Arrange the following amines in order of decreasing basicity:

Explanation

(i) In compound II (pyridine) , the lone pair of electrons on N is present in a `sp^(2)`-orbital while in compounds I (Piperidine) and III (morpholine), the lone pair of electrons on N is present in a `sp^(3)`-orbital. Since a `sp^(2)-`orbital has more s-character (33.33%) than a `sp^(3)`-orbital (25%), therefore, the lone pair of electrons on N is more readily available for protonation in I and III than in II. In other words, compound (II) is less basic than I and III. among I and III,III contains an oxygen atom which has -I-effect. as a result, it will attract the lone pair of electrons on N towards itself. consequently, the lone pair of electrons on N in III is less readily available for protonation than in I. in other words, compound I is more basic than III.
(ii) Compound IV (pyrrole) is aromatic in character. therefore, in accordance with Huckel's rule it has a cyclic cloud of six `pi`-electrons. out of these, four are contributed by two double bonds while the remaining two are the lone pair of electrons on the N-atom. in other words, the lone pair of electrons on N-atom is contributed towards the aromatic sextet formation and hence is not at all available for protonation. therefore, compound IV is the least basic. in fact, it is such a weak base that it is weakly acidic in character and thus reacts with K metal when heated to form the corresponding potassium salt.
Thus, the basicity of the above four compounds decreases in the order: `IgtIIIgtIIgtIV`.