Problem 211 - A Complete Resource Book in Chemistry for JEE Main

Question

The major product obtained on interaction of phenol with sodium hydroxide and carbon dioxide is: [2009] (a) salicylaldehyde (b) salicylic acid (c) phthalic acid (d) benzoic acid

Step-by-Step Solution

Answer

Salicylic acid is the major product.

Step 1: Understand the Reaction Context

The problem involves phenol, sodium hydroxide, and carbon dioxide, which hints at the Kolbe-Schmitt reaction. This reaction is commonly used to introduce a carboxyl group \((COOH)\) into an aromatic ring orthogonal to a hydroxy group, typically resulting in the production of salicylic acid from phenol.

Step 2: Recall the Kolbe-Schmitt Reaction

In the Kolbe-Schmitt reaction, phenol is first converted into a sodium phenoxide ion \((C_6H_5ONa)\) by reacting with sodium hydroxide. The sodium phenoxide ion is then treated with carbon dioxide \((CO_2)\) under pressure and temperature to form salicylic acid after acidification.

Step 3: Identify the Major Product

The Kolbe-Schmitt reaction specifically yields salicylic acid when phenol reacts with sodium hydroxide and carbon dioxide. This is a named reaction known for synthesizing salicylic acid efficiently. Thus, the major product in this reaction is salicylic acid.

Key Concepts

Salicylic Acid Synthesis

The synthesis of salicylic acid using the Kolbe-Schmitt reaction is a classic example of organic chemistry in action. This method is particularly elegant because it allows the functionalization of an aromatic ring by introducing a carboxyl group (COOH) at a specific position. In this case, salicylic acid is synthesized from phenol, utilizing the unique properties of a carboxylation reaction.

The reaction takes phenol o 🡪 (aromatic alcohol) as a starting material.

• First, phenol is transformed into a sodium salt of phenol, known as sodium phenoxide, by treatment with sodium hydroxide (NaOH).
• The sodium phenoxide ion then reacts with carbon dioxide (CO₂).
• This step is crucial as it forms the intermediate that will ultimately convert into salicylic acid.

Finally, this intermediate is treated with an acid, often achieving the desired salicylic acid through a simple protonation reaction. Salicylic acid is notable for its role as a precursor in aspirin production, highlighting its importance both in industrial and pharmaceutical chemistry.

Phenol Reactions

Phenol is an aromatic compound with a hydroxyl group (-OH) directly attached to the benzene ring, giving it special reactivity patterns.

The hydroxyl group influences how phenol behaves in chemical reactions.

• This group makes phenol more acidic than typical alcohols.
• In reactions, this acidity allows it to form phenoxide ions when treated with a base like sodium hydroxide (NaOH).

Phenol's aromatic nature also plays a role in how it interacts with electrophiles. The benzene ring is activated by the hydroxyl group, making it more reactive compared to benzene itself. This activation enables reactions such as the Kolbe-Schmitt reaction. Furthermore, phenol can undergo nitration and halogenation more readily, showcasing its versatility.

This reactivity is a cornerstone in synthetic chemistry, enabling the creation of various derivatives and serving as a building block for more complex molecules.

Sodium Phenoxide Formation

Sodium phenoxide is an anion formed when phenol reacts with a strong base, such as sodium hydroxide (NaOH).

This reaction illustrates the acidity of phenol, which, despite being less acidic than conventional acids, can donate a proton (H⁺) to form the phenoxide ion.

• The hydroxyl group of phenol (-OH) donates its hydrogen ion while accepting a sodium ion from sodium hydroxide, resulting in the formation of sodium phenoxide.
• This ion is more nucleophilic than phenol itself, making it an excellent intermediate for further reactions, such as with carbon dioxide in the Kolbe-Schmitt reaction.

Sodium phenoxide's ability to form easily and react further highlights why understanding its formation is crucial. It is a key player in many reactions, especially those that involve the introduction of functional groups into the aromatic ring, aiding in the synthesis of valuable organic compounds.