SN1, SN2, E1, E2 Reactions Overview

Methyl Substrate

• SN2 Reaction:
  • Example: Methyl bromide
  • Independent of solvent or base

Primary Substrate

• SN2 Reaction:
  • Example: Ethyl bromide
  • If strong unhindered base (e.g., hydroxide)
• E2 Reaction:
  • If bulky base is used
  • Example of bulky base: Tert-butoxide

Secondary Substrate

• SN2 Reaction:
  • Using aprotic solvents (e.g., acetone, DMF, DMSO)
  • With good nucleophiles (e.g., iodide, cyanide)
• SN1 and E1 Reactions:
  • With protic solvents (e.g., water, methanol)
  • Typically favored by protic solvents
• E2 Reaction (Major Product):
  • With strong bases, especially if sterically hindered substrate or base

Tertiary Substrate

• SN1 and E1 Reactions:
  • SN1 favored by protic solvents
  • E1 favored by heat
• E2 Reaction:
  • With strong bases

Example Reactions

Example 1

• Two-bromo-butane with sodium cyanide in acetone
  • Mechanism: SN2
  • Product: Inverted configuration from backside attack

Example 2

• Tert-butylchloride with water
  • Mechanism: SN1 and E1
  • SN1 Product: Alcohol (Tertiary)
  • E1 Product: Alkene (double bond formation)

Example 3

• Tertiary Alkyl Halide with Methanol
  • Mechanism: SN1 and E1
  • SN1 Product: Ether with racemic mixture
  • E1 Product: Mixture of alkenes with varying stability

Mechanism Factors

• Steric Hindrance: Influences SN2 reactions
• Nucleophile Strength: Affects SN2 favorability
• Solvent Type:
  • Protic: Generally favors SN1/E1
  • Aprotic: Favors SN2

Special Cases

• Alkyl Fluorides:
  • Favor Hoffman product due to poor leaving group
  • Transition states resemble carbanions

Comparing Alkyl Halide Reactions

• Two-bromo-3-methylbutane with water: SN1 mechanism
• 2-Bromobutane with acetate ion: SN2 mechanism

Key Takeaways

• Primary Unhindered Halides: Prefer SN2 unless base is bulky
• Secondary Halides: Reaction mechanism depends on solvent and nucleophile/bases
• Tertiary Halides: Do not undergo SN2; typically SN1 or E1
• Steric Hindrance and Base Strength: Critical in determining SN2 vs. E2 pathways

This overview provides a general guideline and exceptions should be considered based on specific reaction conditions and substrates.