Karty CHEM 230 Chapter 9

9. Nucleophilic Substitution and Elimination Reactions

9.1 Overview of SN2, SN1, E2, and E1 Reactions

  • These four reaction types compete for two reasons:

    • They involve substrates with a leaving group.

    • Species acting as nucleophiles can also act as bases, leading to dual functions termed "attacking species."

  • Attacking Species: A species that uses a lone pair to bond with electron-poor atoms (nucleophile) or hydrogen atoms (base).

9.2 Mechanisms of SN2 vs. SN1 and E2 vs. E1

  • SN2 Mechanism: The attacking species acts as a nucleophile, leading to a concerted reaction.

  • SN1 Mechanism: The reaction occurs in two steps, with the nucleophile attacking a carbocation intermediate.

9.3 Rate-Determining Steps

  • Substitution Reactions: The rate of substitution reactions is determined by the energy barrier of the transition state.

  • Elimination Reactions: Similar mechanisms apply with differences based on the type and strength of bases involved.

9.4 Factors Affecting Reaction Rates

Factor 1: Strength of the Attacking Species

  • A stronger nucleophile promotes faster SN2 reactions.

  • Hammond Postulate: Faster reactions occur with lower energy barriers.

    • For example, stronger nucleophiles result in lower energy transition states.

Factor 2: Concentration of Attacking Species

  • Concentration affects reaction rates:

    • High concentrations of strong nucleophiles/base favor SN2/E2 reactions.

    • Low concentrations favor SN1/E1 reactions, particularly for weak nucleophiles/bases.

Factor 3: Leaving Group Ability

  • Better leaving groups enhance reaction rates across all mechanisms.

    • Tosylate (TsO-) is significantly more effective than Cl- as a leaving group.

  • The relative ability for effective leaving groups correlates with their stability in conjugate base forms.

9.5 Solvent Effects

  • Polar Aprotic Solvents: Favor SN2 and E2 reactions by stabilizing the nucleophile while allowing it to be reactive.

  • Polar Protic Solvents: Favor SN1 and E1 reactions due to better stabilization of carbocations.

9.6 Temperature Effects on Reactions

  • Increasing temperature typically favors elimination reactions over substitution.

9.7 Predicting Reaction Outcomes in Competition

  1. Check feasibility based on substrate and leaving group stability.

  2. Determine the strength of attacking species, deciding between nucleophilic or basic action.

  3. Apply tiebreakers based on reaction conditions and substrate structure.

9.8 Regioselectivity

  • Zaitsev's Rule states that the major product of elimination reactions is typically the more substituted alkene, leading to more stable products.

  • Exception: With bulky bases, E2 reactions may yield less substituted products due to steric hindrance.

9.9 Glycosides: Formation and Hydrolysis

  • Glycosides form via reactions involving alcohols (e.g., acetal function) with sugars.

  • Hydrolysis of glycosides requires acid catalysis, yielding free monosaccharides.