Substitution

I. Classification of Organic Reactions

  • There are four general types of organic reactions:

    • Additions

    • Substitutions

    • Eliminations

    • Rearrangements

II. Nucleophilic Substitution Reactions

  • A nucleophilic substitution reaction involves a nucleophile attacking a substrate to form a new compound.

  • Two major types of nucleophilic substitutions:

    • SN1 (Substitution, Nucleophilic, Unimolecular)

    • SN2 (Substitution, Nucleophilic, Bimolecular)

A. SN1 Reaction Mechanism

  • SN1 reactions occur in two steps:

    1. Formation of Carbocation:

    • Leaving group (X) departs, forming a carbocation.

    1. Nucleophile Attack:

    • Nucleophile (Nu) attacks the carbocation, forming the product.

  • The rate of a SN1 reaction depends on:

    • Concentration of the substrate (RX)

    • Rate-determining step involves only one molecule (unimolecular).

  • Reaction Rate Dependence:

    • Rate = k[RX]

    • Doubling the concentration of the substrate affects the rate.

B. SN2 Reaction Mechanism

  • SN2 reactions occur in one concerted step:

    • Nucleophile attacks the substrate as the leaving group departs, leading to a bimolecular mechanism.

  • The rate of an SN2 reaction depends on:

    • Concentration of the substrate (RX)

    • Concentration of the nucleophile (Nu)

    • Rate-determining step involves two molecules (bimolecular).

  • Reaction Rate Dependence:

    • Rate = k[RX][Nu]

    • Doubling the concentration of either the substrate or the nucleophile speeds up the reaction.

III. Substrate Classification for Alkyl Halides

  • Alkyl halides can be classified as:

    • Primary (1°) Alkyl Halide:

      • Has one alkyl group attached to the carbon bonded to the halogen.

    • Secondary (2°) Alkyl Halide:

      • Has two alkyl groups attached to the carbon bonded to the halogen.

    • Tertiary (3°) Alkyl Halide:

      • Has three alkyl groups attached to the carbon bonded to the halogen.

IV. Key Concepts in Nucleophilic Substitution

  • Backside Attack:

    • Characteristic of SN2 where the nucleophile attacks the opposite side of the leaving group, leading to inversion of configuration.

  • Transition State:

    • A temporary state during the reaction where bonds are partially formed and broken.

V. Factors Influencing Mechanism

  • Leaving Group Ability:

    • The appropriateness of the leaving group (X) effects both SN1 and SN2 reactions.

    • Common leaving groups include:

    • F

    • Cl

    • Br

    • I

  • Nucleophile Strength:

    • The nature of the nucleophile (e.g., NH3, OH-, OCH3) affects the rate and mechanism of substitution.

VI. Implications of Mechanisms

  • What does the mechanism tell us about:

    • The Substrate: Influences if SN1 or SN2 occurs based on sterics and stability of intermediates.

    • The Leaving Group: Stronger leaving groups favor both SN1 and SN2 mechanisms.

    • The Nucleophile: A stronger nucleophile favors a quicker reaction in SN2, while in SN1, nucleophile strength plays a lesser role.

    • The Reaction: Determines the kinetics and outcomes of the substitution reactions.