Question

Why The maximum wavelength value for ethene is 175nm where as that for 1,3-butadiene is 217 nm

Answer 1

λmax for ethene (175 nm) would be smaller than λmax for 1,3-butadiene (217 nm). As λmax would be the wavelength of transition.  λmax relies on energy gap between most elevated occupied molecular orbital and least occupied sub-atomic orbital.  

This energy gap is bigger in ethene than that of 1,3-butadiene . This amount of more energy required for transition is given by smaller amount of wavelength.  

This is represented in the given image.

Answer 2

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