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 \(\mathrm{n}\) - hexane.

Step-by-Step Solution

Answer

Statements (a) and (c) are correct.

Step 1: Understand Stereoisomers

Stereoisomers are molecules with the same molecular formula and sequence of bonded atoms, but they differ in the three-dimensional orientations of their atoms in space.

Step 2: Identify Diastereomers

Diastereomers are a type of stereoisomers that are not mirror images of each other. Thus, statement (a) is correct.

Step 3: Physical Properties of Diastereomers

Diastereomers typically have different physical properties such as melting points and solubility because they have different spatial arrangements. Therefore, statement (b) is incorrect.

Step 4: Cis/Trans Isomers and Physical Properties

Cis isomers often have higher melting points due to stronger intermolecular forces caused by polarity, but they have lower solubility than trans isomers. Thus, statement (c) is correct.

Step 5: Enolic Form in Different Phases

The enolic form of \( \mathrm{CH}_{3} \mathrm{COCH}_{2} \mathrm{COCH}_{3} \) is more stable in polar environments. In non-polar environments like gaseous phase or n-hexane, the keto form is generally more stable than the enolic form. Thus, statement (d) is incorrect.

Key Concepts

Diastereomers

Diastereomers are a fascinating subgroup of stereoisomers. Unlike enantiomers, which are mirror images of each other, diastereomers are not mirror images. This distinction is crucial in the world of stereochemistry. Diastereomers come about when molecules have multiple chiral centers. Think of them like siblings who share genetic similarities but are clearly different individuals when you meet them.

What makes diastereomers particularly interesting is that they have different spatial arrangements. This means that, unlike enantiomers which often have very similar chemical and physical properties, diastereomers usually have different physical and chemical properties. This can include differences in boiling points, melting points, and solubility. This variability makes diastereomers useful in many chemical processes and in the pharmaceutical industry, where specific stereoisomers can have vastly different effects on biological systems.

Cis/Trans Isomers

Cis/trans isomers, also known as geometric isomers, are a type of diastereomer found in alkenes where two carbon atoms are double-bonded. This double bond restricts rotation, causing the newly formed isomers. Imagine these as tags on a stationary piece of clothing. If the tags are on the same side, it's a cis isomer; if on opposite sides, it’s a trans isomer.

The physical properties of cis and trans isomers can significantly vary. For example, cis isomers often have higher boiling points. This happens because the polarity in cis isomers leads to stronger intermolecular forces. However, these same forces can lead to lower solubility compared to their trans counterparts. Trans isomers, being more symmetric, tend to pack more closely in a solid state, which is why they often have higher melting points than cis isomers. Understanding how these differences in configuration affect properties is essential for applications in material science and drug formulation.

Enolic and Keto Forms

Compounds that can exist in both keto and enol forms are referred to as tautomers, a special type of isomerism called tautomerism. In the keto form, the compound contains a carbonyl group (C=O), whereas in the enol form it has a hydroxyl group (OH) directly bonded to the carbon with a double bond (C=C). These forms are interconvertible and can exist in equilibrium.

Generally, the keto form is more stable than the enol form. This is due to the strong carbon-oxygen double bond's stability in the keto form. Exceptions occur under certain conditions like in polar environments where the enol form could become more stable due to hydrogen bonding. However, in non-polar environments such as a gaseous phase or n-hexane, the keto form is favored. This knowledge is particularly valuable in understanding reaction mechanisms and chemical stability.

Physical Properties of Isomers

The physical properties of isomers can significantly affect their applications and behaviors in different environments. Isomers like diastereomers can have varied melting points, boiling points, and solubility. These differences are attributable to their distinct spatial arrangements and the resulting intermolecular interactions.

Boiling and melting points relate to the energy required to overcome these interactions between molecules. Isomers that interact strongly with each other, due to better fit or polarity, will typically have higher boiling and melting points. Solubility, on the other hand, is influenced by how well an isomer can interact with a solvent. For example, polar isomers tend to dissolve better in polar solvents due to favorable interactions. By understanding these properties, chemists and industries can predict and manipulate the behavior of various isomers in practical applications.