Write a general rule that can be used to predict the major pro...

Question

Write a general rule that can be used to predict the major product of a Diels–Alder reaction between an alkene with an electron-withdrawing substituent and a diene with a substituent that can donate electrons by resonance depending on the location of the substituent on the diene.

Accepted Answer

Hello, everyone. Let's solve this problem together. It says provide a general principle that may be utilized in anticipating the main product of a deals alter reaction involving an alkene that has a substituent that can withdraw electrons and a dying that has a substituent that based on its placement can donate electrons through resonance. So first, let's review quickly what a Dale's older reaction is, we know it is a four plus two cyclo addition reaction. So that means we are reacting a dying which has two pi bonds or four pi electrons reacting that with a dienno file which has one pi bond reacting or two pi electrons reacting in the presence of heat to form a cyclic product which has one new pie bond and also two new sigma bonds where those original pie bonds in the dying and the dienno file were. So we want to have an alkene that has a substituent with an electron withdrawing group and a dying with an electron donating group. So the dyne is changing based on its position or the product is changing based on the position of the electron donating group in the dying. So first, let's draw our dyno file since that is not changing. So a dino file or our alkene with an electron withdrawing group, an example of an electron withdrawing group is a carbon group, right? So let's draw a single double bond, one, double bond and attach the end of that double bond, we'll add a carbon with a double bond oxygen and a hydrogen. So actually an aldehyde group, right, not a ketone. And we can have resonance here since there is a conjugated dying. So that carbon carbon double bond, those pi electrons can shift between the other two carbons breaking that carbonyl double bond resulting in the oxygen having a negative charge. And the carbon at the other end having a positive charge. So if we draw the resonance hybrid for this diana file, we have a partial negative on the oxygen and a partial positive on the end carbon. OK. Let's look at our dyne which will have an electron donating group. And we'll start with that electron donating group on carbon number one, right, we will have two different versions since our compound is symmetrical. So dying with electron donating group on carbon one. So let's draw our dying in an electron donating group on carbon. Number one can be an O eyl group och two ch three. So we have the lone pairs on the oxygen that can be donated into the dying, shifting those pi electrons. Yeah, to create a new resonance structure where we have a double bond between the carbon and the oxygen and a new double bond between carbons two and three. And we have a positive charge on the oxygen negative charge on the end carbon carbon number four and drawing the resonance hybrid for this. Yeah, we have a partial positive charge on oxygen dotted line all throughout the dying and a partial negative charge on carbon number four. OK. So let's draw what the reaction would look like between these two compounds. And then we will change the position of the electron donating group on the dying and see what that reaction looks like. So, reaction with electron donating group on carbon one of dying, we have our dying and we are going to use the resonance hybrid structures lining up our diana file correctly just like we saw in the original example of the deal's alder reaction. So we have the opposite charges lined up with each other with our three step mechanism creating a product that is cyclic with our new double bond. And we see that the substituent groups are next to each other. They are actually Ortho to one another, right on carbons, one and two of that ring. OK. Let's change the position of that electron donating group on the dying two carbon two and see what the product looks like. Oops. So we have our o ethyl group. Mhm which now looks like this and we will have resonance over carbons and the oxygen atom. So our partial positive charge is still on the oxygen partial negative charges. Now, on carbon one, and if we rotate it to line up with our diana file, right, we want our double bond to line up correctly or rather the charges to line up correctly. Not the double bond because we want that negative charge on the dying to nucleic attack the dia file. So lining up our diana file the same way as the first time oops. So our Diana file has the first arrow to the dying shift that double bond in the dying. And that partial negative charge attacks the partial positive charge of the diana file to form our product. We're now our aldehyde group is still on carbon number two. If we are considering the numbering that we said in the first ring, that wouldn't be the correct nomenclature though, because of the double bond and our ofo group would be per to that aldehyde group, right. So after seeing this example, the general principle that we can say is that if the electron donating group is attached at the end of the dying, it will be adjacent to the electron withdrawing group of the dienno file in the product. If it is not attached to the end of the dying, it will be opposite to the electron withdrawing group of the dienno file in the product. So that is it for this problem. I hope this video is helpful and I'll see you next time.

Write a general rule that can be used to predict the major product of a Diels–Alder reaction between an alkene with an electron-withdrawing substituent and a diene with a substituent that can donate electrons by resonance depending on the location of the substituent on the diene.

Key Concepts

Diels–Alder Reaction

Electron-Withdrawing and Electron-Donating Groups

Regioselectivity in Diels–Alder Reactions

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