Which would have the lowest frequency the carbon-carbon bond in propane (C−C), in propene (C=C), or in propyne (C≡C).
The frequency of vibration of a chemical bond is related to the strength and stiffness of the bond, which in turn is determined by the bond order (the number of shared electron pairs between the two atoms). A higher bond order generally means a stronger and stiffer bond, which vibrates at a higher frequency.
Here are the bond types in the molecules you mentioned:
- Propane (C−C): single bond (bond order of 1)
- Propene (C=C): double bond (bond order of 2)
- Propyne (C≡C): triple bond (bond order of 3)
Triple bonds are stronger and stiffer than double bonds, which in turn are stronger and stiffer than single bonds. Therefore, the ranking of bond strengths and stiffnesses is:
C≡C (strongest and stiffest) > C=C > C−C (weakest and least stiff)
According to this ranking, the vibrational frequency should be:
Frequency of C≡C (highest) > Frequency of C=C > Frequency of C−C (lowest)
Thus, the carbon-carbon single bond in propane (C−C) would have the lowest vibrational frequency compared to the carbon-carbon double bond in propene (C=C) and the carbon-carbon triple bond in propyne (C≡C).