The Chemical Bond
Functional Groups in Organic Chemistry
Nucleophiles and Electrophiles
Conjugated π Systems and Aromaticity
Reactions of Aromatic Compounds
|Nucleophilic aromatic substitution has two possible pathways, addition-elimination and elimination-addition. This group of reactions is in distant second place among arene reactions to the electrophilic aromatic substitution mechanisms in MCAT importance. A student of mine once did report seeing addition-elimination on their first exam, although I cannot recall if she said that the passage in question gave the mechanism. I suspect that the passage would have given many necessary details of the reaction or the population of correct answers would have been too small (there is a statistical ceiling on how hard MCAT questions can be). What I am trying to communicate is that there is a core group of the organic mechanisms that you need to know backwards and forwards for the MCAT. There is a another tier that the mechanism that you need to be familiar with, the mechanisms of which are comprehensible when you are presented with them. And there is a large group of mechanisms in between. Trust your instincts. You learn as much as you can about all of them, but you have to weigh out your time too. |
Let us make a few points about addition-elimination. For addition-elimination to occur with an aryl halide, a strongly electron-withdrawing substituent must be present ortho or para to the leaving group. In electrophilic substitution, the ring acts as an electron pair donor, but in nucleophilic substitution, the ring acts as an electron pair acceptor.