Question

Pentafiluorobenzene has an IR spectrum with a C-H stretching frequency at 3050. cm-1. Use four significant figures for your answer.

Predict the following bond strengths for the pentafluorobenzene: 425.0 kJ/mol for the C–H bond and 510.0 kJ/mol for the C–F bond.

Using the bond strength data above, predict the stretching frequency in wavenumbers (cm–1) of a C–F bond in a sample of pentafluorobenzene.

Answer 1

C-F bond strength is stronger than C-H bond as Fluorine being more electronegative has concentrated electron density around it and giving a partial ionic character to the bond. the partial charges on the atoms are attractive in nature. This is also shown by their bond energies. Energy required to break C-F bond (510 kJ/mol) is higher than that fro C-H bond (425kJ/mol).

Stretching frequency of C-F bond can be measured from Hooke's Law: wavenumber (cm^-1)= 1/2πc × √k⁄μ

C is speed of light

k is force constant

μ is reduced mass

m₁ and m₂ are mass of atoms, Carbon and Fluorine in this case.