Question

How can you not distinguish between vicinal and geminal dihalides?

Answer 1

Geminal and Vicinal dihalides

1. When the two halogen atoms are attached to the same carbon atom and are prepared by addition of Vinyl halide(${\mathrm{CH}}_2=\mathrm{CHX}$) with hydrogen chloride($\mathrm{HCl}$) a Geminal Dihalide is formed.
2. Examples:1,1 -dichloroethane (${\mathrm{CH}}_3{\mathrm{CHCl}}_2$).
3. When the two halogen atoms are on adjacent carbon atoms and are prepared between halogen and alkene a vicinal dihalide is formed.
4. Examples:1,2 dichloroethane. (${\mathrm{ClCH}}_2{\mathrm{CH}}_2\mathrm{Cl}$)
5. They both can give the same product alkene on reaction with alcoholic potassium hydroxide ($\mathrm{KOH}$).
6. Reactions

:$\underset{\mathrm{Geminal}\mathrm{dichloride}}{{\mathrm{CH}}_3{\mathrm{CHCl}}_2\left(\mathrm{l}\right)}+\underset{\mathrm{Potassium}\mathrm{Hydroxide}}{\mathrm{alc}.\mathrm{KOH}\left(\mathrm{l}\right)}\to \underset{\mathrm{Ethene}}{{\mathrm{CH}}_2={\mathrm{CH}}_2}\left(\mathrm{l}\right)$

$\underset{\mathrm{Vicinal}\mathrm{dichloride}}{{\mathrm{ClCH}}_2-{\mathrm{CH}}_2\mathrm{Cl}\left(\mathrm{l}\right)}+\underset{\mathrm{Potassium}\mathrm{Hydroxide}}{\mathrm{alc}.\mathrm{KOH}\left(\mathrm{l}\right)}\to \underset{\mathrm{Ethene}}{{\mathrm{CH}}_2={\mathrm{CH}}_2\left(\mathrm{l}\right)}$

Therefore, Both gives Alkene on reaction with alcoholic Potassium hydroxide ($\mathrm{KOH}$). hence, we cannot distinguish.