Hydrocarbons and Isomers

Hydrocarbons are organic compounds composed exclusively of carbon and hydrogen. They play an essential role in fuels, such as petrol, due to the substantial energy they release during combustion.

Types of Hydrocarbons

• Saturated Hydrocarbons (Alkanes):

  • Characterised by single carbon-carbon bonds, which impart stability and low reactivity.
  • Example: Methane ($\text{CH}_4$) is a basic yet prevalent alkane.

• Unsaturated Hydrocarbons:

  • Alkenes: Feature at least one carbon-carbon double bond, which enhances reactivity compared to alkanes.
  • Alkynes: Contain at least one carbon-carbon triple bond, rendering them more reactive than alkenes.
  • Examples: Ethene ($\text{C}_2\text{H}_4$) for alkenes and Ethyne ($\text{C}_2\text{H}_2$) for alkynes.

Explanation of Isomers

Isomers: Compounds that share the same molecular formula but possess different structures, affecting properties and reactions.

• Chain Isomers:

  • Have identical molecular formulas but differ in carbon chain structures, influencing physical and chemical properties.

• Position Isomers:

  • Retain the same carbon framework, but functional groups or substituents differ in position.

Isomer Type	Example Compound	Molecular Formula	Concise Features + Uses
Chain Isomer	Butane vs. Iso-Butane	$\text{C}_4\text{H}_{10}$	Varying boiling points; useful in fuel formulations.
Position Isomers	1-Bromopropane, 2-Bromopropane	$\text{C}_3\text{H}_7\text{Br}$	Position of bromine alters boiling point and reactivity.

Functional Group Isomers

• Functional Group Isomers: These isomers share the same molecular formula but feature different functional groups, resulting in distinct chemical properties.

Example:

• Alcohol vs. Ether:
  • Chemical Formula: $\text{C}_4\text{H}_{10}\text{O}$
  • 1-Butanol: Includes a hydroxyl group (–OH) and demonstrates higher solubility.
  • Methoxypropane: Comprises an ether linkage (C-O-C), which results in reduced solubility and boiling points.

Chemical Properties and Reactivity

• Saturated Hydrocarbons:

  • Exhibit lower reactivity due to the stability of single bonds.

• Unsaturated Hydrocarbons:

  • Display higher reactivity due to the presence of pi bonds within double and triple bonds.

Common Reactions

• Hydrogenation:

  • This process converts unsaturated hydrocarbons into saturated ones by adding hydrogen.
  • Example Reaction: $\text{C}_2\text{H}_4 + \text{H}_2 \rightarrow \text{C}_2\text{H}_6$

• Halogenation:

  • Involves adding halogens, such as chlorine or bromine, to unsaturated hydrocarbons.

Worked Example: Identifying Isomers

Consider the molecular formula $\text{C}_4\text{H}_{10}$. Let's identify possible structural isomers:

1. Butane (straight chain):

   • CH₃-CH₂-CH₂-CH₃
   • All carbon atoms are in a single line

2. Isobutane (branched chain):

   • CH₃-CH(CH₃)-CH₃
   • Has a methyl branch at the middle carbon

These compounds have identical molecular formulas but different structures, leading to different properties:

• Butane has a boiling point of -0.5°C
• Isobutane has a lower boiling point of -11.7°C

The difference in boiling points is due to the more compact structure of isobutane, which reduces intermolecular forces.