Question

Benzene is converted into para-nitrophenol by using reagents: (I) dil. \(\mathrm{NaOH}\) (II) \(\mathrm{HNO}_{3} / \mathrm{H}_{2} \mathrm{SO}_{4}\) (III) \(\mathrm{Br}_{2} / \mathrm{Fe}\) The most suitable sequence of these reagents are respectively (A) I, II , III (B) I, III , II (C) II, III, I (D) III, II, I

Short answer

The short answer to the question is: \(\text{(D) III, II, I}\).

Step-by-step solution

Identify the reactions with the given reagents

Let's first list the reactions associated with the given reagents: (I) dil. NaOH: It's an ortho-para directing reagent, would not affect benzene ring directly. (II) HNO3 / H2SO4: Nitration of benzene, which would introduce a nitro group (-NO2) to the benzene ring forming Nitrobenzene. (III) Br2 / Fe: Bromination of benzene, which would introduce a bromine atom (Br) to the benzene ring forming Bromobenzene.

Determine the initial reagent to introduce the first substituent

We need to start by introducing either a bromine or a nitro group onto the benzene ring. Since the desired product is para-nitrophenol, we should start with the nitration reaction using HNO3 / H2SO4 as the initial reagent. After this step, we will have nitrobenzene.

Introduce the second substituent

Next, we need to introduce a bromine atom ortho or para to the existing nitro group. We use Br2 / Fe as the second reagent, which will result in the formation of 1-bromo-4-nitrobenzene (para-bromonitrobenzene).

Convert the nitro group to phenol

Finally, to convert the nitro group to phenol, we use dilute NaOH to reduce the nitro group to the corresponding amine group and then undergo diazotisation reaction followed by hydrolysis to give the desired para-nitrophenol. Therefore, the correct sequence of reagents is: 1. HNO3 / H2SO4 2. Br2 / Fe 3. dil. NaOH The correct option is (D) III, II, I.

Key concepts

Nitration of Benzene

Nitration of benzene is a crucial organic chemistry reaction where a nitro group ( O2) is introduced into the benzene ring. This process is achieved using a mixture of concentrated nitric acid ( O3) and sulfuric acid ( 2SO4). The sulfuric acid acts as a catalyst to produce the nitronium ion ( O2^+), which is the active nitrating species.

The reaction is as follows: - Benzene undergoes an electrophilic aromatic substitution reaction with the nitronium ion. - This leads to the formation of nitrobenzene where the O2 group replaces a hydrogen atom in the benzene ring.

The electrophilic aromatic substitution is a two-step process that involves:

• Formation of a sigma complex, an intermediate where the benzene ring becomes temporarily non-aromatic as it forms a bond with the nitronium ion.
• Restoration of aromaticity by loss of a hydrogen ion, yielding nitrobenzene.

This reaction is the first step in synthesizing para-nitrophenol, setting up the benzene ring for further substitution.

Bromination of Benzene

Bromination of benzene involves the introduction of a bromine atom ( 2) into the benzene ring. This reaction requires a catalyst, typically iron (Fe) or ferric bromide ( 3), to generate the bromine cation, +. The benzene then undergoes a similar electrophilic aromatic substitution as observed in nitration.

The process begins with:

• The generation of the bromine cation in the presence of Fe, which is a crucial step as bromine itself is not sufficiently reactive alone.
• The benzene ring interacts with the bromine cation, forming a sigma complex where the + substitutes a hydrogen atom on the benzene ring.
• This intermediate then returns to its aromatic state upon loss of the hydrogen ion, forming bromobenzene.

The outcome of this reaction is the essential para-bromonitrobenzene intermediate, as it places the bromine ortho or para to the previously added O2 group, which is necessary for the desired synthesis pathway toward para-nitrophenol.

Diazotisation and Hydrolysis

Diazotisation is a chemical reaction where an amine group ( H2) from the nitro group is converted into a diazonium salt. This transformation is essential to advance the synthesis of para-nitrophenol.

The steps involved include:

• First, the nitro group (-NO2) is reduced to an amine group ( H2) using a reducing agent such as dil. OH. This must be done before diazotisation can occur.
• The amine group is then treated with nitrous acid ( HO2), formed in situ from sodium nitrite and hydrochloric acid, to give the diazonium salt.
• The diazonium salt is unstable and undergoes hydrolysis, where it reacts with water to produce phenol.

This diazotisation followed by hydrolysis effectively converts the initially formed nitro group into an alcohol group — precisely what we need to achieve para-nitrophenol. This final stage completes the transformation starting from benzene to para-nitrophenol, an important compound in various chemical applications.