Alkenes

1️⃣ Unsaturated Hydrocarbons

• Definition:
  Unsaturated hydrocarbons are hydrocarbons that contain at least one carbon–carbon double bond (C=C) in their molecular structure.
• Examples:
  Ethene (C₂H₄), Propene (C₃H₆), Butene (C₄H₈)
• Contrast with Alkanes:
  Alkanes are saturated hydrocarbons — they contain only single bonds between carbon atoms (C–C).
  Alkenes are unsaturated, with one or more double bonds (C=C).

2️⃣ General Formula & Members of Alkenes

• General Formula of Alkenes: C_nH_2n ​ where n = number of carbon atoms
• First Few Members: NameMolecular FormulaStructural FormulaEtheneC₂H₄CH₂=CH₂PropeneC₃H₆CH₂=CH–CH₃ButeneC₄H₈CH₂=CH–CH₂–CH₃ (1-butene)

3️⃣ Preparation of Alkenes

🔹 From Alcohols by Dehydration

Ethene can be prepared by heating ethanol with excess concentrated sulfuric acid (H₂SO₄) or passing ethanol vapor over heated aluminium oxide (Al₂O₃).

Equation: C₂H₅OH →C₂H₄+H₂O

(Ethanol → Ethene + Water)

4️⃣ Cracking of Hydrocarbons

🔹 What is Cracking?

• Cracking is the process of breaking down long-chain alkanes into shorter alkanes and alkenes using heat and a catalyst.

🔹 Conditions:

• Heat (thermal cracking: ~600–700°C)
• Catalyst (e.g., alumina or silica)

🔹 Example:

C₁₀H₂₂→C₈H₁₈+C₂H₄ ​

🔹 Importance of Cracking:

• Produces short-chain alkenes (e.g., ethene and propene) used in polymer industries
• Converts less useful long-chain fractions of crude oil into more useful fuels

5️⃣ Reactions of Alkenes

Alkenes are more reactive than alkanes due to the presence of the double bond (C=C), which can break and form new bonds easily.

6️⃣ Hydrogen Addition Reaction (Hydrogenation)

Ethene reacts with hydrogen in the presence of a nickel catalyst at 150°C.

Equation: C₂H₄+H₂→C₂H₆

Uses: In the manufacture of margarine from vegetable oils (unsaturated fats → saturated fats)

7️⃣ Bromine Addition Reaction (Test for Unsaturation)

Alkenes react with bromine water, which is orange, and turns it colourless.

Equation: C₂H₄+Br₂→CH₂Br–CH₂Br (Ethene + Bromine → 1,2-dibromoethane)

Observation: Decolourisation of bromine water indicates the presence of a C=C bond.

8️⃣ Steam Addition Reaction (Hydration of Alkene)

Ethene reacts with steam in the presence of a phosphoric acid catalyst at high temperature and pressure.

Equation: C₂H₄+H₂O→C₂H₅OH (Ethene → Ethanol)

Industrial Use: Used to manufacture ethanol from ethene.

9️⃣ Polymerisation of Alkenes

• Alkenes can undergo addition polymerization where many alkene molecules join to form a long-chain polymer.

🔹 Example:

nCH₂=CH₂→[−CH₂−CH₂−]_n ​

(Ethene → Poly(ethene), also called polythene)

🔹 Other Polymers:

📝 Summary

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