question_answer

In the reaction of \[C{{H}_{2}}=C{{H}_{2}}\] and \[HBr\] initial addition occurs of:

A)  indefinite

B)  \[{{H}^{+}}+B{{r}^{-}}\] both at one time

C)  \[{{H}^{+}}\]

D)   \[B{{r}^{-}}\]

Correct Answer: C

Solution :

Key Idea: Alkene undergo electrophilic addition reaction. Electrophile is electron deficient species. During, electrophilic addition electrophile first adds across\[C=C\]or\[C\equiv C\]. (i) \[\overset{+}{\mathop{C}}\,{{H}_{2}}-\overset{-}{\mathop{C}}\,{{H}_{2}}+{{H}^{+}}+B{{r}^{-}}\] (ii)\[CH_{2}^{+}-CH_{2}^{-}+{{H}^{+}}\xrightarrow{{}}CH_{2}^{+}-C{{H}_{3}}\] (iii)\[CH_{2}^{+}-C{{H}_{3}}+B{{r}^{-}}\xrightarrow{{}}C{{H}_{2}}Br-C{{H}_{3}}\] \[\therefore \]Initial addition occur by\[{{H}^{+}}\].