Group Frequencies Associated with Alkynes

A compound containing the structural unit C=C may have either a hydrogen atom next to it (i.e., CR=C— H) or other groups such as CH3, etc. It is convenient to discuss group frequencies for alkynes in terms of both the C=C stretch and, if the C=C—H group is present, the stretching and deformation frequencies of the —C=C—H group. 

The C C structural unit has characteristic absorption frequencies in the region from 2260 to 2100 cm The exact position depends on the groups substituted around the triple bond, and the intensity of the band depends, to some extent, on the position of the triple bond in the molecule. 

For the structural unit RiC=CR2, a characteristic C=C stretching vibration can be found in the 2260-2190 cm region. The intensity of the band will be determined by the groups Ri and R2. If these groups are identical, the stretching motion of the triple bond causes no net change in the dipole moment of the molecule and therefore no absorption band will be observed. If the two R 

It would appear from the fact that so many compounds offer interference in this region that it is very difficult to identify the alkyne structure in a completely unknown material. However, many groups which have a frequency that might be mistaken for an alkyne vibration also have other group frequencies which can be used to identify them. Thus, by a process of elimination it may still be possible to identify an alkyne from a single group frequency. 

The frequencies associated with aromatic rings and the regions in which they occur are as follows  

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