Question

How to distinguish primary secondary and tertiary amines by ir spectroscopy?

Answer 1

In IR spectra,the differentiation is done via the change in position and strength of the peaks due to stretching,bending etc type of vibrational motions of bonds.

In case of the 3 types of amines,the main difference is the no of N-H bonds.For primary amines,there are 2 strong bands at 3400 and 3300cm-1 due to the asymmetric stretching and symmetric stretching vibration respectively of the 2 N-H bonds.

Whereas for secondary aliphatic amines,the 3300 peak is vanishingly weak,on the other hand,an aromatic 2ndary amine gives a strong peak at 3400cm-1.Based on this primary and secondary aromatic and aliphatic amines can be separated.For a tertiary amine,due to the absence of N-H bond,there would be no peak due to N-H stretch.

Also,in primary amines,the N-H scissoring mode appears as a medium to high intensity broad band around 1640 to 1560cm-1.But for aromatic secondary amines,it shifts lower near 1500cm-1.And for aliphatic secondary amines this peak is very weak

Answer 2

Answer:

The N-H wag peak of secondary amines is distinct from that of primary amines in its class.

Explanation:

In order to discriminate between these two amine kinds, one can also use this peak position. Because tertiary amines lack N-H linkages, they lack relevant infrared peaks.