Question

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

Short answer

Salicylic acid (b) is the major product.

Step-by-step solution

Identify the Reaction

When phenol reacts with sodium hydroxide (NaOH) and carbon dioxide (CO2), it undergoes the Kolbe-Schmitt reaction. This is a well-known carboxylation reaction used to produce aromatic acids.

Understand the Process

In the Kolbe-Schmitt reaction, phenol is first converted to sodium phenoxide by sodium hydroxide. Then, carbon dioxide reacts with sodium phenoxide to form salicylic acid once the solution is acidified. The specific reaction is as follows: \( \text{C}_6\text{H}_5\text{ONa} + \text{CO}_2 \xrightarrow{\text{Heat}} \text{HOC}_6\text{H}_4\text{COONa} \rightarrow \text{HOC}_6\text{H}_4\text{COOH} \).

Key concepts

Carboxylation Reaction

A carboxylation reaction is a type of organic chemical reaction that introduces a carboxyl group (-COOH) into a molecule. This reaction is fundamental in organic chemistry and biochemistry for creating carboxylic acids. It often involves the reaction of a substrate with carbon dioxide (CO₂).

In the Kolbe-Schmitt reaction, specifically, carbon dioxide is added to an aromatic compound. Phenol is converted into an intermediate called sodium phenoxide before reacting with CO₂. The presence of heat facilitates the reaction, leading to the formation of a carboxylate salt. This intermediate is subsequently acidified to form the desired acid product. In simple terms, the added CO₂ becomes part of the compound, creating a new carboxylic acid.

Such reactions are essential in industrial and laboratory settings to synthesize various aromatic carboxylic acids, which are crucial in pharmaceuticals and other chemical industries. Carboxylation reactions like the Kolbe-Schmitt reaction are lauded for their efficiency in modifying molecules to create more complex compounds with useful properties.

Phenol and Sodium Hydroxide

Phenol, a fairly acidic compound, can react with bases to form phenolates. When phenol (\(\text{C}_6\text{H}_5\text{OH}\)) is treated with sodium hydroxide (\(\text{NaOH}\)), a strong base, a salt called sodium phenoxide (\(\text{C}_6\text{H}_5\text{ONa}\)) is formed.

This reaction is a key preparatory step in the Kolbe-Schmitt process. In this context, sodium hydroxide deprotonates phenol, which increases the nucleophilicity of the resulting phenoxide ion. This means the phenoxide ion is now more chemically reactive, particularly capable of interacting with carbon dioxide (CO₂) in a subsequent step.

The importance of sodium phenoxide in the reaction lies in its ability to act as a reactive intermediate that facilitates the addition of a carboxylic group to the aromatic ring, specifically at the ortho position relative to the hydroxyl group. Phenol and its interactions with bases are fundamental in organic synthetic pathways to create more advanced materials.

Salicylic Acid Production

Salicylic acid is a significant product of the Kolbe-Schmitt reaction, widely appreciated for its use in medicine and skincare. This aromatic acid is traditionally derived from the carboxylation of phenolic compounds and plays a key role in producing other valuable compounds.

The process begins with the transformation of phenol into sodium phenoxide by reacting with sodium hydroxide. Upon heating with carbon dioxide (\(\text{CO}_2\)), the sodium phenoxide undergoes carboxylation, leading to the formation of an intermediate sodium salicylate. Further acidification converts this intermediate into salicylic acid (\(\text{HOC}_6\text{H}_4\text{COOH}\)).

Salicylic acid is notable for being the precursor to acetylsalicylic acid, commonly known as aspirin, a well-known analgesic and anti-inflammatory drug. In addition to its pharmaceutical applications, salicylic acid is employed in the cosmetic industry for its ability to gently exfoliate skin and treat acne. The production method stemming from the Kolbe-Schmitt reaction remains a robust and efficient pathway in industrial chemistry today.