Lecture Notes: Substitution and Elimination Reactions in Organic Chemistry

Key Topics

• Substitution Reactions (Sn2 mechanism)
• Elimination Reactions
• Mechanistic Steps

Overview of Mechanisms

• Mechanisms: How reactions occur, crucial for understanding organic chemistry.
• Learn 10 basic elementary steps to grasp 90% of reactions.
• Memorization isn’t effective; understanding mechanisms is key.

Types of Reactions

1. Substitution Reaction (Sn2)
   • Involves replacing a halogen in an alkyl halide with a nucleophile.
   • Mechanism: Nucleophile attacks carbon attached to halogen, halogen leaves.
2. Elimination Reaction
   • Removal of a halogen and a hydrogen to form a double bond (alkene).
   • Mechanism: Base abstracts a proton, electron pair forms a double bond, halogen leaves.

Specifics of Substitution Reactions

• Alkyl Halide: Compound with a carbon bonded to a halogen (e.g., Cl, Br).
• Carbon Hybridization: Focus on sp3 hybridized carbons (not sp2).
• Nucleophile: Nucleus-loving species, usually negatively charged.
• Mechanistic Details: Lone pair of nucleophile attacks carbon, breaking carbon-halogen bond.
• Sn2 Mechanism: Bimolecular, concerted reaction; both alkyl halide and nucleophile simultaneously involved.

Example: Sn2 Reaction

• Reaction: Nucleophile replaces halogen in one step (concerted).
• Stereo Specificity: Results in inversion of stereochemistry (R to S or vice versa).
• Backside Attack: Nucleophile attacks from the opposite side of the leaving group.

Specifics of Elimination Reactions

• Base: Abstracts a proton adjacent to the halogen, leading to formation of a double bond.

• Mechanism: Proton abstraction, double bond formation, halogen leaves.

• Example:

  Alkyl halide + Base → Alkene + BH + Halide

• Mixture of Products: Base and nucleophile can be the same, leading to both substitution and elimination products.

Difference Between Base and Nucleophile

• Base: Abstracts protons.
• Nucleophile: Attacks carbon nucleus.

Influence of Halogens

• Electronegativity: Creates electron-poor carbon (positive partial charge), attracting nucleophiles.
• Good Leaving Groups: Halogens (conjugate bases of strong acids are better leaving groups).
• Examples:
  • Iodide (good leaving group, conjugate base of HI with negative pKa).
  • Hydroxide (bad leaving group).

Sn1 Mechanism

• Substitution reaction featuring a two-step process.
• Involves formation of carbocation intermediate (unimolecular first step).
• SN vs. Sn2: Both substitution reactions, but different paths and intermediate states.

Transition States and Reaction Kinetics

• Rate Law: Sn2 reactions are second order; doubling reactant concentrations doubles reaction rate.
• Transition State: Defines concerted mechanism; nucleophile and leaving group must be 180 degrees apart.
• Molecular Orbital Theory: Explains why backside attack is favored (orbital overlap).

Practical Applications

• Predict product formation based on knowing mechanism (Sn2 inversion of configuration).

Conclusion

• Understanding mechanisms (like Sn2) is pivotal to predict outcomes in organic chemistry reactions.
• Focus on learning mechanistic steps and their implications instead of rote memorization.