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Lesson 2: Unsaturated Hydrocarbons – Alkenes (Olefins)

Chemistry

Lesson 2: Unsaturated Hydrocarbons – Alkenes (Olefins)

Overview

Overview

🌳 Concept tree

  1. Unsaturated Hydrocarbons – Alkenes (Olefins)
  2. ├── Structure
  3. ├── Nomenclature
  4. ├── Laboratory preparation of ethene
  5. ├── Physical properties
  6. └── Chemical properties of alkenes
Lesson 2: Unsaturated Hydrocarbons – Alkenes (Olefins) · 1 / 7
Structure

Structure

  • Alkenes are unsaturated open-chain hydrocarbons containing at least one C=C double bond.
  • The double bond consists of one strong sigma (σ) bond and one weaker pi (π) bond, which is easily broken — this is why alkenes are much more reactive than alkanes.
  • They may be considered alkanes minus two hydrogen atoms; general formula CₙH₂ₙ (n ≥ 2).
Lesson 2: Unsaturated Hydrocarbons – Alkenes (Olefins) · 2 / 7
Nomenclature

Nomenclature

  1. Take the longest chain containing the double bond; replace -ane by -ene.
  2. Number from the end nearer the double bond (whatever other groups there are), and put the number of the first carbon of the double bond before the name.
  3. Substituents are named and numbered as for alkanes.
    Examples: CH₂=CH–CH₃ propene; CH₃–CH=CH–CH₂–CH₃ 2-pentene; CH₂=CH–CH(CH₃)–CH₂–CH₃ 3-methyl-1-pentene; CH₂=CH–CH₂–CH₂Cl is 4-chloro-1-butene; (CH₃)₂C=CH₂ 2-methylpropene (isobutene). The common name of ethene is ethylene.
Lesson 2: Unsaturated Hydrocarbons – Alkenes (Olefins) · 3 / 7
Laboratory preparation of ethene

Laboratory preparation of ethene

By dehydration of ethanol→heat ethanol with concentrated sulphuric acid at about 180 °C:

C₂H₅OH —(conc. H₂SO₄, 180 °C)→ CH₂=CH₂ + H₂O

  • It occurs in two steps:
  1. CH₃CH₂OH + HOSO₃H → CH₃CH₂–OSO₃H (ethyl hydrogen sulphate) + H₂O
  2. CH₃CH₂–OSO₃H (heat) → CH₂=CH₂ + H₂SO₄ (the acid is regenerated, so it acts as a catalyst/dehydrating agent)
Apparatus→flask with ethanol/acid mixture (sand or pumice stones added to prevent bumping) → delivery tube → wash bottle with NaOH solution (removes SO₂ and CO₂ formed in side reactions) → collect ethene over water. If the temperature is only 140 °C the product is diethyl ether instead (see alcohols).
Lesson 2: Unsaturated Hydrocarbons – Alkenes (Olefins) · 4 / 7
Physical properties

Physical properties

  • The first alkenes (C₂–C₄) are gases; C₅–C₁₅ are liquids; above C₁₅ solids.
  • They are non-polar, insoluble in water, soluble in organic solvents such as ether, benzene and CCl₄.
Lesson 2: Unsaturated Hydrocarbons – Alkenes (Olefins) · 5 / 7
Chemical properties of alkenes

Chemical properties of alkenes

  1. (exothermic): C₂H₄ + 3O₂ → 2CO₂ + 2H₂O + energy. (Alkenes burn with a smokier flame than alkanes because of the higher carbon percentage.)
  2. The weak π bond breaks, and each carbon of the double bond picks up one atom or group; the product is saturated.
  3. CH₂=CH₂ + H₂ → CH₃–CH₃ (ethane). Each π bond needs one mole of H₂.
  4. CH₂=CH₂ + Br₂ (in CCl₄) → CH₂Br–CH₂Br (1,2-dibromoethane). The red-brown colour of bromine disappears, so this is a test for unsaturation.
  5. CH₂=CH₂ + HBr → CH₃CH₂Br (bromoethane).
  6. For a symmetrical alkene (e.g. ethene) H may go to either carbon — one product.
  7. For an unsymmetrical alkene (e.g. propene) there are two possible products; Markovnikov’s rule decides: when an unsymmetrical reagent (HX or H–OSO₃H) adds to an unsymmetrical alkene, the positive part (H) adds to the double-bond carbon that already carries more hydrogen atoms and the negative part adds to the carbon with fewer hydrogens.
    CH₃–CH=CH₂ + HBr → CH₃–CHBr–CH₃ (2-bromopropane, the major product).
  8. water alone is not able to add (its H–OH bond is too weakly polar to attack the double bond), so a strong acid is needed:
  9. CH₂=CH₂ + H–OSO₃H → CH₃CH₂–OSO₃H (ethyl hydrogen sulphate)
  10. CH₃CH₂OSO₃H + H₂O → CH₃CH₂OH + H₂SO₄
  11. CH₂=CH₂ + H₂O —(H₃PO₄ or H₂SO₄)→ CH₃CH₂OH. Industrially, ethene is hydrated with steam in the presence of phosphoric acid (catalyst) at high temperature and pressure.
  12. Cold dilute alkaline potassium permanganate (KMnO₄ + Na₂CO₃) oxidises alkenes to glycols (diols): the purple colour disappears (a brown MnO₂ precipitate appears). This is used to detect a double bond.
  13. CH₂=CH₂ + [O] + H₂O → HOCH₂–CH₂OH (ethylene glycol, 1,2-ethanediol)
  14. it forms hydrogen bonds with water molecules and prevents them from joining into ice crystals, lowering the freezing point.
  15. Polymerisation. A polymer (Greek: "many parts") is a very large molecule built from many small repeating units called monomers.
  16. is the joining of many unsaturated monomer molecules without losing any atoms.
  17. When ethene is heated under very high pressure (about 1000 atm) with a peroxide initiator, its π bonds open, each carbon gets a free electron, and the molecules join in a long chain: n CH₂=CH₂ → –(CH₂–CH₂)ₙ– polyethylene (polythene). Though ethene has M = 28, polyethylene may have a molecular mass of tens of thousands.
  18. PVC (polyvinyl chloride) from chloroethene CH₂=CHCl; Teflon (polytetrafluoroethene, PTFE) from CF₂=CF₂, non-stick and chemically very resistant; polypropene from propene CH₃CH=CH₂.
  19. A copolymer is formed from two different monomers (the copolymer unit then continues the polymerisation).
Lesson 2: Unsaturated Hydrocarbons – Alkenes (Olefins) · 6 / 7
Part 7

