkenes and alkynes) based on their bond types.
Q2: What is the general formula of alkanes, alkenes, and alkynes?
- Alkanes: CₙH₂ₙ₊₂ (single bonds, saturated)
- Alkenes: CₙH₂ₙ (double bond, unsaturated)
- Alkynes: CₙH₂ₙ₋₂ (triple bond, unsaturated)
Q3: How are alkanes different from alkenes and alkynes?
- Alkanes: Saturated, single bonds only
- Alkenes: Unsaturated, contain at least one double bond
- Alkynes: Unsaturated, contain at least one triple bond
Q4: What is the meaning of saturated and unsaturated hydrocarbons?
- Saturated: All carbon valencies are fully satisfied (maximum hydrogen attached)
- Unsaturated: Carbon valencies are not fully satisfied (contain double or triple bonds)
Q5: How is the nomenclature of aliphatic hydrocarbons done?
It is based on IUPAC rules, considering:
- Length of the carbon chain
- Type and position of double or triple bonds
- Substituents attached to the chain
Q6: Give an example of each type of aliphatic hydrocarbon.
- Alkane: Methane (CH₄), Ethane (C₂H₆)
- Alkene: Ethene (C₂H₄), Propene (C₃H₆)
- Alkyne: Ethyne (C₂H₂), Propyne (C₃H₄)
Q7: Why are hydrocarbons important?
Hydrocarbons are fuels, raw materials for plastics, solvents, and industrial chemicals. They are essential in daily life and industry.
📝 MCQs – Aliphatic Hydrocarbons
1. Which of the following is a saturated hydrocarbon?
(a) Ethene
(b) Ethyne
(c) Ethane ✅
(d) Propene
2. The general formula of alkenes is:
(a) CₙH₂ₙ₊₂
(b) CₙH₂ₙ ✅
(c) CₙH₂ₙ₋₂
(d) CₙHₙ
3. Which hydrocarbon contains a triple bond?
(a) Alkane
(b) Alkene
(c) Alkyne ✅
(d) Aromatic hydrocarbon
4. The IUPAC name of CH₃–CH₂–CH₃ is:
(a) Propane ✅
(b) Propene
(c) Propyne
(d) Methane
5. Unsaturated hydrocarbons include:
(a) Alkanes
(b) Alkenes and Alkynes ✅
(c) Only Alkanes
(d) Only Alkynes
6. Which of the following is correct for saturated hydrocarbons?
(a) They contain double bonds
(b) All carbon valencies are fully satisfied ✅
(c) They contain triple bonds
(d) They are reactive like alkenes
7. The molecular formula of ethyne is:
(a) C₂H₄
(b) C₂H₂ ✅
(c) C₂H₆
(d) C₃H₆
8. Which of the following is a property of alkanes?
(a) Highly reactive
(b) Saturated hydrocarbons ✅
(c) Contains double bonds
(d) Contains triple bonds
9. What type of bond is present in alkenes?
(a) Single bond
(b) Double bond ✅
(c) Triple bond
(d) Both single and triple bonds
10. Which of the following is used as a fuel and solvent?
(a) Alkynes
(b) Alkanes ✅
(c) Alkenes
(d) Aromatic hydrocarbons