74 - Organic Chemistry

Question

The pKa of the conjugate acid of guanidine is 13.6, making it one of the strongest neutral organic bases. Offer an explanation.

Step-by-Step Solution

Answer

The resonance stabilization of the conjugate acid makes guanidine a strong base, as it can readily donate its lone pairs.

 

p K a of conjugate acid

The pKa value is a measure of how strong or weak an acid is. It is also known as the acid dissociation constant of a solution. The stronger the acid, the lower is its pKa value.

 

p K a of guanidine

 

The pKa of the conjugate acid of guanidine is 13.6, which is very high. Thus the acidity of guanidine would be below.

 

The reaction of guanidine with acid occurs as shown below:

                       Conjugate acid of guanidine

The conjugate acid is stabilized due to the resonance delocalization, as shown below:

 

Canonical structures of resonance stabilized conjugate acid

The resonance stabilization makes guanidine a very strong base since the lone pair over nitrogen is readily donated.