[Hazel]

In the following reaction,step 2 and 3,why the water molecule acts as nucleophile instead of the -ve charge halogen group?
As I learned before,the halogen is always a good nucleophile while water molecule is a poor nucleophile.

[macman104]

My understanding of this is that since the molecule is solvated in water so the number of water molecules available compared to the halogen ion is so large.  You should notice that you can get double addition of the halogen to the alkene if you exclude the water. 

In reality, it is possible that in this reaction you may get some double halogenated compound.  But for book work, we indicate that the desired reaction is the halohydrin formation by adding the water and ignore the possible side reaction (note, I'm not 100% sure if we will get the side reaction, but it seems reasonable to assume so).