Problem 133 - A Complete Resource Book in Chemistry for JEE Main

Question

Identify the correct order of reactivity in electrophilic substitution reactions of the following compounds: c1ccccc1 Cc1ccccc1 Clc1ccccc1 O=[N+]([O-])c1ccccc1 (1) (II) (III) (IV) (a) \(\mid>\mathrm{II}>\mathrm{III}>\mathrm{IV}\) (b) IV \(>\mathrm{III}>\mathrm{II}>\mathrm{I}\) (c) \(\mathrm{II}>\mathrm{I}>\mathrm{III}>\mathrm{IV}\) (d) \(\mathrm{II}>\mathrm{III}>\mathrm{I}>\mathrm{IV}\)

Step-by-Step Solution

Answer

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

Step 1: Identify Structural Formulas

First, convert the SMILES notation into structural formulas: - c1ccccc1 corresponds to benzene (Compound I). - Cc1ccccc1 corresponds to toluene (Compound II). - Clc1ccccc1 corresponds to chlorobenzene (Compound III). - O=[N+]([O-])c1ccccc1 corresponds to nitrobenzene (Compound IV).

Step 2: Understand Reactivity in Electrophilic Substitution

Electrophilic aromatic substitution reactions are influenced by the electron density on the aromatic ring. Electron-donating groups (EDGs) increase reactivity by stabilizing the intermediate carbocation, while electron-withdrawing groups (EWGs) decrease reactivity by destabilizing the carbocation.

Step 3: Analyze Each Compound's Electron-Donating/Withdrawing Effects

- Toluene (Compound II) has a methyl group as an EDG, which activates the aromatic ring, increasing reactivity. - Benzene (Compound I) has no substituents and is the baseline for reactivity. - Chlorobenzene (Compound III) has a chlorine atom, which acts as a weak EWG with a slight deactivating effect. - Nitrobenzene (Compound IV) has a nitro group, a strong EWG, significantly decreasing reactivity.

Step 4: Order the Compounds by Decreasing Reactivity

Putting the compounds in order of decreasing reactivity based on the electron-donating and electron-withdrawing effects: 1. Toluene (II) is most reactive. 2. Benzene (I) is next as it has no substituent effects. 3. Chlorobenzene (III) follows as chlorine is a weak deactivator. 4. Nitrobenzene (IV) is least reactive due to the strong deactivation by the nitro group.

Step 5: Match the Order with the Provided Choices

The correct order identified based on reactivity is toluene > benzene > chlorobenzene > nitrobenzene, which matches choice (c): \( ext{II > I > III > IV}\).

Key Concepts

Reactivity of Aromatic Compounds

Understanding the reactivity of aromatic compounds is essential when studying electrophilic substitution reactions. Aromatic compounds like benzene rings are characterized by their conjugated pi-electron systems, which provides stability. This stability makes them less reactive in certain reactions compared to alkanes or alkenes. Therefore, the presence of specific groups can dramatically alter their reactivity. In electrophilic substitution, the aromatic ring acts as a nucleophile, engaging in a reaction where an electron-rich aromatic compound interacts with an electron-deficient species (electrophile). The reactivity of these aromatic compounds predominantly depends on the electron density of the aromatic ring. Compounds with higher electron density react more readily with electrophiles, leading to faster reaction rates. Conversely, when electron density is lowered due to certain substituents, the reactivity reduces, affecting the speed and outcome of the reaction.

Electron-Donating Groups

Electron-donating groups (EDGs) are vital in modulating the reactivity of aromatic compounds by increasing electron density. These groups donate electron density through resonance or inductive effects, enhancing the nucleophilic character of the benzene ring. Common electron-donating groups include alkyl groups, such as the methyl group in toluene.

• Alkyl groups can donate electrons inductively, offsetting the natural electron deficiency in the aromatic ring.
• This donation stabilizes the intermediate carbocation formed during electrophilic substitution reactions, increasing the overall reaction rate.
• Other electron-donating groups include methoxy (-OCH₃) and amino (-NH₂) groups, significant for reactions like nitration, sulfonation, and halogenation.

Incorporating these groups typically enhances reactivity, making the compound more susceptible to electrophilic attack. Therefore, aromatic compounds with EDGs generally react faster in electrophilic aromatic substitution reactions.

Electron-Withdrawing Groups

Contrary to electron-donating groups, electron-withdrawing groups (EWGs) decrease the reactivity of aromatic compounds by reducing electron density in the benzene ring. This reduction occurs either through resonance or inductive effects. For example, nitro (NO₂) groups are potent electron-withdrawing groups that significantly reduce the reactivity of the aromatic ring.

• The nitro group's electronegative nitrogen strongly pulls electron density away, destabilizing the intermediate carbocation.
• This destabilization results in slower reaction rates due to the decreased likelihood of successful electrophilic attack.
• Other electron-withdrawing groups include halogens, carbonyls, and cyano groups, which typically deactivate the benzene ring to varying extents.

Consequently, the presence of EWGs makes electrophilic aromatic substitution reactions less feasible, requiring harsher conditions or resulting in lower yields.

SMILES Notation

The Simplified Molecular Input Line Entry System (SMILES) is a powerful notation to convey complex molecular structures in a linear textual form. SMILES is used widely in cheminformatics to represent and enable database searches of chemical compounds.

• SMILES strings represent molecules by encoding atoms and bonds in a plain-text format.
• For instance, the SMILES string "c1ccccc1" represents benzene, a simple aromatic compound consisting of six carbon atoms in a ring and alternating double bonds.
• To represent toluene, "Cc1ccccc1" is used, where the "C" outside the ring represents the methyl substituent.

Understanding SMILES helps in interpreting chemical structures effectively and efficiently, especially important in database searching, visualization, and computational studies.

Chemical Reactivity Order

Determining the order of chemical reactivity is crucial in predicting the behavior of compounds during reactions. This order often depends on the substituents present on the aromatic ring, which either enhance or inhibit the reactivity.

• The reaction dynamics center around the nature of these substituents: electron-donating or electron-withdrawing.
• For example, the reactivity order of the compounds from the exercise can be explained based on these effects: - Toluene, with a methyl group, increases reactivity the most. - Benzene, without any substituent groups, serves as a baseline. - Chlorobenzene has a chlorine atom that slightly reduces reactivity due to its mild electron-withdrawing effect. - Nitrobenzene, with a strong electron-withdrawing nitro group, is the least reactive.

Recognizing the reactivity order through analyzing substituents is essential for predicting outcomes in organic reactions, ensuring successful planning and execution in synthetic chemistry.