Amongst the following, the strongest base in aqueous medium is ____________.
Basicity of amines depends on its structure and the classification (electron releasing/withdrawing) other groups attached to the adjacent carbon atoms which determine ease of formation of the cation by accepting protons from the acid.
(iii) (CH3)2 NH is the strongest base due to the efficient +I effect(electron releasing effect ) of alkyl(here –CH3). This effect of alkyl group pushes the unshared electron pair(lone pair) more towards the N atoms and thus make them more available for protonation by acid.
• For the same reason (i) CH3NH2 will be less basic than (iii) because it has less of the effect.
• In case of (ii) NCCH2NH2, CN(cyanide C≡N:) is an electron-withdrawing group i.e. it pulls electrons towards itself from the N atom hence making the lone pairs less available for protonation, therefore it is not a strong base.
• In comparison between alkyl and aryl amines, the former is always more basic than the latter because in case of aryl amines:
the -NH2 (amine) group is attached directly to the benzene(aryl) ring. hence the unshared electron pair on nitrogen atom undergoes conjugation (resonance) with the benzene ring and thus it becomes almost unavailable for protonation.
thus (iv) C6H5NHCH3 might be theweakestamongst the given amines.