The λmax value for ethene is 175 nm whereas that for 1,3-butadiene is 217 nm.
Answers
The λmax value for ethene is 175 nm whereas that for 1,3-butadiene is 217 nm, this indicates that 1,3-butadiene energy gap between the highest occupied molecular orbital and lowest unoccupied molecular orbital is smaller than in ethene.
Addition of successive conjugated double bonds progressively narrows the gap between highest occupied molecular orbital and lowest occupied molecular orbital.Thus longer wavelength (lower energy) radiation required for the transition.
Answer: ethene comes under unconjugated chromophore while 1,3-butadiene is a conjugated chromophore. And conjugated chromophore absorbs energy at a longer wavelength as compared to the non conjugated ones. And longer the conjugated system, the higher will be the λmax value.
Explanation: