Question

How could IR spectroscopy distinguish between 1-hexyne, 2-hexyne, and 3-hexyne?

Answer 1

A terminal alkyne (but not an internal alkyne) will show a C–H stretch as a strong, narrow band in the range 3330-3270 `cm^-1`.

Explanation:

The given absorption can be attributed to the stretching of the `C-=C-H` bond (i.e. the `C-H` part of the bond gives rise to the absorption). In principle, this could be used to identify the terminal alkyne from 2-hexyne, and 3-hexyne. How could you be sure? In fact, the only way would be to substitute the terminal hydrogen with deuterium, and see if a new absorption appears at lower frequency. Chemists might have indeed done this maybe 40 years ago. A modern chemist would use Proton NMR spectroscopy, which would directly determine the identity of the alkyne.