Chemistry

19417 Alkyl Halides and Amines17.3.6 Reactivity:Amines are basic and nucleophiles because of non-bonding pair of electrons on nitrogen.The relative availability of this pair of electron and the relative stability of correspondingammonium ion is responsible of basicity of different amines. Consider the following reactions.The strength of a base is expressed in terms of pkb, i.e., pkb = -logkbFor ammonia and methyl amine, the pkb values areSince PKNH3 < PKCH3NH2 methyl amine is a strongerbase than ammonia. It can be explained as under:In ammonia, the pair of electron attracted by s-orbitals of hydrogen atoms where as in CH - 3 NH2, sp 3 —orbital of carbon pushes electrons towards nitrogen.Therefore, the pair of electron on nitrogen is relatively more available in methyl aminethan in ammonia. The methyl ammonium ion, CH - 3 NH + 3 is stabilized due to electron donatinginductive effect of the methyl group. On the other hand, NH4 + ion is not stabilized by hydrogenatoms. Both these factors favor methylamine to a stronger base than ammonia.Higher members show deviation to these arguments. It is because the stabilization of apositive ion also depends upon the extent of salvation, hydrogen bonding and resonancestabilization. Moreover, the availability of non-bonding pair of electrons is also affected by stericfactor in addition to these aspects17.3.7 Reactions of AminesOverviewThe important organic reactions of amines(nucleophiles) are with the commonelectrophiles: Alkyl halides via nucleophilic substitution Aldehydes or ketones via nucleophilicaddition Carboxylic acid derivatives, especiallyacid chlorides or anhydrides, via nucleophilicacyl substitution.