Would the following reactions go to the right or the left? Dra...

Question

Would the following reactions go to the right or the left? Draw the products and label ALL species. Provide the full mechanism.

Reaction diagram showing benzene and BH3 with an arrow indicating direction for acidity ranking.

Accepted Answer

So this one was pretty tricky because this is not your typical Bronsted Lowry acid base reaction. Remember that in Bronsted Lowry, what we do is we exchange a proton. So what that means is that on one side the proton is in one place and on the other side the proton is in another. Okay? And these are in equilibrium with each other and this is what we've been doing over and over again. Well, in this case, I can't use pKa values because this is not a Bronsted Lowry reaction. This is actually Lewis based Lewis acid reaction. And remember that I told you guys that Lewis is the more general definition and it encompasses all reactions. So basically, anytime you have a nucleophile and an electrophile reacting together, that is Lewis acid Lewis base. And that's what this is. So really this is going to be more like the types of reactions that we're gonna see later in orgo where we're just going to see nucleophiles and electrophiles reacting together and the difference is that instead of getting equilibrium, because equilibrium means like you're exchanging one thing for another, all we're going to get is products here. Okay? So what that means is that this question I'm not asking you guys, does it go to the right or the left? It's just going to go it always goes to the right. Your challenge here is to figure out what does the product look like. Okay? And like I said, this is very similar to the type of reactions that you're going to see later on in Orgo where we're just going to say predict the product. Okay? So now that I've said all of that, how can we identify the acid and the base? Okay? And honestly, this one just requires a little bit of memorizing. I already taught you guys something really special about BH 3. Do you guys remember? Remember that boron has an empty p orbital. Okay? And remember there was one other atom that had an empty p orbital. Do you remember that? Also, aluminum. Okay. So if you ever see aluminum, that's another one. And this means that it's going to be an amazing Lewis acid because it's really good to accept it's easy for it to accept electrons. Okay? So this is actually going to be a super strong acid. It's just not a good proton donor. Instead, it's just an electron pair acceptor. Okay? So that's my Lewis acid, so obviously the double bond must be my base. But how can it be a base? Well, remember, double bonds have extra electrons. Okay? So remember that I told you guys that double bonds are good nucleophiles. Okay? So it actually makes sense that this would be my Lewis base. Okay? And if we were using general nucleophile electrophile terms, this would be my nucleophile because it's good at giving away electrons And this would be my electrophile because it's good at accepting electrons. Okay? Cool. So now let's go ahead and draw the arrow. Like I said, usually we would hold off on drawing the mechanism until we know which side is favored. But I already told you the right side is favored because this is just going to be a reaction. It's just going to give me a single product. Okay? So where should the arrow start from? It should start from the negatively charged thing, which is the nucleophile. So what I should do is I should take the electrons from my double bond and put them straight into my p orbital. That means that now I'm going to be making a new bond between the boron and one of the carbons on the double bond. Does that make sense? Now another thing to keep in mind is that how many hydrogens did each carbon on the double bond have before? This one had 1 and this one had 1. Are you guys cool with that? Because that would make the octet for both of those. Now what I want to do is draw what the product I'm going to get is that it's still attached to an H up here, but now it's also attached, it has a new single bond to now it's also attached, it has a new single bond to and then obviously H3. Okay? Is that cool so far? Now the bottom one still only has now this is a single bond in between, not a double bond And now this only has one H. So are there any formal charges that I need to add to this molecule? Actually, there's 2. Okay? You should have added a formal charge positive on this carbon. Why? Because that carbon now has only 3 bonds instead of 4 because the double bond broke and nothing came in to replace it. So that would be what we call carbocation. And then the boron likes to have 3 bonds because it's in the 3rd column or 3rd group, but now it has 4, so the boron should get a negative charge. Okay? And that's actually going to be our final product for now. Okay? For now, because we just learned acid and bases, this is where we're going to stop. But just so you guys know, we just did the first step to a reaction that we're going to learn in chapter 8 that I'm just going to tell you, it's called hydroboration. It's like a really like really big word. Okay? But this is actually the first step in a reaction that we're going learn later because like I told you, later we're going to be dealing with nucleophiles and electrophiles a lot and you're going to see stuff like this all the time. Okay? So hopefully that wasn't too confusing. I was just trying to throw in some broad stedullary and also some lewis. Okay? But just keep in mind that for the difference between lewis is that it always goes to the right, so let me write this down, just so you guys can know. Lewis is gonna always go to the right. Okay? So that's something about it that you just need to know. It's always going to go to the right non equilibrium. Okay? And it's always going to it's always going to just make a single product. Okay? So instead of being like an equilibrium, you're just going to make one product and it's on the right. Cool? And then also, you can't really use pKa values for it because you're not donating protons. Remember the definition of pKa was how likely you are to donate a proton. So since we're not donating protons, you can't use pKa's here, but thankfully you don't have to because like I said, you can just always say it's going to go to the right. All right guys, let's move on to the last question.

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Would the following reactions go to the right or the left? Draw the products and label ALL species. Provide the full mechanism.

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