Basic nature of aromatic amines

Basic nature of aromatic amines

In aniline or other aromatic amines, the lone pair present on nitrogen atom is delocalized with benzene ring by resonance .

      

But anilinium ion is less resonance stabilized than aniline.

   

              

Thus, electron density is less on N atom due to which aniline or other aromatic amines are less basic than aliphatic amines.

Any group   present on benzene ring  that has electron withdrawing effect (– NO₂, – CN, – SO₃H, – COOH – Cl, C₆H₅, etc.) decreases basicity of aniline(Nitroaniline is less basic than aniline as nitro group is electron withdrawing group (– I group) and aniline is more basic than diphenyl amine) .(conjugation decreases basic nature)

An electron repelling effect (– NH₂, – OR, R –, etc.) increases basicity of aniline. Toluidine is more basic than aniline as – CH₃ group is electron repelling group (+ I group).

Further greater the value of K_b or lower the value of pK_b, stronger will be the base.The basic character of some amines have the following order,

R₂ NH > RNH₂ > C₆H₅ CH₂ NH₃ > C₆ H₅ NH₂

N-alkylated anilines are stronger bases than aniline because of steric effect. Ethyl group being bigger than methyl has more steric effect, so N-ethyl aniline is stronger base than N-methyl aniline. Thus, basic character is,

C₆ H₅ N ( C₂ H₅ )₂ > C₆ H₅ NHC₂ H₅ > C₆ H₅ N ( CH₃ )₂ > C₆ H₅ NHCH₃ > C₆ H₅ NH₂ NH₃ > C₆ H₅NHC₂ H₅ > C₆  H₅ NHCH₃ > C₆ H₅ NH₂ > C₆ H₅ BHC₆ H₅                    

       

In Toluidines –                              p-isomer    >   m-   >   o-

Chloroanilines–                             p-isomer   >   m-   >   o-

Phenylene diamines –                 p-isomer    >  m-    >   o-

Nitroanilines–                                 m-isomer >   p-     >   o-

Aniline is less basic than ammonia. The phenyl group exerts –I (inductive) effect, i.e., it withdraws electrons. This results to the lower availability of electrons on nitrogen for protonation.

Ethylamine and acetamide both contain an amino group but acetamide does not show basic nature. This is because lone pair of electrons on nitrogen is delocalised by resonance with the carbonyl group which makes it less available for protonation.

 

 The compounds with least 's' character (sp³-hybridized) is most basic and with more ‘s’ character (sp-hybridized) is least basic. Examples in decreasing order of basicity are,

       

 Electron withdrawing (C₆H₅ –) groups decrease electron density on nitrogen atom and thereby decreasing basicity.

   (CH₃ )₂ NH > CH₃ NH₂ > C₆ H₅ NHCH₃ > C₆ H₅ NH₂        

    CH₃ CH₂ NH₂ > HO ( CH₂ )₃ NH₂ > HO (CH₂ )₂ NH₂       

Electron withdrawing inductive effect of the –OH group decreases the electron density on nitrogen. This effect diminishes with distance from the amino group.

CH₃ CH₂ NH₂ > C₆ H₅ CONH₂ > CH₃ CONH₂