Lecture Notes: Chapter B - Nomenclature of Alkenes, Alkynes, and Benzene Derivatives

Overview

• Chapter Focus: Naming alkenes, alkynes, and benzene derivatives
• Prerequisite: Knowledge from Chapter A
• Core Concept: Identify and properly name organic compounds with carbon double/triple bonds and benzene derivatives

Naming Alkenes and Alkynes

1. Identify the Longest Chain

   • Must include the entire double or triple bond
   • Example: A 6-carbon chain with a double bond -> longest chain is the carbon chain that includes the double bond

2. Determine Parent Name

   • For open chain alkenes: Replace “-ane” with “-ene” (alkene) or “-yne” (alkyne)
   • For rings: If the double bond is within a ring, the ring's name takes precedence (e.g., "cyclobutene")

3. Number the Carbon Chain

   • Start numbering from the end nearest to the double or triple bond
   • Example: Carbon double bond in a chain of 6 -> start numbering to give the double bond the lowest possible number
   • Substituents: Prioritize the double/triple bond over substituents
   • Tie-breaking: Use lowest numbers and substituent rules from Chapter A

Examples

1. Cyclohexene vs. Simple Alkenes

   • Cyclohexene: Double bond implies the name even with more extensive chains
   • Straight chains: Name changes based on carbon count

2. Multiple Bonds

   • Longest chain must include most double/triple bonds possible
   • Use “-a” before “-ene” or “-yne” if multiple present (e.g., Hexa-1,4-diene)

3. Benzene Derivatives

   • Parent Name: Benzene
   • Number to give substituents the lowest possible numbers
   • Alphabetize substituents in name

Special Rules for Benzene Derivatives

1. Common Naming

   • Benzene with one substituent uses the name as a suffix (e.g., nitrobenzene)
   • Multiple substituents follow numbering rules for lowest locants

2. Prefixes for Positions on Benzene

   • Ortho (o-): Groups on 1,2 positions
   • Meta (m-): Groups on 1,3 positions
   • Para (p-): Groups on 1,4 positions

3. Phenol and Benzyl Groups

   • Phenol: Benzene ring as a substituent
   • Benzyl: Benzene ring with an extra carbon before attaching to the main chain

Practical Examples

1. Example Molecules

   • Determining correct numbering for lowest locator numbers
   • Using alphabet for tie-breaking
   • Importance of correct naming in exams and practical scenarios like lab work

2. Trivial Names

   • Important but often misleading
   • Memorize key trivial names like Acetylene (a triple bond, not a double)

Naming Rules Summary

• Prioritize double/triple bonds over substituents
• For compounds with both alkenes and alkynes, prioritize alkenes
• Use correct prefixes and suffixes based on the number and type of bonds
• Correctly identify and number parent chains and substituents

Q&A and Examples: Contains multiple examples for you to practice naming different compounds, including various configurations of benzene derivatives, cycloalkanes, and substituted alkenes and alkynes.

Key Takeaways

• Accurate naming is critical for clear communication in chemistry
• Follow systematic rules for chain length, bond type, and substituent prioritization
• Memorize common trivial names and positions for ease in naming benzene derivatives