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Lewis Acid: Definition, Theory & Examples

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  • 0:01 Definition of a Lewis Acid
  • 0:20 Background & Theory
  • 1:56 Lewis Acids
  • 2:57 Examples of Lewis Reactions
  • 4:29 Lesson Summary
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Lesson Transcript
Instructor: Nicola McDougal

Nicky has taught a variety of chemistry courses at college level. Nicky has a PhD in Physical Chemistry.

This lesson will introduce a new way of looking at acid and base chemistry. We will learn how to identify Lewis acids and how they react with Lewis bases. A quiz will test what we have learned.

Definition of a Lewis Acid

A Lewis acid is any species that can accept a pair of electrons. A Lewis base is a species that can donate a pair of electrons to an electron acceptor. The bond formed in a Lewis acid/base reaction is called a coordinate covalent bond.

Background and Theory

Before delving into the specifics of the Lewis acid/base reaction, let's back up a bit, all the way to the Bronsted-Lowry Acid-Base theory. This theory focuses on acids and bases acting as proton donors and acceptors. However, this theory is very restrictive and sometimes doesn't adequately explain reactions, such as in solids and gases.

In 1923, G.N. Lewis proposed an alternative theory to describe acids and bases. His theory gave us an explanation of acids and bases based on structure and bonding. Through the use of the Lewis definition of acids and bases, chemists are now able to predict a wider variety of acid-base reactions. Lewis' theory used electrons instead of proton transfer and specifically stated that an acid is a species that accepts an electron pair while a base donates an electron pair.

The reaction of a Lewis acid and a Lewis base produces a coordinate covalent bond. You will often see a coordinate covalent bond shown as an arrow instead of a line in a Lewis structure. The arrow points from the donor atom to the acceptor atom.

A coordinate covalent bond is just a type of covalent bond in which one reactant gives its electron pair to another reactant. In this case, the Lewis base donates its electrons to the Lewis acid. The resulting product of this reaction is called an addition compound or more commonly, an adduct.

Lewis Acids

Various species can act as Lewis acids. All cations are Lewis acids since they are able to accept electrons. Examples include copper (Cu2), iron (Fe2+ and Fe3+), and hydrogen ion (H+). An atom, ion, or molecule with an incomplete octet of electrons can accept electrons. Examples include boron trifluoride (BF3) and aluminum fluoride (AlF3).

Molecules where the central atom can have more than eight valence shell electrons (or an expanded octet) can be electron acceptors. Examples include silicon tetrabromide (SiBr4) and silicon tetrafluoride (SiF4). Molecules that have multiple bonds between two atoms of different electronegativities. Examples include carbon dioxide (CO2) and sulfur dioxide (SO2).

Examples of Lewis Acid and Base Reactions

There are many reactions involving Lewis acids and bases. Examine these two examples. The curved arrow shows the direction the electrons travel. A new bond is formed. Can you identify why each Lewis acid can accept electrons?

In the first reaction, boron trifluoride (BF3) can accept a pair of electrons because boron has an incomplete octet (just six electrons). Remember, boron is one of those exceptions to the octet rule! The product formed from this reaction is tetrafluoroborate (BF4). In the second reaction, hydrogen ion (H+) is a cation and can accept a pair of electrons from ammonia (NH3). The product formed is the ammonium ion (NH4+).

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