Question

Compound which loses its optical activity upon standing an enantiomerically pure form of it in a solution of \(\mathrm{NaOEt} ?\) (A) CC1=CC(=O)C(C)(C)CC1 (B) CC(=O)C1CC=C(C)CC1 (C) CC(C)C1CCC=CC1=O (D) Both B and C

Short answer

The compound which loses its optical activity upon standing an enantiomerically pure form of it in a solution of NaOEt is (B) CC(=O)C1CC=C(C)CC1. This compound is susceptible to base-catalyzed racemization due to the proximity of the carbonyl group and stereogenic center, hence can form a racemic mixture with time.

Step-by-step solution

Identifying Chiral Centers

The first step is to identify the chiral centres (stereogenic centres) in each compound. These are carbons that are bonded to four different groups. (A) contains two chiral centers (B) contains one chiral center (C) contains two chiral centers

Pinpointing Susceptible Chiral Centers

The next step is to find which chiral centers are susceptible to base-catalyzed racemization. This would be a chiral centre that would be involved in an elimination or substitution reaction with sodium ethoxide. This is typically a centre that is alpha (adjacent) to a carbonyl (C=O) group. In (A), neither of the chiral centers is alpha to a carbonyl group. In (B), the single chiral center is alpha to a carbonyl group. In (C), neither of the chiral centers is alpha to a carbonyl group.

Decision:

Based on our observations, the compound which loses its optical activity upon standing an enantiomerically pure form of it in a solution of NaOEt is (B) CC(=O)C1CC=C(C)CC1. This compound is susceptible to base-catalyzed racemization due to the proximity of the carbonyl group and stereogenic center hence can form a racemic mixture with time. Thus, the correct answer is B.

Key concepts

Chiral Centers

Chiral centers are a fundamental concept in organic chemistry, referring to carbon atoms within a molecule that are bonded to four different substituents. This diversity in attachments results in molecules that are non-superimposable on their mirror images, much like how your left and right hands are similar but not identical. Imagine holding a model with four different colored balls attached to a central carbon – no matter how you turn it, you can't align it perfectly with its mirror reflection unless two substituents are the same.

In the context of our exercise, identifying chiral centers is crucial because they are the origin of a compound's optical activity – compounds can rotate plane-polarized light, which is how we measure this property. Compound (A) with two chiral centers and compound (C) with the same show potential for optical activity, while compound (B) exhibits this property due to its single chiral center. Optical activity is vital for substances like pharmaceuticals, where the same structure may have different biological effects depending on the orientation of its chiral center.

Racemization

Racemization is a process where the specific configuration at a chiral center is lost, often resulting in a mixture that does not rotate plane-polarized light. Think of it as playing a game where each player starts at a unique position, but over time, everyone ends up in a random spot – the original order is gone. In a chemical sense, the pure enantiomer of a compound can convert to a racemic mixture containing equal amounts of both left- and right-handed forms.

In our textbook solution, we explored how a compound (B) loses optical activity when an enantiomerically pure form stands in a solution of NaOEt. The mechanism behind it is base-catalyzed racemization, specifically when the chiral center is next to a carbonyl group. This proximity allows certain reactions that result in the interconversion between the enantiomers, leading to a racemic mix that nullifies the rotation of polarized light, thereby displaying no optical activity.

Stereogenic Centers

Stereogenic centers are a broader concept than chiral centers. While all chiral centers are stereogenic centers, not all stereogenic centers lead to chirality. A stereogenic center is any point in a molecule where the interchange of two groups leads to a stereoisomer. Stereogenic centers are like decision points on a road map—it matters which path you take because taking a different one will lead you to a unique destination.

In our exercise, the presence of stereogenic centers dictates the optical properties of the compounds. Even though compounds (A) and (C) both have two stereogenic centers, it's important to note that the presence of a carbonyl group near a stereogenic center in compound (B) makes it unique. It's this proximity that permits the compound to undergo racemization under the given conditions. Therefore, knowing the spatial arrangement around these stereogenic centers is pivotal in understanding and predicting the behavior of organic molecules under various chemical reactions.