Question

How could IR spectroscopy be used to distinguish between benzene and cyclohexene?

Answer 1

Cyclohexene has strong aliphatic C-H stretching absorptions at 3000-2850 cm⁻¹. Benzene does not.

The characteristic IR absorptions of benzene are at

3100-3000 cm⁻¹: =C-H stretch
~1950 cm⁻¹: C=C stretch
~1800 cm⁻¹: C=C stretch
1500-1400 cm⁻¹: =C-H in-plane bending
900-650 cm⁻¹: =C-H out-of plane (oop) bending

The characteristic IR absorptions of cyclohexene are

3100-3000 cm⁻¹: =C-H stretch
3000-2850 cm⁻¹: C-H stretch
~1650 cm⁻¹: C=C stretch
~1450 cm⁻¹: CH₂ bend (scissoring)
1000-650 cm⁻¹: =C-H bend

The big difference is that cyclohexene has strong aliphatic C-H stretching peaks at 3000-2850 cm⁻¹.