Arrange the following:

In decreasing order of the pKb values:

C2H5NH2, C6H5NHCH3, (C2H5)2NH and C6H5NH2

Pkb value is the negative logarithm of the basicity constant (Kb) .i.e., pKb = -logKb

Evidently, smaller the value of pKb , stronger is the base (strong tendency to donate electrons)

Aliphatic amines(R-NH2) are more basic(tendency to donate electrons) than aromatic amines(C6H5NH2) because of the following reasons:

In aliphatic amines, alkyl groups are present. Alkyl groups are electron releasing groups, hence they increase the elecron density of N-atom and thus is easily available to donate electrons. This poperty makes aliphatic amines more basic.

In aromatic amines, aryl group is present. Aromatic amine shows resonance:

As a result of resonance, the lone pair of electrons on the nitrogen atom gets delocalized over benzene ring. As a result, electron density on the nitrogen decreases and thus is less easily available to donate electrons making it less basic.

Hence aliphatic amines(R-NH2) are more basic than aromatic amines(C6H5NH2)

Now, in C2H5NH2, one ethyl group(alkyl) is present and in (C2H5)2NH2 two ethyl groups are present. As we know that more alkyl groups are

present, more basic will be the amine.

Hence, (C2H5)2NH2 is more basic than the C2H5NH2

Now in C6H5NH2, due to delocalisation of lone pair of electrons of the N-atom over the benzene ring, makes it less basic than (C2H5)2NH2 and C2H5NH2

Now, in C6H5NHCH3, due to presence of CH3 group, makes it more basic than C6H5NH2 but less basic than (C2H5)2NH2 and C2H5NH2 due to the presence of aromatic ring which is responsible for the delocalisation of lone pair of electrons of N-atom over the benzene ring.

Combining all these facts, the relative basic strength of these four amines decrease in the order:

(C2H5)2NH2 > C2H5NH2 > C6H5NHCH3 > C6H5NH2

Since, a stronger base has a lower pkb value, therefore, pKb values decrease in the reverse order:

C6H5NH2 > C6H5NHCH3 > C2H5NH2 > (C2H5)2NH2