14.2 General Methods of Preparations of Alkanes

This topic explains the general methods used to prepare alkanes in the laboratory and industry.

Main Idea

Alkanes can be prepared by several methods. The book describes three general methods:

• By the hydrogenation of alkenes and alkynes
• By the reduction of alkyl halides
• By the cracking of larger hydrocarbons

Simple explanation: Alkanes can be made by adding hydrogen to unsaturated compounds, reducing alkyl halides, or breaking down larger hydrocarbons.

Core Concept

Summary of Preparation Methods

Detailed Explanation

1. By the Hydrogenation of Alkenes and Alkynes

Definition: Addition of hydrogen molecule across carbon–carbon multiple bonds is called hydrogenation.

• Finely divided nickel (Ni) catalyst
• 200–300 °C temperature
• High pressure
• Alternatively: Pt or Pd at room temperature

• Alkynes have a triple bond (C≡C).
• One molecule of H₂ breaks the triple bond to form a double bond (alkene).
• A second molecule of H₂ breaks the double bond to form a single bond (alkane).

Equations from the book:

Ethyne → Ethene:

CH≡CH + H₂ Ni, 200–300°C→ CH₂=CH₂

Ethene → Ethane:

CH₂=CH₂ + H₂ Ni, 200–300°C→ CH₃-CH₃

Simple explanation: Hydrogen is added to double or triple bonds to make them single bonds — converting alkenes/alkynes into alkanes.

2. By the Reduction of Alkyl Halides

Definition: When an alkyl halide is treated with Zn in the presence of an aqueous acid, an alkane is produced.

• Zinc (Zn)
• Aqueous acid — usually aqueous HCl or CH₃COOH

• Zn reacts with aqueous acid to liberate atomic hydrogen called nascent hydrogen.
• Nascent hydrogen reduces the alkyl halide.
• Addition of nascent hydrogen is called reduction.

Equation from the book:

CH₃-Cl + 2[H] Zn/HCl(aq)→ CH₄ + HCl

Simple explanation: Zinc and acid produce "fresh" hydrogen that replaces the halogen in the alkyl halide with hydrogen, forming an alkane.

3. By the Cracking of Larger Hydrocarbons

Definition: A large hydrocarbon (alkane) molecule breaks into smaller hydrocarbons when heated at high temperatures (450–750 °C) and high pressure. This process is called thermal cracking.

• High temperature: 450–750 °C
• High pressure

Products: - A mixture of alkanes and alkenes.

Example from the book:

When decane is heated at high temperature and high pressure, it breaks down into octane and ethene:

C₁₀H₂₂ → C₈H₁₈ + C₂H₄

- Decane → Octane + Ethene

Simple explanation: Cracking breaks large alkane molecules into smaller, more useful ones.

Intermediate Level

Hydrogenation — In Detail

• Addition of hydrogen across a carbon–carbon multiple bond.
• Converts unsaturated hydrocarbons (alkenes, alkynes) to saturated hydrocarbons (alkanes).

• Catalyst: finely divided nickel (Ni), platinum (Pt), or palladium (Pd)
• Temperature: 200–300 °C (with Ni); room temperature (with Pt or Pd)
• Pressure: high

Examples:

Alkyne → Alkene: CH≡CH + H₂ Ni→ CH₂=CH₂

Alkene → Alkane: CH₂=CH₂ + H₂ Ni→ CH₃-CH₃

Alkyne → Alkane (two steps): CH≡CH + 2H₂ Ni→ CH₃-CH₃

Simple explanation: Hydrogenation adds hydrogen to double/triple bonds, making them single bonds.

Reduction of Alkyl Halides — In Detail

• Addition of nascent hydrogen to an alkyl halide.
• The halogen is replaced by hydrogen.

Reagents: - Zn + aqueous HCl or Zn + aqueous CH₃COOH

How nascent hydrogen is produced: Zn + 2HCl → ZnCl₂ + 2[H]

- The 2[H] is nascent hydrogen — highly reactive atomic hydrogen.

