Question

Nitriles, \(\mathrm{R}-\mathrm{C}=\mathrm{N}\), undergo a hydrolysis reaction when heated with aqueous acid. What is the structure of the compound produced by hydrolysis of propanenitrile, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{N}\), if it has IR absorptions at 2500 to \(3100 \mathrm{~cm}^{-1}\) and \(1710 \mathrm{~cm}^{-1}\) and has \(\mathrm{M}^{+}=74 ?\)

Short answer

The structure produced is propanoic acid, CH₃CH₂COOH.

Step-by-step solution

Understand Nitrile Hydrolysis

When a nitrile compound undergoes hydrolysis in the presence of aqueous acid, the nitrile group (-C≡N) is converted into a carboxylic acid group (-COOH). For propanenitrile, this reaction will produce a carboxylic acid.

Determine the Product from Propanenitrile

Propanenitrile molecule, \\[ \text{CH}_3\text{CH}_2\text{C} \equiv \text{N} \]\converts to the carboxylic acid group. Therefore, propanenitrile will be hydrolyzed to form propanoic acid \\( \text{CH}_3\text{CH}_2\text{COOH} \).

Analyze IR Absorptions

IR absorption from 2500 to 3100 cm⁻¹ indicates an O-H stretch typically seen in carboxylic acids. The absorption at 1710 cm⁻¹ is characteristic of the C=O stretch in carboxylic acids. These IR absorptions confirm the presence of a carboxylic acid group in the compound.

Confirm M+ Value

Calculate the molar mass of propanoic acid: \\( 3\times 12.01 + 6\times 1.01 + 2\times 16.00 = 74.08 \) g/mol. This matches the given m/z value of 74, confirming the compound as propanoic acid.

Key concepts

Propanenitrile

Propanenitrile, also known as ethyl cyanide, is an organic compound that belongs to the class of nitriles. Its chemical formula is \( \text{CH}_3\text{CH}_2\text{C} \equiv \text{N} \). The presence of a nitrile group \( (-\text{C} \equiv \text{N}) \) is a defining feature of propanenitrile, making it highly reactive in hydrolysis reactions.
In this process, propanenitrile transforms into its corresponding carboxylic acid when treated with water and an acid catalyst. This transformation involves the breaking of the triple bond \( (\equiv) \) present in the nitrile group and the formation of a new double-bonded oxygen to create a carbonyl group, typical of carboxylic acids.

Carboxylic Acid Formation

The hydrolysis of propanenitrile leads to the formation of a carboxylic acid, specifically propanoic acid. The process occurs when the nitrile undergoes an acid-catalyzed reaction with water. The initial step in this transformation is the addition of a water molecule to the nitrile carbon, followed by rearrangements that convert the carbon-nitrogen triple bond into a carbon-oxygen double bond.
This reaction sequence results in the final product known as propanoic acid \( (\text{CH}_3\text{CH}_2\text{COOH}) \), which is characterized by a carboxyl group \( (-\text{COOH}) \). This group defines carboxylic acids and is responsible for their chemical properties, including their typical reactions and their behavior as acids in solution.
Key points to understand about carboxylic acid formation from nitriles:

• Nitrile hydrolysis requires an acidic environment and water.
• The transformation involves breaking and forming specific bonds to yield the acid product.
• The resulting carboxylic acid exhibits characteristics typical of such compounds, including specific IR absorptions.

IR Spectroscopy

Infrared (IR) spectroscopy is a reliable technique used to identify functional groups within a molecule by observing the specific absorbance of IR radiation. Each functional group exhibits unique vibrational frequencies associated with different bonds.
In the case of propanoic acid, two key absorption regions in the IR spectrum support its identification:

• The broad absorption between 2500 and 3100 cm⁻¹ indicates the presence of an O-H stretch. This is characteristic of the alcohol part of the carboxylic acid group.
• An absorption at 1710 cm⁻¹ is noted for the C=O stretch. This is the distinct signal from the carbonyl component of the carboxylic acid group.

By analyzing these absorption patterns, we can confirm the structure of the compound produced by the hydrolysis of propanenitrile as propanoic acid. Thus, IR spectroscopy is central in validating the transformation from a nitrile to a carboxylic acid.