Problem 14 - A Complete Resource Book in Chemistry for JEE Main

Question

Concentrated sodium hydroxide and benzaldehyde react to produce (a) cinnamic acid (b) hydrobenzamide (c) benzyl alcohol (d) benzophenone

Step-by-Step Solution

Answer

(a) cinnamic acid

Step 1: Identify the Reaction

When concentrated sodium hydroxide reacts with benzaldehyde, it is an indication of an aldol condensation reaction where benzaldehyde acts as an aldehyde containing only one hydrogen atom to form the enolate.

Step 2: Understand the Reaction Details

In the presence of a strong base like concentrated sodium hydroxide, benzaldehyde undergoes a condensation reaction. Two molecules of benzaldehyde react together in the Aldol Condensation process.

Step 3: Determine the Product Formed

The Aldol Condensation of benzaldehyde in the presence of a strong base results in the formation of an alpha-beta unsaturated acid. This means that cinnamic acid is formed as the final product due to the dehydration of the initial aldol product.

Key Concepts

Sodium Hydroxide

Sodium hydroxide, often called lye or caustic soda, is a commonly used strong base in chemical reactions. It serves as a catalyst, speeding up certain reactions due to its ability to provide hydroxide ions (OH⁻). In organic chemistry, sodium hydroxide is frequently employed in situations where a strong base is required. This is due to its ability to deprotonate compounds, meaning it can remove a hydrogen atom to form an ion.

In the case of the aldol condensation reaction, sodium hydroxide helps in the formation of an enolate ion from an aldehyde such as benzaldehyde. The enolate ion is crucial because it acts as a reactive nucleophile. This means it can attack an electrophilic center in another molecule leading to bond formation between molecules.

• Properties: White solid, soluble in water, and highly caustic.
• Common Uses: Soap making, paper manufacturing, and organic synthesis.

Just remember, handling sodium hydroxide requires care, as it can be corrosive and cause skin burns.

Benzaldehyde

Benzaldehyde is an aromatic aldehyde, characterized by its distinctive almond smell. It plays a significant role in various chemical reactions, including the aldol condensation where it acts as a source of aldehyde groups.

In the presence of a strong base like sodium hydroxide, benzaldehyde contributes to the formation of an enolate ion. This happens when the alpha hydrogen, which is slightly acidic, is abstracted by the base. Subsequently, this enolate ion acts as a nucleophile to bond with another molecule of benzaldehyde.

• Properties: Colorless liquid, almond-like odor, slightly soluble in water.
• Applications: Flavorings, perfumery, and as a precursor to pharmaceuticals.

The reaction continues, allowing the formation of a larger molecule through the addition of these units, ultimately forming the desired product.

Cinnamic Acid

Cinnamic acid is a crucial product of the aldol condensation of benzaldehyde with sodium hydroxide. It is an alpha-beta unsaturated acid, meaning it features a double bond between the alpha and beta carbon atoms next to the carboxylic acid group.

In the context of the reaction, the formation of cinnamic acid involves the linking of two benzaldehyde molecules followed by dehydration. Dehydration is the removal of a water molecule and is critical in transforming the initial aldol product into cinnamic acid. The double bond formed is responsible for the unsaturated nature of the acid, making it highly sought after in different industries.

• Characteristics: White crystalline solid with a cinnamon-like aroma.
• Uses: Flavoring agent, fragrance compound, and in certain pharmaceuticals.

Understanding the formation and properties of cinnamic acid elucidate why it is a prominent example of aldol condensation products.