Problem 19 - Chemistry in 30 days

Question

In a set of reactions, propionic acid yielded a compound ( (D). \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}\) \(\stackrel{\mathrm{SOCl}_{2}}{\longrightarrow}(\mathrm{B}) \stackrel{\mathrm{NH}_{,}}{\longrightarrow}(\mathrm{C}) \stackrel{\mathrm{KOH}, \mathrm{Br}_{\mathrm{t}}}{\longrightarrow}(\mathrm{D})\) The structure of ((D) would be (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NHCH}_{3}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}\) (c) \(\mathrm{CH}_{1} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CONH}_{2}\)

Step-by-Step Solution

Answer

Compound (D) is \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2} \). The correct answer is (b).

Step 1: Convert Propionic Acid to an Acyl Chloride

The reaction of propionic acid \( \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{COOH} \) with \( \mathrm{SOCl}_{2} \) converts it into an acyl chloride \( \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{COCl} \), which is compound (B) in this reaction sequence.

Step 2: Convert Acyl Chloride to an Amide

Next, compound (B) \( \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{COCl} \) reacts with ammonia \( \mathrm{NH}_{3} \) to form an amide \( \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CONH}_{2} \), which is compound (C). This reaction is a typical nucleophilic acyl substitution where \( \mathrm{Cl}^{-} \) is replaced by \( \mathrm{NH}_{2} \).

Step 3: Convert the Amide to an Amine using Hofmann Bromamide Reaction

The amide \( \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CONH}_{2} \) (compound C) undergoes the Hofmann bromamide degradation reaction with \( \mathrm{KOH} \) and \( \mathrm{Br}_{2} \). This reaction removes the carbonyl group, resulting in the formation of a primary amine \( \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{NH}_{2} \), which is compound (D).

Key Concepts

Nucleophilic Acyl Substitution

Nucleophilic acyl substitution is a fundamental mechanism in organic chemistry, especially in the reactions involving carboxylic acid derivatives like acyl chlorides.
When propionic acid (\( \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{COOH} \)) is reacted with thionyl chloride (\( \mathrm{SOCl}_2 \)), it transforms into an acyl chloride (\( \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{COCl} \)). This conversion sets the stage for the nucleophilic acyl substitution process.
The acyl chloride can then undergo substitution when it reacts with ammonia (\( \mathrm{NH}_3 \)). Ammonia acts as the nucleophile in this instance, attacking the carbonyl carbon.
The chloride ion (\( \mathrm{Cl}^- \)) serves as the leaving group, while the nucleophile (\( \mathrm{NH}_2 \)) replaces it, forming the amide (\( \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CONH}_{2} \)).

• Nucleophile: An electron-rich species seeking a positive center.
• Acyl Chloride: Highly reactive intermediate in nucleophilic substitution reactions.
• Leaving Group: The group that departs with a pair of electrons from the substrate.

This mechanism is crucial for creating new amide bonds, a common structural motif in various synthetic processes.

Propionic Acid

Propionic acid, a simple carboxylic acid with the formula \( \text{CH}_3\text{CH}_2\text{COOH} \), is an essential compound in organic chemistry.
It serves as a building block in many chemical syntheses due to its straightforward structure.
The three-carbon acid is well-recognized for its pungent odor, reminiscent of body odor or sour milk.

• Propionic acid is used industrially in the production of preservatives, such as those found in bread and cereals, due to its antimicrobial properties.
• In laboratory settings, it is often utilized to synthesize other compounds, like acyl chlorides, which are more reactive and useful in further reactions.
• Its role in organic reactions: Propionic acid itself undergoes simple transformations to become versatile intermediates in synthetic pathways.

Propionic acid's conversion to an acyl chloride involves replacing the hydroxyl group with a chlorine atom.
This transformation amplifies the reactivity of the molecule, paving the way for further chemical modifications.

Primary Amine Formation

In organic synthesis, the formation of primary amines is a crucial transformation. The Hofmann Bromamide Reaction is a specific reaction that achieves this conversion efficiently from amides.
In this reaction, the amide (\( \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CONH}_{2} \)) is treated with bromine (\( \mathrm{Br}_2 \)) and a strong base, potassium hydroxide (\( \mathrm{KOH} \)).
Here's how it happens:

• Mechanism: The bromamide degradation effectively removes the carbonyl group (\( \mathrm{C=O} \)) from the amide, resulting in a primary amine (\( \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{NH}_{2} \)).
• Product: The reaction yields a primary amine that contains one less carbon than the original amide.
• Simplicity: This reaction is renowned for its simplicity and efficiency in producing amines from readily available amides.

The Hofmann Bromamide Reaction is thus an indispensable tool in organic chemistry, providing a clear pathway to produce primary amines from amides. This process is particularly valuable in pharmaceutical and agrochemical industries, where amines are key intermediates.