Sn2 Reactions and Solvent characteristics

[PainDoc]

I am having trouble figuring out why this statement is true, especially the bolded sentence:

"When the nucleophile and/or electrophile have a formal charge, S_N2 reaction rate is fastest when the solvent is of modest polarity. When the nucelophile and electrophile are both neutral, S_N2 reaction rate is fastest when the solvent is highly polar."

 

[Trave11er]

Look at it this way: we have situation when both reagents and products are neutral and thus destabilized by the charged solvent (green line). The transition state, on the other hand, where there is a significant charge is present is stabilized by polar solvent, so reaction goes faster, because activation energy for the rate determined step is decreased.