Lewis acids and bases

A Lewis acid is a chemical that can accept an electron pair from a Lewis base. A Lewis base is a molecule or ion that can share a pair of electrons to form a dative bond with a Lewis acid. For example, NH₃, which is ammonia, acts as a Lewis base because it has a lone pair of electrons it can share. On the other hand, trimethylborane, written as ((CH_3)_3B) or (Me_3B), is a Lewis acid because it can accept those electrons.

When a Lewis acid and a Lewis base come together, they share the electron pair from the base to form a dative bond. This is important in many chemical reactions. For example, in a reaction between ammonia ((NH_3)) and trimethylborane ((Me_3B)), the lone pair from ammonia forms a bond with trimethylborane, making a new compound called (NH_3 \cdot BMe_3).

Lewis acids and bases are named after the American chemist Gilbert N. Lewis. Sometimes, people use the words nucleophile and electrophile instead of Lewis base and Lewis acid. These words describe how fast a reaction happens.

Depicting adducts

When a Lewis base gives electrons to a Lewis acid, we can show this with an arrow. For example, we can write Me₃B←NH₃ to mean that NH₃ is donating its electrons to Me₃B. Sometimes, we use dots to show the electrons, like Me₃B + :NH₃ → Me₃B:NH₃. Another way is to use a center dot, such as Me₃B·NH₃. This same idea is used for other compounds, like boron trifluoride diethyl etherate, written as BF₃·Et₂O. Even in crystals, like MgSO₄·7H₂O for hydrated magnesium sulfate, the dot shows how water is attached. Once formed, these bonds act like other bonds. Main articles: covalent bonding, ionic bonding

Lewis acids

Lewis acids are substances that can take a pair of electrons from another substance called a Lewis base. They are used in many chemical reactions. Simple examples include boron trihalides, like BF₃, which can bind with a fluoride ion to form BF₄⁻.

More complex Lewis acids, such as AlCl₃, often need to break apart before they can accept electrons. For example, in the Friedel–Crafts alkylation reaction, AlCl₃ accepts a chloride ion to help create a new carbon-based ion. The proton (H⁺) is also a strong Lewis acid, forming bonds with bases like ammonia (NH₃) or hydroxide (OH⁻).

Lewis bases

A Lewis base is a type of chemical that can share a pair of electrons to form a bond. Common examples include ammonia and other similar compounds. These bases are often used in chemistry to help make reactions work better, especially in creating important medicines.

The strength of a Lewis base depends on how easily it can share its electrons. Some simple examples of Lewis bases are things like water and ammonia. These bases are important in many chemical processes, especially in making catalysts that help control reactions in the production of drugs.

Hard and soft classification

Main article: HSAB theory

Lewis acids and bases can be grouped as "hard" or "soft." A hard Lewis acid is small and does not change easily. A soft Lewis acid has larger atoms that change more easily. For example, H⁺ and Zn²⁺ are hard acids, and Ag⁺ and Pt²⁺ are soft acids.

Lewis bases can also be hard or soft. Hard bases, like ammonia and water, hold their electrons tightly. Soft bases, like organophosphines and carbon monoxide, can share their electrons more easily. This classification helps us understand how strong Lewis acid-base pairs are. Hard-hard and soft-soft pairs usually form stronger bonds than hard-soft pairs.

Quantifying Lewis acidity

Scientists have many ways to measure how strong a Lewis acid is. Some of these ways look at changes in special signals from tools, like the Gutmann-Beckett method.

One important model is the ECW model. This model helps us learn about the bond between a Lewis acid and a Lewis base. It gives each acid and base special numbers, called E and C. These numbers show how they connect and help guess how strong the bond will be. The model shows that Lewis acids and bases can’t be ranked in just one way, because their strength changes depending on what they are paired with.

History

The idea of Lewis acids and bases began with a scientist named Gilbert N. Lewis. In 1923, he said an acid is something that can take a pair of electrons from another molecule. This idea came out the same year as another theory called the Brønsted–Lowry acid–base theory. Both theories help us understand acids and bases, but they look at things in different ways.

Later, in 1963, scientists found a way to sort Lewis acids and bases into "hard" and "soft" types. This helps guess how strong their reactions might be. Lewis believed that bases give a pair of electrons, and acids take them. This idea still helps scientists today when they study how different chemicals react with each other.