A halogenation intended to make compound A formed B instead.

Question

(a) Suggest a mechanism for the intended formation of A.

Accepted Answer

All right. Hi, everyone. So this question says that the following reaction was performed to produce compound A but compound B was obtained instead draw a suitable mechanism to show the intended formation of compound A. Now here our starting material is three ethel pencil benzene reacting with elemental bromine in the presence of heat. Now, the fact that we have elemental bromine in the presence of heat indicates that this is going to be an example of free radical Broin nation. So in free radical brominated, we're substituting a given hydrogen in this compound with an atom of bromine. So here we have to go ahead and explain the formation of compound A specifically which is three bromo three ethel pencil benzene. So lets go ahead and get started and recall that all radical mechanisms have three distinct steps. The first of which being initiation. Now, the initiation step is the step in which your first radicals begin to form, which will then react with other materials in your system. In this case, your organic starting material in a sort of chain reaction, right? So the initiation step is the beginning of that chain reaction. Now here the reason why we like to use elemental bromine is because the bond between the two brom means is actually relatively weak because bromine of course is electron. So the presence of heat is essentially a catalyst, right, it provides this bond enough energy to go ahead and break apart to yield two bromine radicals. Now, the bromine radicals are the first radicals which will then react further in the propagation steps. So now in the first propagation step, one of our bromine radicals is going to go ahead and interact with our starting material, right are three ethyl pencil benzene. So I'm gonna going to go ahead and redraw it heres my ethel group. And now we have to take into consideration what our product is supposed to look like after I modify the structure. But there. So if we look at the molecule, we are trying to form compound a our BROM atom is attached to this tertiary o carbon. So because our bromine is attached in that position, the tertiary alch carbon must have been a radical in our hypothetical mechanism. So in our first propagation step, right, the desired radical must form. So this tertiary hydrogen that's attaching here or thats attached is going to interact with one of our bromine radicals from the initiation step. So our hydrogen and our bromine create a bond to make H pr whereas our tertiary carbon is left to create a radical. So here is my benzene ring. Here's my carbon chain that I theo should look a little bit better. And now, like I said previously, that tertiary hydrogen has been removed to yield a radical in its wake in addition to HBR as a by-product, and it's at this point that the resulting radical can react further in our second propagation step. Because here, the second propagation step is the actual addition of bromine onto that tertiary carpet. So here is my intermediate right here is my radical and now the radical is actually going to react with a molecule of elemental bromine to yield the final Broin nation prot because here a radical interacts with element to bromine. And the bond between our two bro means is going to break. So one bromine is going to attach itself to our tertiary carbon like. So right here is our brominated product and now we have another bromine radical left over. So the reason why it's called propagation is because it's essentially a chain reaction. So the molecule of bromine or the bromine radical, excuse me, that's left over after the second propagation step is going to go ahead and react with another molecule of three eop pensyl benzene. And it's going to do the same thing over and over again until all molecules of three eop pensyl benzene have been brominated. So it's at that point that we can introduce our final termination step, which is a fitting name because at this point at the time of termination, right there are no more organic starting material molecules, no molecules of organic starting material to convert any more. They've all been brominated. So it's at this point that all radicals must combine with other radicals so that there are no more left in solution. Now, in this specific mechanism, right, we have one bromine radical left over from our second propagation step, but we also have one more radical from the initiation step. So in our termination step, right, both of these left over bromine radicals can go ahead and combine to go ahead and yield elemental bromine once again. And this is a a reversible process. Now, it's also worth mentioning that there are actually multiple termination steps because all cure radicals can combine with bromine radicals and so on and so forth. But for ST geometric purposes, just to balance out everything I've drawn, I'm only going to draw the formation of BR two from two BR radicals. But here is our final answer, right? This is our complete mechanism. Our first step being initiation, right, in which our first Bromy radicals formed. So here is initiation like I said previously and then, right, in the first propagation step, our bromine radical that created, that was created previously interacts with a molecule of three ethel pencil benzene. The generate are tertiary alch radical. And then in the second propagation step, our tertiary al radical reacted with BR two to generate our brominated product plus an additional bromine radical which then carried this process forward. Now, finally, right in our termination step, our leftover bromine radicals combine together to create BR two, therefore, eliminating any leftover radicals that might be in solution and thus ending the course of the reaction, hence the term termination and there you have it. So with that being said, if you stuck around to the end, thank you so very much for watching. And I hope you found this helpful.

A halogenation intended to make compound A formed B instead.

(a) Suggest a mechanism for the intended formation of A.

Key Concepts

Halogenation Reaction

Reaction Mechanism

Regioselectivity

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