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Arrange the following: (i) in decreasing order of pK_(a) values C_(2)H_(5)NH_(2),C_(6)H_(5)NHCH_(3),(C_(2)H_(5))_(2)NH and C_(6)H_(5)NH_(2). (ii) in increasing order of basic strength C_(6)H_(5)NH_(2),C_(6)H_(5)N(CH_(3))_(2),(C_(2)H_(5))_(2)NH and CH_(3)NH_(2). |
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Answer» Solution :(i) Due to delocalization of lone pair of electrons of the N-atom over the BENZENE RING, `C_(6)H_(5)NH_(2) and C_(6)H_(5)NHCH_(3)` are far less basic than `C_(2)H_(5)NH_(2) and (C_(2)H_(5))_(2)NH`. Further, due to +I-effect of the `CH_(3)` group, `C_(6)H_(5)NHCH_(3)` is little more basic than `C_(6)H_(5)NH_(2)`. among `C_(2)H_(5)NH_(2) and (CH_(2)H_(5))_(2)NH,(C_(2)H_(5))_(2)NH` is more basic than `C_(2)H_(5)NH_(2)` due to greater +I-effect of the two `C_(2)H_(5)` groups and stabilization of its conjugate acid by H-bonding. combining all these facts, the relative basic strength of these four amines decreases in the order: `(C_(2)H_(5))_(2) NH gt C_(2)H_(5)NH_(2) gt C_(6)H_(5)NHCH_(3) gt C_(6)H_(5)NH_(2)`. Since a stronger base has a lower `pK_(b)` value, therefore, `pK_(a)` values decrease in the reverse order: `C_(6)H_(5)NH_(2) gt C_(6)H_(5)NHCH_(3) gt C_(2)H_(5)NH_(2) gt (C_(2)H_(5))_(2)NH`. (ii) We have ALREADY explained in Ans. (i) Above that the relative basic strength of the amines, `C_(6)H_(5)NH_(2),C_(6)H_(5)NHCH_(3) and (C_(2)H_(5))_(2)NH` decreases in the order: `(C_(2)H_(5))_(2)NH gt C_(6)H_(5) NHCH_(3) gt C_(6)H_(5)NH_(2)`. Conversely, the basic strength of these amines increases in the order: `C_(6)H_(5)NH_(2) lt C_(6)H_(5)NHCH_(3) lt (C_(2)H_(5))_(2)NH` Among `CH_(3)NH_(2) and (C_(2)H_(5))_(2)NH`, primarily due to the greater +I-effect of the two `C_(2)H_(5)` groups over one `CH_(3)` group, `(C_(2)H_(5))_(2)NH` is more basic than `CH_(3)NH_(2)`. thus, the basic strength of the four amines increases in the order: `C_(6)H_(5)NH_(2) lt C_(6)H_(5)NHCH_(3) lt CH_(3)NH_(2) lt (C_(2)H_(5))_(2)NH`. |
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