Homologous Series

What is it?

A homologous series is like a family of organic compounds that share the same “special group” of atoms (called a functional group ). Each member of the family has the same type of chemical behavior, but their physical properties (like boiling point) change gradually as the molecules get bigger.

Key Ideas Made Simple

Same “Special Group”:

• All members have the same functional group (e.g., –OH in alcohols, –COOH in carboxylic acids).
• This group decides how they react. For example:
  • All alcohols can react with sodium to produce hydrogen gas.

Formula Pattern:

• Each series follows a general formula .
• Examples :
  • Alkanes: CnH2n+2 (e.g., methane CH4, ethane C2H6).
  • Alcohols: CnH2n+1 OH (e.g., ethanol C2H5 OH).

Chain Length Matters:

• Each member differs by a –CH₂– group (like adding a bead to a necklace).
• Example :
  • Methane (CH4) → Ethane (C2H6) → Propane (C3H8).

Physical Property Trends:

• Boiling points :
  • Smaller molecules (like methane) are gases.
  • Larger molecules (like octane) are liquids or solids (stronger forces between molecules).
• Solubility :
  • Small molecules with –OH or –COOH groups mix with water (e.g., ethanol).
  • Larger molecules don’t mix (e.g., paraffin wax).

Same Chemical Behavior:

• All members react in similar ways.
• Example: All alkenes (with C=C) make plastics when reacted with a catalyst.

Why It Matters

• Predict reactions: If you know how ethanol behaves, you can guess how propanol (C3H7​OH) will react.

• Understand trends: Why is ethanol a liquid but methane a gas? Because ethanol has a larger molecule and hydrogen bonding!

Exam Tip

• Spot the family : Look for the functional group (e.g., –OH = alcohol).
• Link size to properties : “Why does hexane (C6H14) have a higher boiling point than butane (C4H10)?”

Answer: Hexane has a longer chain → stronger forces between molecules → harder to separate.