Question

The following ultraviolet absorption maxima have been measured:

1,3-Butadiene 217 nm

2-Methyl-1,3-butadiene 220 nm

1,3-Pentadiene 223 nm

2,3-Dimethyl-1,3-butadiene 226 nm

2,4-Hexadiene227 nm

2,4-Dimethyl-1,3-pentadiene 232 nm

2,5-Dimethyl-2,4-hexadiene 240 nm

What conclusion can you draw about the effect of alkyl substitution on

UV absorption maxima? Approximately what effect does each added

alkyl group have?

Short answer

The value of ${\mathrm{\lambda }}_{\max }$in the ultraviolet spectrum of dienes becomes larger with increasing alkyl group substitution. Since energy is inversely related to ${\mathrm{\lambda }}_{\max }$, the energy needed to produce ultraviolet absorption decrease with increasing substitution.

Step-by-step solution

Ultraviolet 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 anddata-custom-editor="chemistry" $\mathrm{\lambda }$ the wavelength of absorption.

Factor affecting value of wavelength in UV spectroscopy

The absorption of light in UV spectroscopy promotes the $\mathrm{\pi }-{\mathrm{\pi }}^{*}$electron transition. In order for this transition occur the energy gap between HOMO and LUMO has to correspond to energies of the UV radiation.

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

• The most important factor that will affect the HOMO and LUMO energy gap, and, therefore, the energy and the wavelength of absorbed radiation is the extent of conjugation. The conjugation lowers the difference between HOMO and LUMO and the more extended it is, the lower the energy difference will be.
• The value of ${\mathrm{\lambda }}_{\max }$in the ultraviolet spectrum of dienes becomes larger with increasing alkyl substitution.

Effect added by each alkyl group

Each alkyl substituent cause increase in of 3-6 nm. The value of ${\lambda }_{max}$in the ultraviolet spectrum of dienes becomes larger with increasing alkyl group substitution. Since energy is inversely related to data-custom-editor="chemistry" ${\mathrm{\lambda }}_{\max }$, the energy needed to produce ultraviolet absorption decrease with increasing substitution.