The following results from an answer key to a Sophmore level text (Vollhardt) are bothering me:
A) 2-bromobutane + Cl- ---> Sn2 reaction, when solvent is DMF
B) 2-bromobutane+ Cl- ---> Sn1 reaction, when solvent is nitromethane
C) t-butyl bromide + I- ---> no reaction when solvent is DMF. It says that DMF solvent is not sufficiently polar to support carbocation formation for the Sn1 reaction.
T-butyl bromide is tertiary, whereas the 2-bromobutane is secondary. I am especially confused about t-butyl bromide not reacting at all.
More polar solvent favors Sn1 as far as I know. The dielectric constant of nitromethane is 35.9 vs 38.5 for DMF. That means Nitromethane is less polar, right? So it seems that if anything, DMF should favor Sn1 more than nitromethane? And how is it even possible to remember all these random cases?