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6: Alkyl Halides: Nucleophilic Substitution and Elimination

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  • Learning Objectives

    After reading the chapter and completing the exercises and homework, a student can be able to:

    • classify alkyl halides - refer to section 7.1
    • predict relative boiling points and solubility of alkyl halides - refer to section 7.1
    • discuss the common uses of alkyl halides - refer to section 7.2
    • specify the reagents for the most efficient synthesis of alkyl halides using free-radical halogenation of alkanes (Chapter 5) or allylic halogenation of alkenes with NBS - refer to section 7.3
    • apply the alpha and beta labels to alkyl halides for substitution and elimination reactions - refer to section 7.4
    • determine the rate law & predict the mechanism based on its rate equation or reaction data for SN1, SN2, E1 & E2 reactions - refer to sections 7.5, 7.6, 7.8, 7.13, and 7.15
    • use Zaitsev’s rule to predict major and minor products of elimination reactions including halocyclohexanes - refer to sections 7.14, 7.15, and 7.16
    • predict the products and specify the reagents for SN1, SN2, E1 and E2 reactions with stereochemistry - refer to sections 7.6, 7.7, 7.9, 7.14, 7.15, 7.19
    • propose mechanisms for SN1, SN2, E1 and E2 reactions - refer to sections 7.5, 7.6, 7.7, 7.8, 7.9, 7.13, 7.14, 7.15, 7.19
    • draw, interpret, and apply Reaction Energy Diagrams for SN1, SN2, E1 and E2 reactions - refer to sections 7.5, 7.6, 7.7, 7.8, 7.9, 7.13, 7.14, 7.15, 7.19
    • predict carbocation rearrangements in 1st order reactions - refer to section 7.10
    • explain and apply Hammond's Postulate to substitution reactions - refer to section 7.11
    • explain how the kinetic isotope effect (KIE) can be used to elucidate reaction mechanisms - refer to section 7.17
    • distinguish 1st or 2nd order substitution and elimination reactions - refer to sections 7.12 and 7.18
    • discuss the importance of leaving groups in biological substitution reactions - refer to section 7.20
    • discuss enzymatic elimination reactions of histidine - refer to section 7.21

    • 6.1: 7.1 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.
    • 6.2: Common Uses 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.
    • 6.3: Preparation of Alkyl Halides
      Alkyl halides can be readily synthesized from alkanes, alkenes, and alcohols.  This section expands the ways we can brominate tetrahedral carbons to the allylic position of alkenes.
    • 6.4: Reactions of Alkyl Halides: Substitution and Elimination
      The two major reaction pathways for alkyl halides (substitution and elimination) are introduced.
    • 6.5: 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.
    • 6.6: Characteristics of 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.
    • 6.7: Stereochemistry of the SN2 Reaction
      The SN2 reaction is stereospecific. A stereospecific reaction is one in which different stereoisomers react to give different stereoisomers of the product.
    • 6.8: 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.
    • 6.9: Characteristics of 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.
    • 6.10: Rearrangements of the Carbocation and Sₙ1 Reactions
      Carbocation rearrangements are structural reorganizational shifts within an ion to a more stable state (lower energy).
    • 6.11: The Hammond Postulate and Transition States
      The Hammond postulate states that a transition state resembles the structure of the nearest stable species and helps explain the product distribution differences observed between exergonic and endergonic reactions.
    • 6.12: 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.
    • 6.13: 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.
    • 6.14: 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).
    • 6.15: 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).
    • 6.16: E2 Regiochemistry and Cyclohexane Conformations
      Cyclohexyl halides provides the perfect opportunity to learn and understand the regiochemistry of the E2 reaction and why Zaitsev's Rule does not always apply.   The anti-coplanar orientation of the E2 mechanism can also be see with certain carbon chain diastereomers.
    • 6.17: The E2 Reaction and the Deuterium Isotope Effect
      The bimolecular transition state of the E2 reaction illustrates the effects of bond strength on reaction rates when studying the kinetic isotope effect of deuterium.
    • 6.18: 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.
    • 6.19: Comparing Substitution and Elimination Reactions
      Chemical reactivity patterns can help us determine the most favorable pathway among the closely related SN1, SN2, E1, and E2 mechanisms.
    • 6.20: Biological Substitution Reactions
      A few examples of biochemical SN1 and SN2 mechanisms are introduced with an emphasis on the effects of the leaving group.
    • 6.21: Biological Elimination Reactions
      A few examples of biochemical E1 and E2 mechanisms are introduced.
    • 6.22: Additional Exercises
      This section has additional exercises for the key learning objectives of this chapter.
    • 6.23: Solutions to Additional Exercises
      This section has the solutions to the additional exercises from the previous section.