Introduction to Hydrocarbons

Definition and Classification of Hydrocarbons

• Hydrocarbon Definition: Organic compounds consisting exclusively of carbon ($\text{C}$) and hydrogen ($\text{H}$) atoms.
• Saturated Hydrocarbons (Alkanes):
  • Contain only single carbon-carbon ($\text{C--C}$) bonds with the maximum number of hydrogen atoms.
  • General formula: $\text{C}_n\text{H}_{2n+2}$
  • Examples: Methane ($\text{CH}_4$), Ethane ($\text{C}_2\text{H}_6$), Propane ($\text{C}_3\text{H}_8$).
• Unsaturated Hydrocarbons (Alkenes & Alkynes):
  • Alkenes: Contain at least one double carbon-carbon ($\text{C=C}$) bond. General formula: $\text{C}_n\text{H}_{2n}$ (e.g., Ethene, $\text{C}_2\text{H}_4$).
  • Alkynes: Contain at least one triple carbon-carbon ($\text{C}\equiv\text{C}$) bond. General formula: $\text{C}_n\text{H}_{2n-2}$ (e.g., Ethyne, $\text{C}_2\text{H}_2$).

Crude Oil and Fractional Distillation

• Crude Oil (Petroleum): A naturally occurring, complex physical mixture of hydrocarbons (mainly alkanes, cycloalkanes, and aromatic hydrocarbons) formed over millions of years from marine organisms.
• Niger Delta Crude Oil: Known for being light and low in sulphur content.
• Fractional Distillation Process:
  • Separates crude oil components physically based on differences in boiling points.
  • Vaporized crude oil enters a fractionating column that is hotter at the bottom and cooler at the top.
  • Short-chain hydrocarbons have lower boiling points and condense near the top, while long-chain hydrocarbons have higher boiling points and condense near the bottom.
• Key Fractions by Boiling Range:
  • Below $25\,^\circ\text{C}$: Refinery gases (LPG / cooking gas)
  • $40\text{--}100\,^\circ\text{C}$: Petrol / Gasoline (fuel for cars and generators)
  • $150\text{--}250\,^\circ\text{C}$: Kerosene (lamps, jet fuel)
  • $250\text{--}350\,^\circ\text{C}$: Diesel oil (trucks, generator sets)
  • $350\text{--}400\,^\circ\text{C}$: Lubricating fractions (oils, waxes)
  • Above $400\,^\circ\text{C}$: Bitumen / Residue (roads, roofing)

Cracking and Reforming Processes

• Cracking:
  • Thermal or catalytic breakdown of large, less useful hydrocarbon molecules into smaller, higher-demand fuels and alkenes.
  • Uses heat with a catalyst (catalytic cracking) or high temperature with steam.
  • Reaction equation: $\text{C}_{10}\text{H}_{22} \rightarrow \text{C}_8\text{H}_{18} + \text{C}_2\text{H}_4$
  • Significance: Matches supply to demand for lighter fuels (like petrol) and produces alkenes for petrochemical feedstock.
• Reforming:
  • Rearrangement of hydrocarbon molecular structures without significantly altering molecular size.
  • Converts straight-chain alkanes into branched-chain, cyclic, or aromatic hydrocarbons using heat and a platinum ($\text{Pt}$) catalyst.
  • Reaction example: Straight-chain octane ($\text{C}_8\text{H}_{18}$) $\rightarrow$ 2,2,4-trimethylpentane (iso-octane) + $\text{H}_2$
  • Significance: Improves petrol quality and octane rating, prevents engine knocking, enhances combustion efficiency, and produces hydrogen ($\text{H}_2$) as a valuable by-product.