Alkanes and Alkenes

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Quiz 1:

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Alkanes and Alkenes

A comprehensive summary of Hydrocarbon theory, bonding, and chemical reactions.

1. Alkanes

Definition: Alkanes are a homologous series of hydrocarbons that contain only carbon-carbon single covalent bonds.

Because they contain only single bonds, they are known as Saturated Hydrocarbons. Each carbon atom is bonded to the maximum number of atoms (four).

C H H H H Fig 1: Methane (CH₄)

Physical Properties

  • State: The first four members (Methane to Butane) are gases at room temperature. Larger molecules (like Pentane) are liquids.
  • Trend: Melting and boiling points increase as the molecular size increases.
Name Carbons Formula State (r.t.p)
Methane 1 CH₄ Gas
Ethane 2 C₂H₆ Gas
Propane 3 C₃H₈ Gas
Butane 4 C₄H₁₀ Gas

2. Chemical Properties of Alkanes

Alkanes are generally unreactive because their C–C and C–H bonds are strong. However, they undergo two main types of reactions.

A. Combustion

Alkanes burn in oxygen. This reaction is highly exothermic.

1. Complete Combustion (Excess Oxygen):
Produces Carbon Dioxide and Water.

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)

2. Incomplete Combustion (Insufficient Oxygen):
Produces Carbon Monoxide (CO) and Water.

2CH₄(g) + 3O₂(g) → 2CO(g) + 4H₂O(g)

B. Substitution Reactions

Alkanes react with halogens (like Chlorine) in the presence of Ultraviolet (UV) light. This is a photochemical reaction where a hydrogen atom is replaced by a halogen atom.

CH₄(g) + Cl₂(g) --(UV Light)--> CH₃Cl(g) + HCl(g)

Product: Chloromethane + Hydrogen Chloride.

Note: If chlorine is in excess, further substitution can occur (forming CH₂Cl₂, CHCl₃, etc.).

3. Alkenes

Definition: Alkenes are hydrocarbons containing at least one carbon-carbon double bond (C=C).

They are known as Unsaturated Hydrocarbons.

C C Fig 2: Ethene (C₂H₄)

First member: Ethene (C₂H₄). Methene (CH₂) does not exist because a double bond requires two carbon atoms.

4. Production: Cracking

Alkenes are obtained by Cracking petroleum. This involves breaking down larger alkane molecules into smaller molecules using heat and a catalyst.

Process Conditions:

  • High Temperature (approx. 500°C - 700°C)
  • Catalyst: Aluminium Oxide (Al₂O₃) and Silicon Dioxide (SiO₂).
Long Alkane → Shorter Alkane + Alkene (+ Hydrogen)

Example (Hexane cracking):

C₆H₁₄ → C₄H₁₀ (Butane) + C₂H₄ (Ethene)

Importance of Cracking:

  1. Produces short-chain alkenes (like ethene) for making plastics/ethanol.
  2. Produces Hydrogen (used to make ammonia).
  3. Produces Petrol (short-chain alkanes are in higher demand as fuel).

5. Chemical Properties of Alkenes

Alkenes are more reactive than alkanes due to the double bond. They undergo Addition Reactions where the double bond breaks to become a single bond, and new atoms are added.

A. Bromination (Test for Unsaturation)

Reaction: Addition of aqueous bromine (red-brown).

  • Alkene: Decolourises bromine water rapidly (Red-brown → Colourless).
  • Alkane: No change (remains Red-brown).
C₂H₄(g) + Br₂(aq) → C₂H₄Br₂(l)

Product: 1,2-dibromoethane

B. Hydrogenation (Addition of Hydrogen)

Used to convert vegetable oils into solid margarine.

  • Reagent: Hydrogen gas (H₂)
  • Conditions: 200°C, Nickel (Ni) catalyst.
C₂H₄(g) + H₂(g) → C₂H₆(g) (Ethane)

C. Hydration (Addition of Steam)

Used to manufacture alcohols (ethanol).

  • Reagent: Steam (H₂O)
  • Conditions: 300°C, 60 atm pressure.
  • Catalyst: Phosphoric(V) acid (H₃PO₄).
C₂H₄(g) + H₂O(g) → C₂H₅OH(l) (Ethanol)

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