Chapter 7: Alkyl Halides: Nucleophilic Substitution and Elimination Reactions

Chapter 7: Alkyl Halides: Nucleophilic Substitution and Elimination Reactions

7.1 Introduction to Alkyl Halides

  • Alkyl halides can undergo substitution and elimination reactions.
  • Questions for review:
    • What is the hybridization of each highlighted carbon in the structures shown?
    • Which structure represents an alkyl halide?

7.2 Substitution and Elimination Reactions

  • Alkyl halides can undergo:
    • Substitution reaction: Occurs when reacted with a nucleophile.
    • Elimination reaction: Occurs when reacted with a base.
  • Commonalities between nucleophiles and bases:
    • Both can act as reactants in these reactions.
  • If a reagent can act as a nucleophile or base, substitution and elimination will act as competing reaction pathways.
  • Identification of the substrate in reactions is essential.

7.3 Reasons for Substitution and Elimination Reactions

Reasons why alkyl halides undergo these reactions:
  1. The halogen is electron-withdrawing, generating a partial positive charge on the alpha carbon. This makes it more susceptible to nucleophilic attack.
  2. The halogen acts as a leaving group. A substrate must possess a good leaving group to undergo substitution or elimination reactions.
7.4 Leaving Groups
  • Good leaving groups: Are the conjugate bases of strong acids.
    • Example: Leaving groups like I⁻, Br⁻, and Cl⁻ are good because they come from strong acids.

7.5 Alkyl Halide Nomenclature

  • Alkyl halides are compounds where a carbon group (alkyl) is bonded to a halide (F, Cl, Br, I).
Steps in Naming Alkyl Halides:
  1. Identify and name the parent chain.
  2. Identify the names of the substituents.
  3. Assign a locant (number) to each substituent.
  4. Assemble the name alphabetically.
Common Names
  • Some simple molecules are recognized by their common names:
    • Example: Methylene chloride as a commonly used solvent.

7.6 Alkyl Halide Structure

  • Greek letters are often used to label the carbons of the alkyl group attached to the halide.
  • Reactions occur at the alpha carbon, characterized by branching.
  • Types of alkyl halides: Primary, Secondary, Tertiary.

7.7 Uses of Organohalides

  • Organohalides are included in a variety of products:
    • Insecticides (e.g., DDT)
    • Dyes (e.g., Tyrian purple)
    • Drugs (e.g., anticancer, antidepressants)
    • Food additives (e.g., Splenda)
  • Structure influences function in these applications.

7.8 Nucleophilic Substitution Reactions

  • A substitution reaction requires:
    • Loss of a leaving group.
    • Nucleophilic attack.
Mechanisms:
  1. Concerted Mechanism (SN2): Involves breaking the bond to the leaving group while making a bond to the nucleophile simultaneously.
    • SN2 Reaction Characteristics: Back-side attack; less sterically hindered electrophiles react more readily.
  2. Stepwise Mechanism (SN1): The leaving group departs first, forming a carbocation intermediate, followed by nucleophilic attack.

7.9 Factors Influencing SN2 Reactions

  • Kinetics in SN2 reactions is defined by:
    • Structure of the substrate: Reactivity order is: R−XR-X (where R is carbon group) - CH_3X > 1° > 2° >> 3° (tertiary is unreactive).
    • Nucleophile concentration and reactivity: Rate is directly proportional to nucleophile concentration.
    • Solvent effects: Polar aprotic solvents are favored for SN2 reactions, while protic solvents can hinder reactivity.
    • The nature of the leaving group impacts the reaction speed.

7.10 Kinetics of SN2 Reactions

  • Rearrangement of products due to steric factors in substituents follows this general rate of reaction:
    • Greater steric hindrance decreases the rate of reactivity.
    • Reaction energy diagrams show transition states with maximal steric interference in tertiary substrates.

7.11 Predicting Products of Reactions

To predict what products form in reactions:
  1. Determine the function of the reagent: Is it a nucleophile or a base?
  2. Analyze the substrate: Identify if it is primary, secondary, or tertiary.
  3. Consider relevant regio- and stereochemical requirements: Predict where substitution or elimination occurs, and what the resulting configuration will be.
Stepwise Reactions:
  • In SN1 mechanisms, nucleophiles can lead to mixtures of stereoisomers due to carbocation formation.
  • E1 reactions can yield multiple regioisomers due to carbocation rearrangements.

7.12 Alternatives to Alkyl Halides

  • Alternative substrates: Alkyl sulfonates (e.g., mesylates, tosylates) are good leaving groups used in substitution and elimination reactions.
  • They offer similar reactivities to alkyl halides but are made from alcohols, providing a better leaving group.

7.13 Synthetic Strategies and Retrosynthetic Analysis

  • Organic synthesis involves constructing complex compounds from simpler ones.
  • Retrosynthetic analysis is used to envision how reactions will proceed:
    • Identify bonds to create, determine nucleophile and substrate for reactions, and assess possible outcomes.
Example for Retrosynthetic Analysis:
  • Identify bonds in a target molecule and propose necessary reactions to create these bonds, considering factors like nucleophile strength and conditions needed for reactions to proceed successfully.

Applications and Summary

  • Understanding the mechanisms of nucleophilic substitution and elimination reactions equips chemists to predict outcomes in synthetic organic reactions effectively. Familiarity with structural characteristics of substrates and leaving groups aids in navigating reaction pathways. Practice is essential to mastering these concepts.