Lewis Acids & Lewis Bases, A New Analysis
Lewis acid and Lewis base concept organises and 'explains' the majority
of reaction chemistry that school and university students are expected to be
familiar with. Lewis acid/base reaction chemistry concerns: electron pair
donors, electron pair acceptors, anions, cations, lone-pairs, ligands,
spectator ions, HOMOs, LUMOs, nucleophiles, nucleofuges, electrophiles,
electrofuges, electrophilic & nucleophilic substitution, acid & base catalysed
eliminations, Brønsted acidity, proton abstracting bases, adducts,
complexes, Diels-Alder cycloaddition, curly arrows, and more more more. No other reaction
chemistry is so broad, varied, or central to how we think about and understand chemical reactivity.
In this web book:
acids are RED
bases are BLUE
Watch these videos for a visual story:
Lewis Acids & Lewis Bases: Introduction
Lewis Acids & Lewis Bases: Congeneric Arrays
Introduction to the Lewis Acid/Base Interaction Matrix
This page has been tranlated into Romanian, here.
Lewis Acid/Base Theory
base is a species with an available (reactive) pair
of electrons and a Lewis acid is an
electron pair acceptor.
The simplest reaction is for a Lewis acid
to interact with a Lewis base
to give a Lewis acid/base complex:
Lewis Acid + Lewis Base Lewis Acid/Base Complex
modern theoretical language, the Lewis acid's
LUMO its Lowest Unoccupied Molecular
Orbital interacts with the Lewis
base's HOMO its Highest Occupied
MO to give a bonding molecular
LUMO + HOMO Bonding MO
are often said to have a vacant orbital.
ok, ok, OK already!
is a problem... and it concerns the overuse of the term "acid".
In the very common Lowry-Brønsted system, a [Brønsted] acid is a proton, H+, donor and
a [Brønsted] base is a proton acceptor.
In the Lewis approach, a [Lewis] acid is an electron pair acceptor (LUMO) and a [Lewis] base is an electron pair donor (HOMO).
It transpires that
the Lowry-Brønsted model is a sub-set of the broader Lewis
approach. Analysis shows the proton H+ to be a unique type of Lewis
acid (see below).
Before the material on this page can be appreciated it is essential to understand the difference between a Lewis acid and a Brønsted
acid, and between a Lewis base and a Brønsted base, as discussed in more detail on
another page of this chemogenesis web book, here.
The simplest Lewis
acid plus Lewis base interaction is
complexation and this process can be represented
using both Lewis theory and FMO theory:
Using Lewis theory,
electron accountancy and curly arrows, the lone-pair
of electrons moves from the Lewis base to the Lewis
acid, the electron-pair acceptor, to give a
two electron chemical bond. The curly arrow
represents the movement of the electron-pair:
Using FMO theory,
the Lewis base's highest occupied molecular orbital
or HOMO interacts with the Lewis acid's
lowest unoccupied molecular orbital or LUMO to give a bonding
In The Reaction Flask
In the test tube we experience
many types of reaction that we explain in terms of Lewis acid/base interactions,
including: anions & cations in solution, lone-pairs, ligands, spectator
ions, nucleophiles, nucleofuges, electrophiles, electrofuges, ionic substitution,
addition, elimination & rearrangement, precipitates, Brønsted
acids, proton accepting bases, transition metal complexes, cycloaddition,
All species with an electron pair accepting
(vacant) orbital. All species with full or partial positive charge
behave as Lewis Acids. Lewis
Acid behaviour is found amongst:
Metal cations complexed by ligands
Electrophiles (attacking Lewis acids)
Electrofuges (Lewis Acid leaving group)
Classic electron deficient species such as BF3 and AlCl3
Cationic spectator counter ions
Electron deficient π-systems
which take part in multicentre interactions
species have a pair of electrons to donate,
or an available HOMO. All species with full
or partial charge behave as Lewis Bases.
Lone-pair donation behaviour is found amongst:
Conjugate Brønsted bases
Anionic counter ions
with polarised bonds (methyl iodide, carbonyl functions, etc.) behave
as if they have Lewis acid and Lewis base ends or poles.
The differentiation becomes more pronounced as bond polarisation increases.
Thus, the carbon
atoms of both methyl iodide and a carbonyl function are made delta+
by the electronegative iodine and oxygen atoms.
carbon atoms of both
methyl iodide and carbonyl functions are susceptible to attack by nucleophilic
The question is: Can
we make sense of the complexity of the chemistry we see in the reaction
The answer is yes,
if we start with the arrays of Lewis acids and bases generated from
the Five Hydrogen Probe Experiments, as discussed over the previous few pages of this webbook.
