SN1 Reaction Mechanism and Energy Diagram

SN1 Reaction (Energy Diagram and Mechanism)

• SN1 reactions are a type of nucleophilic substitution.
• In a nucleophilic substitution reaction, one group is replaced by a nucleophile (an electron-rich species).
• A leaving group is the group that leaves.
• There are two main types of nucleophilic substitution: SN1 and SN2.
• SN1 substitution reactions are nucleophilic, unimolecular.
• The first step is the rate-determining step (RDS).
• The bond between the leaving group and the α-carbon breaks, and the leaving group departs.
• This forms a carbocation intermediate.
• The carbocation intermediate is then attacked by the nucleophile in the second step.
• The first step only involves one species (the substrate), so it is unimolecular.

SN1 Reaction Mechanism

• The mechanism of SN1 reaction has two steps.
• The C-X bond forms a carbocation in rate-determining step.
• Heterolysis of the C-Br bond forms a carbocation.
• Nucleophilic attack of acetate (a Lewis base) on the carbocation forms the new C-O bond.
• Carbocation is formed as a reactive intermediate.

SN1 Kinetics

• SN1 reactions exhibit first-order kinetics.
• The reaction is unimolecular; only the alkyl halide is involved.
• The identity and concentration of the nucleophile do not affect the reaction rate.
• The nucleophile does not appear in the rate equation.
• Rate = k[(CH3)3CBr]

SN1 Stereochemistry

• In the second step, the nucleophile can attack from either side.
• This leads to a pair of enantiomers.
• This process is called racemization.
• Loss of the leaving group in the first step generates a planar carbocation which is achiral.
• Nucleophilic substitution forms a racemic mixture of two products.
• With H₂O, a neutral nucleophile, the initial product of nucleophilic substitution (ROH₂) loses a proton to form the final neutral product, ROH.
• The reaction mechanism has two steps, so there are two energy barriers.

When can SN1 occur?

• Stable carbocation intermediate:

  • The reaction intermediate is a carbocation.
  • Stabilizing the carbocation lowers the barrier to reaction for the first step, which enhances SN1 reactions.
  • Tertiary (3°) carbocations are more stable than secondary (2°) carbocations.
  • 2° carbocations are stable enough to form.
  • SN1 is possible.

• Good leaving group:

  • A good leaving group lowers the energy needed to remove it, lowering the barrier to reaction.
  • A good leaving group is essential for SN1 reactions.

The Leaving Group

• In nucleophilic substitution of R-X, the C-X bond is heterolytically cleaved.
• The leaving group departs with the electron pair, forming X⁻.
• The more stable the leaving group (X⁻), the better it is at accepting an electron pair, becoming a weaker base.
• In comparing two leaving groups, the better leaving group is the weaker base.
• For example H₂O is a better leaving group than HO⁻.

Trends in Leaving Group Ability

• Left-to-right across a row of the periodic table, basicity decreases; leaving group ability increases.
• Down a column of the periodic table, basicity decreases; leaving group ability increases.

Good and Poor Leaving Groups

• Good leaving groups are weak bases (e.g., Cl⁻, Br⁻, I⁻).
• Poor leaving groups are strong bases (e.g., OH⁻, F⁻, NH₂⁻).

Sample Problems

• Specific examples of SN1 reactions are presented to illustrate how to draw products and indicate stereochemistry.
  • Example reactions are given and asked to draw the products and indicate the stereochemistry or the relative reaction rates.

Nucleophiles and Bases

• Nucleophiles and bases are similar, having lone pairs or π bonds.
• They differ in what they attack: bases attack protons, and nucleophiles attack other electron-deficient atoms (often carbon).

Solvation by Polar Protic Solvents

• Polar protic solvents effectively solvate both cations and anions.
• Na⁺ cation is solvated via ion-dipole interactions with H₂O molecules.
• Br⁻ anions are solvated by strong hydrogen bonding interactions.

Nucleophilicity in Polar Protic Solvents

• Smaller, more electronegative anions are more strongly solvated, reducing reactivity.
• Nucleophilicity increases down a column of the periodic table as the size of the anion increases (opposite of basicity) in polar protic solvents.

Polar Aprotic Solvents

• These solvents have dipole-dipole interactions but lack O-H or N-H bonds.
• They are incapable of hydrogen bonding.
• Common polar aprotic solvents are listed.