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Chemistry LibreTexts

4: Substitution and Elimination reactions

  • Page ID
    • 4.1: Alkyl Halides - Structure and Physical Properties
      Alkyl halides are classified based upon the structure of the carbon atom bonded to the halogen.  Common names and physical properties are discussed.
    • 4.2: Common Sources of Alkyl Halides
      Halogen containing organic compounds are relatively rare in terrestrial plants and animals, with the thyroid hormones T3 and T4 as notable exceptions.  Alkyl halides are excellent electrophiles and quickly become an o-chem student's best friend for synthetic pathways.
    • 4.3: Reactions of Alkyl Halides- Substitution and Elimination
      The two major reaction pathways for alkyl halides (substitution and elimination) are introduced.
    • 4.4: Characteristic of the SN2 Reaction
      The SN2 mechanism is described mechanistically and kinetically as a one-step (concerted) reaction between two reactants (bimolecular) that inverts the configuration of the carbon at the reactive site.  The terms nucleophile, electrophile, and leaving group are explained by application to SN2 reactions.
    • 4.5: Factors affecting the SN2 Reaction
      In order of decreasing importance, the factors impacting SN2 reaction pathways are the structure of the alkyl halide, the strength of the nucleophile, the stability of the leaving group, and the type of solvent.
    • 4.6: Characteristic of the SN1 Reaction
      In the SN1 reaction, the solvent helps pull apart the halogen and carbon to form a halide and carbocation.  A nucleophile can now form a bond with the carbocation to create a new product.  The mechanism is explained with stereochemistry and reaction kinetics.
    • 4.7: Factors Affecting the SN1 Reaction
      The formation and stability of the carbocation intermediate strongly influence the SN1 mechanism.  The structure of the alkyl halide,  the stability of the leaving group, and the type of solvent influence the reaction pathway.  Since the nucleophile is not involved in the rate determining step, the strength of the nucleophile has low importance.
    • 4.8: Comparison of SN1 and SN2 Reactions
      In comparing the SN1 and SN2 mechanisms, the structure of the alkyl halide (electrophile), the strength of the nucleophile, and the reaction solvent are the primary considerations.  The leaving group will have a similar effect for both reactions, so it is not of interest when comparing the mechanistic pathways.
    • 4.9: Characteristics of the E2 Reaction
      E2, bimolecular elimination, was proposed in the 1920s by British chemist Christopher Kelk Ingold.  In E2 reactions, a beta-hydrogen and the leaving group are eliminated from an alkyl halide in reaction with a strong base to form an alkene.
    • 4.10: Zaitsev's Rule
      Zaitsev's Rule can be used to predict the regiochemistry of elimination reactions.  Regiochemistry describes the orientation  of reactions about carbon-carbon double bonds (C=C).
    • 4.11: Characteristics of the E1 Reaction
      The unimolecular E1 mechanism is a first order elimination reaction in which carbocation formation and stability are the primary factors for determining reaction pathway(s) and product(s).
    • 4.12: Comparison of E1 and E2 Reactions
      The strength of the base is the primary consideration when distinguishing between the E1 and E2 pathways.  The reaction solvent is a secondary consideration.
    • 4.13: Competition between substitution and elimination

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