& Classifying Lewis Acids and Lewis Bases
group elemental hydrides and the hydrogen
probe experiments generate a couple of dozen congeneric
arrays of chemical entities exhibiting some rather general
types of chemical reactivity behaviour:
From this selection of congeneric
arrays, the Lewis acids and Lewis
bases can be selected and further sorted by frontier molecular
orbital topology (shape plus phase information) of the reactive centre:
of Lewis base are recognised:
Hydride ion, H, and hydrogen, H2
Anion Lewis bases
HO, water, H2O:, methylcarbanion,
Electron rich π-systems:
ethene, benzene, etc.
Six types of
Lewis acid are recognised:
Proton Lewis acid
The proton, H+
Group 1 and 2 cations:
Li+, Mg2+, etc.
Ion Lewis acids
Ammonium ion, [NH4]+,
oxonium ion, [OH3]+, etc.
BF3, the carbenium ion, H3C+
Electron poor π-systems:
enones, tetracyanoethylene, etc.
Metal Lewis acids
Cations and bulk
metals of the: transition metals, post-transition metals, lanthanides
Lewis acids & Bases in Detail
The six types of Lewis acid
and four types of Lewis base can be arranged into an interaction matrix
that generates 24 types of Lewis acid/base interaction complex and associated
reaction chemistry, as discussed in detail on the next
page of this webbook.
To learn more about the six
types of Lewis acid and four types
of Lewis base continue to scroll down this page.
Ion, Hydrogen & Helium
H D T
H2 D2 T2
HD HT DT
Search for s-HOMO
Lewis bases species in The Chemical Thesaurus
Lewis bases have spherical 1s or ovoid (peanut) 1s HOMOs which are
devoid of the closed electron shells which hinder complexation (due
to closed-shell/closed-shell repulsion) seen in all other Lewis base
(H, hydride) or neutral (H2,
soft. The softest and most polarisable of all species.
Nearly all of the elements are able to form hydride compounds:
Lewis bases interact with Lewis acids to form a complex they are
deemed to reduce the Lewis acid: s-HOMO Lewis bases are reducing
ion is a strong proton abstracting base and nucleophile. H2 is only very weakly basic such that H2, D2,
T2, HD, HT and DT hardly seem to be basic
at all, yet as they can be protonated to [H3]+ (in the gas phase) they must be Lewis bases.
H2 form metallic complexes.
Helium can be protonated under gas phase conditions to [HHe]+ so it is a Lewis base, an s-HOMO Lews base.
H D T
H2 D2 T2
HD HT DT
Complex Anion Lewis Bases
Symmetry Molecular Anions
Search for complex
anion Lewis bases species in The Chemical Thesaurus
anion Lewis bases have a hypervalent central cation (boron, aluminium
or heavy metal) saturated with anionic Lewis base ligands. Lewis octet
and 18-electron rules are generally satisfied. The HOMO shows high
Lewis base species behave as charged hard spheres that form ionic
charge-controlled complexes (ie act as non-nucleophilic counter
ions), or they behave as donors of hard/soft ligands, X.
Ligand substitution in which a nucleophilic Lewis Base displaces a nucleofugal
ligand is common and ligand symbiosis considerations/effects
are very important.
There are four
subclasses of complex anion Lewis base:
X = Halogen
anion which gives rise to the synthetically useful non-basic,
non-nucleophilic, non-interfering anionic spectator counter ions:
[BF4] [SbF6] [AsF6] [FeBr4]
X = Hydride
ion which gives rise to species which act as donors of nucleophilic
hydride ion, [BH4] and [AlH4],
as long as there is not a Brønsted Acidic proton available
or H2 is generated.
M = Heavy
metal (Fe or Cr as opposed to B or Al). Such complex anions
are much studied in classical inorganic coordination chemistry.
Transition metals centres often exhibit multiple oxidation states.
These are better considered as type 23 Lewis acid/base complexes.
X = Oxygen
heavy metal species with oxygen ligands are commonly used as oxidising
ligand replacement congeneric series are common:
Lobe-HOMO Lewis Bases
|Electron Lone Pair Species
Search for Lobe-HOMO Lewis base species in The Chemical Thesaurus
lone pair species in which the Lewis base HOMO is a px FMO orbital.
