The S_N2 reaction is the concerted, backside displacement of a good leaving group by a nucleophile [see Chapter 12]. Why do nucleophiles attack from the back in S_N2 reactions?

Verified step by step guidance

1. Understand the S_N2 mechanism: S_N2 stands for bimolecular nucleophilic substitution, where the reaction occurs in a single step with the nucleophile attacking the substrate as the leaving group departs.
2. Recognize the structure of the substrate: In S_N2 reactions, the substrate is typically a primary or secondary alkyl halide, where the carbon atom bonded to the leaving group is sp3 hybridized.
3. Consider the steric hindrance: The nucleophile attacks from the backside to avoid steric hindrance from the groups attached to the carbon atom. This backside attack allows the nucleophile to approach the carbon atom more easily.
4. Analyze the electronic factors: The backside attack is favored because it allows the nucleophile to approach the carbon atom along the axis of the C-LG (leaving group) bond, which is the path of least electronic resistance.
5. Understand the transition state: During the S_N2 reaction, the transition state involves a pentavalent carbon with partial bonds to both the nucleophile and the leaving group, leading to inversion of configuration at the carbon center.

Key Concepts

S_N2 Mechanism

The S_N2 mechanism is a type of nucleophilic substitution reaction where a nucleophile attacks an electrophile, resulting in the simultaneous displacement of a leaving group. This reaction occurs in a single concerted step, meaning that bond formation and bond breaking happen at the same time. The '2' in S_N2 indicates that two species are involved in the rate-determining step: the nucleophile and the substrate.

Backside Attack

In S_N2 reactions, nucleophiles attack the substrate from the opposite side of the leaving group, a phenomenon known as backside attack. This orientation is crucial because it allows for optimal overlap between the nucleophile's electron pair and the electrophilic carbon's empty orbital. The backside attack also leads to the inversion of configuration at the carbon center, which is a hallmark of S_N2 reactions.

Sterics and Nucleophilicity

Steric hindrance plays a significant role in S_N2 reactions, as bulky groups around the electrophilic carbon can impede the approach of the nucleophile. Nucleophilicity, or the ability of a species to donate an electron pair, is influenced by both the charge and the steric environment of the nucleophile. Stronger nucleophiles are more likely to successfully perform a backside attack, leading to a faster reaction rate.