Question

Shown below are the structures and UV spectra of toluene and 1- methyl-cyclohexene. Identify which one is which and explain your choices. Your explanation should include a discussion of the structure of the molecule and its molecular orbitals. CH3 CH3 UV/VIS SPECTRUM UV/VIS SPECTRUM Logarithm epsilon uosdo wyeon 240 270 LILLEHLLLLLLLLL 185 190 195 200 205 Wavelength (mm) Wavelength (ml

Answer 1

Graph A corresponds to i methyl cyclo hexene and graph B corresponds to Toluene.

1 methyl cyclohexene has only one pi bond and hence corresponds to one pi bonding molecular orbital and 1 pi antibonding orbital. wheras in toluene there are three alternate pi bonds leading to extensive conjugation. In this compound there are three bonding and three antibonding molecular orbitals. the energies of the molecular orbitals can be represented as below.

energy I'I Ã Bonding orbital ** Anti Bonding orbital

In1- methyl cyclohexane the energy gap between Highest occupied molecular orbital and least unoccupied molecular orbital is lesser when compared to Toluene which has three bonding and three antibonding molecular orbitals. Hence the energy needed for the transition of electron in case of cyclohexene is greater which means it absorbs at lower wavelength when compared to Toluene, when we compare the graphs we find that maximum absorption is at around 190nm in graph A and 260nm in graph B, Hence Graph A having absorption at lower wavelength should correspond to Cyclohexene. Graph B will correspond to Toluene.

Also there are many peaks seen in graph B which are due to the other transitions of electrons from the differeent molecular orbitals present in Toluene.