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Chemistry > Acids, Bases and Salts > What is Lewis Acid?
Acids, Bases and Salts

What is Lewis Acid?

Lewis Acid

It is a chemical species that consist of an empty orbital that is capable of accepting a pair of an electron from a Lewis base for making Lewis adduct. Trimethylborane is capable of accepting a lone pair, therefore, it is a Lewis acid.

On the other hand, in a Lewis adduct, the Lewis acid and the Lewis base share a pair of an electron that the Lewis base furnishes, making a dative bond.

Moreover, in a chemical reaction between NH3 and Me3B, the lone pair from NH3 forms a dative bond an empty orbital of Me3B for making an adduct NH3•BMe3. Thus, the term refers to the various contributions of Gilbert N. Lewis.

The term nucleophile and electrophile are much or less interchangeable with the Lewis base and the Lewis acid, respectively. However, these terms, especially their abstract noun produces nucleophilicity and electrophilicity.

Thus, this emphasizes the kinetic aspect of the reactivity in the reaction, with the Lewis basicity and the Lewis acidity emphasizes the thermodynamic aspect of the Lewis adduct formation.

lewis acid

Simple Lewis Acids

Many studied examples of such Lewis acids are the boron trihalides and the organoboranes, whereas, other compounds exhibit this kind of behaviour:
BF3 + F− → BF4−

In this adduct, all the four fluoride centres that are more accurately, ligands are equivalent.

BF3 + OMe2 → BF3OMe2

Both BF4− and the BF3OMe2 are the Lewis base adducts of boron trifluoride.
In various cases, adduct violates the octet rule, likewise the triiodide anion:

I2 + I− → I3−

The colour’s variability of the iodine casts back the capability of variables of the solvent in order to make adducts with the Lewis acid I2.

In many cases the Lewis acid can bind two Lewis base, a popular example being the formation of the hexafluorosilicate:

SiF4 + 2 F− → SiF62−

Complex Lewis Acids

Most compounds which are Lewis acids require an activation step before production of the adduct with Lewis base. Similarly, popular cases are the aluminum trihalides that are visible widely as Lewis acids.

Aluminum trihalides, unlike the boron trihalides, never exist in the form of AIX3, but as an aggregate and polymers that degrade by the Lewis base. A simpler case is the formation of borane adducts.

Monomeric BH3 doesn’t exist appreciably. So the formation of the borane adducts occurs because of the degradation of diborane.

B2H6 + 2 H− → 2 BH4−
In this type of case, an intermediate B2H7− is capable of isolating.
Many metal complexes of the metal work as Lewis acids, but usually only after they disassociate much weakly bound Lewis base, often water (H2O).

[Mg(H2O)6]2+ + 6 NH3 → [Mg(NH3)6]2+ + 6 H2O

H+ as Lewis acid

The proton (H+) is one of the stronger but it is also one of the Lewis acids that has most complications. Moreover, it is a convention to avoid the fact that a proton is heavily bound to solvent, as a result, the acid-base reactions are visible as the formation of adducts:

  • H+ + NH3 → NH4+
  • H+ + OH− → H2O

Application of Lewis Acids

A typical example of the application of a Lewis acid is in the Friedel-Crafts alkylation reaction. Further, the key step in this is the acceptance by AlCl3 of a chloride ion lone pair, forming AlCl4− and making the strongly acidic, i.e. electrophilic, carbonium ion.

RCl +AlCl3 → R+ + AlCl4−

Lewis bases

A Lewis base is an atomic or molecular species in which the highest occupied molecular orbital (HOMO) is localized highly.

Solved Question for You

Ques. A species which is able to accept a proton is known as:

(A). Acid
(B). Base
(C). Neutral compound
(D). Cation

Ans. (B). Base

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