[gg66]

Can someone please explain why the answer is B and not D in this picture attached in the link?

http://puu.sh/lY7yD/bb7fb5e834.png

[Babcock_Hall]

Can you explain your thinking first?  BTW I see that you drew CH₃⁺.  Can you explain why?

[gg66]

Well I thought that once the I leaves, it's taking the electrons with it, and there would be a positive charge left on the C as it doesn't have a full octet anymore. And since that C only has hydrogens, it's the least stable and would thus react with water the fastest.

[Babcock_Hall]

Haloalkanes can react by one of two mechanisms.  One is as you describe, and it is called the S_N1 mechanism.  However, there is also a mechanism in which the nucleophile gives the carbon atoms a pair of electrons simultaneously (in a concerted manner) with the departure of the electron pair with the halogen.  This is called the S_N2 mechanism.  Did you cover these two in class?

[gg66]

Oh yes, we did! So D is going by SN2 and B by SN1. I'm still not seeing why B is more favored 

[Babcock_Hall]

I think you are correct with respect to matching the mechanism to the substrate.  The only explanation that occurs to me at the moment is that water is a poor nucleophile.  A poor nucleophile will produce slow S_N2 reactions but not S_N1 reactions.  However, my opinion is that it is a difficult question.

[gg66]

Ahh I see, that would make sense! I think that would explain it, thank you!