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) \(\mathrm{I}>\mathrm{II}>\mathrm{III}>\mathrm{IV}\) (b) \(\mathrm{II}>\mathrm{IV}>\mathrm{I}>\mathrm{III}\) (c) \(\mathrm{II}>\mathrm{IV}>\mathrm{III}>\mathrm{I}\) (d) IV \(>\mathrm{II}>\mathrm{III}>\mathrm{I}\)

Step-by-Step Solution

Answer

The correct order of reactivity is option (c) II > IV > III > I.

Step 1: Understand the Concept

The reactivity of haloarenes towards nucleophilic substitution is influenced by electron-withdrawing groups (EWGs) such as nitro groups. These groups enhance the reactivity by stabilizing the anion formed during the reaction. The positioning and number of EWG impact the reactivity, with ortho and para positions significantly increasing the effect compared to the meta position.

Step 2: Evaluate Each Compound

Now, evaluate the given compounds based on the information: - II (2,4,6-trinitrobromobenzene) has nitro groups at all ortho positions, being the most activated towards nucleophilic substitution by OH^-. - IV (2,4-dinitrobromobenzene) has two nitro groups, one at ortho and another at para position, making it highly reactive. - III (p-nitrobromobenzene) has one nitro group at the para position, which makes it quite reactive but less so than compounds with multiple nitro groups. - I (m-nitrobromobenzene) has one nitro group in a meta position, which is the least activating position compared to ortho and para positions.

Step 3: Rank the Reactivity

By analyzing the activating effect of the nitro groups, rank the compounds: 1. II (2,4,6-trinitrobromobenzene) is the most reactive due to three nitro groups, all in activating positions. 2. IV (2,4-dinitrobromobenzene) comes second with two nitro groups in strong activating positions. 3. III (p-nitrobromobenzene) follows with one effective nitro group at the para position. 4. I (m-nitrobromobenzene) is the least reactive with the nitro group in the less activating meta position.

Step 4: Choose the Correct Option

With the ranked order being II > IV > III > I, identify the matching option: (c) II > IV > III > I.

Key Concepts

Electrophilic Substitution

Electrophilic substitution is a fundamental reaction mechanism in organic chemistry, particularly when dealing with aromatic compounds like benzene and its derivatives. In this mechanism, an electrophile, which is an electron-poor species, seeks out the electron-rich aromatic ring to replace an existing substituent, most commonly hydrogen. This reaction is facilitated by the high electron density of the aromatic ring, allowing it to easily attract electrophiles.

Understanding the dynamics of this reaction is crucial, especially when haloarenes—aromatic compounds containing halogen atoms—are involved. These compounds typically undergo electrophilic substitution less readily compared to pure benzene. Why is that? It's because the halogen itself, although slightly deactivating due to its electron-withdrawing nature, can sometimes stabilize the reactants and intermediates through resonance. However, the presence of strong electron-withdrawing groups like nitro groups can significantly affect the reactivity.

• Electrophiles are electron-poor species.
• Aromatic compounds have high electron density, attracting electrophiles.
• Haloarenes exhibit lower reactivity due to deactivating halogens.

Electron-Withdrawing Groups

Electron-Withdrawing Groups (EWGs) play a crucial role in modifying the reactivity of aromatic compounds. These groups, such as nitro groups ( NO_2 ), are capable of pulling electron density away from the aromatic ring. The effect of EWGs is especially prominent in nucleophilic substitution reactions.

The positioning of these groups determines how strongly they affect the reactivity of a compound. EWGs located at the ortho and para positions relative to a leaving group tend to increase the reactivity of haloarenes in nucleophilic substitution reactions. This increase happens because they stabilize the negative charge that develops on the aromatic ring during the intermediate stages of the reaction—a factor essential for the nucleophile's attack.

• EWGs like nitro groups reduce electron density.
• Ortho and para positions enhance reactivity via resonance stabilization.
• Stabilization of charge increases nucleophile attraction.

Nitrobenzene Derivatives

Nitrobenzene derivatives are compounds in which one or more nitro groups ( NO_2 ) are attached to a benzene ring. These groups are strong electron-withdrawing entities, notably used to increase the susceptibility of haloarenes to nucleophilic substitution reactions.

The presence and position of these nitro groups drastically alter the chemical behavior of the benzene ring. As demonstrated in the given exercise, derivatives like 2,4,6-trinitrobromobenzene exhibit high reactivity. This is because of the cumulative effect of three nitro groups strategically positioned to maximize the electron-withdrawing effect.

• Each nitro group withholds electron density from the benzene ring.
• Increased number of nitro groups enhances the reactivity towards nucleophiles.
• Positioning (ortho, meta, para) determines the level of these effects.