Lewis acid and Lewis base

Question

Define Lewis acid and Lewis base.

Answer 1

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.

Lewis Base

Atomic or molecular chemical species having a highly localized HOMO (The 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 pK_a of the corresponding parent acid. Since Lewis bases are electron-rich species that have the ability to donate electron-pairs, they can be classified as nucleophiles. Similarly, Lewis acids can be classified as electrophiles (since they behave as electron-pair acceptors).

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. Thus, these compounds can be classified as Lewis bases.
• The compounds in which Oxygen, Sulphur, 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 (SO₄^2-) 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.