Given absorption maxima for 2,3-dimethyl-2-butene (190 nm) and 2,5-dimethyl-2,4-hexadiene (240 nm), at what wavelength does 1,3,5-hexatriene absorb

258 nm

Explanation:

Light absorption by molecules and subsequent excitation to higher energy levels of the most loosely bound electrons is the subject of study in UV-Visible spectroscopy.

Since π-electrons are most loosely bound in organic molecules, it follows that a study of the wavelength of maximum absorption of a molecule offers reasonable information about the degree of unsaturation in the molecule.

If we consider the compounds; 2,3-dimethyl-2-butene, 2,5-dimethyl-2,4-hexadiene and 1,3,5-hexatriene, we will notice that the wavelength of maximum absorption increases with degree of unsaturation and nature of the double bonds. Conjugated systems tend to shift absorption maxima to longer wavelengths (as we can see in 2,5-dimethyl-2,4-hexadiene and 1,3,5-hexatriene). Absorption data for alkenes usually reflect π to π* transitions.

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