Lewis Acid/Base Interaction Matrix Database


Type 4 Lewis Acid/Base Complexation Chemistry
Protonation of π-Systems

Bonding:

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.

Charge: The charge on a Type 4 complex can be negative, neutral or positive.
Chemistry:

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:

Congeneric Series: There are few congeneric series of real interest as each species is Hückel/FMO unique.
Type 4 Lewis acid/base complex (generic)
Allyl cation (generic)

more here
Benzene

more here
Benzonium ion

more here
Cyclohexene

more here
Cyclopentadiene

more here
Cyclopropene

more here
Diphenylmethane

more here
9-Phenylfluorene

more here
Propene

more here
Toluene

more here


Interactions and reactions classified as:
Type 4 Lewis Acid/Base Complexation Chemistry
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Poster
Nucleophiles & Bases

© Mark R. Leach 1999-


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