Final answer:
In an SN2 reaction occurring at a chiral center, the incoming nucleophile performs a 'backside attack', resulting in the inversion of stereochemistry from R to S or vice versa. Unlike SN1 reactions, SN2 reactions at chiral centers do not lead to racemic mixtures.
Step-by-step explanation:
What Happens When SN2 Occurs at a Chiral Center?
When an SN2 reaction occurs at a chiral center, the result is the inversion of configuration at that center. During an SN2 reaction, the incoming nucleophile approaches the electrophilic carbon from the side opposite to the leaving group. As the nucleophile starts forming a bond with the carbon, the leaving group departs, causing the other three groups attached to the chiral center to move to the opposite side. This creates a situation where the stereochemistry of the molecule is inverted, often referred to as a 'backside attack'. Specifically, if the electrophilic carbon is a chiral center, its configuration will change from R to S or vice versa in the final product.
In contrast to SN1 reactions, which can create a racemic mixture because the intermediate carbocation is planar and can be attacked from either side, SN2 reactions result in a single, inverted product. The substrate plays a crucial role in determining whether an SN2 or an E2 reaction occurs, with methyl and primary substrates typically undergoing SN2 reactions, whereas tertiary substrates are more prone to E2 reactions. This is due to steric hindrance which prevents the nucleophile from approaching the electrophilic carbon in tertiary substrates, prompting an E2 elimination instead.
The concept of a chiral center is vital as it is often the cause for the chirality of a molecule. These centers are typically sp3-hybridized and contain four different substituents, allowing for non-superimposable mirror images. The rearrangement during an SN2 reaction at such chiral centers is what leads to the change in configuration and ultimately the inversion of stereochemistry.