Problem 101 - A Complete Resource Book in Chemistry for JEE Main

Question

Among the following statements on the nitration of aromatic compounds, the false one is (a) the rate of nitration of benzene is almost the same as the hexadeuterobenzene (b) the rate of nitration of toluene is greater than that of benzene (c) nitration is an electrophilic substitution reaction (d) the rate of nitration of benzene is greater than that of hexadeuterobenzene

Step-by-Step Solution

Answer

The false statement is (d): the rate of nitration of benzene is greater than that of hexadeuterobenzene.

Step 1: Identify the Nature of Reaction

Nitration of aromatic compounds involves the introduction of a nitro group (-NO2) into an aromatic ring. This reaction is known as electrophilic aromatic substitution (EAS). Therefore, statement (c) is true: nitration is an electrophilic substitution reaction.

Step 2: Compare Reaction Rates for Benzene and Toluene

Toluene, due to its electron-donating methyl group, activates the aromatic ring, making it more reactive towards electrophilic substitution compared to benzene. Thus, the rate of nitration of toluene is greater than benzene, making statement (b) true.

Step 3: Evaluate Benzene vs. Hexadeuterobenzene

Hexadeuterobenzene is structurally similar to benzene but has deuterium atoms instead of hydrogen. The substitution of deuterium for hydrogen usually results in a negligible kinetic isotope effect for reactions such as nitration, so the nitration rate remains almost the same. Therefore, statement (a) is true: the rate of nitration of benzene is almost the same as hexadeuterobenzene.

Step 4: Determine the False Statement

Since statement (a) is true, the statement that the rate of nitration of benzene is greater than that of hexadeuterobenzene (statement (d)) is false because both have similar rates for this reaction.

Key Concepts

Electrophilic Aromatic Substitution

The nitration of aromatic compounds is a fascinating process that falls under the category of electrophilic aromatic substitution (EAS). This type of reaction involves the replacement of a hydrogen atom on an aromatic ring with an electrophile. In the case of nitration, the electrophile is a nitronium ion (\(NO_2^{+}\)). This ion is generated from nitric acid in the presence of sulfuric acid.

During the reaction, the electrophile approaches the electron-rich aromatic ring. The double bonds, characteristic of benzene, temporarily break to bond with the incoming nitronium ion, forming a sigma complex or p-bonded complex. This intermediate then loses a proton, restoring the aromaticity and completing the EAS process.

• The characteristic feature of EAS is the retention of the aromatic moiety.
• The reaction typically favors positions that are activated by electron-donating groups.
• This process is distinct from nucleophilic aromatic substitution where nucleophiles replace substituents.

Understanding EAS is fundamental to grasping how nitration and related transformations occur, particularly when designing synthetic pathways for more complex organic molecules.

Reaction Rates in Chemistry

Reaction rates in chemistry refer to how fast or slow a reaction occurs. When assessing reaction rates, several factors can influence the speed of a chemical change. Temperature, concentration of reactants, presence of catalysts, and the nature of the substances involved all play a role.

For aromatic compounds like benzene and toluene, the reaction rates are significantly influenced by the structure and substituents attached to the aromatic ring. In the nitration of benzene versus toluene, toluene reacts faster. This is due to its methyl group, which is an electron-donating substituent.

• The methyl group enhances the electron density in the benzene ring, making it more reactive.
• Electron-donating groups generally activate the ring towards an electrophilic attack, speeding up reactions.
• Changes in substituents can drastically change the rate and outcome of the reaction.

Thus, understanding these influences on reaction rates aids in predicting how changes in molecular structure can affect chemical behavior.

Kinetic Isotope Effect

The kinetic isotope effect (KIE) is a fascinating phenomenon observed in chemical reactions where the rate of reaction is affected by the substitution of an atom in the reactants with one of its isotopes. Typically, this involves replacing hydrogen with deuterium, an isotope with a higher atomic mass.

In the context of the nitration of benzene versus hexadeuterobenzene, the kinetic isotope effect plays a role; however, it is usually subtle for such reactions.

• KIE is most pronounced in reactions where the breaking of bonds involving the isotope is the rate-determining step.
• With benzene and hexadeuterobenzene, the similar reactivity suggests a negligible KIE because the C-H or C-D bond is not the major factor in the nitration step.
• Such insights are valuable for designing experiments and understanding mechanistic pathways in organic chemistry.

Comprehending the kinetic isotope effect allows chemists to explore the subtleties of reaction dynamics and aids in developing more nuanced approaches to studying chemical reactions.