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Question
Give a plausible explanation for the following:
Why do primary amines have higher boiling points than tertiary amines?
Solution
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. As a result, extra energy is required to separate the molecules of primary amines. Hence, primary amines have higher boiling points than tertiary amines.
\[\begin{array}{cc}
\ce{H}\phantom{.............}\ce{H}\phantom{............}\ce{H}\phantom{....}\\
\phantom{...}|\phantom{..............}|\phantom{.............}|\phantom{.......}\\
\ce{\text{- - - - - -} N-H \text{- - - - - -} N-H \text{- - - - - -} N-H \text{- - - - - -}}\\
\phantom{...}|\phantom{..............}|\phantom{.............}|\phantom{.......}\\
\ce{\underset{Primary Amine}{R\phantom{.............}R\phantom{............}R\phantom{....}}}
\end{array}\]
\[\begin{array}{cc}
\ce{R}\phantom{....}\\
|\phantom{....}\\
\ce{N-R}\\
|\phantom{....}\\
\ce{\underset{Tertiary amine}{R}}\phantom{....}
\end{array}\]
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