$\mathrm{n}-$Butylamine(I), diethylamine(II) and $\mathrm{N}, \mathrm{N}-$dimethylethyl amine(III) have the same molar mass. The increasing order of their boiling point is

  $$\begin{gathered} {\mathrm{H}}_3\mathrm{C}-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{NH}}_2 \\ \mathrm{n}-\mathrm{Butylamine} (\mathrm{Primary} \mathrm{amine}) \end{gathered}$$

$$\begin{gathered} {\mathrm{CH}}_3{\mathrm{CH}}_2-\mathrm{NH}-{\mathrm{CH}}_2-{\mathrm{CH}}_3\text{, } \\ \mathrm{Diethylamine} (\mathrm{Secondary} \mathrm{amine}) \end{gathered}$$      

$$\begin{gathered} {\mathrm{CH}}_3 \\ | \\ {\mathrm{CH}}_3-\mathrm{N}-{\mathrm{C}}_2{\mathrm{H}}_5 \\ \mathrm{Ethyldimethylamine} (\mathrm{Tertiary} \mathrm{amine}) \end{gathered}$$

• Primary amines have two hydrogen atoms available for hydrogen bond formation, while $2°$ amines have only one hydrogen atom for hydrogen bond formation. 
• $3°$ amines do not have intermolecular association due to the absence of hydrogen atoms. 
• Thus, order of boiling point of isomeric amines is as follows: $3° < 2° < 1 ° \mathrm{or} \mathrm{Ill} < \mathrm{II} < \mathrm{I}.$