Propose mechanisms for the following reactions. Additional pro...

Question

Propose mechanisms for the following reactions. Additional products may be formed, but your mechanism only needs to explain the products shown.

(c) Chemical reaction diagram showing brominated compound converting to multiple products with ethyl alcohol and heat.

Accepted Answer

All right. Hello everyone. So this question is asking us to propose the mechanisms of the reaction given up here. The starting material is an aic Halide reacting with methanol in the presence of heat to produce three distinct products. Note that more products may be formed, but the mechanism needed is the one that shows how the products are formed. All right. So first and foremost, based on the structures of the three products that were given, we can get a sense as to what mechanisms are taking place. And therefore, which ones we need to show, right, which mechanisms we need to show the first two products have essentially replaced the bromine of the starting material with the meth oxy group of methanol. So that implies that the first two products are obtained from a substitution reaction. On the other hand, our third product, our third and final product has a second double bond. And notably, bromine is no longer present in this product. And the meth boxy group is not connecting either or is not present either. So that means that the third and final product is obtained from an elimination reaction because the product of that was another key. So now that we have a sense as to the reaction that's happening here in a more general sense, we can use our very convenient flow chart to get a sense as to what specific mechanisms we're going to use for this question. Recall that the first question of that flow chart was with respect to the strength of the nucleo file. Now our alila hide is the substrate because bromine, if you recall is going to behave as the leaving group here, this means that methanol is going to be our Nile. And because methanol is neutral, it's going to be described here as weak. So weak nucleo files tend to favor either SN one or E one. And so from there, we move on to the second question of the flow chart which is asking about the type of leaving group present. Now, before that, we'd have to look at the carbon atom that's connecting to our leaving group. In this case, Bromy, and we can see that that particular carbon is secondary. So the leaving group is secondary, which based on that information means that our two mechanisms are going to be S and one and E one. And when it comes to SN one and E one reactions, they're both characterized by the formation of a carbo cion intermediate. So let's go ahead and show that to start off our mechanism here. All right. So here I temporarily pause the recording just so I can go ahead and redraw the structure of the starting material. No, the first step in both cases here in both SN one and E one is the formation of that carbo cry. So to start off right, in the presence of the heat that in solution represented here by the symbol delta bromine is going to go ahead and detach itself from a carbon. And when it does, so it's going to take with it the electrons of the bond between carbon and bromine. And so as a result that carbon is left electron deficient, hence the positive charge. So carpet is going to have a positive charge and the neutral bromine atom from the starting material would eventually become a bromide ion. But here, let's focus on the Carbo Caria because as discussed previously, this particular carbo cion can be described as a lilic because the positive charge is on a carbon adjacent to a double bond. And because it's a lyric, let's recall resonance resonance is the ability of a cravo carion to potentially change positions in order to delocalized the charge across a larger area. So in this particular case, we are going to see resons because of the fact that the positive charge is in close proximity to the double wand. And so it's because of that resonance that we see two distinct substitution products and also the elimination product because notice how the location of the double bond in one of our substitution products and also the elimination product has since moved. That's because of resonance. So here the double wand from the starting material can go ahead and move towards where the positive charge is currently depicted. This de localizes the positive charge and gives the Nile more places to attack, hence the different products. So as a result, the positive charge has since moved on to a tertiary carbon where initially it was on a secondary carbon. So because our resonance forms are complete, I'm going to go ahead and enclose them both in brackets. All right. So from here, we can go ahead and move on towards the formation of our substitution and elimination products. So as we move towards the right side of the screen, I'm going to go ahead and showcase the substitution products. However, going towards the left from the rearranged form of our initial carbo cion, let's go ahead and showcase the elimination. So elimination is the removal of a proton from a carbon that is adjacent to the carbo cion itself. Those carbons are referred to as beta carbons, which means that the protons being removed are beta protons. Now based on the structure of the elimination product, the beta hydrogen to be removed is the one on the six member ring as opposed to the one on the five member ring though, keep in mind that multiple products can be produced. So in this particular case, Methanol is going to behave as a base as opposed to a nucleo file, which means that the lone pair of electrons or one of the lone pairs of electrons on oxygen is going to go ahead and remove that beta hydrogen. As a result, the electrons in the bond between the beta carbon and the beta hydrogen are going to go ahead and establish a new double bond between the carbon that has a positive charge. And the beta carbon itself, this results in our elimination product which is a die all king. So from here, as we move towards the right side of the screen, let's go ahead and showcase the substitution mechanisms using both resonance forms of the carbo cion. So now in the case of substitution, methanol is going to behave as a nhi as discussed previously. And so what that means is that the one of the lone pairs of electrons on the oxygen of methanol is going to directly attack the carbo cion which establishes a new bond between oxygen and carbon. So starting off with the second resonance form, I drew here, another molecule of methanol directly attacking the rebo cion using a blue arrow for the elimination. I used pink arrows. So from here, we've now established a new bond between oxygen and carbon though in this particular case, because methanol is neutral and that oxygen already has a hydrogen attached to it. After that new bond is created, oxygen is going to have a positive charge. So an additional step is going to be necessary to deproteinate oxygen and yield a neutro meth oxy group. But let's go ahead and showcase this with the first resonance form too. So for the first resonance form, I have another molecule of methanol and using a red arrow this time, I'm going to go ahead and have it attack that secondary aic Carbocaine. So in the case of both substitution mechanisms, the last step is going to be the same, another equivalent of methanol is necessary to deproteinate the oxygen that currently has a positive charge. This gives the oxygen with a positive charge, an additional pair of electrons that ultimately produces a neutral meth oxy group. And as a result, we have our two substitution products and there you have it. So here is the completed mechanism that showcases the formation of the three products shown earlier. Two came from an essence one reaction in which the carbo cion did undergo resonance and the other was from an E one reaction. So if you watch this video all the way through, thank you so very much for doing so. And I hope you found it helpful.

Propose mechanisms for the following reactions. Additional products may be formed, but your mechanism only needs to explain the products shown.
(c)

Key Concepts

Reaction Mechanisms

Nucleophiles and Electrophiles

Functional Groups

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