Lewis Acids and Bases

Lewis acids and bases are described by the Lewis theory of acid-base reactions as electron pair acceptors and electron pair donors respectively. Therefore, a Lewis base can donate a pair of electrons to a Lewis acid to form a product containing a coordinate covalent bond. This product is also referred to as a Lewis adduct. An illustration detailing the reaction between a Lewis acid and base leading to the formation of a coordinate covalent bond between them is given below.

Lewis Acids and Bases

Lewis acids and bases are named after the American chemist Gilbert Newton Lewis, who also made invaluable contributions in the fields of thermodynamics and photochemistry.

Lewis Acid

Lewis Acids are the chemical species which have empty orbitals and are able to accept electron pairs from Lewis bases. This term was classically used to describe chemical species with a trigonal planar structure and an empty p-orbital. An example of such a Lewis acid would be BR3 (where R can be a halide or an organic substituent).

Water and some other compounds are considered as both Lewis acids and bases since they can accept and donate electron pairs based on the reaction.

Examples of Lewis Acids

Some common examples of Lewis acids which can accept electron pairs include:

  • H+ ions (or protons) can be considered as Lewis acids along with onium ions like H3O+.
  • The cations of d block elements which display high oxidation states can act as electron pair acceptors. An example of such a cation is Fe3+.
  • Cations of metals such as Mg2+ and Li+ can form coordination compounds with water acting as the ligand. These aquo complexes can accept electron pairs and behave as Lewis acids.
  • Carbocations given by H3C+ and other trigonal planar species tend to accept electron pairs.
  • The Pentahalides of the following group 15 elements can act as Lewis acids – Antimony, Arsenic, and Phosphorus.

Apart from these chemical compounds listed above, any electron-deficient π system can act as an acceptor of electron pairs – enones, for example.

Lewis Base

Atomic or molecular chemical species having a highly localized HOMO (Highest Occupied Molecular Orbital) act as Lewis bases. These chemical species have the ability to donate an electron pair to a given Lewis acid in order to form an adduct, as discussed earlier.

The most common Lewis bases are ammonia, alkyl amines, and other conventional amines. Commonly, Lewis bases are anionic in nature and their base strength generally depends on the pKa of the corresponding parent acid.

Examples of Lewis Bases

Examples of Lewis bases which have an ability to donate an electron pair are listed below.

  • Pyridine and the derivatives of pyridine have the ability to act as electron pair donors.
  • The compounds in which Oxygen, Sulfur, Selenium, and Tellurium (which belong to group 16 of the Periodic Table) exhibit an oxidation state of -2 are generally Lewis bases. Examples of such compounds include water and ketones.
  • The simple anions which have an electron pair can also act as Lewis bases by donating these electrons. Examples of such anions include H and F. Even some complex anions, such as the sulfate anion (SO42-) can donate pairs of electrons.
  • The π-systems which are rich in electrons (such as benzene, ethyne, and ethene) exhibit great electron pair donating capabilities.

Weak Lewis acids have strong conjugate Lewis bases. Apart from this, many chemical species having a lone pair of electrons such as CH3 and OH are identified as Lewis bases due to their electron pair donating capabilities.

Chemical Reactions Between Lewis Acids and Bases

Reactions with the H+ ion

The H+ ion acts as a Lewis acid and H2O acts as a Lewis base. The reaction between the water molecule and the proton yields a hydronium ion (H3O+), as illustrated below.

Formation of Hydronium Ion from a Lewis Acid and Base

Here, the oxygen atom donates an electron pair to the proton, forming a coordinate covalent bond in the process. The resulting Lewis acid has a +1 charge associated with it. Another example of a reaction in which the H+ ion acts as a Lewis acid is its reaction with ammonia (NH3) to form an ammonium ion (NH4+).

Formation of Ammonium Ion from a Lewis Acid and Base

In this reaction, the proton receives an electron pair from the nitrogen atom (belonging to the ammonia molecule). The formation of a coordinate covalent bond between the two results in the formation of a Lewis adduct (the ammonium cation).

