Question

Would you expect allene, ${\mathrm{H}}_2\mathrm{C}=\mathrm{C}={\mathrm{CH}}_2$, to show a UV absorption in

the 200 to 400 nm range? Explain.

Short answer

To absorb in the 200-400 nm range, an alkene must be conjugated. Since the double bond of allene aren’t conjugated, allene doesn’t absorb light in the UV region.

Step-by-step solution

Principle of Ultraviolet (UV) spectroscopy

Ultraviolet spectroscopy refers to absorption spectroscopy in part of the ultraviolet and the full adjacent visible region of the electromagnetic spectrum.
• The UV region of the electromagnetic spectrum extends from 100-400 nm.
• The visible region extend from 400-800 nm.
When UV or visible radiation is passed through the solution of an organic compound, it is absorbed by the molecule. As a result of absorption of energy, an electron can be promoted from the ground state orbital to an excited state orbital. Energy and the wavelength are inversely proportional to each other as shown below in the equation.
Energy equation is as follow:
$$\begin{gathered} E=hv \\ E=\frac{hc}{\lambda } \end{gathered}$$
Where E is the energy of the absorption, h is the Planck’s constant, c the velocity of light and 𝛌 the wavelength of absorption.

Types of electronic transition

Excitation of electron occur from bonding or non-bonding molecular orbitals to the antibonding molecular orbitals. The antibonding molecular orbital associated withσ-electron isσ*,with is while the n-electrons are not associated with any antibonding molecular orbital (MO).

Generally, the most probable transition is from the highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO).

Transitions in which $\mathrm{\pi }$electron is excited to an antibonding ${\mathrm{\pi }}^{*}$MO are called transitions.

This type of transition occur in unsaturated compounds containing double and triple bonds and also in aromatic compounds.

Identify the absorption of Allene in UV spectroscopy

For the organic compounds containing unconjugated unsaturated system, the absorption due to transition occur below 200 nm while in conjugated system, absorption occur above 200 nm.

Since the double bond of allene (${\mathrm{H}}_2\mathrm{C}=\mathrm{C}={\mathrm{CH}}_2$)aren’t conjugated, allene doesn’t absorb light in the UV region.