Chapter 3: Problem 127

Write equations for the preparation of the following compounds from benzene. (a) ortho-nitrobenzoic acid or o-nitrobenzoic (b) 1 -bromo-4-trichloromethyl benzene

Short Answer

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Answer: To prepare ortho-nitrobenzoic acid from benzene, the following reactions and reagents are needed: 1. Acylation reaction with acylating agent (CH3COCl) and a Lewis acid catalyst (AlCl3) to form benzoic acid. 2. Nitration reaction with nitration reagent (HNO3/H2SO4) to form ortho-nitrobenzoic acid. To prepare 1-bromo-4-trichloromethyl benzene from benzene, the following reactions and reagents are needed: 1. Chloromethylation reaction with chloromethylation reagent (CH2Cl2) and a Lewis acid catalyst (AlCl3) to form chloromethylbenzene. 2. Bromination reaction with bromination reagent (Br2/AlBr3 or FeBr3) to form 1-bromo-4-trichloromethyl benzene.

Step by step solution

(Preparation of ortho-nitrobenzoic acid from benzene)

To prepare o-nitrobenzoic acid from benzene, we will need to carry out two electrophilic aromatic substitution reactions. In the first reaction, benzene will react with an acylating agent to produce benzoic acid. In the second reaction, benzoic acid will undergo nitration to form the ortho-nitrobenzoic acid. Below is the step-by-step procedure: Step 1: Acylation of Benzene to form Benzoic Acid Benzene + Acylating Agent (\text{CH}_3\text{COCl}) + Lewis Acid Catalyst (\text{AlCl}_3) \rightarrow \text{C}_6\text{H}_5\text{COOH} Step 2: Nitration of Benzoic Acid to form Ortho-nitrobenzoic Acid Benzoic Acid (\text{C}_6\text{H}_5\text{COOH}) + Nitration Reagent (\text{HNO}_3/\text{H}_2\text{SO}_4) \rightarrow \text{o-NO}_2\text{C}_6\text{H}_4\text{COOH} + \text{H}_2\text{O}

(Preparation of 1 -bromo-4-trichloromethyl benzene from benzene)

To prepare 1-bromo-4-trichloromethyl benzene from benzene, we will need to carry out two electrophilic aromatic substitution reactions. In the first reaction, benzene will react with a chloromethylation reagent to produce chloromethylbenzene. In the second reaction, chloromethylbenzene will undergo bromination to form the 1-bromo-4-trichloromethyl benzene. Below is the step-by-step procedure: Step 1: Chloromethylation of Benzene to form Chloromethylbenzene Benzene + Chloromethylation Reagent (\text{CH}_2\text{Cl}_2) + Lewis Acid Catalyst (\text{AlCl}_3) \rightarrow \text{C}_6\text{H}_5\text{CH}_2\text{Cl} Step 2: Bromination of Chloromethylbenzene to form 1-bromo-4-trichloromethyl Benzene Chloromethylbenzene (\text{C}_6\text{H}_5\text{CH}_2\text{Cl}) + Bromination Reagent (\text{Br}_2/\text{AlBr}_3 or \text{FeBr}_3) \rightarrow \text{C}_6\text{H}_4\text{CHBr}_2\text{Cl}

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Benzene Derivatives Preparation

Benzene is a versatile starting material for synthesizing a wide variety of derivatives through electrophilic aromatic substitution (EAS) reactions. The core concept here involves the substitution of a hydrogen atom in the benzene ring with an electrophile, which can introduce various functional groups, such as an acyl, nitro, chloromethyl, or bromo group.

To carry out the substitution, a benzene ring first has to be made more reactive through the interaction with a catalyst, usually a Lewis acid like aluminum chloride (AlCl₃). This step is crucial for the subsequent attachment of a new substituent to the aromatic ring.

The process can be finely tuned to add specific groups at desired positions on the benzene ring.

Acylation of Benzene

In the acylation of benzene, an acyl group (RCO-) is introduced to the aromatic ring. The typical acylating agents are acyl chlorides, such as acetyl chloride (CH₃COCl), and the reaction is facilitated by a Lewis acid catalyst like AlCl₃.

The mechanism involves the formation of a highly reactive acylium ion that acts as the electrophile. This acylium ion interacts with the aromatic ring, resulting in the replacement of a hydrogen atom with the acyl group. Acylation is particularly valuable because it avoids the over-substitution that can occur in alkylation reactions and typically leads to ketone derivatives such as benzoic acid (C₆H₅COOH).

Nitration of Benzoic Acid

Nitration is another common EAS reaction that introduces a nitro group (-NO₂) to the aromatic ring. When benzoic acid undergoes nitration, the addition of the nitro group often occurs at the ortho position due to the carboxylic acid group's directing effect.

The nitration reagent is usually a mixture of nitric acid (HNO₃) and sulfuric acid (H₂SO₄), which produce the nitronium ion (NO₂⁺), the active electrophile in this reaction. Care is needed to control the reaction conditions to prevent the formation of the meta or para isomers unless they are desired.

Chloromethylation of Benzene

Chloromethylation is one of the methods used to introduce a -CH₂Cl group onto the benzene ring, adding both a methylene unit and a chlorine atom. It commonly employs formaldehyde (HCHO) and hydrochloric acid (HCl) or chloromethyl methyl ether (CH₃OCH₂Cl), and a Lewis acid catalyst like AlCl₃.

This reaction involves the formation of a chloromethyl cation that can form an electrophilic bond with the benzene ring. The newly formed chloromethyl group can serve as a reactive handle for further transformations, such as bromination, or can be employed in subsequent functional group conversions.

Bromination of Chloromethylbenzene

Bromination is a straightforward EAS reaction where a bromine atom replaces a hydrogen atom on the aromatic ring. When chloromethylbenzene is treated with bromine (Br₂), in the presence of a catalyst such as aluminum bromide (AlBr₃) or iron(III) bromide (FeBr₃), bromination occurs primarily at the benzene ring positions that are ortho or para to the chloromethyl group.

Due to the presence of the chloromethyl group, which is an activating and ortho/para-directing substituent, the bromine predominantly attaches to the para position relative to it, leading to the formation of compounds like 1-bromo-4-trichloromethyl benzene. This reaction exemplifies the principle that the existing substituents on the benzene ring influence the positions where new substitutions occur.

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Write equations for the preparation of the following compounds from benzene. (a) ortho-nitrobenzoic acid or o-nitrobenzoic (b) 1 -bromo-4-trichloromethyl benzene

Short Answer

Step by step solution

(Preparation of ortho-nitrobenzoic acid from benzene)

(Preparation of 1 -bromo-4-trichloromethyl benzene from benzene)

Key Concepts

Benzene Derivatives Preparation

Acylation of Benzene

Nitration of Benzoic Acid

Chloromethylation of Benzene

Bromination of Chloromethylbenzene

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