What is a Lewis Acid?

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Acids and bases are common terms in chemistry. But a Lewis acid is something different. Certain reactions have the characteristics of acid-base reactions but do not fit the theory of acid-base chemistry put forth by chemists Brønsted and Lowry.

Instead, chemist G.N. Lewis realized that the general concept of acid-base reactions could also include other types of reactions, including proton-transfer reactions. Read on to learn more about what Lewis acids are and whether certain molecules are Lewis acids or bases.

TL;DR (Too Long; Didn't Read)

Lewis considers electron transfer instead of proton transfer to define acids and bases. A Lewis acid accepts an electron pair while a Lewis base donates an electron pair. A Lewis acid is an electron acceptor. It is typically something electron deficient or positively charged.

Lewis did his experiments in 1923 using hydrogen (a positive ion) and hydroxide, (OH-, an anion). According to Brønsted’s theory, the hydroxide ion accepts a proton to form a covalent bond, resulting in water, H20.

In Lewis’s theory, the hydrogen ion is the important component, because it accepts electrons from the hydroxide ion to form the covalent bond.

Definition of Lewis Acid

According to Lewis, a Lewis acid is a chemical species that can form a covalent bond by accepting an electron pair from another chemical species. Many things that are not considered acids can be defined as Lewis acids if they are able to accept electrons. Lewis acids are often described as having vacant orbitals.

There is also a definition of Lewis base. A Lewis base is the opposite, in that it is defined as a species that can form a covalent bond by donating an electron pair to another species.

Metal cations such as AL3+ and FE3+ are Lewis acids. The positive charge of the metal cation attracts electrons.

What Is a Lewis Acid Catalyst?

A Lewis acid catalyst is one that works as all catalysts do. Catalysts increase the rate of chemical reaction. A Lewis acid catalyst increases the reactivity of a substrate by accepting electrons but is not involved in the reaction itself.

Is AlCl3 a Lewis Acid?

A Lewis acid attracts electrons and has vacant orbitals where the attracted electrons can go. With aluminum, a total of 17 valence electrons are present. Since it has an incomplete set of electrons, there is room for another electron. This means ALCl3 is a Lewis acid. AlCl3 can accept electrons.

Is NH3 a Lewis Acid or Base?

NH3, or ammonia, has a lone pair of electrons. It can donate those electrons to chemical species that will accept electrons. Because of this, NH3 is a Lewis base.

When NH3 is neutralized by HCL in water, the NH3 is the electron donor to the hydrogen ion. The result is NH4.

The Lewis and the Brønsted-Lowry concepts describe similar things, but they are different ways of looking at chemical reactions. While the Brønsted-Lowry explanation is a strict way of defining acid-base chemistry, Lewis gave us another view of reactions that can also be described with similar chemistry.