In general terms, the choice of solvent can have a significant effect on the performance of a reaction.

Factors when chosing a solvent are:

• solubility : need to get reagents in the same phase, the molecules need to collide to react !
• usually, the solvent needs to be unreactive towards the reagents

(except in reactions where the solvent is the Nu :  "solvolysis")
• how will the solvent affect the rate of reaction ?

For an S_N2 reaction, the effect of solvent polarity is usually much less, but the ability (or really lack there of) of the solvent to solvate the nucleophile is the important criteria, as shown by the relative rate data for the S_N2 reaction of n-BuBr with N₃^-.
 

POLAR PROTIC SOLVENTS (polar and ability to be H-bond donor)

• have dipoles due to polar bonds
• have H atoms that can be donated into a H-bond
• examples are the more common solvents like H₂O and ROH
• remember basicity is also usually measured in water
• anions will be solvated due to H-bonding, inhibiting their ability to function as a Nu

 

• have dipoles due to polar bonds
• don't  have H atoms that can be donated into a H-bond
• examples are acetone, acetonitrile, DMSO, DMF
• anions are not solvated and are "naked" and reaction is not inhibited

• All nucleophiles will be more reactive in aprotic than protic solvents
• Those species that were most strongly solvated in polar protic solvents will "gain" the most reactivity in  polar aprotic (e.g. F^-).
• Polar aprotic solvents are typically only used when a polar protic solvent  gives poor results due to having a weak Nu, (esp. F^-, ^-CN, RCO₂^-)