Lewis acids and Lewis bases can be defined as chemical species, capable of transferring an electron pair from one species to another by a sort of donation. The result is the formation of a coordinate covalent bond and the compound formed is called an adduct.

general reaction of lewis acid and base

The above figure shows the reaction between Lewis acid and Lewis base with an example, in which the Lewis base (electron donor) attacks the lewis acid (electron acceptor). A coordinate covalent complex is formed this way.

Lewis’s concept of acids and bases was introduced by Gilbert Newton Lewis in 1923. According to Lewis’s concept, a compound capable of attracting a shared pair of electrons from another molecule is called an acid. On the converse, a compound able to donate an electron pair (necessarily a lone pair) to another atom or molecule acts as a base.

Difference between lewis acids and Lewis bases

Lewis acids Lewis bases
It can accept electron pairs It can donate electron pairs
Lewis acids act as electrophiles Lewis bases act as nucleophiles
It uses electrons from LUMO for bonding It uses electrons from HOMO for bonding
All metal ions or atoms act as Lewis acids All anion ions act as Lewis bases
Molecules that contain polar double bonds are Lewis acids Molecules that contain non-polar multiple bonds make Lewis bases
Molecules in which the central atom expands due to its empty d-orbitals e.g. FeCl3 are Lewis acids
All metal oxides that have no d-orbitals e.g. CaO, MgO, etc. are Lewis bases
Examples are 
H+, Cu+, Zn+, Mg2+, Li+, etc
Cl-, OH-, CH3-, NH3, H2O, etc

Lewis Acid

Lewis acid is a substance that is always an electron pair acceptor. Classically, It is described by a trigonal planar structure and an empty p or d-orbital.

Example of lewis acid

  • Protons can be studied as Lewis acid.
  • Cations of d-block elements act as Lewis acid e.g. Cu+, Zn+, etc.
  • Carbocations are the best example of Lewis acid e.g. H3C+, etc.
  • Enones are also good examples.
  • Ligands (complex coordinate metal ions) of Mg2+ and Li+ act as Lewis acids because there is a hybridization status of accepting electrons.

Lewis Base

Lewis bases are electron-rich species that also act as nucleophiles. These are the chemical species that have the most localized highest occupied molecular orbitals (HOMO). These species can donate their electron pairs to Lewis acids for the formation of coordinate covalent bonds or complexes.

Example of lewis base

  • Simple anions like Cl, Br, F. etc.
  • Complex anions of metal such as ferrate, cuprate, plumbate, and argenate (diaminoargentum fluoride).
  • Chemical species which contain lone pairs of electrons, for example, NH3, H2O, HCl, etc.
  • Compounds that contain π-electrons system i.e. ethene, ethyne, benzene, etc.

Lewis adduct

When a Lewis base donates its electrons from the highest occupied molecular orbital, HOMO (high energy) to the lowest unoccupied molecular orbital, LUMO (low energy) of Lewis acid, a coordinate covalent bond or complex is formed called Lewis adduct. It is represented with dots in the center of the complex as a gesture of showing electrons.

For example, trimethyl boron is an electron pair acceptor, a lewis acid whereas ammonia being electron-rich (with nitrogen), is a lewis base. When the electron pair from the nitrogen of ammonia is transferred to the boron of trimethyl boron, a lewis adduct is formed as shown.

Lewis adduct, a product of lewis acid and lewis base

Common reactions between Lewis acids and Lewis bases

The chemical reaction of fluoride ion with boron trifluoride

In this reaction, fluoride anion is an electron donor, and boron from boron trifluoride is an electron acceptor. This means that the fluoride ions act as a Lewis base and boron trifluoride as a Lewis acid. An adduct is formed when Lewis base (F) attacks the Lewis acid (BF3).

reaction of fluoride ion with boron trifluoride

The chemical reaction of the proton with water and ammonia

A hydrogen ion or proton (H+) can react with water and ammonia because of are positively charged species. In fact, a proton (H+) is a Lewis acid being electron deficient or with no electrons at all.

In the case of water, the oxygen atom donates its electron to the proton (H+). As a result of this process, a hydronium ion complex (H3O+) is formed.

reaction of proton with water

As of ammonia, its nitrogen atom acts as Lewis base because of the one pair of electrons present in its p subshells. The adduct, an ammonium ion (NH4+) is formed when this nitrogen atom donates its electron to the proton (H+).

reaction of proton with ammonia

The chemical reaction between ammonia and water

Ammonia acts as Lewis base while the hydrogen of water may act as a Lewis acid. The hydrogen atom accepts electron pairs from the nitrogen atom, and as a result, an adduct is formed. It yields an ammonium ion (NH4+) and a hydroxide ion as a product of this reaction.

reaction between ammonia and water

Related resources

Applications of Lewis acids and Lewis base

Lewis acid

  • They are used in the Friedel-Crafts reactions (acylation and alkylation).
  • Lewis acids have vast applications acting as an electrophile.

