Being naturally lazy, we’d like to condense this a bit for common usage. The term, “pKa of the conjugate acid” is a bit of a mouthful to use on a regular basis. “The pKa Of The Conjugate Acid” Can Be Abbreviated as “pKaH” Since “the stronger the acid, the weaker the conjugate base”, pyridine is therefore a weaker base than piperidine by a factor of 1 million. Since each pKa unit represents a factor of 10, that’s a factor of about 1 million. įrom these pKa values, we can tell that the conjugate acid of pyridine is stronger than the conjugate acid of piperidine by about 6 pKa units. We also have a value for the pKa for the conjugate acid of piperidine.
The pKa for the conjugate acid of pyridine (“pyridinium”) has been measured.
The weaker the acid, the stronger the conjugate base. 
The stronger the acid, the weaker the conjugate base. Recall these two immortal lines of prose, worth reciting to yourself nightly: Using pKa Values To Quantify Basicity of Amines Instead, we can easily use pK a values as an indirect measurement of basicity. Does anyone really want to have to remember a huge set of pK b values too? Hell no! If we had to. But generally, organic chemists’ opinion on this subject is “Screw pK b“.Īfter all, pK a values have been measured for thousands of organic molecules. How can we apply these concepts to measure basicity? Couldn’t we switch things around and define a term, K b, which is the equilibrium for how readily a species combines with H+, define pK b as its negative log, and compare bases by looking at their pK b values? hydroiodic acid, H–I) to 50 and above for certain hydrocarbons (e.g. pK a values range from –10 or so for very strong acids (e.g. We’ve seen that the acidity constant, K a, is a measure of how easily a species dissociates to give H+ and the conjugate base.įor convenience, we use the negative log of K a, called pK a, which is similar to the familiar pH scale but can extend beyond 14 because it’s not strictly limited to aqueous solvent. Here’s pyridine and piperidine in action. By following the bonds that form and break, you can identify each actor.Įvery acid-base reaction can be written as an equilibrium between a forward and reverse reaction. The conjugate base is the species that remains after the acid donates H+ to the base (e.g. The acid is the species which loses H+. The conjugate acid is the species that results after the base forms a bond to H (e.g. The base is the species which donates a lone pair to H+ (also known as a “proton” or “hydrogen nucleus”). There are four actors in every acid-base reaction. Quick Review: Acids, Bases, Conjugate Acids, and Conjugate Basesįirst, let’s quickly review some basicity basics. What Key Factors Govern The Basicity Of Amines?ġ. Some Words Of Caution On pKa Values For Amines. Using pKaH To Determine The Relative Basicities Of Amines. The “pKa Of The Conjugate Acid” Can Be Abbreviated As “pKaH”. Using pKa To Quantify The Basicity Of Amines. Quick Review: Acids, Bases, Conjugate Acids, and Conjugate Bases. In a subsequent post, we’ll then explore some key trends underlying the basicity of amines. PK VALUE HOW TO
In this post we’ll show some specific examples of how to use pKa values to compare the basicity of amines.
The higher the pK a of the conjugate acid, the stronger the base.The best way to quantify the basicity of an amine is to examine the pK a of its conjugate acid.If you said piperidine, congratulations – it is indeed a stronger base.īut why? And how much stronger? How does one go about trying to answer these questions? After all, we can easily compare the strength of acids by examining their pKa’s. Shouldn’t we be able to do the same with bases? How do you measure the basicity of an amine? Case in point: what’s a stronger base: pyridine or piperidine? Quantifying The Basicity Of Amines, Using “The pKa Of The Conjugate Acid”, a.k.a.
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