$S_{N}2$

What main conditions favor an $S_{N}2$ reaction?

1. Structure of the substrate is paramount in allowing enough room for a nucleophile to attack and a leaving group to depart. Favorable structures are $methyl > 1^{o} > 2^{o} > 3^{o}$:

   Figure 41.6: Favorable substrates for an $S_{N}2$ reaction.

   

2. The presence of a ``strong'' nucleophile, i.e. a nucleophile with sufficient negative charge to attack the substrate

How can an $S_{N}2$ reaction be facilitated?

1. Use of polar aprotic solvents
   • A ``polar'' aspect is good to stabilize ions
   • A ``aprotic'' aspect is good because hydrogen bonds do not get in the way of the anion's (nucleophile) attack of the substrate

What is the mechanism for the reaction between methyl bromide and hydroxide?

Figure 41.7: $S_{N}2$ reaction.

• The rate-limiting step is the formation of the transition state.
• Keep in mind:
  • ``Back-side attack'' by nucleophile.
  • The transition state is a shifting of bonds and is the reaction's energy barrier.

How does an $S_{N}2$ reaction affect the stereochemistry of the product?

The backside attack of the nucleophile inverts the configuration of the substrate in the product:

Figure 41.8: Inversion of stereochemistry in $S_{N}2$ reactions: reactant (left) and product (right).

Table 41.1: Summary of $S_{N}1$ & $S_{N}2$ Reactions

	$S_{N}1$	$S_{N}2$
# of steps	2	1
Reaction rate order	1	2
Limiting step	Carbocation formation in the substrate	Transition step formation in an unhindered substrate to allow for attack by a strong nucleophile
Ideal substrate	$3^{o} > 2^{o} > 1^{o} > methyl$	$methyl > 1^{o} > 2^{o} > 3^{o}$
Stereochemistry of product	Racemic	Inverted