question_answer

Which of the following has highest boiling point?

A)  \[{{(C{{H}_{3}})}_{3}}N\]

B)  \[{{C}_{2}}{{H}_{5}}NHC{{H}_{3}}\]

C)  \[C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}N{{H}_{2}}\]

D)  \[N{{H}_{2}}C{{H}_{2}}C{{H}_{2}}N{{H}_{2}}\]

Correct Answer: D

Solution :

                 Amines are polar compounds and with the exception of tertiary amines can form intermolecular hydrogen bonding. As a result amines have higher boiling points. Among the isomeric amines the primary and secondary amines have highest boiling point due to formation of H-bonds. Whereas tertiary amines have lowest boiling point because of their inability to form H-bonds. The boiling point of primary amine is greater than secondary. \[\underset{\begin{smallmatrix}  p-\text{ }amine \\  b.p.:\text{ }323\text{ }K \end{smallmatrix}}{\mathop{C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}N{{H}_{2}}}}\,\,\,\underset{\begin{smallmatrix}  5-amine \\  b.p.:\text{ }307\text{ }K \end{smallmatrix}}{\mathop{C{{H}_{3}}-C{{H}_{2}}-\overset{\begin{smallmatrix}  C{{H}_{3}} \\  | \end{smallmatrix}}{\mathop{N}}\,H}}\,\,\,\,\,\underset{\begin{smallmatrix}  t-amine \\  b.p.:\text{ }276\text{ }K \end{smallmatrix}}{\mathop{{{(C{{H}_{3}})}_{3}}N}}\,\]                 But in \[N{{H}_{2}}C{{H}_{2}}C{{H}_{2}}N{{H}_{2}}\] two amine groups are attached on both side therefore, H-bonding is maximum in this compound. Hence, its boiling point is highest.