In valence bond
terms, the electron pair is in an sp3, sp2 or sp hybrid orbital.
species show directional bonding which implies that the HOMO is
directional, ie lobe shaped.
or neutral with a lone-pair of electrons.
from hard to soft. The hardness of anionic Lobe-HOMO Lewis bases
can be defined with respect to the methyl anion, H3C
the carbon-hydrogen bond length in methane (109pm).
Lewis bases can by definition be protonated and the conjugate Brønsted
acid's proton-to-Lewis base bond length, along with its pKa value, serves to probe the Lewis base's chemistry and behaviour. Bond-length
and pKa data are linear over congeneric series
is a convenient reference Lobe-HOMO Lewis base due to the importance
of carbon in organic chemistry and that so many reactions occur
at carbon centres.
with base-to-H+ bond lengths shorter than 109pm, such as the hydroxide
ion HO (HO–H bond length = 96pm), are deemed to
be harder than the methyl carbanion. Longer bond-length as seen
with iodide, I, (HI bond length = 161pm) equates
bases are the classic electron lone-pair donor species that can act as:
- Proton abstracting
- Ligands around metal complex ions
- Attacking nucleophiles
- Nucleofugal leaving groups
or solvating agents
Lewis acid/base complexes formed by Lobe-HOMO Lewis
bases range from FMO controlled covalent carbon-carbon bonds to charge-controlled
ionic species such as cesium fluoride, CsF.
The atomic lone pair
centre may be embedded in a π-system,
for example the allyl ion, in which case the species can be dual
classified as a Lobe-HOMO and a π-HOMO
Lewis base. When the species reacts via a single atomic centre it is classified as a Lobe-HOMO Lewis
There are several
subclasses of Lobe-HOMO Lewis bases, including:
- Carbanion centres: H3C, enolates, etc., are of huge importance in organic chemistry because the carbanion can act as a nucleophile at a carbon-nulceofuge centre, C-Nfg, such as an alkyl bromide or tosylate. The effect is to increase the length of the carbon skeleton.
The rule is that π-stabilisation makes a carbanion centre less basic but more nucleophilic, as discussed in more detail elsewhere in this chemogenesis webbook, here.
("alpha effect") bases like hydrazine and hydrogen peroxide are rendered more basic
& more nucleophilic by an adjacent lone-pair of electrons. Find α-effect ("alpha effect") Lewis bases in The Chemical Thesaurus.
and polydentate bases have two or more similar Lewis base centres and are commonly employed as ligands in transition metal chemistry. Find bidentate
and polydentate bases in The Chemical Thesaurus.
- Ambident (ambidentate) bases have two dissimilar Lewis base centres and show selectivity. Find ambident and polydentate bases in The Chemical Thesaurus.
There are two
important congeneric planars to be found within electronegative
main group elements and their anions. One is formed
by the X anions and the other by X: neutral lone
Read more about the chemistry of these congeneric planars elsewhere in this webbook, here:
A number of Group 14
elemental hydrides: CH4, SiH4,
GeH4& SnH4, are
rather inert towards Lewis acid and Lewis base reagents. (Species
can be oxidised and they are susceptible to attack by radicals and
can be protonated by super acids to the carbonium ion:
H+ + CH4 > [CH5]+
So methane is
a Lewis base but, like helium, it is an exceedingly feeble proton
abstractor. It follows that: CH4, SiH4, GeH4, SnH4 and related compounds are Lewis bases.
Search for inert species in The Chemical Thesaurus
π-System Lewis Bases
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
Lewis bases have their electrons in their highest occupied molecular orbital, or HOMO, delocalised over two or more p-orbitals.
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:
delta or electron rich π-systems.
Soft when the
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.
behave as π-species
when they undergo FMO controlled multicentre interactions. These most obviously manifest themselves in three situations:
- Stabilisation of the π-system: certain patterns/structures are associated with stability such as 4n+2 π-electrons in a cyclic array, the allyl anion, etc.
- Diels-Alder cycloaddition and other pericyclic interactions, Type 20 Lewis acid/base complexation.
- Formation of π-organometallic complexes, Type 24 Lewis acid/base complexation.
The Proton Lewis Acid
Search for proton Lewis acid species in The Chemical Thesaurus
proton is a point positive charge with a vacant spherical orbital,
the 1s LUMO. This geometry enables the proton to penetrate all types
of Lewis base HOMO topology.
The proton is
the smallest, lightest, hardest and most versatile Lewis acid.
proton is never observed free (in chemistry at least, high energy
high vacuum physics is different). The proton is always passed or
transferred from one Lewis base to another in a concerted Brønsted
acid/base proton transfer reaction.
