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. | Compound | | | | NH3 | | | | | | CH3C≡N |
|---|---|---|---|---|---|---|---|---|---|---|
| pKa | 11.0 | 10.7 | 10.7 | 9.3 | 5.2 | 4.6 | 1.0 | 0.0 | -1.0 | -10.0 |
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). This is the same delocalization that results in activation of a benzene ring toward electrophilic substitution. The following resonance equations, which are similar to those used to explain the enhanced acidity of ortho and para-nitrophenols illustrate electron pair delocalization in p-nitroaniline.
Aniline pKb=9.38 > m-Aminophenol pKb=9.80 The protonation of substituted aniline is inhibited by steric hindrance. When protonated, the nitrogen in the
Ortho effect is an organic chemistry phenomenon where the presence of a chemical group at the at ortho position or the 1 and 2 position of a phenyl ring, relative to the carboxylic compound changes the chemical properties of the compound. This is caused by steric effects and bonding interactions along with polar effects caused by the various substituents which are in a given molecule, resulting
p-Toluidine > m-Toluidine > Aniline > o-Toluidine
Aniline > m-Nitroaniline > p-Nitroaniline > o-Nitroaniline
Aniline > p-Haloaniline > m-Haloaniline > o-Haloaniline
p-Aminophenol pKb=8.50 > o-Aminophenol pKb=9.28 > Aniline pKb=9.38 > m-Aminophenol pKb=9.80
The protonation of substituted aniline is inhibited by steric hindrance. When protonated, the nitrogen in the amino group changes its orbital hybridization from sp2 to sp3, becoming non-planar. This leads to steric hindrance between the ortho-substituted group and the hydrogen atom of the amino group, reducing the stability of the conjugate acid and consequently decreasing the pH of substituted Aniline.
Everything we examined (2)
This check searched the claim as stated. It did not run a separate search for evidence against it.