Question

Suppose you have two unlabeled flasks, and each flask contains one of these two compounds:Can you differentiate the compounds using a UV spectrophotometer? Select the true statement.The lambdamax of the compound with 2 conjugated bonds appears at a longer wavelength than does the lambdamax of the compound with 3 conjugated bonds. The lambdamax of the compound with 3 conjugated bonds appears at a longer wavelength than does the lambdamax of thecompound with 2 conjugated bonds.The two compounds cannot be differentiated by their lambdamax values.

Answer 1

The right answer is: B

"The $\lambda$_max of the compound with 3 conjugated bonds appears at the longer wavelenght than does the one with 2 conjugated bonds"

This is because the higer amount of conjugated bonds, the smaller separation between HOMO and LUMO (electronic frontier orbitals). Therefore lower energy(longer wavelenght) is associated to the electornic transition HOMO LUMO responsable of the UV Vis absorption phenomena.