Problem 213 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH} \frac{\mathrm{Cl}_{2}}{\text { red } \mathrm{P}} \rightarrow \mathrm{A} \stackrel{\text { alc. } \mathrm{KOH}}{\longrightarrow}\) B What is B? (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHO}\) (b) \(\mathrm{ClCH}_{2} \mathrm{CH}_{2} \mathrm{COOH}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COCl}\) (d) \(\mathrm{CH}_{2}=\mathrm{CHCOOH}\)

Step-by-Step Solution

Answer

The final product, B, is \( \mathrm{CH}_{2}=\mathrm{CHCOOH} \) (option d).

Step 1: Identify the Starting Reaction

The starting compound, \( \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{COOH} \), is propanoic acid. The first reaction involves chlorine (\( \mathrm{Cl}_2 \)) and red phosphorus (\( \text{red P} \)). This is a halogenation reaction, which typically substitutes a hydrogen atom adjacent to the carboxylic acid with a chlorine atom, resulting in a substitution product.

Step 2: Perform α-Halogenation of Propanoic Acid

During the first step, the reaction of propanoic acid (\( \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{COOH} \)) with \( \mathrm{Cl}_2 \) and red phosphorus introduces a chlorine atom at the alpha position, forming \( \mathrm{ClCH}_{2}\mathrm{CH}_{2}\mathrm{COOH} \). This product is identified as compound \( A \).

Step 3: Second Reaction with Alc. KOH

When compound \( A \) is treated with alcoholic KOH, it undergoes dehydrohalogenation. This results in the elimination of HCl from the alpha-chloro acid. Removing the hydrogen and chlorine atoms from adjacent carbon atoms forms a double bond, thus producing \( \mathrm{CH}_{2}=\mathrm{CHCOOH} \). This compound is identified as compound \( B \).

Step 4: Identify Compound B

After completing the dehydrohalogenation reaction, compound \( B \) is \( \mathrm{CH}_{2}=\mathrm{CHCOOH} \). This compound matches option (d). Thus, it is the product after the series of reactions.

Key Concepts

Halogenation

In organic chemistry, halogenation refers to the reaction of hydrogen atoms in a compound being replaced by halogen atoms, such as chlorine or bromine. This process is crucial in forming more reactive intermediates for further chemical reactions, such as dehydrohalogenation.
In the provided exercise, halogenation occurs in the presence of chlorine (\( \mathrm{Cl}_2 \)) and red phosphorus. The red phosphorus acts as a catalyst, aiding the halogen to successfully attach to the organic molecule. \( \mathrm{CH}_3\mathrm{CH}_2\mathrm{COOH} \), or propanoic acid, is subjected to halogenation.

• The chlorine atom targets the alpha position, which is the carbon atom adjacent to the carboxylic group.
• This specific type of halogenation, termed alpha-halogenation, allows the conversion of the methyl group adjacent to a carboxylic acid into a more reactive compound.

This reaction step is crucial to prepare the compound for the next step, dehydrohalogenation. Proper understanding of alpha-halogenation is essential as it occurs under specific conditions where the use of red phosphorus plays a significant role.

Dehydrohalogenation

Dehydrohalogenation is an elimination reaction where, typically, a hydrogen atom and a halogen atom are removed from adjacent carbon atoms in a molecule, resulting in the formation of a double bond. This process is significant in the transformation of saturated organic compounds into unsaturated ones, such as alkenes.
In the exercise, compound \( A \) undergoes dehydrohalogenation when treated with alcoholic potassium hydroxide (alc. KOH):

• The reaction initiates as alc. KOH provides a strong base that removes a hydrogen atom adjacent to the alpha-halogen.
• The neighboring chlorine atom is also eliminated, forming hydrogen chloride (HCl) as a by-product.

This process leads to the formation of a double bond between the two previously neighboring carbon atoms. The result is the unsaturated compound \( \mathrm{CH}_2=\mathrm{CHCOOH} \), showcasing a key transformation often used in synthetic organic chemistry to generate alkenes.

Alpha-Chloro Acids

Alpha-chloro acids are a type of carboxylic acid where a chlorine atom is attached to the alpha carbon, the carbon atom directly adjacent to the carboxyl functional group. These acids are intermediates in organic synthesis, especially useful in reactions requiring a good leaving group sourced from the halogen atom.
The formation of an alpha-chloro acid is exemplified in the initial step of the provided exercise, where propanoic acid becomes \( \mathrm{ClCH}_2\mathrm{CH}_2\mathrm{COOH} \).

• These compounds are notably reactive due to the electronegativity of the chlorine atom, which induces chemical changes in subsequent reactions.
• The chlorine provides a pathway for easy removal during dehydrohalogenation, forming a new carbon-carbon double bond.

In synthetic organic chemistry, alpha-chloro acids help facilitate transformations leading to more complex structures. Understanding their reactivity and role simplifies the process of predicting reaction outcomes and designing synthetic pathways.