Class 11 Chemistry - Hydrocarbons Lecture Notes

Introduction to Hydrocarbons

• Hydrocarbons are fundamental compounds in organic chemistry.
• The chapter focuses on hydrocarbons, specifically alkanes, alkenes, alkynes, and aromatic hydrocarbons.
• Previous discussions included naming organic compounds, functional groups, and basic organic reactions.

Types of Hydrocarbons

1. Alkanes: Single bond containing, no functional groups (C and H only).
2. Alkenes: Double bonds between carbon atoms.
3. Alkynes: Triple bonds between carbon atoms.
4. Aromatic hydrocarbons: Compounds that contain at least one aromatic ring.

Study Approach for Hydrocarbons

• General Introduction
• Nomenclature
• Isomerism
• Preparation Methods
• Physical Properties
• Chemical Properties

Alkanes

General Introduction

• Alkanes are hydrocarbons with the general formula CₙH₂ₙ₊₂.
• Example: For n=1 (C₁H₄), it is Methane (CH₄).
• Properties:
  • All carbons in alkanes are sp³ hybridized.
  • Alkanes are also known as paraffins (meaning "little affinity" due to low reactivity).
  • Inert at room temperature; do not react with acids, bases, or substances at room temperature.

Naming Alkanes

• Methane: CH₄
• Ethane: C₂H₆
• Propane: C₃H₈
• Butane: C₄H₁₀
• Can have branched structures, leading to various alkanes.

Isomerism in Alkanes

Types of Isomerism

1. Chain Isomers
   • Same molecular formula but different chain lengths.
   • Observed in alkanes with at least 4 carbon atoms (C₄H₁₀ and greater).
   • Example: C₄H₁₀ can have
     • Butane (n-butane)
     • Isobutane (2-methylpropane)
2. Conformational Isomers
   • Same molecular formula and structure but different spatial arrangements.
   • Due to free rotation around carbon-carbon single bonds.
   • Key conformers:
     • Staggered Conformer: More stable due to minimized torsional strain.
     • Eclipsed Conformer: Less stable due to increased torsional strain.
   • Representations:
     • Newman Projection: Used to visualize conformations of alkanes.
     • Sahaar's Projection: Another method to depict conformations.

Conformational Stability

• Stability is influenced by torsional strain:
  • Minimum in staggered conformer (more stable).
  • Maximum in eclipsed conformer (less stable).

Summary of Isomers

• C₄H₁₀: 2 chain isomers (butane, isobutane).
• C₅H₁₂: 3 chain isomers.
• For larger alkanes, the number of isomers increases.
  • C₆H₁₄: 5 chain isomers.
  • C₇H₁₆: 9 chain isomers.

Conclusion

• Covered the basics of alkanes, their naming, and isomerism.
• Preparation methods will be discussed in the next lecture.

Important Notes

• Chain Isomerism requires at least 4 carbon atoms.
• Conformational Isomers arise from the free rotation of single bonds.
• Use the formulas to identify and differentiate between isomers.