Which of the following on heating with aqueous KOH, produces acetaldehyde?

First identify the type of halogen compound in each option because the product formed with aqueous $$KOH$$ depends on whether the halogen atoms are on the same carbon (geminal), on adjacent carbons (vicinal), or attached in some other functional group.

• $$CH_3COCl$$ is an acid chloride (acyl halide).
• $$CH_3CH_2Cl$$ is a simple 1° alkyl chloride.
• $$CH_2ClCH_2Cl$$ is a vicinal dihalide (two Cl on adjacent carbons).
• $$CH_3CHCl_2$$ is a geminal dihalide (two Cl on the same carbon).

Recall the general reactions with aqueous $$KOH$$:

1. Acyl chloride $$RCOCl$$ + $$KOH_{(aq)}$$ $$\rightarrow$$ carboxylic acid $$RCOOH$$.

2. Alkyl chloride $$R{-}Cl$$ + $$KOH_{(aq)}$$ $$\rightarrow$$ alcohol $$R{-}OH$$ (nucleophilic substitution).

3. Vicinal dihalide $$XCH_2CH_2X$$ + $$2KOH_{(aq)}$$ $$\rightarrow$$ diol $$HOCH_2CH_2OH$$ (glycol formation).

4. Geminal dihalide $$RCHX_2$$ + $$2KOH_{(aq)}$$ $$\rightarrow$$ gem-diol $$RCH(OH)_2$$ $$\rightarrow$$ aldehyde (or ketone) $$RCHO$$ + $$H_2O$$ (the unstable gem-diol loses water).

Apply these patterns to each option:

Option A: $$CH_3COCl$$ $$\xrightarrow{KOH_{(aq)}}$$ $$CH_3COOH$$ (acetic acid), not acetaldehyde.

Option B: $$CH_3CH_2Cl$$ $$\xrightarrow{KOH_{(aq)}}$$ $$CH_3CH_2OH$$ (ethanol).

Option C: $$CH_2ClCH_2Cl$$ $$\xrightarrow{2KOH_{(aq)}}$$ $$HOCH_2CH_2OH$$ (ethylene glycol).

Option D: $$CH_3CHCl_2$$ (gem-dichloride) reacts in two steps:
$$CH_3CHCl_2 + 2KOH_{(aq)} \rightarrow CH_3CH(OH)_2 + 2KCl$$
The gem-diol formed is unstable and eliminates water:
$$CH_3CH(OH)_2 \rightarrow CH_3CHO + H_2O$$

The final product is acetaldehyde $$CH_3CHO$$.

Therefore, the compound that gives acetaldehyde on heating with aqueous $$KOH$$ is:

Option D which is: $$CH_3CHCl_2$$