Question

These are three possible outcomes for a reaction at an assymetric carbon atom (Replacemer of $\mathrm{X}$ by $\mathrm{Y}$ ): The correct statement is/are (A) If $\mathrm{P}$ is the only compound obtained, the process is called inversion of configuration. (B) If $Q$ is the only compound obtained, the process is called retention of configuration. (C) If a $50:50$ mixture of $P$ and $Q$ is obtained, then a Racemic Mixture is obtained. (D) Example of substitution reaction

Short answer

The correct statements are (A) Inversion of configuration occurs when only product P is obtained, (B) Retention of configuration occurs when only product Q is obtained, and (C) a Racemic Mixture is obtained when a 50:50 mixture of P and Q is obtained. Statement D does not hold, as it does not provide a specific example.

Step-by-step solution

Statement A

: If the product P is the only compound obtained, it means the configuration has changed from R to S or vice versa. This process is called inversion of configuration. So Statement A is true.

Statement B

: If only Q is obtained, it indicates that the configuration hasn't changed, either from R to R or S to S, and the process is called retention of configuration. Therefore, Statement B is true.

Statement C

: A racemic mixture refers to a 50:50 mixture of enantiomers. If a 50:50 mixture of P and Q is obtained, it means we have equal amounts of both configurations (R and S), resulting in a racemic mixture. Hence, Statement C is true.

Statement D

: The fourth statement is not a complete or specific description of an example. It just mentions "Example of substitution reaction" without clearly stating a specific reaction. So, Statement D does not hold. Based on the analysis, the correct statements are (A), (B), and (C).

Key concepts

Inversion of Configuration

The inversion of configuration is a fascinating concept in organic chemistry. It refers to a process where a molecule undergoes a change in its spatial arrangement of atoms. Imagine it like flipping an umbrella inside out during a gust of wind. This is commonly seen in reactions at an asymmetric carbon atom.
For instance, consider a molecule with a chiral center represented as having configuration 'R'. If during a reaction the 'R' configuration changes to its mirror image 'S', this process is known as inversion of configuration. This change can occur during some substitution reactions, notably the SN2 reactions.
A key aspect of this process is that it results in a molecule that is a "mirror opposite" to the original. This type of inversion ensures that the three-dimensional structure of the molecule takes its mirror image form. This is why it is sometimes whimsically referred to as the 'umbrella inversion'. The process preserves molecular identity but swaps its configuration.

Retention of Configuration

The term 'retention of configuration' might sound complex, but it's quite straightforward once you get the hang of it. It occurs when the spatial arrangement of a molecule remains unchanged during a chemical reaction. In simpler terms, if a molecule starts with a particular configuration at a chiral center, say 'R', and retains this 'R' configuration even after the reaction, then it's a retention of configuration.
This is typical in some reactions where the chemical transformation does not disturb the geometry of the chiral center. It might happen in indirect substitution reactions where an intermediate is necessarily formed, preserving the original configuration through a path that includes two inversions—resulting effectively in no change at all.
Retention of configuration is essential in the synthesis of many natural products, where maintaining the specific spatial arrangement of atoms is crucial for the biological activity of the molecule. Think of it as maintaining the same arrangement as you move furniture around a room; the furniture pieces stay the same, yet they are rearranged without changing their orientation.

Substitution Reaction

Substitution reactions are a fundamental part of organic chemistry, where one atom or group in a molecule is replaced by another atom or group. Imagine a dance, where dancers change partners without stopping the rhythm of the dance. There are two primary types of substitution reactions: SN1 and SN2.
In an SN2 reaction, the substitution occurs in a single, concerted step and typically involves inversion of configuration, as seen previously. Here, the nucleophile attacks the opposite side of the leaving group, causing the structure to flip.
On the other hand, SN1 reactions proceed via a two-step mechanism where the leaving group departs before the new group is attached, leading to a possibility of retention or racemic mixtures. The intermediate, typically a carbocation, allows for rearrangement, which can complicate the retention/inversion scenario.

• SN2 reactions are more common with primary carbons due to less steric hindrance.
• SN1 reactions are more plausible with tertiary carbons because the carbocation is stable.

Understanding these mechanisms is key in predicting the outcomes of substitution reactions, and therefore, the ultimate configuration of the resulting compounds.