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Question
Give plausible explanation for each of the following:
Why do primary amines have higher boiling point than tertiary amines?
Solution 1
In a molecule of tertiary amine, there are no H−atoms whereas in primary amines, two hydrogen atoms are present. Due to the presence of H−atoms, primary amines undergo extensive intermolecular H−bonding.
\[\begin{array}{cc}
\phantom{.}\ce{H}\phantom{.........}\ce{H}\phantom{.........}\ce{H}\phantom{....}\\
\phantom{........}|\phantom{..........}|\phantom{..........}|\phantom{...........}\\
\ce{\text{- - - - - -}N-H\text{- - - - - -}N-H\text{- - - - - }N-H\text{- - - - - -}}\\
\phantom{........}|\phantom{..........}|\phantom{..........}|\phantom{...........}\\
\phantom{..}\ce{R}\phantom{.........}\ce{R}\phantom{.....}\ce{\underset{Primary amine}{R}}\phantom{}
\end{array}\]
\[\begin{array}{cc}
\ce{R}\phantom{....}\\
|\phantom{....}\\
\ce{N-R}\\
|\phantom{....}\\
\ce{\underset{Tertiary amine}{R}}\phantom{....}
\end{array}\]
As a result, extra energy is required to separate the molecules of primary amines. Hence, primary amines have higher boiling points than tertiary amines.
Solution 2
Due to the presence of two H-atoms on N-atom of primary amines, they undergo extensive intermolecular H-bonding while tertiary amines due to the absence of H-atom on the N-atom do not undergo H-bonding. As a result, primary amines have higher boiling points than tertiary amines of comparable molecular mass.
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