CHEM2100 CH 9 FS2024 Part 1

1. Course Overview

• Upcoming Exams:

  • Exam 4 covering Chapters 9 and 10 on November 6.

    • Material may finish before November 1, but Chapter 10 is the cut-off.

  • ½ Exam covering Chapter 11 on November 20.

• Homework Deadline Change:

  • Chapter 9 homework deadline moved from November 5 to October 29.

2. Introduction to Alkynes

• Importance of Alkynes:

  • Alkynes are characterized by carbon-carbon triple bonds.

  • They are less common than alkenes in laboratories and biology.

  • The study of alkynes will facilitate understanding of organic synthesis strategies.

3. Chapter 9 Content Breakdown

3.1 Naming Alkynes

• Functional Group Nomenclature:

  • Carboxylic acids have the highest priority.

  • Alkynes rank lower than alkenes, alcohols, amines, etc.

• IUPAC Rules:

  • Similar rules for naming alkenes apply to alkynes.

  • Example: Cyclononyne as the smallest cycloalkyne that accommodates a trans-alkene.

3.2 Preparation of Alkynes

• Methods for Preparing Alkynes:

  • Dehydrohalogenation of Vicinal Dihalides:

    • Uses KOH or NaNH2 to yield alkynes from dihalides.

3.3 Reactions of Alkynes

• Key Reactions of Alkynes:

  • Addition of Halogens (HX and X2):

    • Example: Addition of Cl2 and Br2 to alkynes.

  • Hydration:

    • Direct addition (Markovnikov product) vs. Hydroboration-Oxidation (non-Markovnikov product).

3.4 Hydrogenation of Alkynes

• Hydrogenation Reactions:

  • Fully and partially hydrogenated alkynes.

  • Different reagents lead to formation of cis or trans alkenes.

3.5 Oxidative Cleavage of Alkynes

• Cleavage Products:

  • Ozone reacts with alkynes:

    • Internal alkynes yield carboxylic acids.

    • Terminal alkynes yield a carboxylic acid and CO2.

3.6 Acidity of Terminal Alkynes

• Terminal Alkynes:

  • Highly acidic (pKa ~ 44).

  • Acetylide anions can be formed for further reactions.

4. Alkylation of Acetylide Anions

• Preparation and Reactions:

  • Strong bases like n-butyllithium or sodium amide can form alkynyl anions.

  • Alkylation with 1° alkyl halides as a carbon-carbon bond forming reaction.

  • Resulting reactions may lead to multiple alkylations.

5. Jones Synthesis Analogy

• Concept of Organic Synthesis:

  • Similar to planning a road trip, requires a roadmap for successful synthesis.

  • Each step must be strategically planned to achieve desired molecular structures.

6. Conclusion

• Chapter 9 lays foundational knowledge of alkynes essential for understanding organic synthesis and preparing for future chapters.