Question

1. (a) Why does buta-1,3-diene (max 217 nm) have a longer-wavelength (lower-energy) UV-Vis absorption peak as compared to ethene (max 171 nm)? Include an energy level diagram with your answer. (b) What would you expect to happen as you add on to this structure with additional double bonds? Hint: consider the total number of p atomic orbitals contributing to each molecule, as depicted below. ?-? Ethene Butadiene

Answer 1

a) energy gap between the highest occupied molecular orbital(HOMO) and lowest unoccupied molecular orbital (LUMO) in butadience is smaller than in ethylene(ethene) b) addition of successive conjugates double bonds progressive narrows the gap between HOMO and LUMO thus longer wavelength (lower energy) radiation required for the transition