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Saturday, October 20, 2018

LEWIS ACID-BASE CONCEPT:

According to Lewis acid base theory- acids are electron pair accepter (electron deficient) and bases are electron pair donor and the combination of Lewis acid and Lewis base occur through coordinate bond formation.
For example

Others examples of Lewis acid-base neutralization.

Lewis acids:

(1) Cations are act as Lewis acids example, H+ , Ag+ , Cu+2 , Na+ , Fe+2 , Hg+2  etc.
(2) Electron deficient compounds are act as Lewis acids for examples BF3 ,ACl3 ,BCl3 ,FeCl3 etc 
(3)
The compounds  which central atom with available vacant d-orbital  can add additional pairs of electrons even though it already has an octet or more of electrons act as Lewis acids for examples SiCl4,PCl5, SOF4, SbCl3, SbF5 ,SnCl4, IF7, SF4 etc 

(4) Compounds containing multiple bonds for example CO2 , SO2, SO3 ,CS2 etc.

Lewis bases:

(1) Anions are act as Lewis base H- ,CH3- ,NH2- , OH- , X- etc. but all the anions are not Lewis base for example (PCl5)
(2) The Neutral molecules having lone pairs of electrons act as Lewis base for examples NH3 ,R-NH2, R-NH-R, R3-N, H2O , R-OH ,R-O-R , PH3, R-PH2 , R2PH ,R3P , H2S , R-SH , R-S-R etc
(3) Compounds with non polar multiples bond  are also act as Lewis base, example Alkenes(C2H4) , Alkynes(C2H2 ) Dienes, polyenes, benzene, Polynuclear homoaromatic compounds. (Ligand in coordination compounds)

Chemical Reactions according Lewis acid-base concept:

We can now write equations for many reactions that involve a Lewis acid reacting with a Lewis base. The following are a few examples:


Acid-base behavior according to the Lewis theory has many of the same aspects as does acid-base theory according to the Brønsted–Lowry theory.

(1)There is no acid without a base. An electron pair must be donated to one species (the acid) by another (the base).

(2) An acid (or base) reacts to displace a weaker acid (or base) from a compound.

(3)The interaction of a Lewis acid with a Lewis base is a type of neutralization reaction because the acidic and basic characters of the reactants are removed.

An acid-base reaction takes place readily between BF3 and NH3 ,

                                         BF3  +NH3 → H3N:BF3

However, when the product of this reaction is brought into contact with BCl3, a reaction takes place:

                                  H3N:BF3 +BCl3 → H3N:BCl3  + BF3

In this reaction, a Lewis acid, BF3 , has been replaced by essentially a stronger one BCl3 .  And replacement of Lewis  acid by other Lewis acid follow Second order Nucleophilic substitution (SN2).

Merits of Lewis concept:
(1)
The acid/base nature of substance may also defined without any solvent.
(2) It is highly useful to explain coordination compounds in which central metal atom into behave as Lewis acid and ligands behave as Lewis base.
(3) The acid having greater tendency to accept lone pair bare stronger acid and base have greater tendency to donate lone pair are stronger base.

Demerites of Lewis concept: 
(1) It is extremely generalized Concept , almost all the compounds either become acid or base  by this Concept.


Related Questions:

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