Problem 235 - A Complete Resource Book in Chemistry for JEE Main

Question

In the reaction, $$ \mathrm{CH}_{3} \mathrm{COOH} \stackrel{\mathrm{LiAHH}_{4}}{\longrightarrow} \mathrm{A} \stackrel{\mathrm{PCl}_{5}}{\longrightarrow} \mathrm{B} \stackrel{\text { AlC.KOH }}{\longrightarrow} \mathrm{C} $$ the product \(C\) is: (a) Ethylene (b) Acetyl chloride (c) Acetaldehyde (d) Acetylene

Step-by-Step Solution

Answer

The product C is ethylene (a).

Step 1: Analyzing the Conversion from CH3COOH to A using LiAlH4

LiAlH4 is a strong reducing agent. When it reacts with acetic acid (CH3COOH), it reduces it to ethanol (C2H5OH). Therefore, product A is ethanol.

Step 2: Analyzing the Conversion from A to B using PCl5

PCl5 is a common reagent for converting alcohols to alkyl chlorides. Here, it converts ethanol (C2H5OH) to ethyl chloride (C2H5Cl). Thus, product B is ethyl chloride.

Step 3: Analyzing the Conversion from B to C using AlC.KOH

In the presence of a strong base like KOH and aluminum chloride as a catalyst, ethyl chloride (C2H5Cl) undergoes dehydrohalogenation, resulting in the elimination of HCl and forming ethylene (C2H4). Thus, product C is ethylene.

Key Concepts

Reduction Reactions

Reduction reactions involve the gain of electrons or decrease in oxidation state by a molecule, atom, or ion. In organic chemistry, these reactions often result in a reduction of oxygen or addition of hydrogen. LiAlH4, or lithium aluminum hydride, is a powerful reducing agent commonly used to reduce carboxylic acids, esters, and ketones to alcohols.
For instance, in the given exercise, acetic acid (CH3COOH) is reduced by LiAlH4 to ethanol (C2H5OH). This reaction involves the addition of hydrogen atoms, converting the carbonyl group (C=O) into an alcohol group (C-OH), and effectively decreasing the oxidation state of the carbon atom.
Understanding the mechanism of reduction reactions helps in predicting the products formed during chemical processes. Such reductions are essential in synthesizing various organic compounds from more oxidized precursors.

Alcohol to Alkyl Chloride Conversion

The conversion of alcohols to alkyl chlorides is a crucial reaction in organic synthesis. It often uses reagents like phosphorus pentachloride (PCl5), thionyl chloride (SOCl2), or hydrochloric acid (HCl).
In the exercise, ethanol, which is an alcohol, is treated with PCl5 to form ethyl chloride, an alkyl chloride. This transformation occurs via the nucleophilic substitution process, where the -OH group in the alcohol is replaced by a chlorine atom. The reaction with PCl5 is typically understood as:

• The hydroxyl group (-OH) in ethanol substitutes with chloride from PCl5.
• This results in the production of ethyl chloride (C2H5Cl) and by-products like phosphoric acid derivatives.

Such conversions are significant because alkyl chlorides serve as versatile intermediates for further chemical reactions.

Dehydrohalogenation

Dehydrohalogenation is a type of elimination reaction where a hydrogen atom and a halogen atom are removed from an alkyl halide, resulting in the formation of an alkene.
In the provided example, ethyl chloride undergoes dehydrohalogenation in the presence of a strong base like KOH and a catalyst such as aluminum chloride. This reaction facilitates the removal of HCl (hydrogen and chloride ions) from ethyl chloride, yielding ethylene, a simple alkene containing a double bond.
The process follows these conceptual steps:

• The base, KOH, abstracts a proton (H+) from an adjacent carbon atom to the halogen.
• A chloride ion (Cl-) is eliminated from the molecule.
• This leads to the formation of a double bond between carbon atoms, resulting in ethylene (C2H4).

Dehydrohalogenation is an important reaction as it is commonly used to prepare unsaturated hydrocarbons from saturated precursors.

Reagents in Organic Chemistry

Reagents play a crucial role in facilitating organic transformations. They are substances used to cause a chemical change and synthesize new products.
In the reaction sequence provided, reagents like LiAlH4, PCl5, and a combination of AlCl3 and KOH are employed to bring about specific transformations:

• LiAlH4 acts as a reducing agent, converting carboxylic acids to alcohols.
• PCl5 serves as a chlorinating agent, converting alcohols to alkyl chlorides.
• AlCl3 and KOH together facilitate the dehydrohalogenation process, removing HX from alkyl halides to form alkenes.

Understanding the role of different reagents helps in strategically planning synthesis routes for the desired organic compounds. It also aids in predicting the outcomes of chemical reactions.