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Chapter 12: Problem 64

What reagents and/or catalysts are necessary to carry out each conversion? (a) Benzene to nitrobenzene (b) 1,4 -Dichlorobenzene to 2 -bromo-1,4-dichlorobenzene (c) Benzene to aniline

Short Answer

Expert verified
(a) Nitric acid and sulfuric acid; (b) Bromine and iron(III) bromide; (c) Nitric acid, sulfuric acid, and reduction with tin and HCl or hydrogen.

Step by step solution

01

Benzene to Nitrobenzene

To convert benzene to nitrobenzene, a nitration reaction is needed. This involves using a mixture of concentrated nitric acid ( HNO_3 ) and concentrated sulfuric acid ( H_2SO_4 ) as the catalyst. The sulfuric acid acts as a catalyst to generate the nitronium ion ( NO_2^+ ), which is the active electrophile that reacts with benzene.
02

1,4-Dichlorobenzene to 2-Bromo-1,4-Dichlorobenzene

To convert 1,4-dichlorobenzene to 2-bromo-1,4-dichlorobenzene, a bromination reaction is required. This can be achieved using bromine ( Br_2 ) in the presence of a Lewis acid catalyst such as iron(III) bromide ( FeBr_3 ) or iron powder. The catalyst helps in generating the bromonium ion, which facilitates the electrophilic substitution on the benzene ring.
03

Benzene to Aniline

Converting benzene to aniline involves two main steps. First, benzene is nitrated to form nitrobenzene using concentrated nitric acid and sulfuric acid, as described in Step 1. Then, nitrobenzene is reduced to aniline. The reduction is typically carried out using a reagent such as tin ( Sn ) and hydrochloric acid ( HCl ) or using hydrogen gas ( H_2 ) with a metal catalyst like palladium or platinum.

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Key Concepts

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

Nitration Reaction
The nitration reaction is an essential process in organic chemistry where a nitro group is introduced into an aromatic compound. This specific reaction involves benzene and is a classic example of electrophilic aromatic substitution.

To perform the nitration of benzene, a mixture of concentrated nitric acid \((HNO_3)\) and concentrated sulfuric acid \((H_2SO_4)\) is used. \(H_2SO_4\) acts as a catalyst and helps in the formation of the nitronium ion \((NO_2^+)\). This ion is the active electrophile which attacks the benzene ring.

Key points:
  • During nitration, the electrons from the benzene ring bond with the positively charged nitronium ion.
  • This results in the formation of nitrobenzene and the re-aromatization of the ring, with water and sulfuric acid being regenerated.
  • Temperature control is crucial as excessive heat can lead to multiple substitutions, rendering the reaction inefficient.
Bromination Reaction
The bromination reaction is crucial for adding a bromine atom to an aromatic compound such as benzene. This is an electrophilic substitution reaction, much like nitration, but it involves bromine \((Br_2)\) and a catalyst.

For brominating 1,4-dichlorobenzene to 2-bromo-1,4-dichlorobenzene, bromine is used in the presence of a Lewis acid catalyst, such as iron(III) bromide \((FeBr_3)\) or iron powder. The catalyst helps generate a bromonium ion, which serves as the electrophile in the reaction.

Highlights:
  • Initially, \(Br_2\) interacts with \(FeBr_3\), creating a more positively charged species, the bromonium ion.
  • This ion facilitates the electrophilic attack on the aromatic ring of 1,4-dichlorobenzene, allowing a bromine atom to substitute a hydrogen atom.
  • Iron acts as a catalyst and is regenerated at the end of the reaction.
Reduction of Nitrobenzene
The transformation from nitrobenzene to aniline is a type of reduction reaction essential in the synthesis of many aromatic amines. The process involves reducing the nitro group \((NO_2)\) of nitrobenzene to the amino group \((NH_2)\), forming aniline.

There are a few methods to achieve this reduction, but a common approach involves using tin \((Sn)\) and hydrochloric acid \((HCl)\). Another method is to use hydrogen gas \((H_2)\) in the presence of a metal catalyst like palladium or platinum.

Important points:
  • In the \(Sn/HCl\) method, nitrobenzene reacts to form aniline and water as by-products.
  • For catalytic hydrogenation, the hydrogen gas adds across the \(NO_2\) group with the help of a metal catalyst, transforming it into \(NH_2\).
  • The choice of reduction method often depends on the scale of the reaction and available resources.

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Most popular questions from this chapter

Answer true or false. (a) Benzene does not undergo the addition reactions that are characteristic of alkenes. (b) A defining feature of aromatic compounds is that they are highly unsaturated but do not undergo characteristic alkene addition reactions. (c) Nitration of benzene adds a \(-\mathrm{NO}_{2}\) group to one of the carbons of the aromatic ring. (d) Halogenation of an alkene is an addition reaction; halogenation of an arene is a substitution reaction.

In Chapter 20 on the biochemistry of lipids, we will study the three long- chain unsaturated carboxylic acids shown below. Each has 18 carbons and is a component of animal fats, vegetable oils, and biological membranes. Because they consist of a carboxylic acid group on one end and a fatty hydrocarbon chain on the other, they are called fatty acids. How many cis/trans isomers are possible for each? $$\begin{array}{c} \text { Oleic acid } \mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{7} \mathrm{CH}=\mathrm{CH}\left(\mathrm{CH}_{2}\right)_{7} \mathrm{COOH} \\ \text { Linoleic acid } \mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{4}\left(\mathrm{CH}=\mathrm{CHCH}_{2}\right)_{2}\left(\mathrm{CH}_{2}\right)_{6} \mathrm{COOH} \\ \text { Linolenic acid } \mathrm{CH}_{3} \mathrm{CH}_{2}\left(\mathrm{CH}=\mathrm{CHCH}_{2}\right)_{3}\left(\mathrm{CH}_{2}\right)_{6} \mathrm{COOH} \end{array}$$

Draw a structural formula for an alkene with the molecular formula \(\mathrm{C}_{5} \mathrm{H}_{10}\) that reacts with \(\mathrm{Br}_{2}\) to give each product.

(Chemical Connections 12 B) In recycling codes, what do these abbreviations stand for? (a) \(\mathrm{V}\) (b) \(\mathrm{PP}\) (c) PS

Write a structural formula and the name for the simplest (a) alkane, (b) alkene, (c) alkyne, and (d) aromatic hydrocarbon.
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