Lewis acids and bases

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

When a Lewis acid and a Lewis base come together, they share the electron pair provided by the base, creating a dative bond. This process is important in many chemical reactions. For instance, in a reaction between ammonia ((NH_3)) and trimethylborane ((Me_3B)), the lone pair from ammonia forms a dative bond with the empty orbital in trimethylborane, resulting in an adduct called (NH_3 \cdot BMe_3).

Lewis acids and bases are named after the American physical chemist Gilbert N. Lewis, who introduced these ideas. Sometimes, the words nucleophile and electrophile are used in place of Lewis base and Lewis acid. These terms focus more on how fast a reaction happens, while Lewis basicity and acidity look at the stability of the products formed.

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, though they often have a mix of characteristics between covalent and ionic bonding.
Main articles: covalent bonding, ionic bonding

Lewis acids

Lewis acids are substances that can accept 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 give away 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 give away 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 based on whether they are "hard" or "soft." A hard Lewis acid is small and less easily changed, while a soft Lewis acid has larger atoms that are more easily changed. For example, H⁺ and Zn²⁺ are hard acids, and Ag⁺ and Pt²⁺ are soft acids.

Similarly, Lewis bases can also be hard or soft. Hard bases, like ammonia and water, have atoms that hold their electrons tightly, while soft bases, like organophosphines and carbon monoxide, have atoms that can share their electrons more easily. This classification helps us understand how strong different Lewis acid-base pairs will be, with hard-hard and soft-soft pairs usually forming stronger bonds than hard-soft pairs.

Quantifying Lewis acidity

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

One important model is the ECW model. This model helps us understand how strong the bond is between a Lewis acid and a Lewis base. It gives each acid and base special numbers, called E and C, that show how they connect. These numbers help predict how strong the bond will be. The model shows that there isn’t just one way to rank Lewis acids or bases, because their strength can change depending on what they are paired with.

History

The idea of Lewis acids and bases started with a scientist named Gilbert N. Lewis. In 1923, he described an acid as something that can accept 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 a little differently.

Later, in 1963, scientists developed a way to sort Lewis acids and bases into "hard" and "soft" types. This helps predict how strong their interactions might be. Lewis thought that bases donate a pair of electrons, and acids accept them. This idea still helps scientists today when studying how different chemicals react with each other.