Acids and Bases

Three major definitions of acids and bases have influenced the thinking of chemists. In 1884, Svante Arrhenius formulated the first of these definitions. Then, in 1923, independently of each other, Johannes N. Brønsted and Thomas M. Lowry developed the second. The third definition grew from Gilbert Newton Lewis's theory of covalent bonding, which he proposed in 1916.
The first definition, proposed by Svante Arrhenius in his doctoral dissertation, was so revolutionary that he was almost denied his Ph.D. However, in 1903, he received the Nobel Prize in chemistry for his theory. His theory states that a stable ionic compound that is soluble in water will break down, or dissociate, into its component ions. This dissociation, or ionization, of a compound in water, leads to Arrhenius' definition of an acid and a base. An acid is a substance that, when added to water, increases the concentration of hydronium ions, H3O⊕. Because Arrhenius regarded acid-base reactions as occurring only in water, he frequently called the hydronium ion a hydrogen ion, H⊕. An H⊕ ion is a proton, or a hydrogen that is electron-deficient. Thus, a base is a substance that, when added to water, increases the concentration of hydroxide ions, -OH. The following statements summarize his definition.

An Arrhenius acid is a source of H⊕ ion.
An Arrhenius base is a source of -OH ion.

The Arrhenius acid-base theory provided a good start toward understanding acid-base chemistry, but it proved much too limited in its scope.
Brønsted and Lowry developed a more general acid-base definition than that of Arrhenius. Although they considered reactions other than those that take place in aqueous solutions, they still said acids were molecules that donate a hydrogen ion—such as HCl and H2SO4. However, they broadened the definition of bases to include any compound that accepts a proton. The basis of their acid-base definition is that in a reaction a proton transfers between reactants. Thus, acids involving a transfer of H⊕ ions are sometimes called proton acids. According to the Brønsted-Lowry definition, an acid is any molecule or ion that donates a proton to another molecule or ion, and a base is any molecule or ion that receives that proton. The following statements briefly summarize the Brønsted-Lowry definition.
A Brønsted-Lowry acid is a proton donor.
A Brønsted-Lowry base is a proton acceptor.
An example of the Brønsted-Lowry definition is the reaction between hydrogen chloride and sodium hydroxide:

 

In this reaction, HCl is the acid because it is the source of protons, or hydrogen ions; NaOH is a base because the hydroxide ion is the proton acceptor. The following reactions further illustrate the Brønsted-Lowry acid-base definition.

When an acid and a base react with each other, the reactants and products are in equilibrium with each other. Note the two-way arrows. They indicate that this is an equilibrium reaction. That is, the reactants on the left side of the equation are reacting and forming product, and the products on the right side are also reacting and forming the starting reactants. Chemists call the acid and base on the right side of the equation the conjugate acid and conjugate base. The reaction below is labeled to show the conjugate acid and conjugate base.

 

A hydrogen of sulfuric acid (H2SO4) is the acid, and the nitrogen of ammonia (NH3) is the base. They react to form the hydrogen sulfate anion (HSO4-) and the ammonium ion (NH4⊕). The ammonium ion is the conjugate acid of ammonia. The bisulfate ion is the conjugate base of the sulfuric acid.

Like Brønsted and Lowry, G. N. Lewis defined acids and bases in a broader scheme than Arrhenius did. Lewis noted that there are a number of reactions that look like acid-base reactions but do not involve the transfer of a proton. Instead, they involve the interaction of a pair of nonbonding electrons. From that observation, he defined an acid as a molecule that forms a covalent bond by accepting a pair of electrons and a base as a molecule that forms a covalent bond by donating a pair of electrons. Below is a simplified statement of the Lewis definition of acids and bases.
A Lewis acid is an electron-pair acceptor.
A Lewis base is an electron-pair donor.

 

Because a Lewis acid accepts a pair of electrons, chemists call it an electrophile, from the Greek meaning "lover of electrons." They call the base a nucleophile, or "lover of nuclei," because it donates the electrons to a nucleus with an empty orbital. In a chemical reaction, a nucleophile seeks a nucleus, or a positive charge, and an electrophile seeks electrons, or a negative charge. Fundamental to organic chemistry is the fact that nearly all the reactions that you will study are reactions of an acid with a base or, more commonly, of an electrophile with a nucleophile.

 

Chemists use a curved arrow () to show electron movement. A curved arrow points from the electron-rich reactant, the base or nucleophile, toward the electron-poor reactant, the acid or electrophile. Rewriting the previous two reactions using a curved arrow shows the movement of electrons. In each reaction, a pair of nonbonding electrons from a nucleophile reacts with an electrophile to form a bond.

 

All chemical reactions involve orbital interactions. The orbital description of a reaction can help you understand how chemical reactions occur. As you study the various reactions presented in this book, think about the orbitals involved in the reactions. Figure 5.1 is a molecular orbital picture of ammonia reacting with boron trifluoride to form a new bond. Ammonia is a base with a pair of nonbonding electrons. The nitrogen of ammonia is sp3 hybridized. Boron trifluoride is an acid with an incomplete octet of electrons. The boron is sp2 hybridized with an empty p orbital. The reaction occurs when an sp3 orbital of ammonia overlaps with the empty p orbital of boron trifluoride. In the process, the boron becomes sp3 hybridized. With this overlap the two molecules form a new bond.