Periodic Table |
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Lewis Acid/Base Interaction Matrix Database
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π-System Lewis Bases

Electron Rich π-Systems | ![]() If an extended electron rich π-system species reacts via a single atomic centre, for example when an allyl anion is protonated to give propene, the species is better considered behaving as an ambidentate, π-stabilised Lobe-HOMO Lewis base. However, when when the species reacts via its extended π-system directly, for example during Diels-Alder cycloaddition or when forming a π-organometallic complex, the the species should be considered as a π-HOMO Lewis base species. Thus, there is an overlap between π-stabilised Lobe-HOMO and π-HOMO classification. Search for π-HOMO Lewis base species in The Chemical Thesaurus |
FMO Topology: |
π-HOMO Lewis bases have their electrons in their highest occupied molecular orbital, or HOMO, delocalised over two or more p-orbitals. Species require modelling by Hückel-FMO techniques as well as by VB-resonance structure methods. Hückel MO modelling gives rise to whole families of π-structure: polyene ribbons, aromatics, etc.: Indeed, quantum mechanics is all about patterns. A particularly striking manifestation is seen with the polyene system of: 1, 2, 3, 4, 5, 6... conjugated p-orbital systems and how they give rise to the carbanion, allyl anion & pentatrieneyl anion and alkene, 1,3-diene & 1,3,5-triene π-HOMO Lewis bases: ![]() |
Charge: | Negative, δ or electron rich π-systems. |
HSAB: |
Soft when the entire π-system is acting as the Lewis base, but harder when a single atomic centre is involved and the species is behaving as a Lobe-HOMO Lewis base. The allyl anion can behave as (or be considered as) 2 π-electrons delocalised over a 3p orbital function, or as a stabilised 2p orbital carbanion. In this latter case it is better to consider the allyl anion to be behaving as a (harder) Lobe-HOMO Lewis base. |
Chemistry: | Species behave as π-species when they undergo FMO controlled multicentre interactions. These most obviously manifest themselves in three situations:
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Congeneric Series: | ![]() |
π-HOMO Lewis base (generic)![]() |
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Acetaldehyde enolate more here |
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Acetylene more here |
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Acetylene (generic) more here |
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Acetylene, terminal hydrogen (generic) more here |
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Acrylate ion more here |
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Alkene HOMO (generic) more here |
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Alkene, 1,1-disubstituted (generic) more here |
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cis Alkene (generic) more here |
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Alkene, monosubstituted (generic) more here |
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Alkene, tetrasubstituted (generic) more here |
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trans Alkene (generic) more here |
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Alkene, trisubstituted (generic) more here |
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1° Alkyl benzene (generic) more here |
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Alkyl nitrile alpha-carbanion (generic) more here |
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Allene (generic) more here |
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Allene more here |
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Allyl anion more here |
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Allyl anion (generic) more here |
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1° Allylic alcohol (generic) more here |
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2° Allylic alcohol (generic) more here |
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1° Amide, anion (generic) more here |
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Aniline more here |
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Anisole more here |
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Anthracene more here |
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Arsole more here |
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Azide ion more here |
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Azulene more here |
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2,3-Benzanthracene more here |
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Benzene more here |
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Benzene, 1,2-dialkyl (generic) more here |
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Benzene, substituted (generic) more here |
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Benzidine more here |
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Benzoate ion more here |
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Benzofuran more here |
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Benzoic acid more here |
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Benzophenone radical anion more here |
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Benzo[a]anthracene more here |
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Benzo[a]pyrene more here |
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Benzyl anion more here |
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Benzyl anion (generic) more here |
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Benzyne more here |
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Biphenyl more here |
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Butadiene, 1,3-methoxy more here |
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Carbide ion more here |
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Carbon monoxide more here |
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Carboxylate ion (generic) more here |
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Chrysene more here |
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Coumarin more here |
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Cumulene (generic) more here |
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Cyanate ion more here |
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Cyanide ion more here |
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Cycloheptatriene more here |
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Cycloheptatriene anion more here |
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Cycloheptatriene anion (generic) more here |
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1,3-Cyclohexadiene more here |
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Cyclohexenyl anion more here |
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Cyclooctatetrene dianion more here |
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Cyclooctatetrene dianion (generic) more here |
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Cyclopentadiene more here |
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Cyclopentadienyl anion more here |
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Cyclopentadienyl anion (generic) more here |
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Cyclopropene more here |
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Cyclopropenyl anion more here |
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Cyclopropyl anion more here |
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Dibenz[a,h]anthracene more here |
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trans 1,2-Dichloroalkene (generic) more here |
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Diels-Alder adduct (generic) more here |
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beta-Diketone anion (generic) more here |
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N,N-Dimethyl aniline more here |
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Diphenyl amide anion more here |
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Diphenyl ether more here |
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Diphenyl phosphine anion more here |
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Diphenylamine more here |
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Diphenylmethyl anion more here |
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Enamine anion (generic) more here |
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Enolate anion (generic) more here |
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1,12-Ethanoperylene-13,14-dioic anhydride more here |
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Ethylene more here |
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Fluorene anion more here |
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Formate ion more here |
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Furan more here |
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5 Heterocycle (generic) more here |
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6 Heterocycle (generic) more here |
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Heterocycle, polycyclic (generic) more here |
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Hydrazone (generic) more here |
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Imidazole more here |
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Imidazole anion more here |
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Imido anion (generic) more here |
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Imine, dialkyl (generic) more here |
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Imine, N-alkyl alkyl (generic) more here |
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Imine, trialkyl (generic) more here |
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Indole more here |
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Isocyanide (generic) more here |
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Isoquinoline more here |
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Isothiazole more here |
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Ketone, alkyl phenyl (generic) more here |
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Lithium carbide more here |
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N-Methyl aniline more here |
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Naphthalene more here |
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Naphthalene radical anion more here |
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Oxazole more here |
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Pentacene more here |
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Pentadienyl anion more here |
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Pentalene more here |
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Pentalene dianion more here |
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Perylene more here |
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Phenanthrene more here |
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Phenol more here |
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Phenol (generic) more here |
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Phenoxide ion more here |
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Phenoxide ion (generic) more here |
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Phenyl amide anion more here |
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Phenyl anion more here |
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Phenyl anion (generic) more here |
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Phenyl ether (generic) more here |
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Phenylacetylide anion more here |
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9-Phenylfluorene anion more here |
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Phenylphosphine more here |
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Phenylselenide anion more here |
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Phosphole more here |
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Picrate ion more here |
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Polycyclic aromatic (generic) more here |
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Polyene (generic) more here |
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Prop-2-en-1-imine more here |
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Propene more here |
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Purine more here |
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Pyrazine more here |
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Pyrazole more here |
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Pyrene more here |
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Pyridazine more here |
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Pyridine more here |
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Pyridine (generic) more here |
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Pyrimidine more here |
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Pyrrole more here |
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Quinoline more here |
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Selinole more here |
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cis Stilbene more here |
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trans Stilbene more here |
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Styrene more here |
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alpha-Terpene more here |
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Tetrathiafulvalene more here |
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Thiazole more here |
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Thiocyanate ion more here |
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Thiophene more here |
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Thiophenoxide ion more here |
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1,3,5-Triazine more here |
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Triphenylmethyl anion more here |
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Triphenylphosphine more here |
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Vinyl chloride (generic) more here |
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Vinylnaphthalene more here |
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Poster | Nucleophiles & Bases |
© Mark R. Leach 1999 –
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