IR Spectra of 1-Octyne and 4-Octyne

 The IR spectrum of 1-octyne shows characteristics absorptions at 3313 cm-1 and at 2119 cm-1. (b) Neither of this absorptions are seen in the spectrum of 1-octyne.

Terminal alkynes have a characteristic C-H (alkynyl) stretch at around 3300 cm-1 and a C-C triple bond stretch at around 2100 - 2200 cm-1. Internal alkynes cannot exhibit the acetylenic C-H stretch. The very small dipole moment of the disubstituted triple bond limits the stretching and makes the C-C triple band bond not visible.