Reaction Between Ag+ and Ammonia

In this reaction, two Lewis bases form an adduct with one Lewis acid, as illustrated below.

Reaction Between a Lewis Acid and Base Example 1

Here, ammonia acts as a Lewis base and the silver ion acts as a Lewis acid. Each nitrogen atom donates an electron pair to Ag+, resulting in two separate coordinate covalent bonds. The adduct formed from the Lewis acid and base has the chemical formula Ag(NH3)2+.

Reaction Between the Fluoride Ion and Boron Trifluoride

This reaction features the formation of a coordinate bond between the fluoride anion (F) and boron trifluoride (BF3).

Reaction Between a Lewis Acid and Base Example 2

Here, F acts as an electron pair donor whereas BF3 accepts the electron pair. The reaction between the Lewis acid and base results in the formation of an adduct with the chemical formula BF4.

Applications of Lewis Acids and Bases

Some important applications of Lewis acids and bases are provided below.

Lewis acids play a vital role as a catalyst in the Friedel-Crafts reaction –  AlCl3 accepts a lone pair of electrons belonging to the chloride ion leading to the formation of AlCl4 in the Friedel-Crafts alkylation process.

This also leads to the formation of the highly electrophilic carbonium ion which acts as a strong Lewis Acid. The chemical reaction can be written as follows.

RCl + AlCl3 ⟶ R+ + AlCl4

In the field of organic chemistry, Lewis acids are widely used to encourage many cationic or pseudo-cationic chemical reactions.

Lewis bases have immense applications in the modification of the selectivity and the activity of metallic catalysts. For the production of pharmaceuticals, asymmetric catalysis is an important part of enantioselective synthesis. In order to enable asymmetric catalysis, chiral Lewis bases are often used to confer chirality on catalysts.

Several Lewis bases have the ability to form many bonds with Lewis acids. These compounds are also called ‘multidentate Lewis bases’ or ‘chelating agents’ and have a wide range of industrial and agricultural applications.

Frequently Asked Questions

What is the Lewis Definition of Acids and Bases?

Gilbert N. Lewis put forward his definitions of acids and bases in the year 1923. According to this definition, an acid is an electron pair acceptor and a base is an electron pair donor.

Therefore, a Lewis acid can be defined as a chemical entity that can accept a pair of electrons from a Lewis base to form a coordinate covalent bond with it. A Lewis base can be defined as a chemical species in which the highest occupied molecular orbital (HOMO) is highly localized, giving it the ability to donate pairs of electrons.

Does Hydrochloric Acid Qualify as a Lewis Acid?

Hydrochloric acid cannot be classified as a Lewis acid since it cannot accept an electron pair. However, this compound dissociates into its constituent ions, liberating H+ ions (which are considered as Lewis acids).

Due to its inability to accept electron pairs, hydrochloric acid is often referred to as a classical acid rather than a Lewis acid. Also, HCl does not form any adducts when reacted with Lewis bases.

Which Metals act as Lewis Acids?

Metal ions such as Na+, Li+, and Mg2+ can accept pairs of electrons from a donating species since they contain one or more empty orbitals. These ions tend to form coordination compounds by accepting electron pairs from ligands.

Most metal ions exist in a coordinated structure with some ligands. For a Lewis base to donate an electron pair to them, the metal ion must first dissociate from the ligand. The Lewis adducts formed by these ions are generally complexes as well.

List some Examples of Lewis Bases.

A few examples of Lewis bases are listed below.

  • Amines with the general formula R-NH3, such as methylamine.
  • The fluoride ion (F)
  • Ammonia (NH3)
  • Water (H2O)
  • Acetone and many other ketones.

Compounds of the chalcogens (oxygen, sulfur, selenium, and tellurium) in which they exhibit an oxidation state of -2 generally act as Lewis bases.

Thus, the definition, chemical behavior, and the applications of Lewis acids and bases are briefly discussed in this article. To learn more about this definition of acids and bases along with other related topics, register with BYJU’S and download the mobile application on your smartphone.

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