SN1 Rxns

Solvent Polarity

• Polarity in Solvents
  • Solvents can be classified into two categories: polar and nonpolar.
  • Nonpolar solvents are composed only of carbon (C) and hydrogen (H).
  • Example: Hexanes
    • Structure: A string of carbons (C) with hydrogens (H) attached.
    • No significant polar bonds exist; the bonds consist of CC or CH, making the solvent very nonpolar.
  • Problems with Nonpolar Solvents
    • Nonpolar solvents poorly dissolve charged species, which are usually required in substitution reactions (like SN2).
    • Charged species are usually soluble in aqueous solutions due to their polarity, which means they won't dissolve in nonpolar solvents like hexanes.

Polar Solvents

• Types of Polar Solvents
  • Polar solvents can be categorized into polar protic and polar aprotic based on their ability to participate in hydrogen bonding.
  • Polar Protic Solvents
    • Examples: Water and Ethanol
    • These solvents contain oxygen (O), nitrogen (N), or fluorine (F) atoms bonded to hydrogen (H).
    • Capable of hydrogen bonding due to the presence of OH, NH, or FH bonds.
  • Polar Aprotic Solvents
    • Examples: Acetone
    • Contains a polar CO bond but lacks OH, NH, or FH, meaning it cannot participate in hydrogen bonding.
    • Polar aprotic solvents are preferred for SN2 reactions.

Effect of Solvent on Nucleophilicity

• The type of solvent affects nucleophilicity.
• Placing a nucleophile (e.g., Y⁻) in a polar protic solvent decreases its nucleophilicity.
  • Reason: Protic solvents have hydrogen atoms with partial positive charges that can stabilize the negatively charged nucleophile by surrounding it.
    • Stabilization raises the activation energy for the nucleophile to attack the electrophile, slowing down the reaction, particularly in SN2.
• Polar aprotic solvents are better for enhancing nucleophilicity since they do not stabilize the nucleophile as much.
  • Examples of Polar Aprotic Solvents:
    • Dimethylformamide (DMF)
    • Dimethylsulfoxide (DMSO)
    • DMSO has a similar structure to acetone but contains sulfur instead of carbon.
• Reducing solvation of the nucleophile allows better reaction outcomes in SN2 mechanisms.
• Polar aprotic solvents can stabilize the counterion but not the active nucleophile.

SN2 Reactions

• SN2 reactions rely on strong nucleophiles that thrive in polar aprotic solvents.
• Polar aprotic solvents enhance nucleophilicity because they do not stabilize the nucleophile excessively.

SN1 Reactions

• In SN1 reactions, the leaving group departs before the nucleophile attacks.
  • Key difference from SN2: SN1 is a stepwise process, while SN2 is concerted (all happens in one step).
• Mechanism Steps:
  1. Leaving Group Departs
     • Creates a carbocation intermediate (trigonal planar carbon with a positive charge).
  2. Nucleophilic Attack
     • A neutral nucleophile attacks the carbocation, leading to bond formation.
• Deprotonation (if necessary)
  • A base (often the same nucleophile) removes a proton from the product, resulting in a neutral compound.
• The rate-determining step for SN1 reactions is the generation of the carbocation.
  • Factors affecting this rate involve the concentration of the electrophile, NOT the nucleophile (as it is involved in the second step).

Reaction Coordinate Diagram for SN1

• Structure:
  • Start with a starting material -> transitions to a carbocation intermediate -> nucleophile attacks to form a product.
• The highest transition state (carbocation formation) is the rate-determining step.
• The concentration of the electrophile is the only thing that influences the rate of the reaction.

Stereochemistry in SN1 vs SN2

• SN2: Causes inversion of configuration.
• SN1: Results in a racemic mixture of products (some R, some S); due to the planar structure of the carbocation, the nucleophile can attack from either side.

Carbocation Rearrangements in SN1

• Carbocations can rearrange to form more stable carbocation intermediate (e.g., from secondary to tertiary).
  • Hydride Shift: A hydrogen relocates from an adjacent carbon to form a more stable tertiary carbocation.
  • Alkyl Shift: An alkyl group can shift, creating a new bond and a tertiary carbocation.
• Key Concept: Carbocations seek stability, leading to rearrangements that can alter product outcomes.
• Rearrangements happen because creating a more stable carbocation is energetically favorable and occurs faster than nucleophilic attack.

Base Considerations

• In SN1 reactions, the base can often be just water, as it is not critical to have a strong base.
• Deprotonation occurs easily with any available base around after nucleophilic attack.