Lewis acids and bases

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A Lewis acid is a substance that can accept an electron pair. A Lewis base donates an electron pair. When they meet, they form a Lewis adduct, connected by a dative bond in which the base provides the electron pair that the acid accepts. For example, ammonia (NH3) is a Lewis base, and trimethylborane (Me3B) is a Lewis acid; together they form NH3•BMe3.

We often show this with arrows, pointing from the base to the acid to indicate electron donation. Sometimes a center dot is used to show the donated electrons, as in adducts like BF3·Et2O or Me3B·NH3. Adducts can sometimes violate the octet rule, reflecting the variety of bonding situations in Lewis chemistry.

Lewis acids are diverse. They include simple reactants like boron trihalides and phosphorus or antimony halides, as well as metal-containing species. The proton (H+) is one of the strongest Lewis acids, though its behavior depends on the solvent. A classic example is Friedel–Crafts alkylation, where AlCl3 accepts a chloride to create a very reactive electrophile.

Lewis bases are typically electron-rich molecules such as ammonia, amines, or pyridine. They act as nucleophiles in many reactions. The hardness/softness idea (HSAB theory) helps predict how strongly acids and bases will pair: hard acids prefer hard bases, soft acids prefer soft bases, and hard–hard or soft–soft interactions are often stronger.

There are several ways to judge how strongly a Lewis acid or base will interact. Spectroscopic methods (like NMR or IR shifts) and models such as the ECW (electrostatic and covalent) model quantify interaction strength. The ECW model uses parameters that describe electrostatic and covalent contributions to bond formation.

A Lewis base is usually also a Brønsted–Lowry base (it can donate electrons to a proton), but a Lewis acid does not have to be a Brønsted–Lowry acid. The two theories are related but distinct, and HSAB adds another layer to understanding reactivity.

In real chemistry, many adducts form in steps or as aggregates, and some bonds form by “activating” the acid or base first. The idea of a dative (coordinate) bond helps chemists predict what will react and how.

In industry and catalysis, choosing the right Lewis acid or base can steer reactions and even control outcomes like stereochemistry. Chiral Lewis bases are used in asymmetric synthesis to make specific drug-like molecules.