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

Short answer

Statement (d) is false.

Step-by-step solution

Understanding the Question

The problem is asking us to identify the false statement regarding the nitration of aromatic compounds. It provides four different statements to evaluate. We need to understand the basic principles of aromatic nitration to assess the truthfulness of each statement.

Evaluating Statement (a)

Statement (a) claims that the rate of nitration of benzene is almost the same as hexadeuterobenzene. Deuterium is an isotope of hydrogen and has similar chemical properties; however, there are slight isotope effects in reactions, but they are usually small. Therefore, the statement is essentially true as the rate difference is not significant.

Evaluating Statement (b)

Statement (b) mentions that the rate of nitration of toluene is greater than that of benzene. Toluene has a methyl group which is an electron-donating group and activates the benzene ring towards electrophilic substitution, increasing the rate of nitration compared to benzene. This statement is true.

Evaluating Statement (c)

Statement (c) states that nitration is an electrophilic substitution reaction. Nitration of aromatic compounds involves the substitution of a hydrogen atom by a nitro group with the help of electrophiles. Therefore, this statement is correct.

Evaluating Statement (d)

Statement (d) asserts that the rate of nitration of benzene is greater than that of hexadeuterobenzene. Due to the isotope effect, the rate of nitration of hexadeuterobenzene is actually slightly slower than benzene, which contradicts this statement. Thus, statement (d) is false.

Key concepts

Electrophilic Substitution

Electrophilic substitution is a fundamental concept in organic chemistry, particularly when discussing aromatic compounds. It refers to a reaction where an electrophile replaces a substituent, typically a hydrogen, on an aromatic ring. In the context of nitration, the electrophile is often the nitronium ion \(NO_2^+\), which forms in a mixture of concentrated nitric and sulfuric acids.

The aromatic ring, with its delocalized electrons, is rich in electrons and hence attracts the positively charged nitronium ion. This attraction leads to the formation of a sigma complex, which temporarily disrupts the aromaticity of the ring. Following this, a deprotonation step restores aromaticity as the nitro group is attached to the ring.

This type of reaction is widely employed in the synthesis of nitro compounds from aromatic hydrocarbons. Understanding electrophilic substitution is crucial as it lays the foundation for comprehending how different substituents influence the rate and outcome of such reactions.

Rate of Reaction

The rate of reaction is a crucial aspect when examining the nitration of aromatic compounds. It tells us how fast a chemical reaction proceeds. In this context, certain factors influence the nitration rate of different aromatic compounds, like benzene and toluene.

For instance, toluene reacts faster than benzene due to the presence of a methyl group, which is an electron-donating group. This group increases the electron density on the benzene ring, making it more reactive towards electrophiles like the nitronium ion.

Additionally, the reaction conditions such as temperature, concentration of the reactants, and the presence of substituents significantly impact the rate. The rate at which an aromatic compound undergoes nitration provides valuable insights into its reactivity and the influence of substituents on the aromatic system.

Isotope Effect

The isotope effect is an intriguing concept observed in nitration reactions, especially when comparing compounds like benzene and hexadeuterobenzene. This effect arises from differences in the mass of isotopes, which can subtly influence reaction rates.

Deuterium is a heavier isotope of hydrogen, and when present in an aromatic compound like hexadeuterobenzene, the reaction rate may decrease slightly compared to benzene. This is due to the fact that bonds involving deuterium are stronger and take more energy to break than those with hydrogen, leading to a slower reaction rate.

Although the difference is often small, the isotope effect proves to be an essential consideration in precise chemical kinetics and mechanistic studies, enriching our understanding of how isotopic substitution can subtly alter the behavior of chemical reactions.