Acids and Bases: Use of the pKa Table

Discussion: The propensity of a compound to donate a proton is measured as its acid ionization constant, or Ka.  These Ka values cover a wide range of 1010 for the strongest acids such as sulfuric acid to 10-50 for the weakest acids such as methane.  A more convenient scale of acidity is pKa which is the negative logarithm of the Ka (pKa = -log Ka).  Thus a Ka of 1010 becomes a pKa of -10, and a Ka of 10-50 becomes a pKa of 50.  More generally, more negative pKa values correspond to stronger acids and more positive pKa values correspond to weaker acids.  The exact pKa of an acid is a function of molecular structure (i.e., functional groups) and must be determined experimentally.  We know that similar functional groups react in similar ways, so we can estimate pKa values by comparing the structure and functional groups of an unknown with the structure and functional groups of acids whose pKa values are known.
 

Example 1: By comparison with the compounds in Table 2.1 (page 43) of the  Brown text (or Table 3.1, page 105 of Brown and Foote 2nd edition), estimate the pKa of protons in red the following compounds.

a. CH3CH2CH2CO2H (butyric acid)   b. CH3OH2+ (an oxonium ion).
 

Solution 1:

a. Butyric acid is similar in structure to CH3CO2H (acetic acid), in that both are carboxylic acids.  The pKa of acetic acid is 4.78.  Therefore we can estimate the pKa of butyric acid to be approximately 4 - 5.

b. The closest structural match for the oxonium ion in the pKa table is the hydronium ion (H3O+), pKa -1.74.  Thus we can estimate the pKa of CH3OH2+ to be approximately -2.
 

The pKa table is also useful to estimate the position of a proton transfer equilibrium.  Such reactions are common in many aspects of chemistry and biology.  Any equilibrium favors the thermodynamically most stable side.  In the case of a proton transfer reaction, the equilibrium favors the side with the weakest acid and weakest base.  Because a stronger conjugate acid yields a weaker conjugate base, base strength can be determined from the pKa table.
 

Example 2: Determine the position of the equilibrium shown below.







Solution 2: This is a proton transfer equilibrium, so the position can be determined using pKa values.  The acids (proton donors) in this equilibrium are H2CO3 (carbonic acid, pKa 6.36) and NH4+ (ammonium ion, pKa 9.24).  The ammonium ion has the higher pKa than carbonic acid, and is therefore a weaker acid than carbonic acid.  To analyze relative basicity, we need to recall that lower pKa corresponds to a stronger acid, and deprotonation of this stronger acid will give a weaker conjugate base.  The bases (proton acceptors) in this equilibrium are NH3 (ammonia; conjugate acid NH4+, pKa 9.24) and HCO3- (bicarbonate ion; conjugate acid H2CO3, pKa 6.36).  Because H2CO3 is a stronger acid than NH4+, we can deduce that HCO3- is a weaker base than NH3.

Analysis of the relative acid and base strengths suggests that this equilibrium lies to the right.
 

Exercises:

Estimate the pKa of the circled hydrogens.

d. Determine the most acidic hydrogen in the compound shown below.

Determine the position of each of the following equilibria.




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