Question

Benzene diazonium chloride is obtained by the diazotization of aniline It gives two types of reactions in which nitrogen atoms are replaced and reactions in which nitrogen atoms are retained. For the above reaction, the reactivity of the diazonium ion will be maximum when \(\mathrm{G}\) is (a) \(-\mathrm{HSO}_{3}\) (b) \(-\mathrm{Cl}\) (c) \(-\mathrm{OCH}_{3}\) (d) \(-\mathrm{CH}_{3}\)

Short answer

The diazonium ion reactivity is maximum when  -G is  -HSO_3 (option a).

Step-by-step solution

Understanding Diazotization

Diazonium ions are formed through the diazotization of aniline when aniline is treated with nitrous acid (HNO_2), typically generated in situ from sodium nitrite and hydrochloric acid. The reaction produces benzene diazonium chloride, a compound with the characteristic  -N_2^+ group.

Analyzing Reactivity with Substituents

The reactivity of the diazonium ion is influenced by the substituents (G) present on the benzene ring. Electronegative or electron-withdrawing groups increase the stability of the diazonium ion by providing additional resonance or inductive effects that can stabilize the positive charge.

Examining the Options

Let's evaluate each option: -  -HSO_3 is a strong electron-withdrawing group, increasing the diazonium ion's reactivity. -  -Cl is also an electron-withdrawing group, but weaker than  -HSO_3. -  -OCH_3 is an electron-donating group, which destabilizes the diazonium ion. -  -CH_3 is another electron-donating group, further decreasing positive ion stability.

Concluding the Best Option for Maximum Reactivity

Based on the analysis, the maximum reactivity of a diazonium ion occurs with a strong electron-withdrawing group. Thus, among the options provided,  -HSO_3 will maximize the reactivity of the diazonium ion due to its strong electron-withdrawing ability. Therefore, option (a) is the correct choice.

Key concepts

Diazotization of Aniline

The diazotization of aniline is a key reaction in organic chemistry that involves the conversion of aniline (an aromatic amine) into a diazonium ion. This process is triggered by treating aniline with nitrous acid (HNO₂). However, nitrous acid is typically unstable and ephemeral; therefore, it is usually generated in situ using sodium nitrite (NaNO₂) and hydrochloric acid (HCl). During this reaction, aniline is transformed into benzene diazonium chloride. This compound is characterized by the distinctive diazonium group, represented as \(-N_2^+\), which is central to many synthetic applications.
By carrying out diazotization, chemists can introduce sophisticated modifications to the aromatic ring, allowing for subsequent transformations that replace or retain the nitrogen atoms, leading to innovative product formulations. The generation of these highly reactive diazonium ions is crucial for the development of various aromatic derivatives, making this process a cornerstone in synthetic organic chemistry.

Electron-withdrawing Groups

In the context of diazonium ions, electron-withdrawing groups (EWGs) play a substantial role in determining stability and reactivity. Let's break it down:

• Definition: EWGs are groups that attract electrons away from the rest of the molecule due to their high electronegativity or resonance characteristics.
• Effect on Diazonium Ions: These groups stabilize the positive charge of the diazonium ion through resonance or inductive effects, enhancing the overall reactivity of the ion.

Consider the \(-HSO_3\) group. This is a compelling example of a strong EWG. The presence of this group on the benzene ring pulls electron density away, leading to improved stability of the diazonium ion. This heightened stability directly correlates with increased reactivity, making chemical transformations more efficient.
For instance, if \(-Cl\) is attached, it also acts as an EWG, but with less impact than \(-HSO_3\). The level of electronegativity and the group's ability to resonate define how significant its stabilizing influence can be on the diazonium ion, illustrating why not all EWGs have the same magnitude of effect.

Substituent Effects on Reactivity

The group attached to the diazonium ion's aromatic ring significantly impacts its reactivity. This concept is called substituent effects and it involves two main types of groups:

• Electron-donating Groups (EDGs): These groups donate electron density to the diazonium ion, which can destabilize the positive charge. Examples include \(-OCH_3\) and \(-CH_3\). Their presence generally reduces stability and reactivity, often making these ions less favorable for subsequent reactions.
• Electron-withdrawing Groups (EWGs): Opposite to EDGs, these groups withdraw electron density, thus stabilizing the ion and often enhancing its reactivity. The case of \(-HSO_3\) clearly illustrates how EWGs can lead to maximum reactivity of diazonium ions by stabilizing the positive charge.

Understanding substituent effects is essential when designing reactions involving diazonium ions. Depending on the desired transformation, selecting appropriate substituents can significantly influence reaction pathways and outcomes.
This knowledge is integral in fields such as pharmaceuticals and materials science, where precise control over chemical reactions can lead to the synthesis of valuable compounds with specific, desirable properties.