Problem 131 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

The phenol or phenoxide is not formed by: (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~N}_{2} \mathrm{Cl}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{SO}_{3} \mathrm{Na}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{COOH}\)

Step-by-Step Solution

Answer

Phenol or phenoxide is not formed by \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl} \) (option b).

Step 1: Understand the Reaction Conditions for Phenol Formation

Phenol can be formed by hydrolysis reactions. Generally, benzene derivatives like diazonium salts undergo hydrolysis to form phenol. The key reaction is that diazonium salts \[ \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~N}_{2} \mathrm{Cl} \]can be converted into phenol by treating them with water or dilute acid. This is known as diazotization.

Step 2: Assessing Option (a)

For option (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~N}_{2} \mathrm{Cl}\), this compound is a diazonium salt, which can indeed undergo conversion to form phenol on reaction with water.

Step 3: Assessing Option (b)

For option (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl}\), this is chlorobenzene. Under normal conditions, chlorobenzene does not form phenol as chlorine is a poor leaving group and does not easily undergo hydrolysis to form phenol.

Step 4: Assessing Option (c)

For option (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{SO}_{3} \mathrm{Na}\), sodium benzenesulfonate can be converted to phenol through alkaline hydrolysis.

Step 5: Assessing Option (d)

For option (d) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{COOH}\), this structure represents benzoic acid which does not convert to phenol. Hydrolysis reactions on aromatic carboxylic acids do not yield phenols because the carboxylic group will not break down to release a hydroxyl group.

Step 6: Conclude the Non-forming Compound

Based on the steps above, option (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl}\) is the compound where phenol or phenoxide is not easily formed. The chlorine atom in chlorobenzene does not easily leave to form an alcohol group in benign conditions.

Key Concepts

Diazonium Salts

Diazonium salts are fascinating compounds in chemistry that feature a nitrogen-to-nitrogen triple bond. The general formula is \( ext{R-N}_2^+X^- \), where \( R \) represents an aromatic group, typically phenyl, and \( X^- \) is an anion such as chloride (Cl^-). These salts are vital in synthetic chemistry due to their reactive nature. They form from primary amines by the process of diazotization. To achieve this, an amine is treated with nitrous acid (HNO2), often prepared in situ using sodium nitrite (NaNO2) and an acid like hydrochloric acid (HCl).

In reactions, diazonium salts can undergo a variety of transformations. One particularly important reaction is their hydrolysis to form phenol. When you treat a diazonium salt with water or dilute acids, the diazonium group is replaced by a hydroxyl group (-OH). This reaction is crucial for the synthesis of phenols from amines under mild conditions. This hydrolysis is widely exploited in industrial and laboratory synthesis.

Hydrolysis Reactions

Hydrolysis is a chemical reaction involving the breakage of a bond in a molecule using water. It is a critical mechanism for the formation and conversion of various organic compounds. When dealing with aromatic compounds, hydrolysis can lead to quite interesting outcomes if the substituents and conditions are right. Diazonium salts, for instance, undergo hydrolysis to produce phenols.

The general hydrolysis reaction involves water, but catalysts like acids or bases can significantly enhance the reaction rate. During the hydrolysis of diazonium salts, the \( ext{N}_2^+ \) group is replaced by an -OH group, yielding phenol. This phenomenon serves as an excellent example of how organic compounds can transform under such conditions.

• Consider the influence of different leaving groups: some groups leave easily during hydrolysis, while others do not.
• Environmental conditions, such as presence of acids or bases, can drastically affect the outcome of hydrolysis.

Understanding hydrolysis with respect to different substituents is key to predicting and manipulating organic reactions.

Chlorobenzene

Chlorobenzene is a simple aromatic compound featuring a benzene ring attached to a chlorine atom. Its chemical formula is \( ext{C}_6 ext{H}_5 ext{Cl} \). At first glance, it might seem reactive due to the presence of chlorine, but it's deceptively stable under typical conditions, especially concerning hydrolysis.

The chlorinated benzene does not easily form phenol because the chlorine atom acts as a poor leaving group. Chlorine is strongly bonded to the benzene ring, and there isn't an easy pathway to replace the chlorine with a hydroxyl group in a regular hydrolysis reaction. Usually, extreme conditions like high temperatures or the use of specific catalysts are needed to achieve this transformation, which is not feasible in most situations.

• Chlorobenzene's stability is due to the partial double bond character in its C-Cl bond, making it non-reactive to hydrolysis.
• Its resistance to conversion into phenol highlights the importance of the nature of leaving groups in substitution reactions.

This stability accounts for why chlorobenzene is often used in industrial applications where resistance to reaction is valuable.