The proton has
so little mass that it (partially) quantum tunnels between complexed
states, and the
ability of a species to complex with a proton defines Lewis base
acids are all proton/Lewis bases complexes: the proton is the
agent of Brønsted acidity.
The Ka and pKa
of are a measure of Brønsted acid strength with respect to
water. As the Lewis acid H+ remains constant, the terms Ka and pKa
are a measure of a conjugate (Lewis) base's affinity for H+
with respect to the standard Lewis base water, :OH2.
||H+ D+ T+
I & II Metal Cations
Search for s-Lewis
acid species in The Chemical Thesaurus
acids are the cations of the Group I alkali and Group II alkaline
LUMO (2s, 3s 4s 5s & 6s AOs) is superimposed upon a sphere of closed
electron shells which defines the ionic radius of the cation.
hard. Fajan's rules indicate that small highly charged cations, for
example Be2+, are able to polarise anions and give polar
as counter ions or spectator ions to interesting Lewis bases. Very
important biochemical species.
There are two
Group I alkali
metals: Li+ Na+ K+ Rb+ Cs+
II alkali earth metals: Be2+ Mg2+ Ca2+ Sr2+ Ba2+
Onium Ion Lewis Acids
Search for onium ion
Lewis acid species in The Chemical Thesaurus
The onium ion
Lewis acids have a central electronegative atom saturated with Lewis
acid "ligands", usually H+ or alkyl+.
Onium ion Lewis
acids are all proton/X Lobe-HOMO or carbenium ion/X Lobe-HOMO complexes
X = N, O,
F, Ne, P, S, Cl, Ar, As, Se, Br, Kr, Sb, Te, I, Xe
hard, but species behave as a source of hard H+ or the
relatively soft Lobe-LUMO Lewis acid carbenium ion, H3C+.
Onium ions either
form charge-controlled (ionic) complexes or they react by transferring
a ligand to a nucleophilic /basic Lewis base.
If the transferred
ligand is H+, the onium ion acts as a Brønsted
If the transferred
ligand is a carbenium ion Lewis acid, the onium ion is said to be
an alkylating agent.
tetraalkyl ammonium ions, such as [(CH3)4N]+,
can act as spectator cations.
be protonated to the five valent carbonium ion: [CH5]+
nucleophilic substitution reactions at carbon pass through a five
valent carbonium ion transition state:
There is one
onium ion Lewis acid planar:
oxonium and sulfonium ions give rise to many ligand replacement
congeneric series, for example:
[R4N]+ [R3NR']+ [R2NR2]+ [RNR'3]+ [NR'4]+
where R and/or
R' = H, CH3, alkyl, C6H5 etc.
Lobe-LUMO Lewis Acids
Search for lobe-LUMO Lewis acid species in The Chemical Thesaurus
Lewis acid species either have a vacant p orbital (R3C+ or F3B), or
they have an important resonance structure (ie a 'mixed-in' LUMO)
which gives the species considerable vacant p orbital character. Such
Lobe-LUMO centres are polarised delta+.
Hard to soft.
Hardness of Lobe-LUMO Lewis acids is here defined with respect to
the carbon-hydrogen bond length in methane (109pm).
Lewis acids can complex with hydride ion and the corresponding Type
11 Complex's Lewis acid to hydride bond length serves to probe the
Lewis acid's chemistry.
that bond-length data is linear along congeneric series and over
planars. It is convenient to nominate methane as a reference Lobe-LUMO
Lewis acid because of its importance in organic chemistry.
be deconstructed to the carbenium ion Lobe-LUMO Lewis acid and a
Hydride ion Lewis base. The H3C+-to-H
bond length, ie methane's C-H bond length of 109pm, can be used
as a reference point with which to compare to other Lobe-LUMO Lewis
with 'Lewis acid-to-H' bond lengths shorter than
109pm (such as the hydroxy cation HO+, HOH bond length
= 96pm) are deemed to be harder than the carbenium ion.
Lewis acids react via concerted SN2 mechanisms.
These reactions exhibit transition state symbiosis. Hard nucleofuges,
such as fluorosulfonate FSO2O,
render the Lobe-LUMO centre harder, and soft nucleofuges, such as
iodide ion I, render the centre softer.
susceptible to attack by nucleophilic Lewis bases and they may be
actively electrophilic. Lobe-LUMO Lewis acids increase the extent
of the sigma-skeleton when they complex with nucleophiles.
