13.9 : IR 周波数領域: アルキンとニトリルの伸縮

Triple bonds are stronger than double bonds and single bonds. An alkyne's triple bond vibrates at a higher stretching frequency than an alkene's double bond and an alkane's single bond.

For similar reasons, the C≡N of nitriles exhibit higher stretching frequencies than the C=N and the C–N.

A C≡N connected to a double bond or an aromatic ring absorbs at a lower frequency due to their π electrons being in conjugation.

The C≡C and the C≡N stretching frequencies appear in the diagnostic region of the IR spectrum.

In addition to the C≡C, terminal alkynes show a sharp C–H stretching vibration peak at about 3300 cm⁻¹.

The C≡C stretch of terminal alkynes is more intense than the corresponding stretch in internal alkynes. This is because stretching the triple bond in terminal alkynes causes a large change in dipole moment, whereas in internal alkynes, the change is negligible.

Symmetrical internal alkynes do not show C≡C stretching absorptions due to no change in the dipole moment.