Question

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).

PLEASE HELP ME

Answer 1

The question is a little confusing, but what they're saying is on the left you've got the HOMO of hexa-1,3-5-triene. On the right you have the excited state of hexa-1,3,5-triene, which has a different HOMO.Notice that the p orbitals are slightly different (in particular, the ones on the end!).

I have drawn the MO diagram of ethene for you. I've labeled the HOMO and the LUMO for you. Notice that you are supposed to list the LUMO of ethene as part of your response.

Notice that ethene has two pi electrons. Thus, when drawing the MO diagram, you fill in the lowest energy orbital in which all the p-orbital lobes are in phase (that is, all the shaded/nonshaded parts line up). This gives you a LUMO where the orbitals are out of phase (see how the top lobes are shaded and unshaded). This LUMO is what you should draw as a response for LUMO of ethene.

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I have drawn the MO interaction figure myself for you, in order to highlight the differences. First, look at the figure from the left:

Notice that the phases of the HOMO and LUMO match up on one end but not on the other. Since there is one bonding interaction and one antibonding interaction, the reaction does not proceed. There is no net bonding interaction.

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Bonding interaction (Phases match) Bbnding interaction (Phases match)

Notice that for the figure from the right, there are two bonding interactions now, because both phases match when the HOMO and LUMOs interact.

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What does all this mean? Well, it means two things. (a) the reaction is thermally forbidden and (d) the reaction is photochemically allowed.

How do you know this? The answer is pretty simple. If the ground-state HOMO (i.e. non-excited state) has overall bonding interactions, then the reaction is thermally allowed. If there are no net bonding interactions in the ground-state HOMO, then the reaction is thermally forbidden. Notice that for the system given in the problem, this is the case.

If the excited-state HOMO has overall bonding interactions, the reaction is photochemically allowed because in order to get to this HOMO, you had to promote an electron with light. Notice that this is the case for this problem. If there are no net bonding interacions in the excited-state HOMO, then the reaction is photochemically forbidden.

Hope this helps!

Answer 2