The most important organic nitrogen compounds are amines and amides. Amine nitrogens can be joined to alkyl or aryl groups (in which case the amines are called anilines). They all have lone pairs on nitrogen and may have hydrogen atoms on nitrogen too. As nitrogen is less electronegative than oxygen, you should expect amines to be less acidic and more basic than alcohols. And they are. The pKa values for the protonated amines are about 10 (this value is about 0 for water and alcohol) and the pKa values for amines acting as acids are very high, something like 35 (compared with about 15 for an alcohol). So, ammonium salts are about as acidic as phenols and amines will be protonated at pH 7 in water. This is why amino acids (p. 167) exist as zwitterions in water.

Removing a proton from an amine is very difficult as the anion (unfortunately called an ‘amide’ anion) is very unstable and very basic. The only way to succeed is to use a very strong base, usually an alkyllithium. The ‘anion’ then has a N–Li bond and is soluble in organic sol vents. This example, known as LDA, is commonly used as a strong base in organic chemistry. The basicity of amines as neutral compounds is measured by the pKa of their conjugate acids—so, for example, the pKa associated with the protonation of triethylamine, a commonly used tertiary amine, is 11.0.

مواضيع ذات صلة

Substituents affect the pKa

The ‘pKas’ of bases

Delocalization of the negative charge stabilizes the conjugate base

Constructing a pKa scale

The choice of solvent limits the pKa range we can use

An acid’s pKa depends on the stability of its conjugate base

A graphical description of the pKa of acids and bases

The definition of pKa

The ionization of water

Water can behave as an acid or as a base

Every acid has a conjugate base

The pH scale and pKa