Do "poor" leaving groups form ions of poor stability?

1 Answer
May 24, 2015

The general idea is that "poor" leaving groups have a strong nucleophilicity, or a strong "desire" to not bring its electrons with it and allow the bond to break.

For example, #I^-# is quite a good leaving group because it is pretty large (#196# pm, compared to #F^-#, which is #133# pm), meaning its internuclear distance is far and the bonding interactions are weak. That's why #HI# (pKa #~~ -9#) is a stronger acid than #HF# (pKa #~~ 3.14#). As an example on the other extreme, a methyl cation (#CH_3^+#) is one of the worst leaving groups there is.

You can determine how "poor" of a leaving group it is by knowing the pKa of the acid related to the leaving group. The pKa of #CH_4# is somewhere around 50~60, which makes it a very strong base. That alone tells you that it's almost always going to want to stay attached to the rest of the compound.

Also, as a general rule, weaker acids tend to be more stable than stronger ones (weaker acid, stronger basicity in the bonds, tends more to maintain the interaction, doesn't dissociate as easily), so since the conjugate acid of a strong base is a strong acid, the #CH_3^+# leaving group must then be really unstable and reactive.