Question

Benzamide on reaction with \(\mathrm{POCl}_{3}\) gives (a) aniline (b) chlorobenzene (c) benzyl amine (d) benzonitrile

Short answer

The product is benzonitrile (d).

Step-by-step solution

Analyze the Given Reactants

We are given benzamide and asked to predict the product when it reacts with \( \mathrm{POCl}_{3} \). Benzamide has the chemical formula \( \text{C}_6\text{H}_5\text{CONH}_2 \). \( \mathrm{POCl}_{3} \) (Phosphorus oxychloride) is a reagent often used for dehydration reactions.

Consider the Typical Reaction

\( \mathrm{POCl}_{3} \) acts as a dehydration agent which can convert amides into nitriles. This reagent removes the elements of water from the amide group (\( \text{CONH}_2 \)), pushing it towards a nitrile (\( \text{CN} \)) functionality. Therefore, benzamide will undergo a transformation into a benzonitrile.

Identify the Reaction Outcome

After understanding the typical role of \( \mathrm{POCl}_{3} \) with amides, we conclude that the reaction will convert the \( \text{CONH}_2 \) group in benzamide to a \( \text{CN} \) group. As a result, benzamide is transformed into benzonitrile with the chemical formula \( \text{C}_6\text{H}_5\text{CN} \).

Select the Correct Option

Based on the analysis, the correct product formed from the reaction between benzamide and \( \mathrm{POCl}_{3} \) is benzonitrile. From the given options (a) aniline, (b) chlorobenzene, (c) benzyl amine, and (d) benzonitrile, the correct answer is (d) benzonitrile.

Key concepts

Benzamide

Benzamide is a chemical compound essential for students to understand, especially those interested in organic chemistry. It is an organic compound with the formula \( \text{C}_6\text{H}_5\text{CONH}_2 \). This compound is part of the amide group, which features a carbonyl group (a \( \text{C}=\text{O} \) double bond) attached to a nitrogen atom. This versatile molecule forms the basis of many reactions in synthetic chemistry.Benzamide is derived from benzoic acid, where the hydroxyl group \( (\text{OH}) \) is replaced by an amine group \( (\text{NH}_2) \). This transformation makes it reactive under various conditions, especially in dehydration reactions, given the presence of the \( \text{CONH}_2 \) group.Some characteristics of benzamide include:

• Being a solid at room temperature with a white crystalline appearance.
• Possessing a melting point of about 130-132°C.
• Having solubility in water and organic solvents like ethanol.

Phosphorus Oxychloride (POCl3)

Phosphorus oxychloride, denoted as \( \mathrm{POCl}_3 \), is a highly reactive chemical widely used in organic synthesis. It serves primarily as a dehydration agent, crucial for transforming amides into nitriles among various other reactions.This compound comprises one phosphorus atom surrounded by one oxygen and three chlorine atoms. The unique combination contributes to its strong oxidizing and dehydrating properties, enabling the removal of water elements from molecules like benzamide.Key features of \( \mathrm{POCl}_3 \) include:

• A liquid at room temperature, facilitating its handling in laboratory settings.
• Having a pungent odor and the ability to cause irritation, requiring careful handling with proper protective equipment.
• Frequently used not only for amide dehydration but also in making esters, particularly aryl and alkyl phosphates.

In the reaction with benzamide, \( \mathrm{POCl}_3 \) effectively drives the conversion to a nitrile by removing water from the \( \text{CONH}_2 \) group, demonstrating its vital role in this transformation.

Benzonitrile Formation

The transformation of benzamide into benzonitrile is an excellent example of a dehydration reaction that illustrates how functional groups can be interconverted through chemistry.Benzonitrile formation involves the conversion of the amide group \( \text{CONH}_2 \) into a nitrile group \( \text{CN} \), facilitated by \( \mathrm{POCl}_3 \). This conversion occurs when \( \mathrm{POCl}_3 \) removes the elements of water (\( \text{H}_2\text{O} \)) from the amide, effectively dehydrating it to form the nitrile.The reaction proceeds as follows:

• The \( \text{CONH}_2 \) group of benzamide first interacts with \( \mathrm{POCl}_3 \).
• This interaction strips away the water components (one molecule of \( \text{H}_2\text{O} \)).
• The remaining atoms rearrange to form benzonitrile \( \text{C}_6\text{H}_5\text{CN} \), characterized by the \( \text{CN} \) group.

Understanding this reaction gives insight into substituent group changes and the nature of chemical transformations. Benzonitrile, with its robust \( \text{CN} \) bond, showcases how altering one functional group to another can affect the chemical properties and reactivity of a compound.