SN2 Reactions

Definition

SN2 reactions, or bimolecular nucleophilic substitution reactions, are a type of organic reaction where a nucleophile attacks the backside of a carbon atom bearing a good leaving group, resulting in the inversion of stereochemistry at that carbon center.

5 Must Know Facts For Your Next Test

1. SN2 reactions occur through a backside attack mechanism, where the nucleophile approaches the carbon bearing the leaving group from the opposite side.
2. The backside attack results in the inversion of stereochemistry at the carbon center, converting a molecule with one stereoisomer to the opposite stereoisomer.
3. SN2 reactions are favored when the carbon center is primary (CH3-) or secondary (CH2-), as tertiary (CH-) centers are too sterically hindered for the nucleophile to approach.
4. Stronger nucleophiles and weaker leaving groups promote SN2 reactivity, as the nucleophile can more easily displace the leaving group.
5. SN2 reactions are commonly observed in biological substitution reactions, the synthesis of crown ethers, and the preparation of amines.

Review Questions

• Explain how the backside attack mechanism of SN2 reactions leads to the inversion of stereochemistry at the reaction center.
  • In an SN2 reaction, the nucleophile approaches the carbon atom bearing the leaving group from the opposite side, or the backside, of the molecule. This backside attack causes the leaving group to depart, resulting in the inversion of the stereochemistry at the reaction center. The new nucleophile becomes bonded to the carbon in the opposite orientation compared to the original molecule, effectively converting one stereoisomer to the other.
• Describe the role of SN2 reactions in biological substitution reactions, the synthesis of crown ethers, and the preparation of amines.
  • SN2 reactions are commonly observed in biological substitution reactions, such as the displacement of halides in DNA and the substitution of hydroxyl groups in carbohydrates. In the synthesis of crown ethers, SN2 reactions are used to cyclize linear polyethers into macrocyclic structures. Additionally, the preparation of amines often involves SN2 reactions, where a nucleophilic amine displaces a leaving group (e.g., halide or sulfonate) from an alkyl or benzyl substrate to form the desired amine product.
• Analyze the factors that influence the feasibility of an SN2 reaction, including the nature of the nucleophile, leaving group, and carbon center.
  • The feasibility of an SN2 reaction is influenced by several factors. Stronger nucleophiles, such as alkoxide or thiolate ions, are more likely to participate in SN2 reactions compared to weaker nucleophiles. Leaving groups that are better stabilized, such as halides or sulfonate esters, also promote SN2 reactivity by facilitating their departure from the carbon center. Additionally, the nature of the carbon center plays a crucial role, as SN2 reactions are favored at primary or secondary carbon centers, which are less sterically hindered, rather than at tertiary carbon centers. The interplay of these factors determines the likelihood and rate of an SN2 substitution reaction.

Related terms

Nucleophile: A nucleophile is a species that donates electrons and attacks an electrophilic carbon center in a substitution or addition reaction.

Leaving Group: A leaving group is a functional group or atom that is displaced in a substitution reaction, typically a halide or a weak base.

Stereochemistry: Stereochemistry refers to the spatial arrangement of atoms in a molecule and how this affects its chemical and physical properties.