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Type 4 Lewis Acid/Base Complexation Chemistry

When a proton complexes with a hydrocarbon π-system (as opposed to complexing with a single heteroatom atomic centre) the conjugated π-system is reduced in length by one p-orbital to form a Hückel distinct MO system: 2p alkenes are protonated to give give 1p carbenium ions, 6p benzene is protonated gives the corresponding 5p carbenium ion, etc.

Bonding in the complexes is covalent.

When a proton complexes with a hydrocarbon π-system the conjugated π-system is reduced in length by one p-orbital to form a Hückel distinct MO system: 2p alkenes give 1p carbenium ions, 6p benzene gives the corresponding 5p carbenium ion, etc.

If there is a heteroatom Lewis base centre present, it is likely that protonation will occur at that centre. For example, 2p carbonyl functions protonate on oxygen and 6p (aromatic) pyridine protonates on the nitrogen lone pair which is not part of the conjugated π-system:

While the action of a strong acid on benzene is null, benzene is deuterated with deutero sulfuric acid, D₂SO₄. In time the benzene will become fully (or per) deuterated: