Map sapling leaming Consider the cycloaddition of hexa-1,3,5-triene with ethene.
ID: 836973 • Letter: M
Question
Map sapling leaming Consider the cycloaddition of hexa-1,3,5-triene with ethene. The HOMO and excited-state HOMO (HOMo*) are shown below. Drag the atomic orbitals (AOs) to generate the LUMOs of ethene. Label the type of overlap from each orbital interaction indicated via the arrow(s) HOMO hexa-1,3,5-triene HOMO* hexa-1,3,5-triene bonding anti-bonding anti-bonding 8-8 LuMo LUMO ethene LUMO ethene bonding anti-bonding Select the true statement(s). This reaction is thermally forbidden This reaction is thermally allowed This reaction is photochemically forbidden. This reaction is photochemically allowed.Explanation / Answer
To predict whether a pericyclic reaction is allowed or not under given condition, Woodward and Hoffmann proposed following set of rules based on conservation of orbital symmetry concept.
A thermal pericyclic reaction is allowed in the ground state, when the total number of (4q + 2)s and (4r)a components is odd.
Otherwise, if the total of (4q + 2)s and (4r)a components is even, the pericyclic reaction is allowed in the excited state i.e., under photochemical conditions.
An alkene is a (4q + 2)s component, since it has 2 ? electrons and is approaching the triene suprafacially.
There are 6 ? electrons in triene, which is making bonds in suprafacial manner, it is a (4r)scomponent.
So, there is one (4q + 2)s component and there are no (4r)a components.
Hence, the total number of (4q + 2)s and (4r)a components = 1 + 0 = 1, an odd number.
Therefore, the given reaction is thermally allowed in ground state when both the components are approaching suprafacially.