Backside Attack

Definition

A backside attack is a type of nucleophilic substitution reaction where the attacking nucleophile approaches the carbon atom from the opposite side of the leaving group. This orientation of the attack is a key characteristic that distinguishes the SN2 reaction mechanism from the SN1 reaction mechanism.

5 Must Know Facts For Your Next Test

1. The backside attack in the SN2 reaction results in the inversion of stereochemistry at the carbon atom where the substitution occurs.
2. Backside attack is a characteristic of the SN2 reaction mechanism, where the nucleophile approaches the carbon atom from the opposite side of the leaving group.
3. The backside attack in the SN2 reaction is a concerted mechanism, meaning the nucleophile attacks the carbon atom at the same time the leaving group departs.
4. Backside attack is important in the halogenation of alkenes, where the addition of a halogen (X2) to an alkene occurs via an SN2-like mechanism.
5. Backside attack is also a key feature of the ring-opening reactions of epoxides, where a nucleophile attacks the epoxide ring from the opposite side of the oxygen atom.

Review Questions

• Explain how the backside attack in the SN2 reaction mechanism leads to the inversion of stereochemistry at the carbon atom.
  • In the SN2 reaction, the nucleophile approaches the carbon atom from the opposite side of the leaving group, resulting in a backside attack. This backside attack causes the leaving group to depart from the carbon atom, while the nucleophile simultaneously binds to the carbon. This process occurs in a concerted, single-step mechanism, leading to the inversion of stereochemistry at the carbon atom where the substitution takes place.
• Describe the role of backside attack in the halogenation of alkenes via the addition of a halogen (X2).
  • The halogenation of alkenes, where a halogen (X2) is added to an alkene, occurs through an SN2-like mechanism. In this process, the backside attack of the nucleophilic halide ion on the carbon atom of the alkene leads to the formation of a haloalkane product. The backside attack is crucial in this reaction, as it ensures the addition of the halogen occurs in a stereospecific manner, resulting in the overall retention of stereochemistry.
• Analyze the importance of backside attack in the ring-opening reactions of epoxides, and explain how this mechanism contributes to the selectivity of the reaction.
  • In the ring-opening reactions of epoxides, a nucleophile attacks the epoxide ring from the opposite side of the oxygen atom, resulting in a backside attack. This backside attack is essential for the ring-opening process, as it allows the nucleophile to displace the oxygen atom and break the epoxide ring. The backside attack also determines the regiochemistry of the product, as the nucleophile will preferentially attack the less hindered carbon of the epoxide ring. This selectivity, driven by the backside attack mechanism, is crucial in controlling the outcome of epoxide ring-opening reactions.

Related terms

Nucleophilic Substitution:

A reaction in which a nucleophile replaces a leaving group on a carbon atom, resulting in the substitution of one group for another.

SN1 Reaction:

A nucleophilic substitution reaction that occurs in two steps, where the first step involves the formation of a carbocation intermediate, and the second step involves the attack of the nucleophile.

Stereochemistry:

The study of the spatial arrangement of atoms in molecules and the effect of this arrangement on the chemical and physical properties of the molecule.