Question

Select the incorrect statement. (A) Meso tartaric acid is more stable than optically active tartaric acid (B) Meso-butane- 2,3 -diol is more stable than optically active butane- 2,3 -diol (C) Meso-2,3-dideuterio butane is more stable than optically active 2,3-dideuterio butane (D) Meso-1,3-dichloro cyclo hexane is more stable then optically active 1,3 -dichloro cyclohexane

Short answer

The incorrect statement is (D) Meso-1,3-dichloro cyclo hexane is more stable than optically active 1,3-dichloro cyclohexane. This is because meso-1,3-dichloro cyclohexane does not have a plane of symmetry, unlike the other options.

Step-by-step solution

Understanding meso and optically active compounds

Meso compounds have two or more stereocenters with an internal plane of symmetry, resulting in a non-optically active compound. On the other hand, optically active compounds can rotate plane-polarized light due to the absence of a plane of symmetry.

Comparing the stability of meso and optically active tartaric acid (Option A)

Meso tartaric acid has a plane of symmetry, due to which it is more stable than optically active tartaric acid. This is because the internal interactions tend to have a stabilizing effect on the molecule. Option A is a correct statement.

Comparing the stability of meso and optically active butane-2,3-diol (Option B)

Meso-butane-2,3-diol also has a plane of symmetry, meaning it is more stable than its optically active counterpart. This is due to the intramolecular hydrogen bonding that further stabilizes the meso compound. Option B is a correct statement.

Comparing the stability of meso and optically active 2,3-dideuterio butane (Option C)

Meso-2,3-dideuterio butane, similar to previous options, has a plane of symmetry, providing increased stability to the molecule as compared to its optically active counterpart. Option C is a correct statement.

Comparing the stability of meso and optically active 1,3-dichloro cyclohexane (Option D)

Meso-1,3-dichloro cyclohexane, contrary to the other options, does not have a plane of symmetry, meaning it is NOT more stable than its optically active counterpart. As a result, Option D is an incorrect statement. Therefore, the correct answer is: (D) Meso-1,3-dichloro cyclo hexane is more stable than optically active 1,3-dichloro cyclohexane.

Key concepts

Chirality in Organic Chemistry

Chirality is a fundamental concept in organic chemistry that refers to the geometric property of a molecule having a non-superimposable mirror image. The presence of chirality in a molecule is typically due to one or more stereocenters, which are atoms, most commonly carbon, attached to four different substituents. When light interacts with chiral molecules, it can be rotated; such molecules are said to be optically active. Molecules without this property are termed achiral.

For instance, meso compounds, while having stereocenters, possess an internal plane of symmetry, making them achiral. These compounds do not rotate plane-polarized light as their mirror images are superimposable due to the symmetry. Understanding chirality is essential when comparing the stability, reactivity, and biological significance of different molecules in organic chemistry.

Intramolecular Interactions

Intramolecular interactions are forces that occur within a single molecule, influencing its stability and structure. They include hydrogen bonding, ionic interactions, and London dispersion forces. These forces are significant in the context of meso compounds, which often exhibit intramolecular hydrogen bonding due to symmetrical arrangements of functional groups.

When considering the stability of meso compounds against their optically active counterparts, it's often the intramolecular interactions that confer increased stability. For example, meso-butane-2,3-diol benefits from intramolecular hydrogen bonds, which stabilize the molecule better than in optically active diols where such interactions may be absent or less favorable.

Role of Intramolecular Hydrogen Bonding

Hydrogen bonding is particularly potent as an intramolecular force, largely affecting boiling points, solubility, and overall molecular stability. This is a key reason why meso compounds are typically more stable; they can form more stable ring-like structures through hydrogen bonding, leading to lowered energy states and increased resilience to external forces.

Comparing Compound Stabilities

Comparing the stabilities of different organic compounds is pivotal for predicting their behavior in chemical reactions, their suitability for particular applications, and understanding their biological roles. Stability is generally associated with the energy of a compound—the lower the energy, the greater the stability.

Meso compounds, which are achiral, often exhibit greater stability than their chiral counterparts due to their symmetrical structure leading to favorable intramolecular interactions. However, this is not a universal rule, as shown in the provided exercise: meso-1,3-dichloro cyclohexane does not have a plane of symmetry and therefore might not be more stable than its optically active version.

Factors Influencing Stability

Several factors contribute to the stability of organic compounds, including:

• Bond strengths and lengths
• Intramolecular forces and potential intramolecular hydrogen bonding
• The presence of an internal plane of symmetry, reducing strain within the molecule
• Electronic effects such as resonance and hyperconjugation

Comparing stabilities involves assessing these factors and how they contribute to the overall energy profile of a compound. By doing so, chemists can make informed predictions about which isomer or molecular form is more stable under certain conditions.