Lewis acid

Lewis acid is a concept in chemistry that describes a class of chemical substances characterized by their ability to accept an electron pair from another molecule, which is termed a Lewis base. This definition was introduced by Gilbert N. Lewis in 1923, expanding the traditional concept of acids and bases. Unlike the Brønsted–Lowry acid-base theory, which defines acids as proton donors, the Lewis theory describes acids based on the acceptance of electron pairs.

Characteristics

A Lewis acid is typically a molecule or an ion that has an incomplete octet of electrons, making it capable of accepting an electron pair to achieve a more stable electronic configuration. Common examples of Lewis acids include metal cations like Fe³⁺, Al³⁺, and Cu²⁺, and certain compounds like BF₃ and AlCl₃. These acids can form coordinate covalent bonds with Lewis bases, which are electron pair donors.

Applications

Lewis acids are widely used in organic chemistry and industrial chemistry for various applications. They play a crucial role in catalyzing reactions such as Diels-Alder, Friedel-Crafts, and others. In these reactions, the Lewis acid acts to stabilize intermediates or activate substrates, enhancing the reaction rate and selectivity.

Examples

• BF₃ (Boron trifluoride): Often used as a catalyst in organic reactions.
• AlCl₃ (Aluminum chloride): Commonly employed in Friedel-Crafts reactions for the alkylation and acylation of aromatic compounds.
• TiCl₄ (Titanium tetrachloride): Utilized in the production of polyolefins through Ziegler-Natta polymerization.

Reactivity and Selectivity

The strength of a Lewis acid can vary widely and is influenced by several factors, including the positive charge of the acid and the electronegativity of the atom to which the electron pair is being donated. Stronger Lewis acids are typically more electrophilic, making them more reactive towards electron-rich Lewis bases.

Environmental and Safety Considerations

Handling of Lewis acids requires careful consideration due to their reactivity and potential for corrosive behavior. Safety measures include the use of appropriate protective equipment and proper storage conditions to prevent accidental exposure and reactions.

See Also

• Acid
• Base
• Catalysis
• Electrophile
• Nucleophile