Problem 25 - A Complete Resource Book in Chemistry for JEE Main

Question

Tautomerism will be exhibited by (a) \(\mathrm{R}_{3} \mathrm{CNO}_{2}\) (b) \(\mathrm{RCH}_{2} \mathrm{NO}_{2}\) (c) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}\) (d) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CNO}\)

Step-by-Step Solution

Answer

Compound \( \mathrm{RCH}_{2} \mathrm{NO}_{2} \) exhibits tautomerism.

Step 1: Understand Tautomerism

Tautomerism is a type of isomerism where molecules with the same molecular formula exist in dynamic equilibrium due to the transfer of a proton, usually involving functional groups such as keto-enol, nitro-aci, and amine-imine tautomers.

Step 2: Assess Each Compound for Tautomerism

Evaluate the given compounds to determine if they have structural features that allow for tautomeric shifts, particularly looking for active hydrogen atoms and functional groups capable of participating in such shifts.

Step 3: Analyze \\ (a) \\ \( \mathrm{R}_{3} \mathrm{CNO}_{2} \)

This compound lacks active hydrogens adjacent to the nitro group, making tautomerism unlikely. Tautomerism in nitro compounds typically requires a proton source next to the nitro group.

Step 4: Analyze \\ (b) \\ \( \mathrm{RCH}_{2} \mathrm{NO}_{2} \)

This compound has two hydrogens adjacent to the nitro group. It can participate in nitro-aci tautomerism, where one of the hydrogens can shift, forming an aci-nitro structure.

Step 5: Analyze \\ (c) \\ \( \left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH} \)

This is a simple secondary amine without adjacent nitro or keto groups, making it incapable of tautomerism related to nitro or keto-structures.

Step 6: Analyze \\ (d) \\ \( \left(\mathrm{CH}_{3}\right)_{3} \mathrm{CNO} \)

This compound is structurally similar to compound (a) and lacks the necessary protons adjacent to the nitro group required for tautomerism.

Step 7: Conclusion

Based on the analysis, only compound \( \mathrm{RCH}_{2} \mathrm{NO}_{2} \) can exhibit tautomerism due to the presence of hydrogens adjacent to the nitro group that allow for proton migration.

Key Concepts

Isomerism

Isomerism is a fascinating aspect of chemistry, referring to molecules that share the same chemical formula but have different structural arrangements. This means that although the atoms in isomers are the same, the way they are bonded or arranged can vary, leading to different chemical properties.

• Types of Isomerism: The two broad categories include structural isomerism and stereoisomerism. Structural isomerism involves differences in the connectivity of atoms, while stereoisomerism involves differences in the orientation of atoms in space.
• Tautomerism: A special type of structural isomerism where the isomers (called tautomers) can readily interconvert, typically through the transfer of a proton (H⁺) and a change in single and double bonds. This phenomenon is dynamic, meaning the conversion between the two forms occurs continuously.

Understanding these concepts is crucial in deciphering why certain molecules behave in specific ways during chemical reactions.

Proton Transfer

Proton transfer is the movement of a proton (H⁺) from one molecule, or part of a molecule, to another. This process is central to many biochemical reactions and is an essential component of tautomerism. In a chemical reaction involving proton transfer:

• The proton donor (usually responsible for providing the proton) often has a freely movable hydrogen atom.
• The proton acceptor (which receives the proton) typically contains a functional group that can stabilize the charge received from the proton transfer.

This movement can significantly affect the stability and properties of molecules. In tautomerism, especially, this ability allows molecules to shift from one isomeric form to another, altering their chemical behavior and reactions.

Functional Groups

Functional groups are specific groupings of atoms within molecules that determine the characteristics and chemical reactivity of those molecules. They are like the decision-makers in chemical reactions, dictating how a molecule will behave. Some of the key functional groups include:

• Hydroxyl Group (-OH): Found in alcohols such as ethanol.
• Carbonyl Group (C=O): Found in aldehydes and ketones.
• Carboxyl Group (-COOH): Present in carboxylic acids.
• Nitro Group (-NO₂): Important in nitration reactions and found in nitro compounds.

In the context of tautomerism, the presence of certain functional groups is crucial for enabling the tautomeric shift, allowing for the dynamic equilibrium between isomers.

Nitro Compounds

Nitro compounds are organic molecules containing one or more nitro groups (-NO₂). These compounds are commonly used in explosive materials, pharmaceuticals, and as intermediates in various chemical syntheses. Here are some key points about nitro compounds:

• Structure: Nitro groups consist of nitrogen atom bonded to two oxygen atoms and are generally attached to carbon chains.
• Reactivity: They can participate in reactions such as nitration, reduction, and tautomerism.
• Tautomerism in Nitro Compounds: Involves the shift of a proton adjacent to the nitro group, forming an aci-nitro tautomer. This shift is facilitated by the presence of hydrogen atoms near the nitro group, as seen in the compound \(\mathrm{RCH}_2\mathrm{NO}_2\).

Understanding nitro compounds and their behavior is crucial for many industrial applications and chemical reaction processes.