Structure of Alkenes

Key Learning Objectives

• Describe the structure of alkenes.
• Explain why alkenes are unsaturated.
• Draw the displayed formula of alkenes.

Alkenes Overview

• Alkenes are a type of hydrocarbon.
• They contain at least one double covalent bond between carbon atoms.

General Formula

• Alkenes have the general formula: CₙH₂ₙ.
• This formula allows calculation of hydrogen atoms if the number of carbon atoms (n) is known.

Examples

• Ethene
  • Molecular formula: C₂H₄
  • Displayed formula includes a double bond between the carbon atoms.
• Propene
  • Three carbon atoms (C₃H₆): 3 carbons x 2 = 6 hydrogens.
• Butene
  • Four carbon atoms (C₄H₈): 4 carbons x 2 = 8 hydrogens.
  • Displayed formula can place the double bond between any two adjacent carbon atoms at GCSE level.
• Pentene
  • Five carbon atoms (C₅H₁₀): 5 carbons x 2 = 10 hydrogens.
  • Double bond placement flexibility similar to butene.

Unsaturation of Alkenes

• Alkenes are considered unsaturated.
• This is due to having two fewer hydrogen atoms than corresponding alkanes with the same number of carbon atoms.
• Example:
  • Ethane (alkane) vs. Ethene (alkene)
  • Both have two carbon atoms.
  • Ethane: C₂H₆
  • Ethene: C₂H₄

Importance for Exams

• Familiarity with the structures of ethene, propene, butene, and pentene is crucial for exams.

Additional Resources

• Practice questions are available in the associated vision workbook, with further exercises to reinforce learning.