Nucleophilic Aromatic Substitution, Diazonium Salts and Heterocycles
Rings can also be attacked by nucleophiles, but only when the intermediate anion has somewhere to put its charge. Diazonium chemistry then supplies a leaving group on a ring that nothing else can install.
What a learner can do afterwards
- States what a ring needs before a nucleophile will attack it and names the intermediate
- Contrasts the addition and elimination route with the benzyne route and gives the evidence that separates them
- Draws the formation of a diazonium salt and lists three groups it can be replaced by
- Predicts the position of attack on a named heterocycle and justifies it from the intermediate
1 · Read
A benzene ring normally turns nucleophiles away, but strong electron withdrawing groups next to the leaving group change the picture. The nucleophile adds to give a settled anion called the Meisenheimer complex, and then the leaving group departs. That two step path is the addition elimination route, and it needs those withdrawing groups ortho or para to the leaving group.
The benzyne route is a different machine for rings with no activating groups. A very strong base pulls HX off the ring to leave a strained triple bond, and the nucleophile can then add at either end. The proof is labelling: a carbon 14 mark at the leaving carbon ends up split over two spots, which only the symmetric triple bond intermediate explains.
Diazonium chemistry opens a third door. A primary aromatic amine, made by reducing a nitro group, meets nitrous acid at ice temperature to give a diazonium salt. The N2 group then leaves cleanly to install Cl, Br, CN, I, OH, or H, so one amine trades into many groups in two steps.
For heterocycles, always draw the intermediate and ask which atom carries the charge. Pyrrole undergoes electrophilic attack at C2 because that ion spreads the charge over more atoms including nitrogen. Pyridine is sluggish toward electrophiles and is attacked at C3, while nucleophiles hit its C2 and C4 where the negative charge can rest on nitrogen.
Withdrawing groups invite addition elimination, strong base opens the benzyne path, and diazonium salts trade one amine for many groups.
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.