03/03

Recap of Previous Class

• Focus on substitution reactions:

  • Substitution: New nucleophile substitutes for the leaving group.

  • Reaction occurs in a concerted fashion, where nucleophilic attack and leaving group departure happen simultaneously.

  • Example: Consider nucleophile attacking the alpha carbon in substitution reactions:

    • Nucleophile attacks, bond forms, leaving group departs.

• Classification of reaction as SN2:

  • SN2: Bimolecular reaction (two species in the rate determining step).

  • Characterized by inversion of stereochemistry at the alpha carbon due to backside attack by nucleophile.

  • Understanding stereochemistry assignment is crucial; not tested together with substitution reactions.

Reaction Rate Analysis

• Experimental determination of reaction rates:

  • Rate of SN2 reaction depends on the concentration of both substrate and nucleophile.

  • Rate correlation example: Increase concentration increases reaction rate.

  • Key point: Do not confuse SN1 and SN2 classification based on reaction steps—source of confusion often is the one-step notation of SN2.

Nature of Sodium in Reactions

• Sodium (Na) recognized as a counter or spectator ion in organic chemistry.

• Sodium’s positive charge means that associated species (like OCH3–) carries a negative charge, indicating nucleophilicity.

• Reactions with counterions discussed with respect to byproducts.

Transition to SN1 Mechanism

• Introduction to SN1 reaction:

  • Characterized by a stepwise mechanism compared to SN2.

  • First step involves leaving group departing to form a carbocation intermediate.

  • Rate determining step only depends on the substrate concentration, not nucleophile concentration.

  • Rate of reaction hence identified as unimolecular.

• Importance of carbocation stability:

  • Stability trend: Tertiary > Secondary > Primary > Methyl.

  • Hyperconjugation is the concept used to explain stability of carbocations through neighboring R groups donating electron density.

Mechanism Steps for SN1 Reaction

• Steps:

  1. Loss of Leaving Group: Produces carbocation.

  2. Nucleophilic attack by water or neutral nucleophile forms a new bond, creating a new intermediate.

  3. Proton Transfer Step: Additional step to form the neutral final product.

  • Importance of showing both intermediates and byproducts in mechanism equations.

Key Points on Stereochemistry in SN1

• Carbocations result in loss of stereochemical information at the reactive center (alpha carbon).

• Formation of racemic mixture if chirality is present.

• Concept of racemization discussed; significant in understanding stereochemistry in reactions.