Lecture Notes: What Makes a Good Leaving Group

Introduction

• Importance of Leaving Group:
  • Polarizes the bond to the carbon (electrophile).
  • Leaves with an extra set of electrons (bonding electrons to the carbon).

Traits of a Good Leaving Group

1. Electron Withdrawing:

   • Must be electronegative to polarize the bond to carbon.
   • Example: Halogens (e.g., chlorine, bromine, iodine) are good leaving groups because they are more electronegative than carbon and polarize the bond, making carbon more electropositive and a better electrophile.
   • Other good leaving group atoms: Oxygen, Nitrogen, Sulfur.

2. Stability with Additional Electrons:

   • Must be stable with extra electrons, typically are weak bases.
   • Weak Bases as Good Leaving Groups:
     • Conjugate bases of strong acids: Chloride ion, Bromide ion, Iodide.
     • Other ions: Phosphates, Sulfonates, Sulfates (resonance stabilized).

3. Neutral Molecules:

   • Neutral molecules are good leaving groups because they do not have a charge to stabilize once they leave.
   • Examples include Water, Alcohols, Amines, Thioethers, Phosphenes.
   • Note: A neutral leaving group becomes anionic upon leaving, while a positively charged leaving group becomes neutral.

4. Polarizability:

   • Polarizable atoms are large and have "squishy" electron clouds, allowing electrons to move around and stabilize the extra electrons.
   • Polarizability helps increase orbital overlap in the transition state, lowering the energy of the transition state and speeding up the reaction.

Conclusion

• A good leaving group is characterized by its ability to withdraw electrons, stability with extra electrons, being neutral, and having polarizability.
• These traits contribute to the efficiency and speed of nucleophilic substitution reactions.