Rep:Mod:xv114
Exercise 1: Reaction between butadiene and ethylene
Reaction Scheme
Butadiene and ethylene react in Diels Alder as shown in Scheme 1:
File:XNV REACTION SCHEME FOR REACTION 1
Molecular orbital
The highest occupied molecular orbitals (HOMO) and lowest occupied molecular orbitals (LUMO) of both butadiene and ethylene shown in Figure 1, can overlap to a give 4 different transition state MOs, which correlates with those illustrated on the MO diagram, Figure 2. From the MO diagram, it can be seen that it is the HOMO of butadiene that interacts with LUMO of ethylene. The reverse is also possible with π* of ethylene overlapping with ψ3 of butadiene, however the energy difference between them is larger than between ψ2 and π orbitals. Thus the overlap is not as good.
Symmetry of MO of both reactants needs to be the same in order for overlap to occur and in turn the reaction would be 'allowed'. I.e. Only symmetric orbital can overlap with another symmetric orbital, not with asymmetric. This explains why only LUMO of butadiene and HOMO of ethylene can overlap with each other due to both having the same symmetry.Thus, the orbital overlap integral for a symmetric-asymmetric interaction would be zero, whereas that for symmetric-symmetric or asymmetric-asymmetric interaction would be non-zero.
Bond Length
Exercise 3: Diels Alder vs Cheletropic
Xylene and sulphur dioxide can undergo either Diels-Alder or cheletropic reaction, forming six membered ring and a five membered ring respectively, as shown in the scheme below.
