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Arrange the following in the increasing order of their basic strength CH_(3)NH_(2), (CH_(3))_(2)NH, (CH_(3))_(3)N, NH_(3) |
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Answer» `NH_(3) lt (CH_(3))_(3)N lt (CH_(3))_(2)NH lt CH_(3)NH_(2)` the reason why the actual order is different from the expected order can be explained as follows : The basicity of an amine in aqueous solution does not entirely depend upon the electron density on the N-atm but also depends upon the STABILITY of the conjugate acid formed by accepting a proton from the solution. The stability of the conjugate acid, in turn, depends upon the extent of H-bonding. Obviously, greater the number of H-atoms on N-atom, more stable is the conjugate acid. thus, the conjugate acid of a `1^(@)` amine is the most stable since it has three H-atoms which can form H-bonds with `H_(2)O`, the conjugate acid of the `2^(@)` amine is less stable since it has two H-atoms while that of the `3^(@)` amines is the least stable since it has only one H-atom which can form H-bonds with `H_(2)O` as shown below :  Thus, on the basis of the stability of the conjugate acid ALONE, the basic strength of amines in aqueous solution FOLLOW the order : `1^(@)` Amine `gt 2^(@)` Amine `gt 3^(@)` Amine, i.e., `RNH_(2) gt R_(2)NH gt R_(3)N` In actual practice, these two opposing factors balance each other in case of `2^(@)` amines. This makes `2^(@)` amines to be strongest `3^(@)` Amines are weaker bases than `2^(@)` amines since their conjugate acids are less stable than those of `2^(@)` amines while `1^(@)` amines are less basic than `2^(@)` amines since the electron density on the N-atom is less and hence the lone pair of ELECTRONS is less easily available for protonation. |
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