ELECTROPHILLIC ADDITION REACTION - AN EXAMPLE

When 3-methyl pent-1-ene is treated with HBr, H+ attacks first as an electrophile. This creates a secondary carbocation .

This is followed by an hydride shift to generate a more stable tertiary carbocation.

Each  carbocation can get attacked by bromine nucleophile to give a final addition product.

In doing so , in secondary carbocation , a new chiral center is created at C -2.This gives to two configurations at this carbon .

why ? 

Since a carbocation is planar ,nucleophile can attack from the top as well as bottom giving raise to two new configurations.

However in the given case two chiral centers are present.One chiral centre has a fixed configuration and doesent change during reaction but the other changes to give diastereomers,

The MAJOR product does not have chiral center and therofore ACHIRAL .