Acids and bases can be defined as Arrhenius acids and bases that release hydrogen (H+) ions or hydroxide (OH-) ions when dissolved in water, Brønsted-Lowry acids and bases that accept or donate protons in a solution, or Lewis acids and bases that accept or donate electron pairs in acid-base reactions.
An Arrhenius acid is a substance that releases hydrogen (H+) ions when dissolved in water. An Arrhenius base is a substance that releases hydroxide (OH-) ions when dissolved in water. Sodium hydroxide (NaOH) in the presence of water dissociates into Na+ and OH- ions. The release of hydroxide, OH-, indicates the substance is a base. Hydrochloric acid (HCl) in the presence of water dissociates into H+ and Cl-. The release of a proton, an H+ ion, indicates the substance is an acid. Water dissociates into both a hydroxide ion and a proton. However, the proton will bond to a water molecule to form a hydronium ion, H3O+.
A Brønsted-Lowry acid is a compound that can donate a proton to another compound in solution. A Brønsted-Lowry base is a compound that can accept a proton from another compound in solution. Brønsted-Lowry acids are considered proton (H+) donors, similar to the Arrhenius method. However, Brønsted-Lowry bases are considered proton (H+) acceptors, which is distinct from the Arrhenius method. A proton donor is a compound that removes a proton from another compound or solvent molecule. A proton acceptor is a compound that gives up a proton to another compound or solvent molecule.
The Lewis method categorizes acids and bases based on the transfer of electrons (an electron pair). A Lewis acid is an electron-pair acceptor in a Lewis acid-base reaction. An electron-pair acceptor is a substance that accepts a pair of nonbonding electrons. A Lewis base is an electron-pair donor in a Lewis acid-base reaction. An electron-pair donor is a substance that donates a pair of nonbonding electrons.
Lewis bases are any substance with a lone electron pair or multiple bond. A carbanion is a negatively charged carbon with three bonds and one lone electron pair. Carbanions are Lewis bases. Lewis acids are substances that contain an electron-deficient atom, such as boron and aluminum, or a carbocation. A carbocation is a positively charged carbon with three bonds and no lone pairs of electrons. Carbocations are Lewis acids.
In organic chemistry Lewis bases are often referred to as nucleophiles, and Lewis acids are often referred to as electrophiles. A nucleophile is a molecule or ion rich in electrons that donates a pair of electrons that forms a covalent bond. An electrophile is a molecule or ion that accepts electrons to form a covalent bond. In many reactions, the electron density of a nucleophile (lone pair of electrons or bond) will attack an electrophilic atom to form a chemical reaction.
Lewis Acid-Base Reaction
In water, releases H+
In water, releases OH-
Arrhenius acids and bases are defined in terms of what they release in water. Brønsted-Lowry acids and bases are defined in terms of proton acceptance and donation, and Lewis acids and bases are defined in terms of electron-pair acceptance and donation.