11.4 The SN1 Reaction

Substitution reactions are key in organic chemistry, with SN1 reactions following a unique two-step mechanism. These reactions involve a carbocation intermediate, leading to a mix of stereoisomers when chiral substrates are involved.

SN1 reactions favor tertiary alkyl halides and polar protic solvents, contrasting with SN2 reactions. Factors like leaving group ability and carbocation stability influence SN1 reactions, which follow first-order kinetics and often occur through solvolysis.

SN1 Reaction Mechanism and Factors

Mechanism of SN1 reactions

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• SN1 reaction proceeds through a two-step mechanism
  • Step 1 (rate-determining): Slow unimolecular dissociation of the leaving group forming a planar carbocation intermediate
    • Rate depends only on the concentration of the substrate (tertiary alkyl halide)
    • Follows first-order kinetics: rate = $k[substrate]$
  • Step 2: Fast nucleophilic attack on the carbocation from either side forming the substitution product
    • Carbocation is sp2 hybridized allowing for attack from either face (top or bottom)
    • Results in a mixture of stereoisomers (racemic mixture) if the substrate is chiral (has a stereocenter)

Stereochemistry in SN1 reactions

• SN1 reactions on chiral substrates result in a mixture of stereoisomers (racemic mixture)
  • Planar carbocation intermediate allows for equal probability of nucleophilic attack from either side (top or bottom face)
  • Leads to a 50:50 mixture of enantiomers (R and S configurations)
• Stereochemistry at the reaction center is lost due to the planar nature of the carbocation intermediate
  • Original stereochemical information is not retained in the product
• SN1 reactions do not exhibit stereochemical inversion unlike SN2 reactions
  • SN2 reactions proceed with backside attack and inversion of stereochemistry

SN1 vs SN2 reaction factors

• Substrate structure
  • SN1 favored by tertiary alkyl halides and other substrates that form stable carbocations (t-butyl bromide)
    • Increased substitution stabilizes carbocations through hyperconjugation and inductive effects
  • SN2 favored by primary and secondary alkyl halides and other substrates with less hindered reaction centers (methyl bromide)
    • Less substitution reduces steric hindrance allowing for easier backside attack by the nucleophile
• Solvent effects
  • SN1 favored by polar protic solvents (water, ethanol)
    • Stabilize the carbocation intermediate through solvation and hydrogen bonding
    • Assist in the dissociation of the leaving group (bromide, chloride)
  • SN2 favored by polar aprotic solvents (DMSO, acetone)
    • Solvate cations (Na+, K+) without solvating the nucleophile increasing its reactivity
    • Do not stabilize the carbocation intermediate disfavoring SN1

Additional Factors Affecting SN1 Reactions

• Leaving group ability: The better the leaving group (e.g., tosylate, bromide), the faster the SN1 reaction proceeds
• Carbocation stability: More stable carbocations lead to faster SN1 reactions due to easier formation of the intermediate
• Solvolysis: SN1 reactions often occur through solvolysis, where the solvent acts as the nucleophile
• Rate law: The rate of an SN1 reaction depends only on the concentration of the substrate, following first-order kinetics