Question

Question: Which highlighted bond in each pair absorbs at a higher wavenumber?

Short answer

Answer

Step-by-step solution

Absorption of IR radiations

A molecule can absorb IR radiations if the vibrations in the molecule cause a net change in the dipole moment of the molecule.

The vibrational frequency of the mole determines the frequency of IR radiation absorbed.

IR absorption causes an increase in the amplitude of vibration of the molecule.

Region of IR absorbed by a bond

The wavenumber of IR radiation absorbed by a bond depends on its bond strength and atomic mass.

Stronger bonds vibrate at a higher frequency. Therefore, the wavenumber of radiation absorbed increases with the increase in bond strength.

Bonds with lighter atoms vibrate at a higher frequency. Therefore, the wavenumber of radiation absorbed increases with an increase in the atomic mass of bonded atoms.

Comparing wave number of absorbed radiation

(a) Triple bonds are stronger than double bonds. Therefore, of the highlighted bonds, the triple bond absorbs at a higher frequency.

Tripe bond absorbs at a higher wavenumber

(b) Deuterium atom is heavier than the hydrogen atom. Therefore, a bond containing a hydrogen atom absorbs at a higher frequency.

Carbon-hydrogen bond absorbs at a higher wavenumber