question_answer

Gem dihalides on treatment with alcoholic KOH give

A)  alkyne                                 

B)  alkene

C)  alkane                                 

D)  All of these

Correct Answer: A

Solution :

                                Key Idea Gem dihalide When the two halogen atoms are attached to the same carbon atom. Vicinal dihalide When the two halogen atoms are on adjacent carbon atoms. Gem dihalide or vicinal dihalide gives alkyne on treating with alcoholic KOH \[\underset{vicinal\text{ }dihalide}{\mathop{\begin{align}   & C{{H}_{2}}-Cl \\  & | \\  & C{{H}_{2}}-Cl \\ \end{align}}}\,\]               or            \[\underset{gem\text{ }dihalide}{\mathop{C{{H}_{3}}CHC{{l}_{2}}}}\,\]                        \[\xrightarrow{Alco.\text{ }KOH}HC\equiv CH+2HCl\] Note - Gem dihalide on hydrolysis with (aq.) KOH can produce aldehydes and ketones.                  \[\xrightarrow{-{{H}_{2}}O}C{{H}_{3}}CHO\] While vicinal dihalide gives alcohol with (aq.) KOH.                 \[\begin{matrix}    C{{H}_{2}}-X  \\    |  \\    C{{H}_{2}}-X  \\ \end{matrix}\xrightarrow{(aq.)\,KOH}\begin{matrix}    C{{H}_{2}}OH  \\    |  \\    \underset{glycol}{\mathop{C{{H}_{2}}OH}}\,  \\ \end{matrix}\] Therefore, gem and vicinal dihalide can be distinguished by (aq.) KOH.