Question

Which compound, 2-bromo-2-methylpropane or 2-chloro-2-methylpropane, reacted faster in SN1 experiment? What were the relative rates of the two reactions? Which is better leaving group, Br or Cl and are these results consistent with the relative basicities of these two ion? Briefly explain

Answer 1

Good leaving groups are always weak bases. This can be analyzed using the strength of their conjugate acid. The conjugated acid of Br- is HBr and corresponding one of Cl- is HCl. We know that HBr has a pKa of -8.7 and pKa of HCl at -6.3. Thus HBr is strong acid than HCl, which implies Br- is a weak base than Cl-. Or Br- is a good leaving group than Cl-.

Thus 2-bromo-2-methylpropanes SN1 reaction will be faster than 2-chloro-2-methylpropane reaction. That is rate is higher for Br-substituted compound.