There are three
subclasses of Lobe-LUMO Lewis acids. Members of each subclass have
the property that they complex with, or are attacked by, nucleophilic
Trivalent boron and aluminium species, BF3
and AlCl3, and enium ions of the type R3C+,
RO+, Br+. The methyl cation carbenium ion,
H3C+, is a useful reference species.
A + B -> A-B complexation reactions, where B is a nucleophile,
There are several
And many congeneric
Where Nfg = nucleofuge or Lewis base leaving group.
to SN1 and SN2 nucleophilic
substitution: H3C-I, (H3C)3C-Cl,
H3C-OTs, epoxides etc.
carbon) centre attached to the nucleofuge is rendered delta+:
stabilise the carbenium ion centre:
a number of nucleofugal leaving groups, including halide ions, sulfate,
tosylate, triflate, etc.
of a leaving group how easily it is displaced correlates
with the pKa of Lewis base's conjugate acid.
Thus, an Nfg with a strong conjugate Brønsted acid, such
as bromide ion (HBr) is a good leaving group and is easily displaced.
are found with three membered rings with an O, N, S, etc. heteroatom,
and that are susceptible to nucleophilic attack and ring opening:
functional groups susceptible to nucleophilic addition, or nucleophilic
addition-followed-by-elimination, which leads to net substitution.
carbon centres of imines, carbonyls, alpha,beta-unsaturated carbonyls,
rich source of congeneric series.
π-LUMO Lewis Acids
If an extended electron poor π-system species reacts via a single atomic centre, for example when a benzyl cation is reduced to toluene, the species is better considered behaving as an ambidentate, π-stabilised Lobe-LUMO Lewis acid.
However, when when the species reacts via its extended π-system directly, for example during Diels-Alder cycloaddition or when forming a π-organometallic, the the species should be considered as a π-HOMO Lewis base species. Thus, there is an overlap between π-stabilised Lobe-LUMO and π-LUMO classification.
Search for π-LUMO Lewis acid species in The Chemical Thesaurus
cationic hydrocarbon π-systems
and those neutral but electron deficient π-functions
which participate in concerted multicentre reactions.
MO theory gives rise to whole families of π-structure:
polyene ribbons, aromatics, etc. Each system is FMO unique. π-Species
must be considered at the Hückel level, as well as by VB resonance
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 cabenium ion, allyl cation & pentatrieneyl cation and alkene, 1,3-diene & 1,3,5-triene π-LUMO Lewis acids:
or delta+ electron poor π-systems.
Species behave as π-LUMO Lewis acid species when they undergo FMO controlled multicentre interactions. These most obviously manifest themselves in three situations:
- Stabilisation of the π-system: certain patterns/structures are associated with stability such as 4n+2 π-electrons in a cyclic array, the allyl cation, etc.
- Diels-Alder cycloaddition and other pericyclic interactions, Type 20 Lewis acid/base complexation.
- Having non-nucleophilic complex anion Lewis base anions to stabilise the π-LUMO Lewis acid specie.
series, but the chloronitrobenzene series can be considered congeneric
with respect to the nucleophilic displacement of Cl by a nucleophile:
Heavy Metal Lewis Acids
Post-Transition, Lanthanide & Actinide Cations & Bulk Metals
Search for heavy metal Lewis acid species in The Chemical Thesaurus
vacant lobe-shaped p, d or f orbitals which may rehybridize on complexation.
Many orbitals available for back-bonding.
Pearson's original analysis remains
Hard to soft.
Pearson states in his early HSAB publications, see elsewhere in this webbook, that transition metal
ions of high oxidation state are harder than those of low oxidation
metals exhibit variable oxidation state and their complexes are generally
series, although periodicity is seen down groups:
PATTERNS IN REACTION CHEMISTRY
central part of the chemogenesis analysis the identification
of the five reaction chemistries, the classification of Lewis
acids and Lewis bases and the formation of the Lewis acid/base
interaction matrix has been published as a poster and a
book available from Meta-Synthesis.
Click on the poster (above) to
read the text
and see the diagrams.
the Patterns in Reaction Chemistry poster: A1 size (590mm
x 840mm), full colour, laminated (encapsulated) and dispatched
in a poster tube. Special web price: £14.95 (US$28.50,
€22.20) including world wide postage... due to an astonishing increase in postal rates we have had to withdraw this offer. Read more here.
is 96 page softback, the CD-ROM is Mac & PC compatible and
the poster is folded.
more information? Contact firstname.lastname@example.org
Pearson's HASB Principle
Lewis Acid/Base Interaction Matrix
© Mark R. Leach 1999-
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