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) \(\mathrm{IV}>\mathrm{II}>\mathrm{III}>\mathrm{I}\)

Step-by-Step Solution

Answer

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

Step 1: Understanding the Question

The problem asks for the order of reactivity of different nitrobromobenzene compounds towards hydroxide ions (\( OH^- \)). Reactivity in the context of nucleophilic aromatic substitution reactions is influenced by the electron-withdrawing nature and position of substituents (like nitro groups). We must arrange the given compounds from most to least reactive.

Step 2: Analyzing the Compounds

I. m-Nitrobromobenzene (meta position) has one nitro group. III. p-Nitrobromobenzene (para position) has similar reactivity but with the nitro group at the para position, enabling resonance stabilization. II. \(2,4,6\)-Trinitrobromobenzene has three nitro groups and IV. \(2,4\)-Dinitrobromobenzene has two nitro groups. More nitro groups generally increase reactivity due to stronger electron-withdrawing effects.

Step 3: Predicting Reactivity

\(2,4,6\)-Trinitrobromobenzene (II) is expected to be the most reactive as three nitro groups create a strong electron-deficient environment, which attracts \( OH^- \) ions more strongly. \(2,4\)-Dinitrobromobenzene (IV) follows because with two nitro groups, it's highly activated. Among single nitro group variants, para (III) typically is slightly more reactive than meta (I) due to possible resonance stabilization.

Step 4: Arranging in Order

From the analysis, the predicted order based on the number and position of nitro groups is: \(2,4,6\)-Trinitrobromobenzene (II) > \(2,4\)-Dinitrobromobenzene (IV) > p-Nitrobromobenzene (III) > m-Nitrobromobenzene (I).

Key Concepts

Reactivity Order

When considering nucleophilic aromatic substitution reactions, determining the order of reactivity is crucial. This order depends heavily on the position and number of electron-withdrawing groups present in a compound. Particularly for nitrobromobenzene derivatives, the ability to attract nucleophiles, like hydroxide ions, increases with more electron-withdrawing nitro groups.

In the problem, four different nitrobromobenzene compounds are presented. Observing the functionality of each compound:

• Compounds with multiple nitro groups enhance reactivity due to stronger electron-withdrawing effects.
• The meta, para, and ortho positions can influence the compound's susceptibility to nucleophilic attack, with para often being more favorable.

The infographic order from the exercise confirms that more nitro groups and their strategic positioning enhance the reactivity towards ( OH^{-} ) . This places ( 2,4,6- ) trinitrobromobenzene at the top of the list, reflecting its highest capability to participate in these reactions.

Nitrobromobenzene Compounds

Nitrobromobenzene compounds are unique in their aromatic structure as they carry both a bromine atom and nitro groups on a benzene ring. The nitro groups significantly affect these compounds' chemical properties and reactivities.

Each compound's name in the exercise - m-nitrobromobenzene, p-nitrobromobenzene, ( 2,4- ) and ( 2,4,6- ) trinitrobromobenzene - gives insight into both their structure and functional possibilities:

• **m-Nitrobromobenzene (meta):** The nitro group is positioned at the meta location related to bromine, often exerting a different resonance influence compared to ortho and para.
• **p-Nitrobromobenzene (para):** Here, the nitro group in the para position can engage in resonance, increasing compound reactivity marginally.
• ** ( 2,4- ) and ( 2,4,6- ) Trinitrobromobenzene:** These derivatives have multiple nitro groups, significantly enhancing their electron-withdrawing capabilities.

The nitro groups' affinity for withdrawing electrons ultimately leads to changes in the rate and occurrence of reactions with different nucleophiles like hydroxide ions.

Electron-Withdrawing Groups

In nucleophilic aromatic substitutions, the presence of electron-withdrawing groups is a central factor influencing chemical reactivity. These groups, such as nitro groups in nitrobromobenzene compounds, can substantially alter the electronic environment of the aromatic ring.

Features of electron-withdrawing groups:

• They pull electron density away from the benzene ring, making it more susceptible to attack by nucleophiles.
• The effect is cumulative; more groups lead to greater withdrawal of electron density and higher reactivity.
• Their position on the ring (ortho, meta, or para) influences their effectiveness in stabilizing negative charges and participating in resonance.

When comparing nitrobromobenzene derivatives, more nitro groups (electron-withdrawing) equate to increased reactivity towards substitution by nucleophiles like hydroxide ions. This makes compounds like ( 2,4,6- ) trinitrobromobenzene more reactive than those with fewer nitro groups.

Reactivity Towards Hydroxide Ions

The interaction between aromatic compounds and nucleophiles is specifically vital in understanding nucleophilic aromatic substitution. Hydroxide ions ( OH^{-} ) serve as a strong nucleophile, especially reactive towards electron-deficient aromatic compounds.

To enhance reactivity to hydroxide ions, several factors regarding the aromatic compound come into play:

• The electron deficiency created by nitro groups attracts OH^{-} ions more, facilitating the substitution process.
• Compounds with multiple nitro groups will show stronger affinity towards OH^{-} because of increased electron deficiency.
• The spatial arrangement of these groups also helps in determining the ease of attack, with para and ortho positions usually offering an advantage over meta.

Therefore, understanding why ( 2,4,6- ) trinitrobromobenzene stands at the top for reactivity towards OH^{-} ions is crucial. It is predominantly due to extensive electron withdrawal provided by multiple nitro groups creating an optimum environment for nucleophilic attack.