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 (d) are correct.

Step 1: Understanding Stereoisomers

Stereoisomers are compounds with the same molecular formula and sequence of bonded atoms, but which differ only in the three-dimensional orientations of their atoms in space. Enantiomers are stereoisomers that are mirror images of each other, while diastereomers are not mirror images.

Step 2: Evaluating Statement (a)

Statement (a) says, "The stereoisomers which are not mirror images are diastereomers." As defined, diastereomers are indeed stereoisomers that are not mirror images, so this statement is correct.

Step 3: Evaluating Statement (b)

Statement (b) suggests that diastereomers have the same physical properties like melting point and solubility. This is incorrect because diastereomers usually have different physical properties.

Step 4: Analyzing Cis vs Trans Isomers in Statement (c)

Cis isomers typically have higher boiling points but can have lower melting points due to less efficient packing in the solid state. However, their solubility can often be higher due to polarity. Thus, statement (c) is generally incorrect: cis isomers may not necessarily have higher melting points and lower solubility than trans isomers.

Step 5: Evaluating Statement (d)

For the compound \(\mathrm{CH}_{3}\mathrm{COCH}_{2}\mathrm{COCH}_{3}\) (acetylacetone), the keto-enol tautomerism depends on the solvent and conditions. In non-polar environments like \(n\)-hexane and gaseous phases, the enolic form is more stabilized due to intramolecular hydrogen bonding. Thus, statement (d) is correct.

Key Concepts

Stereoisomers

Stereoisomers are an intriguing aspect of chemistry, where molecules have the same molecular formula but differ in the spatial arrangement of atoms. This variance is significant because it can lead to vastly different chemical properties and biological activities. In the realm of stereochemistry, understanding stereoisomers is fundamental.

• Both enantiomers and diastereomers fall under the category of stereoisomers.
• Stereoisomers are essential in fields like pharmaceuticals, where the different arrangements can affect drug activity in the body.

Understanding these differences is not just academic—it has real-world implications. Moreover, identifying stereoisomers involves delving into the 3D configuration of molecules, making it a cornerstone for chemists to master.

Enantiomers

Enantiomers are a specific type of stereoisomer characterized by their mirror-image relationship. These molecules cannot be superimposed on each other, much like your left and right hands. This unique property leads to interesting phenomena known as chirality.

• Each enantiomer can rotate plane-polarized light, leading to its classification as either a dextrorotatory or a levorotatory compound.
• Enantiomers often exhibit different behaviors in chiral environments, such as biological systems.

This is particularly crucial in pharmacology, as one enantiomer of a drug might be therapeutically active, while the other could be inactive or even harmful.

Diastereomers

Diastereomers are a fascinating subset of stereoisomers that are not mirror images of each other. Unlike enantiomers, diastereomers can have multiple chiral centers allowing for more diversity in their structure and properties.

• These stereoisomers can have completely different physical properties, such as melting points and solubilities.
• Identification and separation of diastereomers are critical in synthesizing pure compounds in chemical processes.

Their distinct properties make diastereomers important for both analytical and synthetic chemistry, helping chemists achieve the desired result in their studies.

Cis-trans isomers

Cis-trans isomers, also known as geometric isomers, occur due to the restricted rotation around double bonds or within ring structures. The difference lies in the positioning of substituents relative to a reference plane.

• Cis isomers have substituents on the same side, leading to potential issues with molecular packing.
• Trans isomers, with substituents on opposite sides, often pack more efficiently, influencing melting points and solubility significantly.

Such differences impact the physical properties and behavior of substances, sometimes affecting chemical reactivity as well. Understanding these nuances is vital in both industrial applications and theoretical chemistry research.

Keto-enol tautomerism

Keto-enol tautomerism represents a dynamic equilibrium between two structural isomers: the keto form and the enol form. This concept is pivotal in understanding various chemical reactions and properties of compounds.

• The keto form generally prevails in polar environments, while the enol form is stabilized in non-polar environments due to intramolecular hydrogen bonding.
• This tautomeric shift is crucial in mechanisms such as esterification and aldol condensation.

The ability to influence tautomer stability by changing the environment or chemical conditions is a powerful tool in the hands of chemists, enabling them to control and predict chemical reactivity in complex reactions.