The Pka of an Acid

The pKa of an acid " is defined as the negative logarithm (base 10) of Ka" (Wade, pg. 24). The Ka is called the acid-dissociation constant, and it indicates the strength of the acid. "The stronger the acid, the more it dissociates, giving a larger value of Ka" (Wade, pg. 23). The larger the Ka, the smaller the pKa, which means that, the smaller the pKa, the stronger the acid. So when comparing structure A) meta-cyanophenol, and structure B) para-cyanophenol, we first look at their pKa values in order to their acidity. Structure A has a pKa value of 8.61, and structure B has a pKa value of 7.95; so by comparing these two values we can see that 7.95 is the lower number, so therefore structure B) para-cyanophenol is the stronger acid.

In some cases, we are asked to compare the acidity of two compounds without being given a pKa; in these cases we must look at: conjugate bases, structure/stability, and resonance. In order to get the conjugate bases of each compound we must deprotonate the Oxygen molecule, giving it a negative charge, and making it a base.

The cyano group itself when in acid form, is just a Carbon molecule triple bonded to a Nitrogen molecule (with one lone pair). However in both CB-A and CB-B there are multiple forms of resonance. Resonance is when "the negative charge of a conjugate base may be delocalized over two or more atoms"(Wade, pg. 29). CB-A has two resonance structures, however none of them involve the cyano group. In the CB-A resonance structures the double bonds just shift around the benzene ring. CB-B has three resonance structures, one involving the cyano group. In the resonance structure involving the cyano group, the electrons from the triple bond shift to form a double bond between Carbon 4 and the Carbon of the cyano group. This leaves only a double bond between the Carbon and the Nitrogen, giving the Nitrogen two sets of lone pairs and a negative charge.

The more resonance structures a compound has, the more stable it is. The more stable it is, the less reactive, meaning that it is a weak base, and if something is a weak base, that means that it is a strong parent acid. So because CB-B is more resonant stable, it is the weaker CB meaning it is the stronger acid.