Part 7

  • 📖

🔍 In-Depth Explanations, Deductions & Multi-Book Insights:

Chemistry Thanaweya Amma (2026/2027)

  • Lesson 2 – Unsaturated Hydrocarbons – Alkenes (Olefins)
  • Detailed Academic Lesson Overview:
  • Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond (C=C, consisting of one strong σ bond and one weak π bond, general formula CnH2n).
  • They undergo characteristic addition reactions governed by Markovnikov's rule.

🌳 Hierarchy Classification & Conceptual Roadmap

Alkenes (Olefins – CnH2n)

├── 1. Laboratory Preparation of Ethene (C2H4)

  • │ └── Dehydration of Ethanol (C2H5OH) using conc. H2SO4 at 180°C via ethyl hydrogen sulfate

├── 2. Addition Reactions across Double Bond

Comparison aspectMain ideaKeywords
│ ├── Hydrogenation (Ni catalyst at 150-300°C)C2H4 + H2 → C2H6 (Hardening of vegetable oils)C2H4C2H6Hardeningvegetable
│ ├── HalogenationAddition of Br2 dissolved in CCl4 (Red color of bromine discharges)AdditiondissolvedCCl4color
•│ ├── Hydrohalogenation (HX addition)Hydrohalogenationaddition
│ │ ├── Symmetrical alkenesAny carbon receives Hcarbonreceives
│ │ └── Unsymmetrical alkenes
  • Markovnikov's Rule (H adds to rich carbon
  • X to poor carbon)
MarkovnikovRuleaddsrich
│ └── Catalytic HydrationC2H4 + H2O (dil. H2SO4 at 110°C) → C2H5OHC2H4H2SO4C2H5OH
•└── 3. Oxidation & Baeyer's Test and PolymerizationOxidationBaeyerTestPolymerization
├── Baeyer's TestEthene + alkaline KMnO4 → Ethylene glycol (turns purple KMnO4 colorless)EthenealkalineKMnO4Ethylene
└── Addition Polymerization
  • n C2H4 (High P
  • catalyst) → Polyethylene [PE]
C2H4HighcatalystPolyethylene
•📋 Master Teacher's Blackboard Rules & Exam SecretsMasterTeacherBlackboardRules
RULE 1
  • Markovnikov's Rule: When an asymmetric reagent (like H-Br) adds to an asymmetric alkene
  • the hydrogen atom attaches to the carbon atom with the greater number of hydrogen atoms
Markovnikovasymmetricreagentlike
RULE 2
  • Baeyer's test (alkaline KMnO4) is a diagnostic test for the presence of the carbon-carbon double bond
  • the purple color disappears with the formation of colorless ethylene glycol
BaeyertestalkalineKMnO4
RULE 3
  • Ethylene glycol (1
  • 2-ethanediol) is used as an antifreeze in car radiators in cold climates because its two -OH groups form strong hydrogen bonds with water molecules
  • preventing water crystals from aggregating
Ethyleneglycolethanediolused
📊 Detailed Comparison MatrixDiagnostic Tests for Carbon-Carbon Double BondsDiagnosticTestsCarbonDouble
Reagent Used Condition Observation with Alkene (Ethene) Observation with Alkane (Ethane)
Bromine in Carbon Tetrachloride (Br2 / CCl4) Room temp, dark Red bromine color discharges completely Red color persists (No reaction)
Alkaline Potassium Permanganate (Baeyer's) Aqueous alkaline solution
  • Purple color disappears
  • forms ethylene glycol
 Purple color persists
Acidic Hydration (H2O + H2SO4) 110°C Forms ethanol No reaction
Lesson 2: Unsaturated Hydrocarbons – Alkenes (Olefins) · 7 / 7