JAMB Chemistry Note- Aliphatic Hydrocarbons

Definition of Aliphatic Hydrocarbons

Aliphatic hydrocarbons are organic compounds consisting of carbon and hydrogen atoms arranged in straight chains, branched chains, or non-aromatic rings. These hydrocarbons are classified into three main homologous series:

1. Alkanes (saturated hydrocarbons).
2. Alkenes (unsaturated hydrocarbons with at least one double bond).
3. Alkynes (unsaturated hydrocarbons with at least one triple bond).

1. Alkanes

General Formula: C_nH_2n+2

Alkanes are saturated hydrocarbons containing only single covalent bonds between carbon atoms.

Properties:

Physical Properties:

• State: C₁-C₄: Gases (e.g., methane, ethane).
• C₅-C₁₇: Liquids (e.g., pentane).
• C₁₈ and above: Solids (e.g., paraffin wax).
• Boiling and Melting Points: Increase with molecular size due to stronger van der Waals forces.
• Solubility: Insoluble in water but soluble in nonpolar solvents like benzene.

Chemical Properties:

• Combustion: Complete combustion produces carbon dioxide and water: CH₄ + 2O₂ → CO₂ + 2H₂O. Incomplete combustion produces carbon monoxide or soot.
• Substitution Reactions: Alkanes react with halogens under UV light via a free-radical mechanism: CH₄ + Cl₂ →^UV CH₃Cl + HCl.
• Test for Alkanes: Alkanes do not decolorize bromine water or acidified potassium manganate(VII), showing they are saturated hydrocarbons.

2. Alkenes

General Formula: C_nH_2n

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

Properties:

Physical Properties:

• Similar to alkanes but generally have lower boiling points due to fewer hydrogen atoms.
• Nonpolar and insoluble in water but soluble in organic solvents.

Chemical Properties:

• Combustion: Similar to alkanes but produce a smokier flame due to incomplete combustion.
• Addition Reactions:
  • Hydrogenation: Adds hydrogen to convert an alkene to an alkane: C₂H₄ + H₂ →^Ni C₂H₆.
  • Halogenation: Adds halogens (e.g., bromine) to form dihalides: C₂H₄ + Br₂ → C₂H₄Br₂.
  • Polymerization: Forms long-chain polymers: n(C₂H₄) → -(CH₂-CH₂)-_n (polythene).
• Test for Alkenes: Alkenes decolorize bromine water (orange to colorless) and acidified potassium manganate(VII) (purple to colorless), showing they are unsaturated.

3. Alkynes

General Formula: C_nH_2n-2

Alkynes are unsaturated hydrocarbons containing at least one carbon-carbon triple bond (C≡C).

Properties:

Physical Properties:

• Nonpolar, insoluble in water, and soluble in organic solvents.
• Similar to alkenes but slightly higher boiling points.

Chemical Properties:

• Combustion: Produces carbon dioxide and water with a very smoky flame.
• Addition Reactions: Reacts with hydrogen to form alkenes or alkanes: C₂H₂ + 2H₂ →^Ni C₂H₆.
• Test for Alkynes: Terminal alkynes react with ammoniacal silver nitrate (Tollen’s reagent) to form a white precipitate of silver acetylide, distinguishing them from alkenes.

Fractional Distillation of Petroleum

Petroleum is a complex mixture of hydrocarbons, including alkanes, alkenes, cycloalkanes, and aromatic hydrocarbons. To separate its components, fractional distillation is used.

Process of Fractional Distillation:

1. Heating the Crude Oil: Crude oil is heated to about 350°C in a furnace, turning most of it into vapor.
2. Introduction into a Fractionating Column: The vaporized oil enters a tall fractionating column where the temperature decreases from the bottom to the top.
3. Separation by Boiling Point:
   • Larger molecules (higher boiling points) condense at the bottom.
   • Smaller molecules (lower boiling points) rise higher before condensing.

Major Fractions and Their Uses:

Fraction	Boiling Range (°C)	Uses
Gases	Below 30	Fuel for cooking and heating.
Gasoline (Petrol)	30–200	Fuel for cars.
Kerosene	200–300	Jet fuel, cooking stoves.
Diesel	250–350	Fuel for trucks and engines.
Lubricating Oil	300–400	Machinery lubrication.
Residue (Bitumen)	Above 400	Road surfacing and roofing.

Cracking

To meet the demand for lighter, more useful fractions like gasoline, cracking is employed.

Types of Cracking:

• Thermal Cracking: High temperature (450–750°C) and pressure (70atm). Produces smaller alkenes and alkanes.
• Catalytic Cracking: Uses a catalyst (e.g., zeolite) at lower temperatures (450°C). Produces more branched alkanes and aromatic hydrocarbons, which have higher octane numbers.

Octane Number

The octane number measures the quality of gasoline. High octane numbers indicate better anti-knock properties in engines.
Example: Iso-octane has an octane number of 100, while n-heptane has an octane number of 0.