Question

The IR \(\mathrm{C} \equiv \mathrm{C}\) stretching absorption in symmetrical alkynes is usually absent. Why?

Short answer

Answer: The stretching absorption of the carbon-carbon triple bond in symmetrical alkynes is absent in IR spectroscopy because it does not result in any change in the molecule's dipole moment, making it infrared-inactive.

Step-by-step solution

Understand infrared (IR) spectroscopy and molecular vibrations

Infrared (IR) spectroscopy is an analytical technique used to identify and study molecules by analyzing their interaction with infrared radiation. When IR radiation interacts with a molecule, it causes the molecule's bonds to vibrate. These vibrations include stretching and bending of bonds, and they occur at specific frequencies depending on the bond type and the atoms involved.

Learn about the selection rules for IR spectroscopy

Not all vibrations in a molecule can be observed using IR spectroscopy. For a vibration to be IR active, it must cause a change in the overall molecular dipole moment. This change in dipole moment allows the molecule to interact with the electromagnetic field of the infrared radiation, thus absorbing energy and causing the bond to vibrate. If a molecular vibration does not result in a change in the dipole moment, it is said to be infrared-inactive, and will not be observed in an IR spectrum.

Examine the structure of symmetrical alkynes

In symmetrical alkynes, the carbon-carbon triple bond (C≡C) is situated in the center of the molecule. This means that the alkyne has the same substituents attached to both carbon atoms involved in the triple bond. An example of a symmetrical alkyne is ethyne (HC≡CH).

Analyze the C≡C stretching vibration in symmetrical alkynes

When the C≡C bond in a symmetrical alkyne undergoes a stretching vibration, both carbon atoms move away from or toward each other along the bond axis. However, because the alkyne is symmetrical, this vibration does not lead to a change in the overall dipole moment of the molecule.

Understand why the C≡C stretching absorption is absent in symmetrical alkynes

Since the stretching vibration of the C≡C bond in symmetrical alkynes does not result in a change in the dipole moment, it cannot be observed in an IR spectrum, as it is infrared-inactive. This is why the IR stretching absorption for the C≡C bond in symmetrical alkynes is usually absent. In summary, the stretching absorption of the carbon-carbon triple bond in symmetrical alkynes is absent in IR spectroscopy because it does not result in any change in the molecule's dipole moment and is thus considered infrared-inactive.