Q.Two isomers (A) and (B) have the same molecular formula C₂H₄Cl₂. Compound (A) reacts with aqueous KOH to give compound (C) of molecular formula C₂H₄O. Compound (B) reacts with aqueous KOH to give compound (D) of molecular formula C₂H₆O₂. Identify (A), (B), (C) and (D).

Step 1. Both isomers share the formula C2H4Cl2, and the two structural possibilities are the gem-dihalide (both Cl on one carbon, CH3-CHCl2, ethylidene chloride) and the vic-dihalide (Cl on adjacent carbons, ClCH2-CH2Cl, ethylene dichloride).

Step 2. Compound (A) gives (C), C2H4O, with aqueous KOH. A gem-dihalide's hydrolysis gives an unstable geminal diol that collapses (loses water) to an ALDEHYDE OR KETONE -- and CH3CHO, acetaldehyde, has exactly the formula C2H4O, so (A) must be the gem-dihalide, ethylidene chloride, and (C) is acetaldehyde.

Step 3. Compound (B) gives (D), C2H6O2, with aqueous KOH. A vic-dihalide's hydrolysis gives a stable GLYCOL directly -- and HOCH2-CH2OH, ethylene glycol, has exactly the formula C2H6O2, so (B) must be the vic-dihalide, ethylene dichloride, and (D) is ethylene glycol. …

Use the PRODUCT formulas as the deciding clue: a C2H4O product (one oxygen, matching a carbonyl) signals the gem-dihalide route, while a C2H6O2 produ …

• Swapping (A) and (B) -- it is specifically the GEM-dihalide (both halogens on one carbon) that gives the aldehyde/ketone (fewer oxygens, C2H4O), while the VIC-dihalide (halogens on adjacent carbons) gives the glycol (more oxygens, …