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Nov 1, 2018

Electronic Theory of Acids and Bases

Electronic Theory:

Each of the concepts had its own limitations. The need was therefore felt for a concept which can cover all the earlier aspects of acids and bases. Lewis, (1923) explains the acid-base phenomenon not in terms of ionic reactions but in terms of electronic structure of the acid and base along with the formation of the coordinate covalent bond.
Lewis Acid and Base Definition and Examples
Lewis Structure of NH₃
A base was to be considered according to Lewis as an electron pair donor and an acid as an electron pair acceptor. 
An acid therefore combines with a base leading to a coordinate link from the base to the acid.
An acid must have a vacant orbital into which an electron pair donated by a base can be accommodated. A base, then, is a substance which has at least one unshared (lone) pair of electrons.
Lewis Acid and Base Definition and Examples
Lewis Representation of Acid and Base
According to Lewis theory, the process of neutralisation is simply the formation of a coordinate bond between an acid and a base. The neutralisation product, termed as coordinate complex or adduct, may be either non-ionisable or may undergoes dissociation or condensation reaction depending on its stability.
Lewis Acid and Base Definition and Examples
Acid-Base Neutralisation Reactions

Bronsted Concept Vs Lewis Concept:

According to Bronsted Concept, a base is any molecules or ion which accepts proton. Lewis Concept recognises that the base is an electron pair to donate to H⁺. So that during neutralization a coordinate bond is formed between the donor atom of the base and the proton.
H⁺ + :NH₃        HNH₃
Thus all Bronsted bases are bases in Lewis sense also.

Arrhenius Concept Vs Lewis Concept:

According to Arrhenius, an acid when dissolved in water dissociate into hydrogen ions and anions, that is it release hydrogen ions (proton). Since the proton can receive electron pair from a base. Thus all the Arrhenius acids are also acids under Lewis definition.

Solvent System Concept Vs Lewis Concept:

The solvent system concept recognizes that ammonium chloride should behave as an acid in liquid ammonia. It may be noted that ammonium ion is but an ammoniated proton and the proton can accept a lone pair of electron from a base. Therefore an ammonium ion in liquid ammonia is also an acid in Lewis sense.
The solvent system considers amide ion is a base in liquid ammonia.The amide ion is capable of donating a lone pair to an acid, hence this is a base in Lewis sense.
The reaction between the ammonium ion and amide ion in liquid ammonia is represented in Lewis sense as follows:
NH₃ + [H⁺ + NH₂⁻] NH₄⁺ + NH₃
This reaction shows that amide ion is stronger donor for the proton then ammonia.

Classification of Lewis Acids and Bases:

A wide variety of compounds are recognized as acids and bases according to Lewis. Any Lewis acid must contain at least one empty orbital in the valence shell of one of its atoms to accept an electron pair from a Lewis base.

(i) Molecules containing a central atom with an incomplete octet:

Typical examples of this class of acids are electron deficient molecules such as alkyl and halides of Be, B and Al
Some reactions of this types of Lewis acid with Lewis base are,
F₃B (Acid) + O(C₂H₅)₂ (Base)    F₃BO(C₂H₅)₂ (Adduct)
Cl₃Al(Acid)  +  NC₅H₅(Base)    Cl₃AlNC₅H₅(Adduct)
Me₃B(Acid)  +  N₂H₄(Base)    Me₃BN₂H₄(Adduct)
Explain - NH₃ behaves as a base but BF₃ as an Acid.
In NH₃, the central N atom have lone pair of electrons This lone pair coordinate to empty orbital, is termed as a base according to Lewis concept. 
The compounds with less than an octet for the central atom are Lewis acids. In BF₃B in BF₃, contain six electrons in the central atom thus it behaves as an acid.
F₃B(acid)  +  :NH₃(base)  ⇆  F₃BNH₃(adduct)
Explain why tri-covalent phosphorus compounds can serve both as Lewis acids and also as bases?
Tri-covalent phosphorus compounds like PCl₃ have a lone pair of electrons in phosphorus. This lone pair may coordinate to a metal ion thus allowing the compound to serve as a Lewis base. 
Ni + 4PCl₃    [Ni(PCl₃)₄]⁰
Again the quantum shell of phosphorus has provision for d-orbitals which can receive back donated electrons from electron reach low oxidation state of a metal ion. In this latter case tri-covalent phosphorus compound serves as a Lewis acid.

