Reaction Mechanisms

SN1 Reaction Mechanism (Substitution Nucleophilic Unimolecular)

Key feature: The rate-determining step involves only one molecule (the substrate), hence unimolecular. It usually occurs with tertiary or stabilized carbocations.

Step 1: Formation of the Carbocation (Rate-Determining Step)

  1. The leaving group (often a halide, like Cl⁻, Br⁻, or I⁻) departs from the substrate.

  2. This generates a carbocation at the carbon atom.

  3. This step is slow and determines the reaction rate.

Example:

(R3C−Br)→R3C++Br−(R3​C−Br)→R3​C++Br−

Important points:

  • The stability of the carbocation is critical: tertiary > secondary > primary.

  • Polar protic solvents (like water or alcohols) stabilize the carbocation and leaving group.

Step 2: Nucleophilic Attack

  1. The nucleophile (like H₂O, ROH, or CN⁻) attacks the carbocation.

  2. This forms a new bond between the carbon and the nucleophile.

  3. This step is fast compared to the first.

Example:

R3C++H2O→R3C−OH2+R3​C++H2​O→R3​C−OH2+​

Step 3: Deprotonation (if necessary)

  1. If the nucleophile is neutral (like H₂O), it may carry an extra proton after attack.

  2. A base (often solvent) removes the proton, giving the neutral product.

Example:

R3C−OH2+→R3C−OH+H+R3​C−OH2+​→R3​C−OH+H+

Key Characteristics of SN1 Reactions

  • Unimolecular rate-determining step: Rate depends only on the substrate.
    Rate=k[substrate]Rate=k[substrate]

  • Carbocation intermediate: Can lead to rearrangements (hydride or alkyl shift).

  • Stereochemistry: Usually produces a racemic mixture due to attack from either side.

  • Favored by polar protic solvents and tertiary carbons.

SN2 Reaction Mechanism (Substitution Nucleophilic Bimolecular)

Key feature: The rate-determining step involves both the substrate and the nucleophile at the same time — hence bimolecular. It usually occurs with primary or secondary carbons, rarely tertiary.

Step 1: Nucleophilic Attack and Leaving Group Departure (Concerted)

  1. The nucleophile attacks the carbon that is bonded to the leaving group.

  2. This attack occurs from the side opposite to the leaving group (backside attack).

  3. Simultaneously, the leaving group departs.

  4. There is no intermediate carbocation; the process is concerted (happens in one step).

Example:

CH3CH2Br+OH−→CH3CH2OH+Br−CH3​CH2​Br+OH−→CH3​CH2​OH+Br−

Important points:

  • The carbon undergoes inversion of configuration at the stereocenter (like an umbrella flipping inside out).

  • The reaction rate depends on both the substrate and the nucleophile:

Rate=k[substrate][nucleophile]Rate=k[substrate][nucleophile]

Step 2: Formation of the Product

  1. The nucleophile is now fully bonded to the carbon.

  2. The leaving group has left completely.

  3. The product forms with inverted stereochemistry at the reaction center.

Key Characteristics of SN2 Reactions

  • One-step mechanism: No carbocation intermediate.

  • Backside attack: Leads to inversion of stereochemistry (Walden inversion).

  • Favored by polar aprotic solvents (like DMSO, acetone) because they don’t stabilize nucleophiles.

  • Works best with primary > secondary > tertiary substrates (tertiary is too hindered).

E1 Reaction Mechanism (Unimolecular Elimination)

Key feature: The rate-determining step involves only the substrate, just like SN1, because a carbocation intermediate is formed.

Step 1: Formation of the Carbocation (Rate-Determining Step)

  1. The leaving group departs from the substrate, forming a carbocation.

  2. This step is slow and determines the reaction rate.

Example:

(CH3)3C−Br→(CH3)3C++Br−(CH3​)3​C−Br→(CH3​)3​C++Br−

Important points:

  • Carbocation stability is crucial: tertiary > secondary > primary.

  • Polar protic solvents stabilize both the leaving group and the carbocation.

Step 2: Deprotonation by Base

  1. A weak base (often the solvent itself) removes a proton (H⁺) from a β-carbon (a carbon adjacent to the carbocation).

  2. The electrons from the C–H bond move to form a double bond (π bond), producing an alkene.

Example:

(CH3)3C++H2O→(CH3)2C=CH2+H3O+(CH3​)3​C++H2​O→(CH3​)2​C=CH2​+H3​O+

Key Characteristics of E1 Reactions

  • Unimolecular rate-determining step: Rate depends only on the substrate:

Rate=k[substrate]Rate=k[substrate]

  • Carbocation intermediate: Can undergo rearrangements (hydride or alkyl shifts).

  • Product: Usually more substituted alkene is favored (Zaitsev’s rule).

  • Stereochemistry: Not stereospecific; the double bond can have E/Z isomers.

  • Favoured by: Weak bases, polar protic solvents, tertiary or stabilized carbocations.

E2 Reaction Mechanism (Bimolecular Elimination)

Key feature: The reaction is concerted (one step) and involves both the substrate and the base, hence bimolecular.

Step 1: Base Abstraction of a Proton

  1. A strong base abstracts a proton (H⁺) from a β-carbon (the carbon adjacent to the one bearing the leaving group).

  2. This occurs simultaneously with the leaving group departing from the α-carbon.

Step 2: Formation of the Double Bond

  1. As the proton is removed and the leaving group departs, the electrons from the C–H bond move to form a C=C double bond.

  2. This forms the alkene in a single, concerted step—no carbocation is formed.

Example:

CH3CH2Br+OH−→CH2=CH2+H2O+Br−CH3​CH2​Br+OH−→CH2​=CH2​+H2​O+Br−

Key Characteristics of E2 Reactions

  • One-step, concerted mechanism: No carbocation intermediate.

  • Anti-periplanar geometry: The proton being abstracted and the leaving group must be opposite each other in the same plane for optimal overlap.

  • Strong base required: Common bases include OH⁻, OR⁻, or bulky bases like t-BuO⁻.

  • Favored by: Primary, secondary, or even tertiary substrates with strong bases.

  • Stereochemistry: Follows anti-periplanar requirement, which affects the geometry of the resulting alkene.

  • Product preference: Often follows Zaitsev’s rule, unless a bulky base favors the less substituted (Hofmann) product.