Chem 3418 Organic Chemistry Laboratory I Acid Base Review
Chem 3418 Organic Chemistry Laboratory I Acid Base Review
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]
and
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
pKa =-logKa
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
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]
and
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
pKa =-logKa
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