Lewis base

  • In the modification of selectivity and activity of the metallic catalyst, a lewis base may be used to make complexes, etc.
  • Production of pharmaceuticals involves lewis bases as the main product.
  • Asymmetric catalysis has an important role in the enantioselective synthesis, which involves lewis base properties.
  • Lewis bases may be used to grant chirality on the catalysts.
  • They act as chelating agents or multidentate Lewis bases.

Key takeaway(s)

Lewis acid vs Lewis base

Concepts Berg

Which metals act as Lewis acids?

Metal ions such as sodium (Na1+), magnesium (Mg2+), and cerium (Ce3+), etc act as Lewis acids because they have empty orbitals. They can coordinate with bases to form adducts. This means all metal ions that have one or more empty orbital can be Lewis acid.

Does hydrochloric acid qualify as a Lewis acid?

Yes, hydrochloric acid is qualified as Lewis acid because it can accept electron pairs from other compounds like H2O. When water reacts with HCl, it donates its electron to hydrochloric acid.

Is ethyl acetate a Lewis base?

Ethyl acetate is a Lewis base. It can donate its electron pair to Lewis acids. The result of this reaction is the formation of a Lewis adduct.

BF3 is Lewis acid or base?

BF3 is a Lewis acid. Boron trifluoride accepts electron pairs from the Lewis bases. Boron is metal and has fewer electrons in outermost shells. That’s why it is termed a Lewis acid.

Identifying Lewis acids and bases in reactions?

Identifying the Lewis acid and bases is very simple by noting which is electron acceptor and electron donor. Lewis acid can accept electron pairs while Lewis base can donate.

Is SO2 a Lewis acid or base?

Sulfur dioxide (SO2) is a very good Lewis acid. The reason is that it contains a pi-electronic system due to the multiple bonds and larger number of electrons. The pi-electrons can be easily shifted towards the more electronegative atom which is oxygen in this case, as a result, sulfur becomes relatively more positive and accepts electron pairs.

Classify the following as Lewis acid or Lewis base NH4, NH3?

Ammonia (NH3) is a Lewis base while ammonium ion (NH4) is a Lewis acid. When the H+ ion reacts with ammonia. A result of this reaction NH3 donates electron pairs to the proton (H+) and the formation of NH4 ion takes place which acts as a nucleophile.

Why is carbon dioxide a Lewis acid?

Carbon dioxide (CO2) is a very good Lewis acid. The reason is that it contains a pi-electron system due to the double bonding and sp-hybridization. The pi-electrons can be easily shifted towards the more electronegative atom which is oxygen in this case. As a result, carbon becomes more positive or electrophilic to accept electron pairs.

On what factors does the strength of Lewis acid depend?

The strength of Lewis acid depends on its electrophilic nature. The more positive charge on the metal ion shifts it towards the highly acidic character e.g. Na+, K+ are weak acids as compared to Al3+, Fe3+.

The factors affecting this strength are:

  1. Bond polarity
  2. Bond strength

Is water an acid or base?

Pure water is neither acidic nor basic. When two moles of water dissociate as a result hydronium ion (H3O+) and hydroxyl ion (OH) are formed. Hydronium ions have an acidic character while hydroxyl ions are basic. The amount of (H3O+) ions and (OH) ions are always equal in pure water, which cancels the effect of each other, so water is a neutral substance.

Why is pyridine more basic than pyrrole?

Pyridine and pyrrole have lone pairs on the nitrogen atoms. Lone pairs are localized in pyridine while they are in resonance in pyrrole. Pyridine can donate its electron pair easily due to localization while pyrrole cannot donate electron pairs that easily because of being in resonance and delocalized. That is why, pyridine is more basic than pyrrole.

What is common in acid and base?

Both acid and base are electrolytes. When dissolved in water they give ions and conduct electricity.

Are acids anions and bases cations?

Acid is an electron pair acceptor which is a character of the cation. The base is an electron-pair donor, that’s why it is an anion.

Which is more acidic, SO3 or SO2?

SO3 has more acidic behavior than SO2 due to a greater ionization constant and a smaller pKa value. Moreover, sulfur trioxide has a higher oxidation state for sulfur than sulfur dioxide, making SO3 more acidic than SO2.

Which is more basic pyridine, triethylamine, or ammonia?

Pyridine > ammonia > triethylamine

Pyridine has a planar structure which increases its electron pair donation ability. Ammonia and triethylamine have tetrahedral structures. Due to the bulky appearance of the ethyl group in triethylamine, a steric hindrance establishes as compared to hydrogen atoms in ammonia, which makes triethylamine less basic than ammonia.