Mechanisms

S_N1 Substitution Nucleophilic - Unimolecular

The SN1 mechanism involves two steps:  

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

   • The leaving group (LG) dissociates from the substrate, forming a positively charged carbocation intermediate.
   • The rate of the reaction depends only on the concentration of the substrate ([R-LG]), not the nucleophile, as the nucleophile is not involved in the rate-determining step.
   • Rate = k[R-LG]
   • Carbocation stability plays a crucial role: tertiary (3°) > secondary (2°) > primary (1°) > methyl, due to increasing carbocation stability through inductive effects and hyperconjugation.
     
     For example, if you placed tert-butyl chloride in water it would undergo the following substitution, replacing the Cl atom with water.
     
2. Nucleophilic Attack on the Carbocation:
   • • The nucleophile rapidly attacks the planar carbocation to form the final substituted product.
     • Since the carbocation is planar, the nucleophile can attack from either side, leading to no stereochemical preference and often resulting in a racemic mixture if the starting material is chiral.
       
       A water molecule will eventually attach the cation to form an alkoxonium, which is subsequently deprotonated to form the corresponding alcohol. 
       

Key Characteristics of SN1 Mechanism:

• Nucleophile: The strength of the nucleophile is not crucial since it does not influence the rate of the reaction.
• Stereochemistry: The intermediate carbocation is planar, leading to loss of stereochemical information. If the starting material is chiral, the product will typically be a racemic mixture.
• Solvent: SN1 reactions are favored in polar solvents, especially polar protic solvents like water or alcohols. These solvents stabilize the carbocation intermediate and solvate the leaving group, making its dissociation easier.

S_N2 Substitution Nucleophilic - Bimolecular

The SN2 mechanism is a single-step, concerted reaction that follows the rate law:

1. Single-Step Mechanism (Bimolecular Nucleophilic Substitution):

   • The nucleophile directly attacks the substrate from the opposite side (Back side attack) of the leaving group. This backside attack leads to the simultaneous breaking of the bond to the leaving group and the formation of the bond to the nucleophile.
   • Since this is a concerted process, the reaction proceeds in a single step without forming intermediates.
   • Rate = k[R-LG][Nu⁻]
   • The rate depends on both the concentration of the substrate ([R-LG]) and the nucleophile ([Nu⁻]).

Key Characteristics of SN2 Mechanism:

• Nucleophile:

  • A strong nucleophile is required to attack the substrate effectively, as it directly affects the reaction rate.
  • Nucleophiles with a negative charge or lone pairs (e.g., OH⁻, CN⁻, or I⁻) are typically more effective.
  • Nucleophile sterics are important.

• Solvent:

  • SN2 reactions are favored in polar aprotic solvents (e.g., DMSO, acetone, DMF). These solvents stabilize the nucleophile less, allowing it to remain highly reactive.

• Stereochemistry:

  • The reaction results in complete inversion of configuration (Walden Inversion) at the reactive carbon center, making it stereospecific.

• Substrate:

  • Sterically accessible substrates (e.g., methyl or primary carbons) react faster. Secondary carbons are slower, and tertiary carbons are essentially unreactive in SN2 due to steric hindrance.

• Rate Law:
  Rate = k[R-LG][Nu⁻]
  The reaction rate depends on both the nucleophile and the substrate, making it bimolecular.

Why backside attack?

1. LG is in the way for frontside attack
2. Backside has the required empty antibonding orbital to accept electrons from Nu (Nucleophile)