There is one problem... and it concerns the overuse of the term "acid".

In the very common Lowry-Brønsted system, an acid is a proton, H⁺, donor and a base is a proton acceptor.

This is very different to the Lewis approach where an acid is an electron pair acceptor and a base is an electron pair donor.

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.

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 the chemogenesis web book, here.

The question is: Can we make sense of the complexity of the chemistry we see in the reaction flask?

The answer is yes, if we start with the arrays of Lewis acids and bases generated from the Five Hydrogen Probe Experiments.

The hydride ion is able to interact with all types of Lewis acid.

When s-HOMO Lewis bases interact with Lewis acids to form a complex they are deemed to reduce the Lewis Acid: s-HOMO Lewis Bases are reducing agents.

The hydride 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.

H– and H2 form metallic complexes.

H^–  D^–  T^–
H2  D2  T2
HD  HT  DT

Complex anion 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 complexes.

X = Oxygen heavy metal species with oxygen ligands are commonly used as oxidising agents.

Families of ligand replacement congeneric series are common:

Typical electron 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.

On complexation species show directional bonding which implies that the HOMO is directional, ie lobe shaped.

Species range 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).

All Lobe-HOMO 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 and planars.

Methane, CH4, 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.

Congeneric species with base-to-H+ bond lengths shorter than 109pm, such as the hydroxide ion HO^– (HOH 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 with softness.

Lobe-HOMO Lewis bases are the classic electron lone-pair species that act as:

• Proton abstracting (Brønsted) Bases
• Ligands (lig. or X)
• Nucleophiles (Nu)
• Nucleofuges (Nfg)
• Non-interfering spectator anions
• Co-complexing or solvating agents

Complexes can range from FMO controlled covalent carbon-carbon bonds to charge-controlled ionic species such as cesium fluoride, CsF.

The lone pair centre may be embedded in a p-system, for example the allyl ion, in which case the species can be dual classified as a Lobe-HOMO and a p-HOMO Lewis base.

There are several subclasses of Lobe-HOMO Lewis bases, including:

• alpha-Effect bases like hydrazine and hydrogen peroxide are rendered more basic & nucleophilic by an adjacent lone-pair
• Ambident (ambidentate) bases have two dissimilar Lewis base centres and show selectivity
• Bidentate and polydentate bases have two or more similar Lewis base centres

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 pair species.

The 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 diradicals.)

However, methane 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.

p-System Lewis bases have their 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 p-structure: polyene ribbons, aromatics, etc.:

Soft when the entire p-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) a 2p electron 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.

The proton is the smallest, lightest, hardest and most versatile Lewis acid.

However, the 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 character.

Brønsted 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.

The s-Lewis Acids are the cations of the Group I alkali and Group II alkaline earth metals.

The shell-like LUMO (2s, 3s 4s 5s & 6s AOs) is superimposed upon a sphere of closed electron shells which defines the ionic radius of the cation.

There are two s-LUMO series:

Group I alkali metals: Li⁺  Na⁺   K⁺   Rb⁺   Cs⁺
Group II alkali earth metals: Be²⁺   Mg²⁺  Ca²⁺   Sr²⁺   Ba²⁺

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 where:

X = N, O, F, Ne, P, S, Cl, Ar, As, Se, Br, Kr, Sb, Te, I, Xe

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 Acid.

If the transferred ligand is a carbenium ion Lewis acid, the onium ion is said to be an alkylating agent.

High symmetry tetraalkyl ammonium ions, such as [(CH3)4N]⁺, can act as spectator cations.

Methane can be protonated to the five valent carbonium ion: [CH5]⁺

Second order nucleophilic substitution reactions at carbon pass through a five valent carbonium ion transition state:

There is one onium ion Lewis acid planar:

Ammonium, phosphonium, 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.

Hard to soft. Hardness of Lobe-LUMO Lewis acids is here defined with respect to the carbon-hydrogen bond length in methane (109pm).

All Lobe-LUMO 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.

It transpires 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.

Methane can 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 acids.

Congeneric species 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.

Many Lobe-LUMO 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.

Species are 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 Lewis bases.

Vacant p-orbital Lewis Acids
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.

Species undergo A + B -> A-B complexation reactions, where B is a nucleophile, Nu:

There are several congeneric series:

And many congeneric dots:

R-Nfg Complexes
Where Nfg = nucleofuge or Lewis base leaving group.

Species susceptible to SN1 and SN2 nucleophilic substitution: H3C-I, (H3C)3C-Cl, H3C-OTs, epoxides etc.

The (usually carbon) centre attached to the nucleofuge is rendered delta⁺:

Alkyl groups stabilise the carbenium ion centre:

There quite a number of nucleofugal leaving groups, including halide ions, sulfate, tosylate, triflate, etc.

The liability 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.

Further examples are found with three membered rings with an O, N, S, etc. heteroatom, and that are susceptible to nucleophilic attack and ring opening:

p-Heteroatom Functions
Polarised p-bonded functional groups susceptible to nucleophilic addition, or nucleophilic addition-followed-by-elimination, which leads to net substitution.

The delta⁺ carbon centres of imines, carbonyls, alpha,beta-unsaturated carbonyls, etc.:

Delocalised cationic hydrocarbon p-systems and those neutral but electron deficient p-functions which participate in concerted multicentre reactions.

Hückel MO theory gives rise to whole families of p-structure: polyene ribbons, aromatics, etc. Each system is FMO unique. p-Species must be considered at the Hückel level, as well as by VB resonance techniques.

Species behave as p-species when they undergo FMO controlled multicentre interactions.

However, if they react via a single centre, for example the complexation of the benzyl cation with chloride to give benzyl chloride, species are better considered as ambidentate Lobe-LUMO Lewis acids.

Susceptible to pericyclic cycloaddition with p-HOMO Lewis bases.

Few congeneric series, but the chloronitrobenzene series can be considered congeneric with respect to the nucleophilic displacement of Cl– by a nucleophile:

Hard to Soft. Pearson states in his early HSAB publications that transition metal ions of high oxidation state are Harder than those of low oxidation state.

Few congeneric series, although periodicity is seen down groups: