Question

Which of the following compounds is going to absorb UV light at the largest nm numerical value?

a) Ethene
b) Octa-1,3,5,7-tetraene
c) Buta-1,3-diene
d) Hex-1,3,5-triene
e) Hexa-1,4,-diene

Answer 1

Octa-1,3,5,7-tetraene, due to its extensive conjugated pi-system, is expected to absorb UV light at the largest nm numerical value compared to the other listed compounds.

Step-by-step explanation:

The absorption of UV light by organic molecules is related to the presence of conjugated pi-systems. The more extensive the pi-conjugation, the narrower the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), leading to absorption at longer wavelengths. Among the given compounds, Octa-1,3,5,7-tetraene has the most extensive conjugated system, which would result in the longest wavelength of UV absorption compared to the other compounds listed.

As per LibreTexts™, with increasing conjugation, such as in 1,3,5-hexatriene, there is an increase in the wavelength of UV light absorption. Therefore, it is logical to deduce that Octa-1,3,5,7-tetraene, with its four double bonds in a conjugated system, absorbs UV light at a wavelength even longer than 1,3,5-hexatriene's 258 nm.