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 are named after the American chemist Gilbert Newton Lewis, who also made invaluable contributions in the fields of thermodynamics and photochemistry.
Lewis Acids are nothing but 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.
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.
Applications of Lewis Acids and Bases
Some important applications of Lewis acids and bases are provided below.
One important application of Lewis acids is 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 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.
Thus, many Lewis acids and bases have a wide range of applications in the field of chemistry.