Organic Chemistry 2: Alkyl Halides and Nucleophilic Substitution

Overview

• Focus on alkyl halides and nucleophilic substitution reactions
• Discuss reasons for nucleophilic substitution in alkyl halides
• Review of nucleophiles
• Effect of alkyl halide type on reaction rates
• Review rate equations and kinetics of SN1 and SN2 reactions

Nucleophilic Substitution Reactions

• General Overview:
  • Involves replacing halogen (leaving group) with a nucleophile
  • Forms a bond between nucleophile and carbon in the alkyl halide
  • Halogen becomes a good leaving group (often noted as LG or X)
• Why Alkyl Halides Undergo These Reactions:
  • Polarization of the carbon-halogen bond due to halogens' electronegativity
  • Carbon-halogen bond becomes polar covalent

Factors Influencing Reaction Rates

• Halogens:
  • Ignore fluoroalkanes due to strong C-F bond
  • Reactivity increases from chlorine to iodine due to decreasing bond strength
• Electrophiles and Nucleophiles:
  • Electrophile: Electron-poor, seeks electrons
  • Nucleophile: Electron-rich, seeks nuclei, can be negatively charged or neutral
  • Interaction between electrophiles and nucleophiles drives substitution

Nucleophile Characteristics

• Electron-rich species that can donate a pair of electrons
• Examples: Hydroxy anion (negative), Ammonia (neutral with electron pair), Alkenes (double bonds as nucleophiles)
• Trends in Nucleophilicity:
  • Negative ions are better nucleophiles than neutral molecules
  • Electronegativity affects nucleophilicity: Increases up a column; better nucleophiles down the column
  • Across a row, increasing electronegativity decreases nucleophilicity

Effect of Alkyl Group on Substitution

• Alkyl Halide Types:
  • Methyl, primary, secondary, tertiary defined by number of alkyl groups around carbon
• Reaction Rate Trends:
  • Steric hindrance: More alkyl groups decrease reaction rate
  • Tertiary alkyl halide reactions are slower
• Experimental Findings:
  • Secondary alkyl bromides as a standard
  • Primary alkyl bromides react faster than secondary; methyl bromides fastest
  • Tertiary alkyl bromides react very slowly

SN1 vs SN2 Reactions

• General Findings:
  • Methyl, primary, secondary: Likely SN2 (bimolecular, depends on both reactants)
  • Tertiary: Likely SN1 (unimolecular, depends only on alkyl halide concentration)
• Experimental Kinetics:
  • Rate equations used to determine reaction order
  • SN2: Rate depends on both alkyl halide and nucleophile
  • SN1: Rate depends only on alkyl halide
• Molecularity:
  • SN2: Second order, bimolecular
  • SN1: First order, unimolecular

Conclusion

• Understanding the types of alkyl halides and their reactions is crucial for determining SN1 and SN2 mechanisms
• Focus on identifying alkyl halide type to predict reaction mechanism in this module