13.9 : IR Frequency Region: Alkyne and Nitrile Stretching

Both alkyne (C≡C) and nitrile (C≡N) functional groups contain triple bonds and show stretching absorptions around the wavenumber range of 2100 to 2300 cm⁻¹ in the diagnostic region of the IR spectra.

Comparing the stretching vibrational frequency of C≡C triple bonds with that of double and single bonds, it is evident that C≡C triple bonds exhibit a higher stretching frequency than C=C double and C–C single bonds. Similarly, the C≡N triple bond exhibits higher stretching absorption than the C=N and C–N bonds. The higher stretching is because the carbon atoms involved in triple bonds are sp hybridized in alkyne and nitrile. Therefore, they form shorter and stronger bonds with higher stretching absorptions.

C≡N bonds connected to a double bond or an aromatic ring absorb at a lower frequency due to their π electrons being conjugated.

Due to the difference in the dipole moment, the C≡C triple bond in the terminal alkynes vibrates at a higher stretching absorption frequency than unsymmetrical internal alkynes. In the terminal alkynes, the stretching of the triple bond causes a significant change in dipole moment, resulting in a large vibrational frequency. In contrast, in internal alkynes, the change in bond dipole moment due to C≡C bond stretching is negligible. The terminal alkynes have ≡C–H stretching absorptions at 3300 cm⁻¹.