Problem 183 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following on heating with aqueous KOH, produces acetaldehyde? \([\mathbf{2 0 0 9}]\) (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}\) (b) \(\mathrm{CH}_{2} \mathrm{ClCH}_{2} \mathrm{Cl}\) (c) \(\mathrm{CH}_{3} \mathrm{CHCl}_{2}\) (d) \(\mathrm{CH}_{3} \mathrm{COCl}\)

Step-by-Step Solution

Answer

(c) CH_3CHCl_2 produces acetaldehyde.

Step 1: Understand the Reaction

Heating with aqueous KOH typically leads to a nucleophilic substitution reaction. Here, chlorine atoms will be replaced by hydroxyl groups, leading to the formation of alcohols, which can subsequently undergo elimination.

Step 2: Evaluate Each Option

Consider each compound's structure to determine what product is formed upon reacting with aqueous KOH: (a) Ethyl chloride (CH_3CH_2Cl) will likely form ethanol but not acetaldehyde. (b) Ethylene dichloride (CH_2ClCH_2Cl) may form ethylene glycol but not acetaldehyde. (c) 1,1-Dichloroethane (CH_3CHCl_2) can lose two HCl molecules to form acetaldehyde. (d) Acetyl chloride (CH_3COCl) typically forms acetic acid or related compounds but not acetaldehyde.

Step 3: Identify the Correct Product Formation

Through option analysis, compound CH_3CHCl_2 reacts with aqueous KOH to form acetaldehyde. The process is as follows: KOH replaces Cl atoms with OH groups, forming a geminal diol, which then loses water (a process called pinacol-pinacolone rearrangement) to produce acetaldehyde.

Key Concepts

Nucleophilic substitution

In the world of chemistry, nucleophilic substitution is a common and fascinating reaction mechanism. It involves the replacement of an atom or a group in a molecule by a nucleophile. A nucleophile is an atom or molecule with a free pair of electrons or at least one pi bond. They are attracted to positive charges and seek to donate electrons to another compound.
In the context of our problem, when compounds like Halides are treated with aqueous KOH, the hydroxyl ion, which is a strong nucleophile, takes the place of the halide ions. For example, in the compound 1,1-Dichloroethane \( \text{CH}_3\text{CHCl}_2 \), the chlorine atoms are substituted by hydroxyl ions \( \text{OH}^- \). This is an important initial step in the conversion of the compound to acetaldehyde.

• This reaction is crucial as it sets the stage for further transformations.
• It involves a two-step mechanism.
• The reaction typically proceeds via S_N2 mechanism, resulting in an inversion of configuration at the carbon center.

These steps illustrate the critical initial phase in complex organic transformations.

Geminal diol formation

Following nucleophilic substitution, the intermediate formed is termed a geminal diol. A geminal diol contains two \( -\text{OH} \) (hydroxyl) groups attached to the same carbon atom. It's called 'geminal' derived from the Latin word "geminus" which means "twin".
When 1,1-Dichloroethane \( \text{CH}_3\text{CHCl}_2 \) is reacted with aqueous KOH, the resultant structures after nucleophilic substitution are \( \text{CH}_3\text{CH(OH)OH} \). This geminal diol is an unstable compound, generally not favored in many reaction conditions. It tends to dehydrate rapidly to form more stable carbonyl compounds, which in this case, leads to acetaldehyde.

• Geminal diols are usually intermediate steps, appearing fleetingly during reactions.
• Their formation is crucial despite their short presence.
• The instability is primarily due to steric hindrance and electronic instability.

Understanding geminal diols is a stepping stone to grasping further organic reaction mechanisms.

Pinacol-pinacolone rearrangement

The final step in the formation of acetaldehyde from 1,1-Dichloroethane involves a chemical process known as the pinacol-pinacolone rearrangement. This type of rearrangement is critical because it involves the transformation of a vicinal or geminal diol to a carbonyl compound with the help of an acid catalyst.
In our scenario, the geminal diol structure \( \text{CH}_3\text{CH(OH)OH} \) loses a molecule of water leading to the formation of acetaldehyde \( \text{CH}_3\text{CHO} \). During this rearrangement, one of the hydroxyl groups leaves as a water molecule, leaving behind a carbocation. This carbocation is then rapidly stabilized to form a carbonyl compound like acetaldehyde.

• This rearrangement is essential for converting complex substrates to simpler, more reactive ones.
• It forms the backbone of many synthetic transformations in organic chemistry.
• The reaction conditions often require an acid catalyst for the dehydration step.

Understanding how pinacol rearrangement works provides valuable insight into the mastery over complex reaction sequences often encountered in organic chemistry.