Acids and bases may be defined by more than one method. Arrhenius acids and bases are described by their ability to dissociate protons and hydroxide ions. Arrhenius acids dissociate and donate protons, and Arrhenius bases dissociate and donate hydroxide ions. A Brønsted-Lowry acid is a proton donor, similar to the Arrhenius definition. However, a Brønsted-Lowry base is considered a proton acceptor. Lastly, Lewis acids are defined as electron acceptors (electrophiles), and Lewis bases are electron donors (nucleophiles).
In organic chemistry, the common definition of an acid or a base is a proton donor, which is a compound that gives up a proton to another compound or solvent molecule, or a proton acceptor, which is a compound capable of removing a proton from another compound or solvent molecule.
The acid dissociation constant, Ka, indicates the amount of acid that will dissociate in solution. The base dissociation constant, Kb, indicates the amount of base that will dissociate in solution. The pKa and pKb determine which direction a reaction will proceed. Acid-base reactions exist at equilibrium, a state in which the rates of the forward and reverse reaction are equal. The position of this equilibrium is determined either by comparing the pKas of the reactant acid and the conjugate acid of the base or by comparing the molecular structures of the two acids. The equilibrium will favor the weaker acid, as it has a lower potential energy and is therefore a more stable species.
There are four effects, known as ARIO (atom effect, resonance effect, inductive effect, and orbital effect), that may be used with pKa to predict the equilibrium and direction of a reaction. These effects focus on bond stability and the species' ability to break or make a bond. Acid-base reactions generally start with a proton transfer step in the mechanism. ARIO and pKa are used to predict the equilibrium and direction of a reaction.
At A Glance
- Acids and bases can be defined as Arrhenius acids and bases, Brǿnsted-Lowry acids and bases, or Lewis acids and bases.
- Acid-base reactions most often exist in a state of equilibrium in which the rates of the forward and reverse reactions are equal.
- The direction that an acid-base reaction proceeds can be determined by comparing the reactant acid to the conjugate acid of the other component. This results in the reaction proceeding toward the acid with a higher pKa.
- ARIO can also be used to predict acid-base reactions based on the ARIO effects of acid and conjugate acid.
- The effect of the atom connected to the hydrogen can affect the acidity of that hydrogen. The periodic trends are used to determine the acidity.
- If a conjugate base has resonance structures, it will be a more stable (weaker) conjugate base and therefore formed from a stronger acid.
- The inductive effect comes from the presence and location of electronegative atoms. The inductive effect stabilizes (weakens) a conjugate base; therefore, it was formed from a strong acid.
- The hybridization of the atom connected to the hydrogen also helps explain qualitatively the strength of an acid. The more s character in the acid, the stronger the acid. An sp hybridized orbital (alkyne) is more acidic than an sp2 hybridized orbital (alkene), which is more acidic than an sp3 hybridized orbital (alkane).
- Typical acid-base reactions involve a proton transfer mechanistic step. The arrows show how the hydrogen atom breaks off the acid and generates a conjugate base. Proton transfer reactions are very fast.