Basicity of Amines
- A review of basic acid base concepts should be helpful to the following discussion.
- Like ammonia, most amines are Brønsted and Lewis bases, but their base strength can be changed enormously by substituents.
- It is common to compare basicity's quantitatively by using the pkas of their conjugate bases rather than their pKb's. Since pKa + pKb = 14, the higher the pKa the stronger the base, in contrast to the usual inverse relationship of pKa with acidity.
- Most simple alkyl amines have pKa's in the range 9.5 to 11.0, and their water solutions are basic (have a pH of 11 to 12, depending on concentration).
- The first four compounds in the following table, including ammonia, fall into that category.
- The last five compounds (colored cells) are significantly weaker bases as a consequence of three factors.
- The first of these is the hybridization of the nitrogen.
- In pyridine the nitrogen is sp2 hybridized, and in nitriles (last entry) an sp hybrid nitrogen is part of the triple bond.
- In each of these compounds (shaded red) the non-bonding electron pair is localized on the nitrogen atom, but increasing s-character brings it closer to the nitrogen nucleus, reducing its tendency to bond to a proton.
- to bond to a proton.
- Secondly, aniline and p-nitroaniline (first two green shaded structures) are weaker bases due to delocalization of the nitrogen non-bonding elect
- Secondly, aniline and p-nitroaniline (first two green shaded structures) are weaker bases due to delocalization of the nitrogen non-bonding electron pair into the aromatic ring (and the nitro substituent).
- The electron pair delocalization is accompanied by a degree of rehybridization of the amino nitrogen atom, but the electron pair delocalization is probably the major factor in the reduced basicity of these compounds.
- A similar electron pair delocalization is responsible for the very low basicity (and nucleophilic reactivity) of amide nitrogen atoms (last green shaded structure).
- Although 4-dimethylaminopyridine (DMAP) might appear to be a base similar in strength to pyridine or N,N-dimethylaniline, it is actually more than ten thousand times stronger, thanks to charge delocalization in its conjugate acid.
- The structure in the gray box shows the locations over which positive charge (colored red) is delocalized in the conjugate acid. This compound is often used as a catalyst for acyl transfer reactions.
- Finally, the very low basicity of pyrrole (shaded blue) reflects the exceptional delocalization of the nitrogen electron pair associated with its incorporation in an aromayic ring .
- Indole (pKa = -2) and imidazole (pKa = 7.0), see above , also have similar heterocyclic aromatic rings.
- Imidazole is over a million times more basic than pyrrole because the sp2 nitrogen that is part of one double bond is structurally similar to pyridine, and has a comparable basicity.
- Although resonance delocalization generally reduces the basicity of amines, a dramatic example of the reverse effect is found in the compound guanidine (pKa = 13.6).
- Here, as shown below, resonance stabilization of the base is small, due to charge separation, while the conjugate acid is stabilized strongly by charge delocalization.
- guanidine is exceptionally
- Guanidine is protonated in physiological conditions. This conjugate acid is called the guanidinium cation, [CH6N3]+. The guanidinium cation has a charge of +1. It is a highly stable cation in aqueous solution due to the efficient resonance stabilization of the charge and efficient solvation by water molecules. As a result, itspKa is 13.6[7] meaning that guanidine is a very strong base in water.
- Consequently, aqueous solutions of guanidine are nearly as basic as are solutions of sodium hydroxide.
Strong bases have weak conjugate acids, and weak bases have strong conjugate acids.
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