Consider the following compounds ethanamine (i); diethylamine (ii); aniline (iii) and $N$ - methylbenzenamine (iv) The correct decreasing order of pKb values of the compounds is:

The basicity of amines is inversely proportional to their $pK_b$ values. Higher basicity corresponds to a lower $pK_b$ value.

Comparing the basicity of the given compounds:
1. Diethylamine (ii) is a secondary aliphatic amine. It is the most basic due to the electron-donating inductive effect of two ethyl groups and favorable solvation.
2. Ethanamine (i) is a primary aliphatic amine. It is less basic than diethylamine due to fewer electron-donating groups.
3. $N$-methylbenzenamine (iv) is an aromatic amine where the lone pair on nitrogen is delocalized into the benzene ring, reducing its availability for protonation. However, the methyl group provides a slight electron-donating effect compared to aniline.
4. Aniline (iii) is a primary aromatic amine where the lone pair is delocalized into the benzene ring. It is the least basic among the given compounds.

The order of basicity is: (ii) > (i) > (iv) > (iii).

Since $pK_b$ is inversely proportional to basicity, the decreasing order of $pK_b$ values is: (iii) > (iv) > (i) > (ii).

Answer: iii $>$ iv $>$ i $>$ ii