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Changing the Carbon Skeleton - Exercise 18

Exercise 12.7 - Changing the Carbon Skeleton

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Introduction

In this video, Exercise 12.7 focuses on changing the carbon skeleton in multi-step synthesis, specifically by adding or removing carbon atoms from a molecule. This involves techniques such as using terminal alkynes and ozonolysis.

Main Points
  1. Adding Carbons:

- Terminal Alkyne Method:

     - Treat a terminal alkyne with sodium amide, a strong base, to convert it into a carbon nucleophile.
     - The nucleophile then undergoes an SN2 reaction with an alkyl halide, forming a new carbon-carbon bond.
     - Example: Ethyl group (CH3CH2) with a leaving group reacts with the nucleophile to add carbons to the molecule.
  1. Example Reaction - Adding Carbons:

- Initial molecule has two carbons.

- Final molecule has five carbons.

- Steps:

     1. Treat with sodium amide to form the carbon nucleophile.
     2. Add an alkyl halide to extend the carbon chain.
     3. Repeat the process to add more carbons if needed.
  1. Removing Carbons:

- Ozonolysis Method:

     - Use ozone (O3) followed by dimethyl sulfide (DMS) for alkenes or water for alkynes.
     - This reaction breaks the double bond and replaces it with double bonds to oxygen, effectively removing carbons from the chain.
     - Example: An alkene treated with O3 and DMS breaks into two molecules with carbonyl groups.
  1. Example Reaction - Removing Carbons:

- Initial molecule has eight carbons.

- Final molecule has seven carbons, with an oxygen replacing one carbon.

- Steps:

     1. Use ozonolysis (O3 followed by DMS) to break the double bond.
     2. The molecule splits, removing one carbon.
  1. Additional Insights:

- These techniques are fundamental in organic synthesis for modifying the carbon skeleton of molecules.

- They enable the construction of complex molecules by strategic addition or removal of carbon atoms.

Conclusion

Understanding how to change the carbon skeleton is essential for multi-step synthesis in organic chemistry. By using terminal alkynes and sodium amide for adding carbons and ozonolysis for removing carbons, chemists can efficiently modify molecular structures to achieve desired outcomes.

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