$S_{N}1$

What main condition favors an $S_{N}1$ reaction?

The relative stability of the carbocation. Relative carbocation stabilities are $3^{o} > 2^{o} > 1^{o} > methyl$. As seen in Figure 41.1 below:

Figure 41.1: Relative stabilities of carbocations.

How can the creation of the carbocation be facilitated?

1. The more substituted a carbocation is, the greater the delocalization of charge and the greater the stabilization of the carbocation.
2. Use of polar protic solvents:
   • A ``polar'' aspect is good to stabilize ions.
   • A ``protic'' aspect is good for solvation: through hydrogen bonds, solvation stabilizes the transition to (1) the cation (carbocation) and (2) to the anion (halide leaving group).
3. Weak bases make good leaving groups.

What is the mechanism for the reaction between $tert$-butyl bromide and water?

Figure 41.2: $S_{N}1$ reaction: Step 1.

• Rate-limiting step: formation of the carbocation
• Keep in mind:
  • Stability of the carbocation is paramount.
  • Carbocations are extremely reactive.
  • The more stable the leaving group, the more likely the carbocation will be formed.
  • Chirality is lost: carbocations have a trigonal planar structure.

  Figure 41.3: $S_{N}1$ reaction: Step 2.

  

• Nucleophilic attack - in this example water will be our nucleophile.
• Keep in mind:
  • Charges still need to be equilibrated.

  Figure 41.4: $S_{N}1$ reaction: Step 3.

  

• Equilibration of charge.

How does an $S_{N}1$ reaction affect the stereochemistry of the product?

The carbocation has a trigonal planar structure that allows a nucleophilic attack to occur from either side. This results in a racemic product with no optical activity as seen in Figure below:

Figure 41.5: Trigonal planar stereochemistry of carbocations.