Reduction reaction: CH₃Cl + 2[H] → CH₄ + HCl

Simple explanation: Zinc and acid make "fresh" hydrogen that replaces the halogen with hydrogen.

Cracking — In Detail

• Breaking large hydrocarbon molecules into smaller ones.
• Also called thermal cracking.

• Temperature: 450–750 °C
• Pressure: high

• Alkanes (smaller)
• Alkenes
• Sometimes hydrogen

Example: C₁₀H₂₂ → C₈H₁₈ + C₂H₄

• Converts less useful large alkanes into more useful smaller ones.
• Produces alkenes for making plastics and other chemicals.
• Used in petroleum industry.

Simple explanation: Cracking is how the petroleum industry turns heavy oils into petrol and useful chemicals.

Comparison of Methods

Important Points

• - Three general methods to prepare alkanes:
• 1. Hydrogenation of alkenes and alkynes
• 2. Reduction of alkyl halides
• 3. Cracking of larger hydrocarbons
• - Hydrogenation: addition of hydrogen across carbon–carbon multiple bonds; catalyst Ni (or Pt/Pd); 200–300 °C and high pressure.
• - Reduction: alkyl halide + Zn + aqueous acid → alkane; involves nascent hydrogen.
• - Cracking: large alkane breaks into smaller alkanes and alkenes; high temperature (450–750 °C) and high pressure.
• - Equations:
• - CH≡CH + H₂ → CH₂=CH₂ → CH₃–CH₃
• - CH₃Cl + 2[H] → CH₄ + HCl
• - C₁₀H₂₂ → C₈H₁₈ + C₂H₄

Remember

• - Hydrogenation = add H₂ to alkene/alkyne → alkane (Ni catalyst, 200–300 °C).
• - Reduction = alkyl halide + Zn/HCl → alkane (nascent hydrogen).
• - Cracking = large alkane → smaller alkanes + alkenes (high temp + high pressure).
• - Equations:
• - CH₂=CH₂ + H₂ → CH₃–CH₃
• - CH₃Cl + 2[H] → CH₄ + HCl
• - C₁₀H₂₂ → C₈H₁₈ + C₂H₄

Common Mistakes

• 1. Confusing hydrogenation with hydration. Hydrogenation adds H₂; hydration adds H₂O.
• 2. Forgetting the catalyst in hydrogenation. Ni (or Pt/Pd) is needed.
• 3. Thinking reduction uses hydrogen gas. Reduction of alkyl halides uses nascent hydrogen (from Zn + acid).
• 4. Confusing cracking with distillation. Cracking breaks molecules; distillation separates them.
• 5. Forgetting the products of cracking. Cracking gives smaller alkanes and alkenes.
• 6. Thinking alkynes add only one H₂. Alkynes can add two molecules of H₂ — first to alkene, then to alkane.

Quick Revision

• General methods of preparation of alkanes:
• Addition of hydrogen molecule across carbon–carbon multiple bonds.
• Conditions: finely divided nickel at 200–300 °C and high pressure; or Pt/Pd at room temperature.
• CH≡CH + H₂ → CH₂=CH₂
• CH₂=CH₂ + H₂ → CH₃–CH₃
• Alkyl halide treated with Zn in presence of aqueous acid.
• Zn + aqueous acid → nascent hydrogen (atomic hydrogen).
• Nascent hydrogen reduces alkyl halide.
• CH₃Cl + 2[H] → CH₄ + HCl
• Large alkane breaks into smaller hydrocarbons at high temperature (450–750 °C) and high pressure.
• Called thermal cracking.
• Produces a mixture of alkanes and alkenes.
• C₁₀H₂₂ → C₈H₁₈ + C₂H₄

60-Second Revision

• Three methods: hydrogenation, reduction, cracking.
• Hydrogenation: alkene/alkyne + H₂ → alkane (Ni, 200–300 °C).
• Reduction: alkyl halide + Zn/HCl → alkane.
• Cracking: large alkane → smaller alkanes + alkenes.
• Warning: reduction of alkyl halides uses nascent hydrogen (from Zn + acid), not H₂ gas.