Methods Used to Measure Weak Acid Strength

1 Methods Used to Measure Weak Acid Strength 

Many methods have been employed to determine the pKaS of very weak organic acids. In fact, pKaS of alkanes near 45 and 50 have even been determined. The techniques are all built upon incremental comparisons. That is, the pKa of a new compound under analysis is always determined using a compound whose pKa is known. The structure whose pKa is known was at some point analyzed relative to another compound whose pKa was initially known. These comparisons are repeatedly done until finally a comparison can be made to a structure whose pfC., was determined in water. 

In this approach, compounds that are sufficiently basic that they can be used to depro-tonate weak acids just outside of the water limit, but whose conjugate acids are just acidic enough to be measurable in water, are the key to linking all organic acids to a water pKa scale. Two of the most important compounds used in this way are the acids cyclopentadiene and 9-phenylfluorene, whose pfCgS are 16.0 and 18.5, respectively. A carbon acidity scale based upon 9-phenylfluorene was developed by Cram, and named the MSAD scale (named after McEwen, Streitwieser, Applequist, and Dessy). This scale is the one commonly used to define pKa values of near 18 to near 40. Values above 40 can be determined using an electrochemical method developed by Breslow. 

There are cases when the add-base equilibrium required to get a true thermodynamic difference between the compoimds being compared is difficult to establish. The add-base equilibrium is not established rapidly, and instead one determines a kinetic acidity. Kinetic acidities are relative rates for proton transfer, which can in some cases be related to thermodynamic acidities. 

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