Problem 94 - A Complete Resource Book in Chemistry for JEE Main

Question

Identify (a) in the following sequence of reactions. (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COCl}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHClCHO}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{OH}) \mathrm{CH}_{2} \mathrm{Cl}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COCH}_{2} \mathrm{Cl}\)

Step-by-Step Solution

Answer

Compound (a) is an acyl chloride acting as a precursor.

Step 1: Analyze Compound (a)

Compound (a), \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COCl}\), is called butanoyl chloride. This is an acyl chloride derived from butanoic acid (\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH}\)). It undergoes reactions typical of acyl chlorides, such as nucleophilic acyl substitution.

Step 2: Identify Reaction Type

When acyl chlorides react with water or alcohols, they form carboxylic acids or esters, respectively. However, looking at the provided sequence, compound (a) will likely undergo transformation via an intermediate reaction that aligns with the structure of the other compounds (b), (c), and (d).

Step 3: Examine Sequence and Structures

Observe compounds (b), (c), and (d). Compound (b), \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHClCHO}\), appears to be an aldehyde with chlorine, possibly indicating a transformation from an alcohol or an acid chloride. Compound (c), \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH(OH)CH}_{2}\mathrm{Cl}\), indicates an alcohol with chlorine. Compound (d), \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COCH}_{2} \mathrm{Cl}\), suggests a ketone with chlorine.

Step 4: Predict Reaction Pathway from (a) to (b)

To move from \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COCl}\) (an acyl chloride) to \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHClCHO}\), consider potential reagents or conditions. Reducing agents could convert acyl chlorides to aldehydes. Therefore, compound (a) must undergo reduction of the carbonyl group to convert to an aldehyde with an additional chlorine atom (as seen in (b)).

Step 5: Identify Role of Compound (a)

Given the transformations, the role of \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COCl}\) is as the starting material that undergoes nucleophilic substitution followed by reduction to form \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHClCHO}\). This transformation aligns with acyl chloride reactivity.

Key Concepts

Acyl Chlorides

Acyl chlorides are reactive organic compounds containing a carbonyl group bonded to a chlorine atom. They're also known as acid chlorides and are usually derived from carboxylic acids by replacing the -OH group with a chlorine atom. Acyl chlorides have the general formula RCOCl, where R represents an alkyl or aryl group.

• Due to their reactivity, acyl chlorides are excellent candidates for nucleophilic acyl substitution reactions.
• In these reactions, the chlorine atom is replaced by a nucleophile, such as water, alcohols, or amines, forming carboxylic acids, esters, or amides, respectively.
• These compounds serve as key intermediates in organic synthesis, allowing the formation of a variety of derivatives.

The reactivity of acyl chlorides is primarily due to the polar carbonyl group, which attracts nucleophiles to the carbon center. This makes acyl chlorides particularly versatile in organic reactions.

Butanoyl Chloride

Butanoyl chloride, with the chemical formula \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COCl}\), is a specific type of acyl chloride. It is derived from butanoic acid, a four-carbon carboxylic acid. Its structure comprises a butane chain and a reactive acyl chloride group, making it useful for several transformations in organic chemistry.

• Butanoyl chloride is a colorless liquid with a pungent odor and is often used as a reagent in the laboratory for synthesizing more complex molecules.
• In nucleophilic acyl substitution, butanoyl chloride can react with nucleophiles like alcohols and water to form esters and acids, respectively.
• Such reactions are often carried out under controlled conditions due to the corrosive and volatile nature of butanoyl chloride.

Because of its efficient reactivity, butanoyl chloride is pivotal in research and industrial applications, contributing to the formation of a wide range of organic compounds.

Reduction Reactions

Reduction reactions involve the addition of electrons or hydrogen to a molecule or the removal of oxygen. In the context of acyl chlorides, reduction involves transforming the carbonyl group to a less oxidized form, such as an aldehyde or an alcohol.

• Reducing agents, such as lithium aluminum hydride (LiAlH4), are commonly used to convert acyl chlorides to aldehydes.
• This reaction is crucial in converting butanoyl chloride to the corresponding aldehyde by reducing the carbonyl group while retaining the chlorine atom.
• Careful handling of reducing agents is essential due to their reactivity and potential hazards.

Overall, reduction reactions expand the versatility of acyl chlorides, enabling the formation of more complex molecules essential in both synthetic and industrial chemistry.