1,3-butadiene has a wavelength max at 217 nm. Each additional conjugated double bond makes what difference to the wavelength max?
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Conjugated systems, which consist of alternating double and single bonds, allow for delocalization of π electrons. This delocalization lowers the energy gap between the ground state and excited state of the electrons. Show more…
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Calculate the wavelength at which the compound below is expected to have maximum absorption. For reference, the expected absorption of butadiene is shown below: 214 nm
Madhur L.
The following ultraviolet absorption maxima have been measured: 1,3-Butadiene 2-Methyl-1,3-butadiene $220 \mathrm{nm}$ 1,3 -Pentadiene $223 \mathrm{nm}$ $2,3-$ Dimethyl-1,3-butadiene $226 \mathrm{nm}$ $2,4-$ Hexadiene $227 \mathrm{nm}$ $2,4-$ Dimethyl-1,3-pentadiene $232 \mathrm{nm}$ $2,5-$ Dimethyl- 2,4 -hexadiene $240 \mathrm{nm}$
Exercise 13A.11(a) on p.556. The two compounds 2,3-dimethyl-2-butene (1) and 2,5-dimethyl-2,4-hexadiene (2) are to be distinguished by their ultraviolet absorption spectra. The maximum absorption in one compound occurs at 192 nm and in the other at 243 nm. Match the maxima to the compounds and justify the assignments. Hint: Recall our discussion of the particle-in-a-box and its application to conjugated polyenes. b) Better results for the orbital energies of conjugated polyenes can be obtained from H6ckel Theory, which is described in Chap. 10. Let's consider ethene and butadiene. Both of these molecules exhibit π*←π transitions upon absorption of UV light. Using the results of H6ckel theory illustrated in Figs. 10E.1 and 10E.2, determine which molecule will absorb the higher frequency radiation. You do not need to carry out H6ckel calculations! Just use the results.
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