![homo lumo diels alder homo lumo diels alder](http://www.chemistry-blog.com/wp-content/uploads/2011/09/Orbitals.png)
This does not happen with 100% efficiency, so only some of the alkenes will become excited. The alkene is now in an "excited state".įigure PR3.6. Irradiating an alkene with UV light leads to promotion of an electron from the LUMO to the HOMO. In fact, there is a way around that problem. Qualitative molecular orbital picture of addition reaction. The concerted reorganization of bonding possible for the Diels Alder reaction can't happen here.įigure PR3.5. If the p orbitals on one end are in phase, the p orbitals on the other end must be out of phase. The HOMO on one alkene and LUMO on the other alkene do not overlap so that bonds can form between the two ends. The transition state would be very high in energy.Īnother problem shows up if we look at the orbital interactions in a addition reaction. In fact, the transition state would be more like antiaromatic cyclobutadiene. You might also say that the benzene-like transition state that stabilizes the pathway through a Cope or Diels Alder reaction isn't possible in a addition. Four-membered rings do occur in nature despite their strain energy. That is true, but it may not be reason enough to prevent the reaction from happening. You might say that the four-membered ring would be much more strained than the six-membered ring formed by the Diels Alder reaction. There are a couple of reasons why, and you may be able to suggest some at this point. However, addition reactions don't occur without special circumstances. If this reaction occurred, two alkenes would come together to form a four-membered ring. In contrast, the addition of one regular alkene to another regular alkene would be called a addition reaction. A 2-carbon unit on one molecule interacts with a 4-carbon unit on another molecule. In that way, these carbon atoms at the ends of the diene and dienophile are able to bond with each other.Ī Diels Alder reaction is sometimes called a addition reaction. It is important that those orbitals are able to overlap with each other to form an in-phase interaction. Pay attention to the p orbital drawings on the carbons that will bond to each other to form the six-membered ring. Qualitative molecular orbital picture of the Diels Alder reaction. As it happens, the LUMO on one molecule has the correct symmetry such that it can overlap and form a bonding interaction with the HOMO on the other molecule.įigure PR3.3. In the Diels Alder reaction, we can think of an interaction between the LUMO on one molecule and the HOMO on the other.
![homo lumo diels alder homo lumo diels alder](https://www.pianetachimica.it/computer/Diels_Alder/HOMO-LUMO.gif)
Once again, the reaction can be thought of in terms of a reorganization of electrons between these two molecules. Explain why using the expression for free energy of a reaction, DG = DH - T DS. The forward reaction is favoured at low temperature, whereas the retro reaction is favoured at high temperature.Draw curved arrows for the retro-Diels Alder reaction.For example, a retro-Diels Alder reaction is shown below. The reversible process is usually named the same way as the forward reaction, but with the prefix "retro". Many pericyclic reactions are reversible. The diene and dienophile in the Diels Alder reaction. The alkene is referred to as a "dienophile" it reacts with the conjugated pair of double bonds.įigure PR3.2. It always occurs between an alkene on one molecule and a conjugated diene on the other molecule. Unlike the Cope and Claisen rearrangements, this reaction often occur intermolecularly (between two molecules). \( \newcommand\)Īs noted previously, the Diels Alder reaction is a classic example of a pericyclic reaction.