(c) Based on what you know about the relative stabilities of a...

Question

(c) Based on what you know about the relative stabilities of alkyl radicals and benzylic radicals, predict the product of addition of HBr to 1-phenylpropene in the presence of a free-radical initiator.
(d) Propose a mechanism for this reaction.

(d) Propose a mechanism for this reaction.

Accepted Answer

All right. Hello everyone. So this question is asking us to predict the product of the addition of H Pr to Bute one in one ill Benzi in the presence of a free radical initiator show the mechanism that's initiation and propagation for the reaction. Now, recall first and foremost that this starting material is an all keen and we're reacting this keen with H pr. Now in normal conditions, this is a Markov Niko process, meaning bromine would be attached to the more substituted carbon of the starting double bond. However, because this reaction proceeds through a radical mechanism in the presence of that radical initiator, this actually makes the product anti Mark honey cup, meaning it's hydrogen that is attached to the more substituted carbon or the more stable carbon. Instead, this is because bromine or electro or excuse me, the bromine radical, which is our electro must add on to the more or sorry, the less substituted carbon to allow for a more stable carbon radical. So with this in mind, let's draw out our starting material we have but one in one ill benzene. So that's a benzene ring with a four carbon chain attached to it. Now, within this four carbon chain, there is a double bond in between carbons, one and two. In other words, between the carbon atoms that are closest to the benzene ring. Now the reason I'm drawing out the strutting material towards the bottom of the screen here is because I want to make space for the initiation step on the top. Now, in the context of this particular reaction, recall that peroxides are a very common free radical initiator. So a peroxide is characterized by this oxygen to oxygen single bond that is relatively weak. So in the presence of heat or light, we'll use heat. In this example, the bond between the two oxygen atoms in the peroxide is subsequently going to break. So one electron from the bond goes to each oxygen and notice how I'm using fishhook or single headed arrows to showcase the movement of this one electron. So one electron goes to each oxygen from the peroxide which creates two radicals. So one of these radicals can then go ahead and react with a molecule of HBR, the radical is going to break the bond between hydrogen and booming. One electron connects are oxygen radical to hydrogen and the other goes to Bromy. So this ultimately creates an alcohol as a by-product and our bromine radical, which is the aforementioned electro file. And from here, we can go ahead and proceed to the propagation step involving our starting material because now in the propagation step, our bromine radical is going to interact with our starting material. But I want to discuss this concept of anti my comedy competition in a little bit more detail and with a little bit more nuance because in this case, the bromine radical is going to interact with the pi bond that is adjacent to the benzene ring in our substituent. Here. After this pi bond breaks both carbons from this keen will create secondary radicals. But one of them is still more stable than the other because notice how carbon, one of the substituent is a benzo carbon, considering it's directly adjacent to a benzene rink, recall that benzo radicals and benzole Cary CS are incredibly stable due to their ability to participate in resonance with the benzene ring. So because of this, the benzo position is going to create a more stable radical, which means that that is where the bromine radical is not going to attack. Rather, the bromine radical is going to attach itself to the second carbon of the ke. So not the benzo position. So switching colors again. Now, the pi bond between carbons, one and two of the substituent is subsequently going to break with one electron combining with the bromine radical to create a bond between carbon two and bromine, the other electron goes to carbon one to create that stabilized or resonant stabilized benic radical. So from here and I'm going to go ahead and make space a little bit we have a benzo radical which is going to then react with an additional molecule of H pr. So here we have H pr maybe I should move this off to the side a little bit. But once again, the bond between hydrogen and bromine is going to break one electron from this bond is going to be used to create a bond between the benzo radical and hydrogen while the other is going to regenerate a bromine radical. So now we have hydrogen attached to the benzi like position. I paused the recording for a little bit because I realized that I needed to make some space. But now we've actually gone ahead and completed our propagation phase because this new bromine radical that has been regenerated is going to repeat this process. Hence the term propagation, meaning that we also have our product. So our product here is two bromo beetle benzene and there you have it. So very briefly, I'm going to scroll up here. So you can see both steps of the mechanism shown, right? We have initiation followed by propagation. So then I'm going to scroll down once more. So you can see the product. And with that being said, if you watch this video all the way through, thank you so very much for doing so. And I hope you found this helpful.

(c) Based on what you know about the relative stabilities of alkyl radicals and benzylic radicals, predict the product of addition of HBr to 1-phenylpropene in the presence of a free-radical initiator.
(d) Propose a mechanism for this reaction.

Key Concepts

Alkyl and Benzylic Radicals

Free Radical Addition Mechanism

Regioselectivity in Radical Reactions

Watch next

(c) Based on what you know about the relative stabilities of alkyl radicals and benzylic radicals, predict the product of addition of HBr to 1-phenylpropene in the presence of a free-radical initiator.(d) Propose a mechanism for this reaction.

Key Concepts

Alkyl and Benzylic Radicals

Free Radical Addition Mechanism

Regioselectivity in Radical Reactions

Watch next

(c) Based on what you know about the relative stabilities of alkyl radicals and benzylic radicals, predict the product of addition of HBr to 1-phenylpropene in the presence of a free-radical initiator.
(d) Propose a mechanism for this reaction.