Question

What is the major product of the addition reaction below?

`CH_3CH_2CH_2CH=CH_2 + HBr ->`

Answer 1

`"2-bromopentane"`

Explanation:

The electron-rich olefin adds to the electrophile, the protium ion, as a first step in the reaction:

`RCH=CH_2 + HBr rarr RC^(+)HCH_3 + Br^-`

The second step is of course bond formation between nucleophilic `Br^-` and the carbocation to form the 2-substituted alkane. The `2""^@` carbocation as shown is stabilized with respect to the `1""^@` carbocation, `RCH_2-CH_2^+`, and the reaction tends to proceed thru this more stable intermediate. This is classic Markownikow addtion.