In: Chemistry
Answer-1) Pi-Molecular orbitals of 1,3,5-Hexatriene are shown in diagram. HOMO and LUMO also shown.
Answer-2) Cycloaddition reactions are kind of pericyclic reaction which is strictly governed by MO symmetry rule. Rule state that the MO with same symmetry only overlaps and this orbital symmetry is conserved in the product also. Based on this we can see in diagram that LUMO of Hexatriene and HOMO of maleic anhydride do not match at one side and hence it is [6+2] cycloaddition is thermally forbidden.
Answer-3) Yes, 1, 3, 5-Hexatriene can undergo [4+2] cycloaddition reaction as we can see in diagram. It’s a thermally allowed reaction and well known as Diels-Alder reaction.