Question

Calculate the wavelength of light that will be absorbed when a it electron in hexa-1,3,5-triene, CH2=CH−CH=CH−CH=CH2, is promoted from the highest occupied level to the lowest unoccupied level. The average C−C bond length in hexatriene can be taken to be 144 pm. Compare your answer with the experimentally observed wavelength of 258 nm.

Answer 1

To calculate the absorbed wavelength of light in hexa-1,3,5-triene, we can compare it to similar compounds. As conjugated pi systems become larger, the energy gap narrows, resulting in longer absorbed wavelengths.

Step-by-step explanation:

To calculate the wavelength of light absorbed when an electron in hexa-1,3,5-triene is promoted from the highest occupied level to the lowest unoccupied level, we need to consider the energy gap between these levels.

As conjugated pi systems become larger, the energy gap for a lambda - star transition becomes narrower, resulting in longer absorbed wavelengths of light. In this case, the observed wavelength of light absorbed is 258 nm.

To calculate the wavelength in hexa-1,3,5-triene, we can compare it to similar compounds.

For example, 1,3-butadiene absorbs UV light with a wavelength of 217 nm. Since hexa-1,3,5-triene is larger and more conjugated, we can expect the absorbed wavelength to be even longer, probably closer to the observed value of 258 nm.