Distinction between pairs of compounds
1, 1-Dichloroethane and 1, 2-dichloroethane.

Explanation

1, 1-Dichloroethane (gem dihalide) on hydrolysis with KOH (aq) gives acetaldehyde which on treatment with 2, 4-dinitro phenylhydrazine gives yellow ppt. of ethanal 2, 4-dinitro phenylhydrazone. 1, 2-Dichloroethane forms ethane 1, 2-diol with KOH (aq) which does not give yellow ppt. with 2, 4-dinitrophenyl hydrazine.
`underset("1, 1-dichloroethane")(CH_(3)CHCl_(2))+2KOH(aq)overset(Delta)rarrCH_(3)CHO+2KCl+2H_(2)O`

`undersetundersetunderset("1, 2-Dichloroethane")(CH_(2)Cl" ")("| ")(CH_(2)Cl)+2KOH(aq)overset(Delta)rarrundersetundersetundersetunderset(("Ethylene glycol "))(" Ethane 1, 2-diol ")(CH_(2)OH" ")("| ")(CH_(2)OH+2KCl)`
`CH_(2)OH-CH_(2)OHoverset("2, 4-dinitrophenylhydrazine")rarr"No action"`