Question

How will you convert benzene into (i) \(\quad p\) -nitrobromobenzene (ii) \(m\) - nitrochlorobenzene (iii) \(p-\) nitrotoluene (iv) acetophenone?

Short answer

React benzene through nitration, followed by different halogenation or alkylation steps for each compound.

Step-by-step solution

Nitration of Benzene

Begin with benzene ( C_6H_6 ) and perform a nitration reaction. Treat benzene with a mixture of concentrated nitric acid ( HNO_3 ) and sulfuric acid ( H_2SO_4 ) at a temperature of about 50°C. This introduces a nitro group ( NO_2 ) on the benzene ring to form nitrobenzene ( C_6H_5NO_2 ).

Halogenation of Nitrobenzene for p-nitrobromobenzene

To produce p-nitrobromobenzene, you need to perform a bromination on nitrobenzene. Use bromine ( Br_2 ) in the presence of iron (III) bromide ( FeBr_3 ) as a catalyst. Since the nitro group is meta-directing but p-orientation is needed, bromination will mainly occur at the para position relative to the nitro group due to sterics, resulting in p-nitrobromobenzene.

Halogenation of Nitrobenzene for m-nitrochlorobenzene

To form m-nitrochlorobenzene, chlorination is required. Treat nitrobenzene with chlorine ( Cl_2 ) in the presence of ferric chloride ( FeCl_3 ). The nitro group is a meta-directing group, which guides the chlorine to the meta position, producing m-nitrochlorobenzene.

Alkylation of Nitrobenzene for p-nitrotoluene

Next, produce p-nitrotoluene from nitrobenzene using Friedel-Crafts alkylation. React nitrobenzene with methyl chloride ( CH_3Cl ) in the presence of aluminum chloride ( AlCl_3 ). Since the nitro group is meta-directing, but the bulky substituent drives p-addition, you end up with p-nitrotoluene after purification.

Acetylation of Benzene

For the preparation of acetophenone, perform a Friedel-Crafts acylation on benzene. Use acetyl chloride ( CH_3COCl ) along with aluminum chloride ( AlCl_3 ) as the catalyst. The reaction will introduce an acyl group ( R-CO ) on the benzene ring, forming acetophenone.

Key concepts

Nitration

Nitration is a chemical process used to introduce a nitro group (\(NO_2\)) into an organic compound. For benzene, this process involves the substitution of a hydrogen atom on the ring with a nitro group. This transformation occurs by treating benzene (\(C_6H_6\)) with concentrated nitric acid (\(HNO_3\)) in the presence of sulfuric acid (\(H_2SO_4\)), which acts as a catalyst.
Nitric acid provides the nitronium ion (\(NO_2^+\)) necessary for the reaction, while sulfuric acid facilitates the generation of this ion and enhances the yield.
The mixture is typically heated to about 50°C, ensuring the reaction's effectiveness without causing decomposition of the benzene or the acids. The result of this process is nitrobenzene (\(C_6H_5NO_2\)), which can serve as a precursor for further chemical reactions.

• Nitration introduces a strong electron-withdrawing group.
• This affects future reactions by directing new substituents primarily to the meta position.

Halogenation

Halogenation refers to the introduction of a halogen atom into an organic compound, such as benzene. In this context, we're looking specifically at the addition of bromine or chlorine to nitrobenzene.
The procedure requires the presence of a halogen (either \(Br_2\) or \(Cl_2\)) and an appropriate catalyst. For bromination, iron (III) bromide (\(FeBr_3\)) acts as the catalyst. For chlorination, ferric chloride (\(FeCl_3\) is used.
In both cases, the nitro group on the benzene ring is meta-directing, meaning it typically guides the halogen to the meta position relative to itself. However, steric effects can allow some freedom for para-positioning with bromination due to the bulkiness. This results in products like \(p\)-nitrobromobenzene and \(m\)-nitrochlorobenzene.

• Halogenation requires careful control of conditions to achieve the desired positional selectivity.
• Meta-directing groups influence the halogenation outcome significantly.

Friedel-Crafts Alkylation

Friedel-Crafts alkylation is an electrophilic aromatic substitution that introduces an alkyl group onto an aromatic ring. This process involves the reaction of an alkyl halide with benzene in the presence of a strong Lewis acid catalyst like aluminum chloride (\(AlCl_3\)).
In the case of forming \(p\)-nitrotoluene from nitrobenzene, the alkylating agent is methyl chloride (\(CH_3Cl\)). The nitro group is inherently meta-directing, yet due to steric effects from the bulky new group and possible rearrangements, para-substitution can occur after purification.
This reaction forms the toluene derivative, \(p\)-nitrotoluene.

• Alkylation can lead to further reactions if careful conditions are not maintained.
• It is crucial to consider the influence of existing groups on the aromatic ring during the reaction.

Friedel-Crafts Acylation

Friedel-Crafts acylation, similar to alkylation, involves an electrophilic substitution on an aromatic ring. However, this process introduces an acyl group, such as \(R-CO\). The reaction requires an acyl halide and a Lewis acid catalyst, typically aluminum chloride (\(AlCl_3\)).
When benzene undergoes acylation with acetyl chloride (\(CH_3COCl\)), acetophenone is formed. The acyl group attaches to the aromatic ring, substituting a hydrogen atom with a ketone functional group.
This process is advantageous because the introduced acyl group is less prone to further reaction, making it more stable compared to alkyl substituents.

• Acylation decreases the ring's reactivity, reducing the likelihood of multiple substitutions in one reaction step.
• The process also produces ketones, which are useful in various chemical syntheses.