Question

Draw a structural formula for the principal product formed when benzamide is treated with cach reagent. (a) \(\mathrm{H}_{2} \mathrm{O}, \mathrm{HCl}\), heat (b) \(\mathrm{NaOH}, \mathrm{H}_{2} \mathrm{O}\), heat (c) \(\mathrm{LiAlH}_{4}\), then \(\mathrm{H}_{2} \mathrm{O}\)

Short answer

Answer: The principal products of the reactions are (a) benzoic acid, (b) sodium benzoate, and (c) aniline.

Step-by-step solution

Identify the functional group of benzamide

Benzamide is an organic compound with the molecular formula \(\mathrm{C}_{7} \mathrm{H}_{7} \mathrm{NO}\). It contains a benzene ring and an amide functional group. The structural formula of benzamide can be written as \(\mathrm{C}_{6} \mathrm{H}_{5} - \mathrm{CONH}_{2}\). Step 2: Reactions of benzamide with each reagent

Determine the reactions of benzamide with each reagent

(a) \(\mathrm{H}_{2} \mathrm{O}, \mathrm{HCl},\) heat: Amides react with acids and heat to form carboxylic acids. So, benzamide reacts with \(\mathrm{H}_{2} \mathrm{O}, \mathrm{HCl},\) heat to form benzoic acid. (b) \(\mathrm{NaOH}, \mathrm{H}_{2} \mathrm{O},\) heat: Amides react with bases and heat to form carboxylate salts. So, benzamide reacts with \(\mathrm{NaOH}, \mathrm{H}_{2} \mathrm{O},\) heat to form sodium benzoate. (c) \(\mathrm{LiAlH}_{4}\), then \(\mathrm{H}_{2} \mathrm{O}\): Amides react with lithium aluminum hydride (\(\mathrm{LiAlH}_{4}\)) to form amines. So, benzamide reacts with \(\mathrm{LiAlH}_{4}\), then \(\mathrm{H}_{2} \mathrm{O}\) to form aniline. Step 3: Drawing structural formulas for each product

Draw the structural formula for the products

(a) Benzoic acid: The principal product formed when benzamide reacts with \(\mathrm{H}_{2} \mathrm{O}, \mathrm{HCl},\) heat is benzoic acid. The structural formula for benzoic acid is \(\mathrm{C}_{6} \mathrm{H}_{5} - \mathrm{COOH}\). (b) Sodium benzoate: The principal product formed when benzamide reacts with \(\mathrm{NaOH}, \mathrm{H}_{2} \mathrm{O},\) heat is sodium benzoate. The structural formula for sodium benzoate is \(\mathrm{C}_{6} \mathrm{H}_{5} - \mathrm{COO}^{-} \mathrm{Na}^{+}\). (c) Aniline: The principal product formed when benzamide reacts with \(\mathrm{LiAlH}_{4}\), then \(\mathrm{H}_{2} \mathrm{O}\) is aniline. The structural formula for aniline is \(\mathrm{C}_{6} \mathrm{H}_{5} - \mathrm{NH}_{2}\).

Key concepts

Amide Functional Group

Understanding the amide functional group is pivotal in organic chemistry. This group consists of a carbonyl carbon double-bonded to an oxygen atom and single-bonded to a nitrogen atom, often denoted as -CONH₂. Amides exhibit properties distinct from other carbonyl-containing functional groups due to the presence of nitrogen.

Benzamide, as an example, contains a benzene ring attached directly to an amide group. This structural arrangement influences its chemical reactivity, allowing it to undergo several reactions such as hydrolysis and reduction. Knowing the behavior of the amide group helps us to predict the outcomes when benzamide is introduced to different reagents.

Carboxylic Acid Formation

Carboxylic acids are organic compounds bearing the -COOH functional group. Converting amides to carboxylic acids is a process known as hydrolysis, which can be acid-catalyzed, as seen when benzamide is heated in the presence of water and hydrochloric acid.

The resulting product, benzoic acid, carries the quintessential carboxylic acid structure with the formula C₆H₅-COOH. Understanding the conditions needed for this transformation — specifically the use of acids and heat — is key for students who may encounter similar reactions with different amides in both laboratory settings and theoretical problems.

Carboxylate Salt Synthesis

Synthesis of carboxylate salts is another interesting reaction of amides under alkaline conditions. When benzamide interacts with sodium hydroxide and water upon heating, a reaction takes place that strips the amide of its amine group, replacing it with an oxygen atom to form a carboxylate group.

The end product is sodium benzoate, consisting of a sodium ion, Na⁺, and a benzoate anion, C₆H₅-COO^-. This type of reaction exemplifies how the acidic proton of the carboxylic acid is replaced by a metal cation, creating a salt. Recognizing the role of the base in facilitating this substitution is critical for predicting the outcome of base-promoted amide reactions.

Amine Synthesis from Amides

The transformation of amides to amines is a significant type of chemical reaction that can be achieved using reducing agents such as lithium aluminum hydride (LiAlH₄). In this reaction, the amide's carbonyl group is reduced, and the nitrogen retains two of its hydrogen atoms, leading to the creation of an amine.

When benzamide is treated with LiAlH₄ followed by water, we get aniline, a primary amine where the benzene ring is directly bonded to an NH₂ group. This reaction is crucial for students recognizing the powerful reducing abilities of LiAlH₄ in converting amides into amines, a fundamental step in synthetic organic chemistry.