Problem 87 - A Complete Resource Book in Chemistry for JEE Main

Question

Identify the product in the following sequence 3,4,5-Tribromoaniline \(\frac{\text { (i) diazotization }}{\text { (ii) } \mathrm{H}_{3} \mathrm{PO}_{2}}\) ? (a) \(3,4,5\)-Tribromobenzene (b) \(3,4,5\)-Tribromonitrobenzene (c) \(2,4,6\)-Tribromobenzene (d) \(1,2,3\)-Tribromobenzene

Step-by-Step Solution

Answer

The product is (a) 3,4,5-Tribromobenzene.

Step 1: Understand the Starting Compound

The compound given is 3,4,5-Tribromoaniline. This means it is an aniline derivative where the amino group (\(-NH_2\)) is attached to a benzene ring, and the benzene ring is substituted with bromine atoms at positions 3, 4, and 5.

Step 2: First Reaction - Diazotization

Diazotization is the process where an amino group in an aromatic amine is converted into a diazonium salt. This is typically carried out under cold conditions using nitrous acid (\(HNO_2\)), often generated in situ from sodium nitrite (\(NaNO_2\)) and an acid. So, 3,4,5-tribromoaniline upon diazotization will form 3,4,5-tribromo benzene diazonium salt.

Step 3: Second Reaction - Reaction with H3PO2

The diazonium salt formed in the previous step is now reacted with hypophosphorous acid (\(H_3PO_2\)). This specific reaction is known as reductive deamination where the diazonium group is replaced by a hydrogen atom, resulting in the removal of the amino group. Thus, 3,4,5-tribromo benzene diazonium salt converts to 3,4,5-tribromobenzene.

Step 4: Identify the Product in the Sequence

The final product after both reactions is 3,4,5-tribromobenzene, meaning the diazonium group has been replaced by a hydrogen atom at the amino position.

Key Concepts

Diazotization

Diazotization is a fascinating chemical reaction that plays an important role in organic chemistry, especially when dealing with aromatic amines. This process involves converting an aromatic amine into a diazonium salt. These salts are incredibly useful intermediates in a variety of organic reactions.

During diazotization, an amine group (-NH_2) in an aromatic compound is converted to a diazonium group (-N_2^+). This transformation typically occurs under cold conditions to stabilize the diazonium salt. For this reaction, nitrous acid (HNO_2) is usually generated in situ by mixing sodium nitrite (NaNO_2) with a mineral acid.

• This process is crucial because diazonium salts are versatile intermediates; they can readily undergo substitution reactions where the diazonium group is replaced by various other groups.
• Wide applications in the synthesis of dyes and pigments, as well as in drug manufacture.

The spontaneous conversion to diazonium salts is particularly vital in aromatic compounds, helping further manipulate the structure and function of these chemicals in industrial settings.

Reductive Deamination

Reductive deamination is a chemical transformation that follows diazotization in the sequence of reactions. After the formation of a diazonium salt, further reactions can replace the diazonium group with a hydrogen atom, effectively removing the amine group from the original compound.

This reaction is achieved using hypophosphorous acid (H_3PO_2) as the reducing agent. Through reductive deamination, the diazonium group undergoes a series of electron transfer steps that lead to its replacement by hydrogen, frequently simplifying the molecular structure.

• This process is pivotal because it completes the transition from an amine to a hydrocarbon, replacing functional groups with very stable and unreactive hydrogen atoms.
• Reductive deamination is utilized not only in laboratory settings but also in large-scale industrial processes.

It represents a powerful method for altering aromatic compounds, making it possible to synthesize various derivatives from common starting materials efficiently and with high specificity.

Aromatic Compounds

Aromatic compounds are a special class of organic molecules characterized by their stabile ring-like structure, where electrons are delocalized over a series of atoms, typically carbon atoms, forming a benzene-like ring. These compounds play a central role in organic chemistry due to their unique stability and reactivity.

One of the most interesting phenomena associated with aromatic compounds is their ability to undergo electrophilic substitution reactions, a process involving the substitution of hydrogen atoms on the aromatic ring with other functional groups.

• This delocalization of electrons gives aromatic compounds exceptional stability, known as 'aromatic stability.'
• While benzene is the simplest of aromatic compounds, there are numerous derivatives like toluene, phenol, and aniline that demonstrate varied chemical properties and uses.

In the context of the reaction sequence studied, the understanding of aromatic chemistry is essential. We observe how modifications of the aromatic structure enable the creation of new materials with potentially valuable properties, showcasing the versatility and importance of this class of organic compounds in both natural and synthetic chemistry.