Lecture Notes: Organic Chemistry - IUPAC Nomenclature of Cyclic Compounds

Introduction

• Topic: Naming of cyclic compounds
• Compound Types: Allocyclic and aromatic compounds
• Emphasis on understanding nomenclature rules, especially for allocyclic compounds

Allocyclic Compounds

• Definition: Organic compounds with carbon atoms connected in a cyclic structure (e.g., triangle, square, hexagon shapes)
• Key Rule: Use of prefix "cyclo"
  • Example: Cyclopropane (Cyclic structure with 3 carbon atoms)
  • Cyclobutane (4 carbon atoms)
  • Cyclohexane (6 carbon atoms)

Nomenclature Rules

1. Rule #1: Always include "cyclo" as a prefix for allocyclic compounds
2. Rule #2: Number branches alphabetically when more than two branches exist
   • Example: Ethyl and Methyl groups
     • Ethyl (E) is alphabetically prior to Methyl (M)
     • Numbering starts from ethyl-containing carbon
3. Rule #3: Highly branched carbon should receive the lowest number
   • Example: If multiple branches exist on one carbon, rank according to branches

Examples and Variations

• Simple Branches: Assign numbers starting from the carbon with substituent groups
  • Example: 1-methyl-cyclohexane
• Multiple Branches: Prioritize alphabetically and consider branching
  • Example: 3-ethyl-1,1-dimethyl-cyclohexane
• Functional Groups: Functional groups receive priority in numbering over cyclic bonds
  • Example: Cyclohexane-1-ol (Alcohol group present)

Side Chains vs. Main Chain

• Rule #4: If a side chain has more carbons than the ring, it becomes the main chain
  • Example: 1-cyclopentylhexane
• Rule #5: If a side chain possesses unsaturation or a functional group, it is the main chain
  • Example: 3-cyclopentylprop-1-ene

Functional Group Priority

• Functional groups override cyclic structures in terms of naming priority
  • Example: Cyclohexylbut-2-en-1-ol

Special Cases

• Carboxylic Acid: Cyclohexane carboxylic acid, not cyclohexane-1-oic acid
• Aldehyde: Cyclohexane carbaldehyde

Practice and Consistency

• Recommendation: Practice multiple examples to master IUPAC nomenclature
• Organic chemistry requires consistent revisions to maintain knowledge

Upcoming Topics

• Focus on aromatic compounds and special cyclic structures like bicyclic and spiro compounds

Conclusion

• Subscribe for more videos on organic chemistry
• The next lecture will cover aromatic compounds and more complex cyclic structures