(ii) Molecules Containing a Central atom with Vacant d-Orbitals:

The central atom of the halides such as SiX₄, GeX₄, TiCl₄, SnX₄, PX₃, PF₅, SF₄, SeF₄, TeCl₄, etc. have vacant d-orbitals. These substances can, therefore, accept an electron pair from the Lewis base to accommodate in their vacant d-orbital and can thus form adducts with a number of halide ions and organic bases. These substances is thus Lewis acids.
These halides are vigorously hydrolyzed the appropriate HX. The hydrolysis reaction take place through the formation of an unstable adduct(intermediate) with H₂O.
Bi-positive tin can function both as a Lewis acid and a Lewis base. Explain?
The Lewis representation of SnCl₂ shows a lone pair on tin through it is as yet short of an octet. Ligands, particularly donor solvents with lone pairs, may coordinate to tin giving complexes. Here SnCl₂ behaves as a Lewis acid.
Again interaction of platinum group metal compounds with SnCl₂ (SnCl₃⁻) as donor leads to the formation of coordinate complexes. As for examples,
[(Ph₃P)₂PtCl(SnCl₃)], [RuCl₂(SnCl₃)₂]⁻². These are examples of Lewis base behavior of SnCl₂.
Can SiCl₄ and SnCl₄ function as Lewis acids?
Both silicon and tin are members of Group IV of the periodic table and their quantum shells admit of d-orbitals. As a result they can expand their valence shell through SP³d² hybridisation and can give rise to six coordinate complexes. In fact complex like [SiCl₄{N(CH₃)₃}₂], [SiCl₂(Py)₄]Cl₂, [SnCl₄(Bpy)] are known. Thus SiCl₄ and SnCl₄ can function as Lewis acids.

(iii) Simple Cations:

Theoretically all simple cations are potential Lewis acids. Reactions of some cations as Lewis acids with Lewis base are,
Lewis acid + Lewis base Adduct or Addition Compounds
Ag⁺ + 2(:NH₃)      [NH₃AgNH₃]⁺ 
Cu⁺² + 4(:NH₃)     [Cu(NH₃)₄]⁺²
Co⁺³ + 6(:OH₂)      [Co(OH₂)₆]⁺³
Li⁺ + :OHCH₃      [LiOHCH₃]
The Lewis acid strength or coordinating ability of the simple cations are increases with-
(a) An increases in the positive charge carried by the cation.
(b) An increases in the nuclear charge for atoms in any period of the periodic table.
(c) A decreases in ionic radius.
(d) A decreases in the number of the shielding shells.
Evidently the acid strength of simple cations increases for the elements on moving from left to right in a period and from bottom to top in a group on the periodic table.
Strength of Lewis acid increasing:
Fe⁺²ㄑFe⁺³( Positive charge increases from +2 to +3)
K⁺ㄑNa⁺ (On moving from bottom to top in a group)
Li⁺ㄑBe⁺² (On moving from left to right in a period)
Arrange these oxides in order of their acidic nature : N₂O₅, As₂O₃, Na₂O, MgO.
Acidic oxides react with water to give oxoacids. The higher the oxidation number and the higher the electronegativity the grater the central element will force to reaction with water to give the oxoacid. Of all these oxoacids nitrogen has the highest oxidation number and electronegativity. Thus the order of acidic nature and oxidation number are-
Acidic nature: Na₂OMgOAs₂O₃N₂O₅
Oxidation number of Oxide forming element: +1  +2  +3  +5.  

(iv) Molecules having a multiple bond between atoms of dissimilar electronegativity:

Example of these class are CO₂, SO₂ and SO₃. In these compounds the oxygen atom are more electronegative than sulphur or carbon atom. As a result the electron density of π-electrons is displaced away from carbon to sulphur atoms which are less electronegative than oxygen, towards the O-atom.Thus carbon and sulphur are electron deficient able to accept the electron pair from a Lewis base such as OH⁻ ions to from dative bond.
Lewis Acid and Base Definition and Examples
Examples of Dative Bond
Bisulphate ion can be viewed both as an acid and a base. Explain.

Bisulphate ion is HSO₃⁻. It may lose a proton to give the conjugate base SO₃⁻², thus behaving as an acid. Again it may add on a proton to give the conjugate acid, thus showing its base character.
SO₃ Behaves as an acid and H₂O as a base. Explain.
SO₃ like BF₃ has less than an octet,and will be termed as an acid according to Lewis concept.
But in H₂O oxygen atom contain lone pairs to donate the Lewis acid.
(v) Elements with an Electron sextet:
Oxygen and sulphur contain six electrons in their valence shell and therefore regarded as Lewis Acids. The oxidation of SO₃⁻² to SO₄⁻² ion by oxygen and S₂O₃⁻² ion by sulphur are the acid-base reactions.
SO₃⁻²  +  [O]   ⇆   [OSO₃]⁻²
SO₃⁻²  +  [S]       [SSO₃]⁻²

Utility of Lewis Concept:

(i) This concept includes those reactions in which no proton are involved.
(ii) Lewis concept is more general than the Bronsted-Lowery concept(that is Protonic Concept) in that acid base behavior is not dependent on the presence of one particular element or on the presence or absence of a solvent.
(iii) It explains the long accepted basic properties of metallic oxides and acidic properties of non-metallic oxides.
(iv) This theory explain many reactions such as gas-phase high temperature and non-solvent reaction as neutralisation process.