CH3 - CH - CH2 - C6H5
l
Br
These 2 protons are different as they are next to a chiral centre.
If we use a fischer projection..
C6H5 C6H5
| |
H - C- Z Z- C - H
| |
H - C - Br H- C - Br
| |
CH3 CH3
We see that when we substitute the H atom with Z, we can get 2 different diasteromers (not mirror images), and so they behave differently.