Question

The structure of the molecule cyclohexene is shown below:

Does the absorption of ultraviolet light by cyclohexeneoccur at shorter wavelengths than in benzene? Explain.

Short answer

Yes, the absorption of UV light by cyclohexene occurs at a shorter wavelength than in benzene.

Organic compounds possessing“conjugated double bond systems” have a large influence on peak wavelengths and absorption intensities. The absorption spectra shows that the peak wavelengths tend to shift toward the longer wavelength region with larger conjugated systems and the absorption peaks are larger.

Step-by-step solution

Step 1:Transition ofthe π electron

When a photon is absorbed, the $\pi$electron from a conjugated system readily transition to their excited state.$\mathbf{\pi }*$

Transition

Transition is the “switching of an electron” from one state of motion to another. The electrons $\pi$ in a conjugated system are more easily transitioned than those electrons that make the molecular framework.

Relation between transition energy and wavelength

Light exhibits properties of both waves and particles (photons).

The energy of a photon is expressed as,

$\frac{hc}{\lambda }$

Where,

• h is Planck’s constant,
• cis the speed of light, and
• $\lambda$is the wavelength.

Transition is expressed at the energy of photons absorbed by the electrons. When a photon has relatively low energy, localid="1663693012310" $hc/\lambda$value is small and therefore the $\lambda$value is relatively high and is observed as the absorption wavelength.

In more conjugated compounds, peaks are seen in regions where $\lambda$is large i.e., longer

The more conjugated a compound system is, the moreπ electrons get delocalized and hence, the longer wavelength is shown in the absorption spectrum.

Therefore, absorption of UV-light by benzene occurs at a longer wavelength than in cyclohexene.