Animated Solution for Chemistry - Carbonyl Compounds: Which of the following on heating with aqueous KOH, produces acetaldehyde?

Unmasking Acetaldehyde

Welcome to an exciting journey through organic chemistry! Today, we are tackling a classic problem that tests your understanding of reagents and reaction mechanisms. We need to identify which of the given compounds yields acetaldehyde when heated with aqueous $KOH$.

Before we dive into the options, let's establish what our reagent does. Aqueous $KOH$ is a fantastic source of hydroxide ions ($O{H}^{-}$). In an aqueous medium, these ions are highly hydrated and act as strong nucleophiles. Their primary job is to seek out electron-deficient carbon atoms and replace leaving groups, such as halogens, through a nucleophilic substitution reaction (typically $S_{N}2$).

Let's systematically evaluate our options:

Option (a): $C{H}_{3}COCl$ This is acetyl chloride, an acid derivative. When treated with aqueous $KOH$, it undergoes rapid hydrolysis to form acetic acid ($C{H}_{3}COOH$), which then forms a salt in the basic medium. No acetaldehyde here.

Option (b): $C{H}_{3}C{H}_{2}Cl$ This is ethyl chloride, a simple primary alkyl halide. The $O{H}^{-}$ ion will smoothly replace the chlorine atom via an $S_{N}2$ mechanism, yielding ethanol ($C{H}_{3}C{H}_{2}OH$). Again, not our target molecule.

Option (c): $C{H}_{2}ClC{H}_{2}Cl$ Here we have a vicinal dihalide (chlorines on adjacent carbons). Aqueous $KOH$ will substitute both chlorine atoms, resulting in ethane-1,2-diol, commonly known as ethylene glycol. Still no acetaldehyde.

Option (d): $C{H}_{3}CHC{l}_2$ This is ethylidene dichloride, a geminal dihalide (both chlorines on the same carbon). When aqueous $KOH$ attacks, it replaces both chlorine atoms with hydroxyl groups, forming an intermediate known as a gem-diol: $C{H}_{3}CH(OH{)}_2$.

Now, here is the crucial catch! A single carbon atom cannot stably accommodate two $-OH$ groups. The highly electronegative oxygen atoms possess lone pairs that repel each other fiercely, creating severe electronic and steric strain.

To relieve this immense instability, the gem-diol spontaneously undergoes dehydration. It loses a water molecule ($-H_{2}O$), and the remaining oxygen forms a stable double bond with the carbon atom.

And there we have it! The resulting molecule is acetaldehyde.

By understanding the behavior of gem-diols, we can confidently conclude that option (d) is the correct answer.

A Quick Tip: Always pay close attention to the solvent! If the question had specified alcoholic $KOH$ instead of aqueous, the reagent would have acted as a strong base, leading to an elimination reaction and producing an alkyne instead of a substitution product. The solvent changes everything!