Problem 111 - A Complete Resource Book in Chemistry for JEE Main

Question

In the reaction sequence, \(\mathrm{Z}\) will be Glycerol \(\stackrel{\mathrm{KHSO}_{4} / \Delta}{\longrightarrow} \mathrm{X} \stackrel{\mathrm{Zn}-\mathrm{Hg} / \mathrm{conc} \mathrm{HCl} / \Delta}{\longrightarrow}\) \(\mathrm{Y} \stackrel{\mathrm{NBS} / \mathrm{CCl}_{4}}{\longrightarrow} \mathrm{Z}\) (a) 1, 2-dibromopropane (b) 1-bromopropane (c) 2-bromopropane (d) 3-bromopropene

Step-by-Step Solution

Answer

The correct answer is (c) 2-bromopropane.

Step 1: Dehydration of Glycerol

Glycerol is treated with KHSO₄ at elevated temperatures (indicated by Δ) which leads to the dehydration of glycerol, forming an unsaturated compound. In this case, it specifically forms acrolein due to the removal of water molecules.

Step 2: Reduction of Acrolein

Acrolein, the unsaturated aldehyde, undergoes Clemmensen Reduction with Zn-Hg in the presence of concentrated HCl under heat. This reduces the acrolein to propanal.

Step 3: Bromination to Form Product Z

Propanal is reacted with N-bromosuccinimide (NBS) in the presence of CCl₄, leading to substitution at the allylic position, resulting in 2-bromopropane.

Key Concepts

Acrolein Formation

Acrolein is a key intermediate formed during the dehydration of glycerol. This process involves removing water molecules from glycerol and is facilitated by heating with potassium hydrogen sulfate ( KHSO_4 ). Under these conditions, glycerol undergoes a transformation through the loss of two molecules of water to form acrolein, which is an unsaturated aldehyde with the chemical structure CH₂=CH-CHO. This reaction is crucial as acrolein serves as a vital starting compound for various chemical processes. Here’s why acrolein formation is both interesting and important:

• Acrolein is a simple unsaturated aldehyde and is quite reactive, which makes it a versatile compound in chemical reactions.
• It features prominently in organic syntheses as it can easily undergo further chemical transformations to form a wide range of useful compounds.
• The process of glycerol dehydration helps in understanding how complex molecules can be broken down into simpler, more reactive species.

Acrolein itself is often utilized in the manufacture of various polymers and other chemical intermediates, showcasing its significant industrial relevance.

Clemmensen Reduction

The Clemmensen Reduction is a classical chemical reaction used to convert carbonyl groups into methylene groups ( CH_2 ) by reducing ketones or aldehydes in a distinctly effective way. In the context of this exercise, acrolein undergoes Clemmensen Reduction to form propanal. This process employs zinc amalgam (Zn-Hg) and concentrated hydrochloric acid (HCl) while heating. Here's why Clemmensen Reduction is significant:

• It is particularly useful for compounds that are sensitive to strong bases as the reaction takes place under acidic conditions.
• Clemmensen Reduction provides a straightforward route to convert unsaturated aldehydes into alkanes, as seen with acrolein’s transformation into propanal.
• This method holds industrial importance where specific reductions are preferred over others because of its selectivity and efficacy.

This reduction is integral to many synthesis pathways where maintaining the integrity of delicate functional groups is essential while reducing the carbonyl functionality effectively.

N-bromosuccinimide Reaction

N-bromosuccinimide (NBS) is a powerful brominating agent prominently used in organic chemistry for selective bromination processes, especially at allylic or benzylic positions. In the exercise, NBS provides the brominating agent necessary to transform propanal into 2-bromopropane. This reaction occurs in an organic solvent such as carbon tetrachloride (CCl_4) and is selective for allylic bromination, which is important due to the following reasons:

• It offers gentle and controlled bromination conditions, minimizing over-reaction or harsh conditions that might degrade sensitive substrates.
• NBS is favored for its ability to conduct bromination in the presence of light or radical initiators, giving it flexibility in use for different synthetic requirements.
• For compounds like propanal, NBS allows for efficient substitution at desired sites without affecting other parts of the molecule.

The use of N-bromosuccinimide is widespread in organic synthesis due to its reliability and ability to provide high selectivity in transformations, aiding in the synthesis of diverse organic compounds.