[jurbit]

Stage one undergrad here. Just been going over Sn1 and Sn2 reactions and it's not hard, except my lecturer showed (attached) as part of a slide and it's confused me.

Rate increases as stability of the intermediate increases, right? And stability increases the more the charge is spread out, so would the molecule with the phenyl group have a faster reaction?

[hollytara]

If you look at the rates of benzyl chloride vs t-butyl chloride, then relative rate is t-Bu: benzyl 10000:1   So the data yo chow is consistent. 

Although we usually think of the charge delocalization as being important there are other factors.  Tertiary cations are more stable than benzyl cations.  Part of this is because there is relief of steric strain when the tertiary cations become planar.