Hydrocarbons - Result Question 25
####27. $H_3 C-CH-CH=CH_2+HBr \to A$
$ CH_3 $
$A$ (predominantly) is :
[2008]
(a)
(b)
(c)
(d)
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Solution:
- (b) The formed $2^{\circ}$ carbocation will rearrange to a more stable $3^{\circ}$ carbocation as:
CCC(C)C CCC+C CCC(C)(C)Br
$ \begin{aligned} & \text{ 2-Bromo-2-methyl butane } \\ & \text{ (Major product) } \\ & H_3 C \\ & H_3 C / \underset{\text{ 3- methyl-1-butene }}{CH-CH=CH_2} \end{aligned} $
Of the two possibilities, $2^{\circ}$ carbocation is more stable so the product of the reaction is expected to be predominantly formed by $2^{\circ}$ carbocation
i.e.
i.e. 2- Bromo-3-Methylbutane
Some electrophilic addition reaction form products that are clearly not the result of the addition of electrophile to the $s p^{2}$ carbon bonded with the most hydrogen and the addition of a nucleophile to the other $s p^{2}$ carbon.
The unexpected product results from a rearrangement of carbocation intermediate. Please note that all carbocation do not rearrange.
