Definition In general, an acid is a substance that can donate a proton (H+) and a base is a substance that can accept a proton. Any proton in an organic molecule can potentially be donated. The most acidic proton in a molecule would be donated first. Any lone pair in an organic molecule can act as the proton acceptor. An acid (HA) reacts with a base (in this case H2O) to form the conjugate base of the acid (A-) and the conjugate acid of the base (H3O+).
HA + H2O
H 3O + + A -
The equilibrium constant for an acid-base reaction is expressed as a dissociation constant (Kd ), with the concentrations of the reactants in the denominator and the concentrations of the products in the numerator.
Kd = [ H3O+][A-] [HA][ H2O]
Most organic acids are weak. They do not ionize (dissociate) very much in water. As a result, in dilute solutions of organic acids, the concentration of water is essentially unchanged by the ionization process. It is more convenient in most cases to leave the concentration of water out of the equation, and to define a new constant, the acid ionization constant, Ka. The equation for Ka incorporates the concentration of water as a constant value.
Ka = [ H3O+][A-] [HA]
Ka = Kd [H2O]
Organic acid ionization constants can cumbersome to work with because they are often very small numbers. Acetic acid, for example, has a Ka is value of about 0.00001 (or 10-5), phenol has a Ka of 10-10. It is usually more convenient to use a logarithmic scale to handle these numbers. Accordingly, Ka values are transformed to pKa values using the formula
Thus, the pKa for acetic acid becomes approximately 5. The pKa of phenol is 10. The pKa values of molecules can be used to classify them according to their relative strengths as an acid. Remember, an acid’s pKa is a measure of its ability to transfer a proton to water. But because the pKa scale is a negative log scale, stronger acids have lower pKa values. Weaker acids ionize less in aqueous solution. They have smaller Ka
values and therefore higher pKa values. Strong acids are defined as those having pKa values less than 0, weak acids are defined as those having pKa values greater than 0. Strong acids are substantially dissociated or ionized in water resulting in a much higher concentration of A- versus the concentration of HA. Weak acids are not very dissociated in water so have a higher relative concentration of HA and a lower concentration of A-. Pka values for a molecule refer to the most acidic proton in the molecule. The stronger the acid the weaker (more stable) the conjugate base. Factors that Influence Acid Strength There are several key factors that influence the strength of an organic acid. These factors are electronegativity, polarizability, resonance effects and hybridization. Electronegativity values for the elements increase from left to right within each row (period) of the periodic table. Therefore, the electrons in an H-O bond are more strongly attracted toward the oxygen than they would be toward the nitrogen atom in an H-N bond. As a result, the proton is held less tightly by an oxygen atom than by a nitrogen atom, making H-O protons more acidic (easier to remove) than H-N protons. The pKa range for alcohols is about 15-18, the pKa range for amines is about 38-42. Polarizability or ability of valence shell electrons to be distorted relates to the distribution of valence electrons around an atom or molecule. Valence electrons in large atoms are further away from the nucleus (n=3, n=4 primary quantum level for example). Valence electrons around a larger atom are not held as closely to the nucleus as those in smaller atoms. Protons that form bonds using the outer shell electrons of a large atom are more acidic than those bonded using outer shell electrons of a smaller atom. For this reason, the acidity of a proton in a bond of the type H-X increases as the atomic...