Problem 127 - A Complete Resource Book in Chemistry for JEE Main

Question

Identify the correct statements (a) The stereisomers which are not mirror images are diastereomers. (b) Diasteteomers have the same physical properties like melting point and solubility (c) Cis isomer has higher melting point and lower solubility than the corresponding trans isomer (d) \(\mathrm{CH}_{3} \mathrm{COCH}_{2} \mathrm{COCH}_{3}\) exists more in enolic form in gaseous phase or in n- hexane.

Step-by-Step Solution

Answer

(a) and (d) are correct.

Step 1: Understanding Stereoisomers

Stereoisomers are molecules with the same molecular formula and sequence of bonded atoms, but different 3D orientations. When stereoisomers are not mirror images, they are called diastereomers. Therefore, statement (a) is correct.

Step 2: Analyzing Physical Properties of Diastereomers

Diastereomers differ in physical properties such as melting point and solubility because they have different spatial arrangements. Thus, statement (b) is incorrect since diastereomers do not necessarily have the same physical properties.

Step 3: Comparing Cis and Trans Isomers

Cis and trans isomers often differ in physical properties. Generally, cis isomers have higher melting points due to symmetry and higher solubility; however, this can vary based on the specific compounds involved. So, statement (c) might not always be true.

Step 4: Evaluating Enol Form Stability

The compound CH3COCH2COCH3, known for keto-enol tautomerism, typically exists more in the keto form as it is more stable. However, in non-polar solvents like n-hexane, the enolic form can be slightly more stable due to hydrogen bonding or solvent effects, making statement (d) potentially correct in this context.

Key Concepts

Stereoisomers

Stereoisomers are a fascinating class of molecules that, despite having the same molecular formula, can exist in different three-dimensional configurations. This means that while the atoms in stereoisomers are connected in the same sequence, their spatial arrangements differ, leading to different properties. A common example of stereoisomers are diastereomers, which are stereoisomers that are not mirror images of each other.

For instance, the compounds that exhibit chirality often result in different stereoisomers. A simple analogy is imagining how your left hand cannot perfectly overlay your right hand—though they look similar, they are not identical. Identifying and understanding the concept of stereoisomers is crucial in organic chemistry because it affects how molecules interact in biological systems.

It's important to remember that these differences in three-dimensional structure do not affect the molecular formula, only the orientation. Thus, stereoisomers are vital in determining the function and interaction of molecules.

Physical Properties of Diastereomers

Diastereomers are a type of stereoisomer characterized by different spatial arrangements that result in different physical properties. Unlike enantiomers, which are mirror images of each other and typically have identical physical properties, diastereomers present unique characteristics.

Key physical properties such as melting points, boiling points, and solubility can vary dramatically between diastereomers.

• For example, one diastereomer may dissolve well in water, while another may not.
• Similarly, their melting and boiling points may be noticeably different, making it easier to separate these substances in the laboratory.

Understanding these differences is crucial for applications in the pharmaceutical industry, where selective diastereomers can result in drugs with specific desired effects. These varying properties are an essential aspect of why diastereomers are important in chemistry.

Cis-Trans Isomers

Cis-trans isomerism is a specific type of stereoisomerism commonly found in alkenes and cycloalkanes. These isomers differ in the positioning of substituents around a double bond or a ring structure. In cis isomers, substituents are on the same side, whereas, in trans isomers, they are on opposite sides.

The physical and chemical properties of cis and trans isomers can differ significantly. For example:

• Cis isomers often have higher boiling points due to stronger intermolecular interactions.
• They may also possess different solubilities.

However, the generalization that cis isomers always have higher melting points and lower solubility compared to their trans counterparts isn't always valid. The specific molecular environment and interactions can reverse or alter these trends. Thus, understanding each specific pair of cis-trans isomers is necessary to accurately determine their properties.

Keto-Enol Tautomerism

Keto-enol tautomerism is a chemical equilibrium between a keto form and an enol form. It commonly involves carbonyl compounds like aldehydes and ketones. The keto form, which has a carbonyl group (C=O), typically predominates due to its greater stability. However, the enol form can become more stable under certain conditions.

For example, in non-polar solvents such as n-hexane, hydrogen bonding or solvent effects can stabilize the enolic form. This balance between the two forms is a fascinating aspect of tautomerism, showcasing the flexibility in molecular structure and stability based on the surrounding environment.

In biological and chemical processes, recognizing when a compound favors the enol form can be crucial for understanding reactivity and mechanism.