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11: Reactions of Alkyl Halides- Nucleophilic Substitutions and Eliminations

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    448645
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    Learning Objectives

    After you have completed Chapter 11, you should be able to

    • fulfill all of the detailed objectives listed under each individual section.
    • use the reactions studied in this chapter with those from earlier ones when designing multistep syntheses.
    • use the reactions and concepts discussed in this chapter to solve road map problems.
    • define, and use in context, the key terms introduced.

    In this course, you have already seen several examples of nucleophilic substitution reactions; now you will see that these reactions can occur by two different mechanisms. You will study the factors that determine which mechanism will be in operation in a given situation, and examine possible ways for increasing or decreasing the rates at which such reactions occur. The stereochemical consequences of both mechanisms will also be discussed. Elimination reactions often accompany nucleophilic substitution; so these reactions are also examined in this chapter. Again you will see that two different mechanisms are possible, and, as in the case of nucleophilic substitution reactions, chemists have learned a great deal about the factors that determine which mechanism will be observed when a given alkyl halide undergoes such a reaction.

    • 11.0: Why This Chapter?
      Nucleophilic substitution and base-induced elimination are two of the most widely occurring and versatile reactions in organic chemistry, both in the laboratory and in biological pathways. We’ll look at them closely in this chapter to see how they occur, what their characteristics are, and how they can be used. We’ll begin with substitution reactions.
    • 11.1: The Discovery of Nucleophilic Substitution Reactions
      Nucleophilic substitution reactions are one of the most common and versatile reaction types in organic chemistry.
    • 11.2: The Sₙ2 Reaction
      In almost all chemical reactions, there is a direct relationship between the rate at which the reaction occurs and the concentrations of the reactants. When we measure this relationship, we measure the kinetics of the reaction.
    • 11.3: Characteristics of the SN2 Reaction
      Now that we know how SN2 reactions occur, we need to see how they can be used and what variables affect them. Some SN2 reactions are fast, and some are slow; some take place in high yield and others in low yield. Understanding the factors involved can be of tremendous value.
    • 11.5: Characteristics of the SN1 Reaction
      Just as the SN2 reaction is strongly influenced by the structure of the substrate, the leaving group, the nucleophile, and the solvent, the SN1 reaction is similarly influenced. Factors that lower ∆G‡, either by lowering the energy level of the transition state or by raising the energy level of the ground state, favor faster SN1 reactions. Conversely, factors that raise ∆G‡, either by raising the energy level of the transition state or by lowering the energy level of the reactant, slow down the
    • 11.6: Biological Substitution Reactions
      Unlike what typically happens in the laboratory, however, the substrate in a biological substitution reaction is usually an organodiphosphate rather than an alkyl halide. Thus, the leaving group is the diphosphate ion, abbreviated PPi, rather than a halide ion. In fact, it’s useful to think of the diphosphate group as the “biological equivalent” of a halide. The dissociation of an organodiphosphate in a biological reaction is typically assisted by complexation to a divalent metal cation.
    • 11.7: Elimination Reactions- Zaitsev's Rule
      Elimination reactions are more complex than substitution reactions for several reasons. One is the problem of regiochemistry. Elimination reactions almost always give mixtures of alkene products, and the best we can usually do is to predict which will be the major product. According to Zaitsev’s rule, base-induced elimination reactions generally (although not always) give the more stable alkene product—that is, the alkene with more alkyl substituents on the double-bond carbons.
    • 11.8: The E2 Reaction and the Deuterium Isotope Effect
      The E2 reaction (for elimination, bimolecular) occurs when an alkyl halide is treated with a strong base, such as hydroxide ion or alkoxide ion (RO–). It is the most commonly occurring pathway for elimination.
    • 11.9: The E2 Reaction and Cyclohexane Conformation
      Anti periplanar geometry for E2 reactions is particularly important in cyclohexane rings, where chair geometry forces a rigid relationship between the substituents on neighboring carbon atoms.
    • 11.10: The E1 and E1cB Reactions
      Unimolecular Elimination (E1) is a reaction in which the removal of an HX substituent results in the formation of a double bond. It is similar to a unimolecular nucleophilic substitution reaction (SN1) in various ways. One being the formation of a carbocation intermediate. Also, the only rate determining (slow) step is the dissociation of the leaving group to form a carbocation, hence the name unimolecular. Thus, since these two reactions behave similarly, they compete against each other.
    • 11.11: Biological Elimination Reactions
      All three elimination reactions—E2, E1, and E1cB—occur in biological pathways, but the E1cB mechanism is particularly common. The substrate is usually an alcohol rather than an alkyl halide, and the H atom removed is usually adjacent to a carbonyl group, just as in laboratory reactions.
    • 11.12: A Summary of Reactivity - SN1, SN2, E1, E1cB, and E2
      SN1, SN2, E1, E1cB, E2—how can you keep it all straight and predict what will happen in any given case? Will substitution or elimination occur? Will the reaction be bimolecular or unimolecular? There are no rigid answers to these questions, but it’s possible to recognize some trends and make some generalizations.
    • 11.13: Chemistry Matters—Green Chemistry
      It may never be possible to make organic chemistry completely benign, but awareness of the environmental problems caused by many chemical processes has grown dramatically in recent years, giving rise to a movement called green chemistry. Green chemistry is the design and implementation of chemical products and processes that reduce waste and attempt to eliminate the generation of hazardous substances.
    • 11.14: Key Terms
    • 11.15: Summary
      The reaction of an alkyl halide or tosylate with a nucleophile/base results either in substitution or in elimination. The resultant nucleophilic substitution and base-induced elimination reactions are two of the most widely occurring and versatile reaction types in organic chemistry, both in the laboratory and in biological pathways.
    • 11.16: Summary of Reactions
      The summary of reactions involving alkyl halides covers nucleophilic substitutions (SN1 and SN2) and eliminations (E1 and E2). Factors such as substrate structure and nucleophile nature affect these pathways. Nucleophilic substitutions replace the halide with a nucleophile, while eliminations lead to alkene formation. Understanding these mechanisms is essential for predicting alkyl halide reactions.
    • 11.4: The SN1 Reaction
      Some reactions can’t be taking place by the SN2 mechanism and occurr by an alternative substitution mechanism called the SN1 reaction, for substitution, nucleophilic, unimolecular. In contrast to the SN2 reaction has a rate that depends only on the alkyl halide concentration and is independent of the nucleophile concentration. In other words, the process is a first-order reaction; the concentration of the nucleophile does not appear in the rate equation.


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