Problem 122 - A Complete Resource Book in Chemistry for JEE Main

Question

The decreasing order of reactivity of I. m-nitrobromobenzene II. 2, 4, 6-trinitrobromobenzene III. p-nitrobromobenzene IV. and 2,4 -dinitrobromobenzene towards \(\mathrm{OH}\) ions is (a) \(\mathbb{I}>\mathrm{II}>\mathrm{III}>\mathrm{IV}\) (b) \(\mathrm{II}>\mathrm{IV}>\mathrm{I}>\mathrm{III}\) (c) \(\mathrm{II}>\mathrm{IV} \geq \mathrm{III}>\mathrm{I}\) (d) \(\mathrm{IV}>\mathrm{II} \geq \mathrm{III}>1\)

Step-by-Step Solution

Answer

Option (b): II > IV > I > III.

Step 1: Understanding Reactivity

Reactivity of aromatic halides towards \(\)OH ions is influenced by electron-withdrawing groups. Nitro groups (\(\)NO extsubscript{2}) increase reactivity by stabilizing the intermediate carbanion.

Step 2: Analyzing the Effect of Nitro Groups

The more nitro groups present, especially at ortho and para positions to the halogen, the greater the reactivity towards \(\)OH ions due to increased electron-withdrawing effect.

Step 3: Comparing the Compounds

II, 2, 4, 6-trinitrobromobenzene, with three nitro groups, is more reactive than others. IV, 2, 4-dinitrobromobenzene, has two nitro groups, enhancing its reactivity compared to I and III. I has a meta nitro group, less effective, and III has a para nitro group.

Step 4: Arranging in Decreasing Order of Reactivity

Based on the number and position of the nitro groups, the decreasing order of reactivity is: \(II > IV > III > I\). The correct answer is option (b) \(II>IV>I>III\).

Key Concepts

Electron-withdrawing groups

Electron-withdrawing groups are key players in affecting the reactivity of aromatic compounds, especially in reactions with nucleophiles like hydroxide ions (\(\mathrm{OH}^-\)). These groups, such as nitro groups (\(\mathrm{NO_2}\)), can increase the overall reactivity of a compound by pulling electron density away from the aromatic ring. This makes the carbon atoms in the ring more positively polarized, thereby making the aromatic ring more susceptible to nucleophilic attack.
Some common electron-withdrawing groups include:

• Nitro groups (\(\mathrm{NO_2}\))
• Carboxyl groups (\(\mathrm{COOH}\))
• Cyanides (\(\mathrm{CN}\))

When these groups are present on an aromatic ring, they stabilize the formation of a negative charge on the ring during a reaction transition state, thus enhancing the reactivity towards nucleophiles.

Nitro groups

Nitro groups (\(\mathrm{NO_2}\)) are particularly powerful electron-withdrawing groups. They effectively draw electron density from the aromatic ring due to the strong electronegativity of the nitrogen and the resonance opportunities provided by the oxygen atoms. The effect of nitro groups is notably influenced by their position on the aromatic ring. When situated at the ortho (adjacent) or para (opposite) positions relative to the halogen, they maximize their electron-withdrawing influence due to greater resonance stabilization. This increased stabilization assists in the formation of reaction intermediates, such as carbanions, during reactions with \(\mathrm{OH}^-\) ions. Each additional nitro group exponentially compounds this effect, making compounds like 2,4,6-trinitrobromobenzene highly reactive in comparative analyses.

Aromatic halides

Aromatic halides are aromatic compounds that include a halogen substituent, such as bromine or chlorine, attached to the benzene ring. These compounds are crucial when discussing reactivity as they often participate in nucleophilic aromatic substitution reactions.In these reactions, the presence of electron-withdrawing groups can greatly enhance the reactivity of the aromatic halide. The halogen atom itself can act as a catalyst by leaving the aromatic nucleus more receptive to attack after vacating its position. This is due to the tendency of aromatic rings with strong electronegative substituents, like nitro groups, to stabilize the ring upon introduction of a nucleophile like hydroxide ions (\(\mathrm{OH}^-\)).When examining the reactivity of aromatic halides, it's notable that the position of the substituted halogen relative to other substituents affects outcomes. The ortho and para positions are especially reactive when accompanied by strong electron-withdrawing groups.

Carbanion stabilization

Carbanion stabilization is a crucial concept in understanding the reactivity of aromatic compounds with electron-withdrawing groups. A carbanion is an anion in which carbon carries a negative charge. In the context of nucleophilic aromatic substitution, carbanion intermediates often form during the reaction sequence. Electron-withdrawing groups, like nitro groups, help stabilize this negative charge through delocalization, increasing the likelihood of reaction completion. Essentially, the ability of these groups to stabilize a carbanion can make a substantial difference in whether a reaction occurs at a noticeable rate or not. Through resonance, electrons can shift into the ring, spreading the negative charge more evenly throughout the aromatic system. Therefore, compounds with multiple nitro groups are often more reactive because they provide additional pathways for electron delocalization, effectively lowering the activation energy required for the reaction.