Electronic Transition Expected in Butadiene
Electronic Transition Expected in 1,3-Butadiene (C4H6)
The energy order of molecular orbitals of butadiene (CH2=CH-CH=CH2) is shown below-
Butadiene is a conjugated system having two two π-bonds. Two π-molecular orbitals of two alkene groups interact to form two ℼ1 & π2 bonding orbitals and two π* molecular orbitals of two alkene groups interact to form two π*3 & π*4 antibonding orbitals. Hence, the energy of π2 is raised but that of π*3 is lowered. So, the energy gap becomessmall between filled ℼ2 and vacant π*3. As a result, lesser amount of energy can cause π2 → π*3 transition. Hence, the absorption in butadiene appears at higher λmax at 217nm than that in ethylene at λmax 170nm due to conjugation.
In conjugated dienes, π → π* transition results in K band, In the longer conjugated dienes like carotene, λmax goes to visible range i.e. at higher wavelength towards red light.
Electronic Transition Expected in Ethene
Electronic Transition Expected in Formaldehyde