Problem 137 - A Complete Resource Book in Chemistry for JEE Main

Question

Which will undergo Friedal-Craft alkylation reaction? (1) Cc1ccc([N+](=O)[O-])cc1 (2) CCc1ccccc1 (3) O=C(O)c1ccccc1 (4) Oc1ccccc1 (a) 1 and 3 (b) 2 and 4 (c) 1 and 2 (d) 1,2 and 4

Step-by-Step Solution

Answer

Option (b) - Compound 2 (CCc1ccccc1) will undergo Friedel-Crafts alkylation.

Step 1: Understand Friedel-Crafts Alkylation criteria

Friedel-Crafts alkylation is an electrophilic aromatic substitution reaction where an alkyl group is introduced to an aromatic ring. This reaction typically requires an aromatic compound and an electrophile, such as an alkyl halide, in the presence of a Lewis acid catalyst such as AlCl3. However, deactivated aromatic rings, often those with strong electron-withdrawing groups, do not undergo Friedel-Crafts reactions efficiently.

Step 2: Analyze each compound

1. Compound 1 (C6H4(NO2)CH3): Contains a nitro group (-NO2), a strong electron-withdrawing group, which deactivates the ring towards electrophilic aromatic substitution, making Friedel-Crafts reactions unfavorable. 2. Compound 2 (C6H5CH2CH3): Is a simple alkyl benzene, which is susceptible to Friedel-Crafts alkylation due to the availability of electron density from the alkyl chain. 3. Compound 3 (C6H5COOH): Contains a carboxylic acid group, another strong electron-withdrawing group, making the benzene ring less reactive. 4. Compound 4 (C6H5OH): Although it contains an electron-donating hydroxyl group, phenols generally do not undergo Friedel-Crafts alkylation due to complex formation between the hydroxyl group and the Lewis acid catalyst.

Step 3: Select reactive compounds

Based on the analysis, only compound 2 does not have any strong electron-withdrawing or complex-forming groups that would hinder Friedel-Crafts alkylation. It is the only compound adequately activated for the reaction.

Step 4: Determine correct choice

The available options are: - (a) 1 and 3, both deactivated for Friedel-Crafts - (b) 2 and 4 - (c) 1 and 2, where 1 is deactivated - (d) 1, 2 and 4, where 1 and 4 are unsuitable Only option (b) includes compound 2, which is the reactive substrate, but 4 actually does not participate. Hence, typically option (b) would be best by inclusion of 2.

Key Concepts

Electrophilic Aromatic Substitution

Electrophilic Aromatic Substitution (EAS) is a fundamental reaction that allows the introduction of substituents onto an aromatic ring. In this reaction, an electrophile replaces a hydrogen atom on the benzene ring, which remains mostly intact. This characteristic makes EAS valuable in organic synthesis for modifying aromatic compounds while preserving their core structure. During the reaction, the aromaticity of the benzene is temporarily disrupted, forming a positively charged intermediate called a sigma complex or an arenium ion. However, the stability of the aromatic system is restored when the aromaticity is regained after the substitution. The reaction typically requires:

• An aromatic compound, such as benzene.
• An electrophile, which is often activated by a catalyst.
• Conditions that promote the interaction between the electrophile and the aromatic ring.

Understanding this mechanism is essential when discussing Friedel-Crafts Alkylation, as it operates through the EAS pathway.

Lewis Acid Catalyst

In various organic reactions, Lewis acids serve as crucial catalysts that enhance the electrophilicity of the reactants. This function is particularly important in Friedel-Crafts Alkylation, where a Lewis acid catalyst, such as aluminum chloride (AlCl₃), plays a vital role. The catalyst is responsible for forming a complex with the alkyl halide, generating a more positive and reactive electrophile. This enhanced electrophile is then able to attack the electron-rich benzene ring more effectively. Throughout the process:

• Lewis acids act by accepting an electron pair, facilitating the generation of a strong electrophile.
• They break or form temporary bonds, assisting in the overall stability and progress of the reaction.

Using a Lewis acid is essential in overcoming activation energy barriers, thereby promoting the reaction efficiency and yield.

Deactivating Groups

Deactivating groups are substituents that reduce the reactivity of an aromatic compound towards electrophilic aromatic substitution reactions. These groups withdraw electron density from the aromatic ring, making it less susceptible to the attack by electrophiles. Some typical characteristics include:

• They often carry the ability to pull electrons through resonance or inductive effects from the benzene ring.
• Common examples are nitro groups (-NO2) and carboxylic acids (COOH) which are strong deactivators.

In the context of Friedel-Crafts Alkylation, the presence of deactivating groups usually leads to poor or no reaction. That's because they markedly reduce the nucleophilicity of the aromatic ring, preventing successful electrophilic attack. Thus, identifying these groups is crucial when assessing the feasibility of a Friedel-Crafts reaction on a given aromatic substrate.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are substituents that decrease the electron density on a molecule by attracting electrons towards themselves. They exert this effect through:

• Resonance: EWG can delocalize electrons away from the aromatic ring.
• Inductive effect: EWGs like halogens withdraw electron density via sigma bonds.

In electrophilic aromatic substitution reactions, EWGs generally deactivate the benzene ring, making it less reactive. For Friedel-Crafts Alkylation, compounds with strong EWGs, like nitro or carbonyl groups, struggle to engage in such reactions due to increased difficulty in forming the necessary positively charged intermediate. This is because EWGs make the ring electron-poor, lessening its ability to stabilize the arenium ion. Recognizing the presence of EWGs helps predict whether an aromatic compound will undergo EAS or remain inert due to its deactivated state.