SN2 Exercises Leaving groups

SN2 Exercises - Leaving Groups

Introduction

In this video, James from Master Organic Chemistry explores how to determine the products of substitution reactions by analyzing the leaving groups. The video provides examples and explains the rationale behind choosing the best leaving group to predict the reaction products.

Main Points/Arguments

• Determining the Leaving Group:

- The key to predicting the product of a substitution reaction is identifying the better leaving group.

- The leaving group is the weaker base, which can be determined using a pKa table.

• Example 1: Four-Carbon Chain with OCH3 and Cl Groups:

- Reaction Setup: The nucleophile CN- attacks either carbon 1 (displacing Cl-) or carbon 4 (displacing OCH3-).

- Analysis:

    - Using a pKa table, Cl- is identified as a better leaving group than OCH3- because it is a weaker base.
    - Product: CN- attacks carbon 1, displacing Cl- and forming CH3O-(CH2)3CN.

- Mechanism:

    - CN- performs a backside attack on carbon 1, breaking the C-Cl bond and forming a C-CN bond.

• Example 2: Four-Carbon Chain with Cl and Br Groups:

- Reaction Setup: The nucleophile NaSCH3 attacks either carbon 1 (displacing Br-) or carbon 4 (displacing Cl-).

- Analysis:

    - Using a pKa table, Br- is identified as a better leaving group than Cl- because it is a weaker base.
    - Product: NaSCH3 attacks carbon 1, displacing Br- and forming CH3S-(CH2)3Cl.

- Mechanism:

    - NaSCH3 performs a backside attack on carbon 1, breaking the C-Br bond and forming a C-S bond.

• Example 3: Four-Carbon Chain with SCH3 and OTS Groups:

- Reaction Setup: The nucleophile NaOH attacks either carbon 1 (displacing OTS-) or carbon 4 (displacing SCH3-).

- Analysis:

    - Using a pKa table, OTS- (tosylate anion) is identified as a better leaving group than SCH3- because it is a weaker base.
    - Product: NaOH attacks carbon 1, displacing OTS- and forming OH-(CH2)3SCH3.

- Mechanism:

    - NaOH performs a backside attack on carbon 1, breaking the C-OTS bond and forming a C-OH bond.

Conclusion

By analyzing the relative basicity of potential leaving groups, one can determine the products of substitution reactions. The best leaving group is the weakest base, which can be identified using a pKa table. Understanding this concept is crucial for predicting reaction outcomes in organic chemistry.