Electronic and structural features that influence basicity

5 Electronic and structural features that influence basicity 

Oxygen is more electronegative than nitrogen, so its electrons are less likely to be donated to a proton. Neutral oxygen bases are generally very much weaker than nitrogen bases, but as we shall see later, protonation of an oxygen atom is important and the first step in many acid-catalysed reactions, especially carbonyl compounds. 

Electron-donating groups on nitrogen are going to increase the likelihood of protonation, and help to stabilize the conjugate acid. They thus increase the basic strength. 

The pATa values for the amines ammonia, methy-lamine, dimethylamine, and trimethylamine are 9.2, 10.6, 10.7, and 9.8 respectively. The electron-donating effect of the methyl substituents increases the basic strength of methylamine over ammonia by about 1.4 pATa units, i.e. by a factor of over 25 (10 = 25.1). However, the introduction of a second methyl substituent has a relatively small effect, and the introduction of a third methyl group, as in trimethylamine, actually reduces the basic strength to nearer that of methylamine. 

When pATa values are measured in the gas phase, where there are no hydrogen bonding effects, they are found to follow the predictions based solely on electron-donating effects. In water, mono-, di-, and tri-alkylated amines all tend to have rather similar pATa values, typically in the range 10-11. 

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ELECTRONIC AND STRUCTURAL FEATURES THAT INFLUENCE BASICITY... 

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