Calculating Radical Yields - Video Tutorials & Practice Problems

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Most professors just want you to be able to determine the major product of a radical halogenation. However some are going to get a little more crazy!

Here I’ll teach you how to predict the exact percentages of monohalogenation products for differing halogens at room temperature. Again, only some professors will ask you for this. Be sure to refer to your class notes to see if this is something you should learn!

1

concept

Explaining relative rates of halogenation.

Video duration:

4m

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Alright, guys. So in these videos are gonna be going through a much more rigorous explanation of how to calculate the major product for radical college nation reaction. Now, the reason that I'm even recording these videos is because your textbook goes through this more in depth explanation and I want to make sure that we have all our bases covered now, understand that this might be beyond the scope of what many professors require for their students. So I'm gonna leave it up to you to check with your professor to see if they want you to calculate these ratios or not. All right, so let's just move right into the lesson. So everyone knows by now the general trend of radical stability, which is that tertiary radicals are more stable than secondaries and secondaries are more stable than primaries. And that's been enough for us. So far, we've been able to get by and just predict the major product that way. But if we want to calculate the exact percentages of major and minor products, we're gonna need equations. We're gonna need some kind of method quantitative method to do that, and it turns out that we can calculate that knowing the relative rate of halogen nation at a certain temperature. So here you see, this is a very important little, uh, table here. These are the relative rates of different types of halogen nations at 25 degrees Celsius or room temperature. Let's just go through these numbers really quick. So you understand what it's meaning. Okay, So remember how we discussed how coronation is very un selective kind of makes bad decisions everywhere, and domination makes these awesome decisions. Well, this can be quantified, and the difference has to do with these halogen ation rates. Notice that chlorination actually does prefer tertiary is okay, so it's not. It has good intentions. Okay? The problem is that it doesn't prefer tertiary is very much on Lee. Slightly. It likes thio halogen eight tertiary, about five times more than primaries and only a little bit more than secondaries. So you can see how coronation you're gonna get a lot of products everywhere. Okay, Now, if you look at Bram in ation, bro nation is much more selective because Brahma nation likes to court to brominated tertiary carbons 1600 times more than primaries. So you can imagine that That's what we called rumination highly selective versus chlorination, which you basically call non selective. Okay, it has to do with the fact that the difference between the between the relative rates are much bigger when you get to Bram in ation. Okay, so here we can go back to our definitions and weaken state That chlorination is non selective because the difference between the relative rates is very small, whereas Brahman ation is highly selective because the difference between the relative rates is very large. Okay, now, these ratios that I'm giving you here are Onley valid at one temperature, and that's room temperature. Okay, because that's what the That's the temperature that these experiments were conducted under. Okay, if we jack up the temperature for this reaction, the ratios between the different types of selectivity become, Can you guess smaller Okay, meaning that there's less difference between primary secondary tertiary. Why the more heat you add to any reaction, the more ambient energy there's going to be, so the less selective it's gonna be, it's gonna wind up. Even rumination can be core since its, um, bad decisions at a high enough temperature. Okay, awesome. It's getting hot in here. All right, so that's basically the concept behind these tape, this table. Okay, Now, one note of caution for you guys. You might be flipping through a textbook and you might see slightly different values here, like you might see it as one and then 4.2 and 5.5. I don't really care about the details. The reason I chose these numbers is because these are the safest round ist numbers that I could find from a combination of sources. I looked online and I looked through Ah, few different textbooks. And these numbers just seem to make the most sense. There's no point in teaching you with complicated numbers, but in general the range is air. Correct. It's 145 about there and then about 2600. Okay, if you're professor teaches you different values, by all means, do not argue with him. Just go with his values. Everything that I'm saying is still gonna pertain to that, even if you have to use different values

2

example

Draw all of the monochlorination products and calculate percentage yields.

Video duration:

4m

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Alright, guys, let's go through this question. It says draw all of the Monaco chlorination products of butane and calculate the percentage yields of each product. Okay, so first of all, I just want to show you this table that I made this table is gonna make it very easy to calculate stuff just by filling in blank spaces. But the point isn't that you should have this at your exam. The point is that you should learn through this and then be able to do it on your own. Okay, so we'll take this one as a worked example. First of all, butane looks like that bold ID molecule there. And the first thing you have to do is you have to calculate or count up the different types of proteins that you have, because remember that every type of proton has a different relative rate. So as we can see, um, I split it up into primary protons and secondary protons. And what we can see is that after counting them up, you actually have six primary ages and you have four secondary ages. Does that make sense? So far? Pretty easy. Now I notice that there's no slot for tertiary guys because there's no tertiary is here. But if I did have tertiary is I'd have to add another row. Makes sense. Right? Awesome. So now we're gonna plug in the relative rate. Where do I get that number? From my table from my table. So yeah. So relative rate of primary chlorination. He says, mono chlorination. That's where this becomes important. So I'm gonna look at the numbers, and it's one and four. So I'm gonna put that in here one and four. Wait. I know. That's right. That's right. Perfect. Awesome. Okay, so I have six at a rate of 14 at a rate of four. Okay. Now what I do is a multiply these numbers together. Okay, So what I'm gonna get is I'm going to get six here. Okay? So six times one equal six. That's the relative reaction. Okay. And then four times four is 16. Okay, so that means that literally I'm getting this, That racial, you see, 6/16 that's going to be the final answer. But I need to put this in a percentage yield way. So we we do. That is, we add up these two numbers. That means that that some of these two numbers is 22. And that's going to be the number that I put in my in my denominator. Okay, so I'm gonna put 22 as the bottom of my fraction for each one and then for the top. I'm just gonna put the actual relative amount. So that means that six out of was were went to the primary. So the fraction yield with 6/22 and the fraction yield of my secondaries with 16/22. Now, these ratios came up a little bit too difficult to calculate in our heads. So let's use a calculator. Um, you could also use This is pretty simple math. So you could use your iPhone if you wanted to. So I'm an iPhone guy. Sorry. So six divided by 22 gives me, um, 27.2% or 0.3. Let me around and then divided by 22. Gives me 72. 7%. Did it? Looks good. All right, so now I draw my products out, And what you can see is that for my primary product with where I get a chlorine here. Okay, I'm going to get 27.3% of that one. And then for my secondary, where I put my chlorine here, I'm gonna get 72.7% of that. Now, Notice that chlorination in this case actually was fairly selective. 72% went for the better option. But still, when you talk in chemistry terms, this is not good enough. And this is this difference is enough for professor to say non selective because no professor or no chemist wants to get 27 of their 27% of their product as a waste. That would be considered a low yield reaction. So even though it does seem kind of selective, it's actually not a selective as we need it to be to make this reaction worthwhile. Got it? Awesome. So hopefully this little like lay out that I made helps helps it to make sense. Eventually, I want you guys feel to do this without a layout. Just know what the next steps are. But for right now, let's go ahead and do some practice. Problems were in the first one. I'll help you along, and then in the second one will take the training wheels off. So let's go ahead and work on the first problem

By multiplying the number of equivalent hydrogens by its relative rate of halogenation, we can obtain a “relative reaction” rate. We compare the relative reactions to determine the percentage breakdown for each product.

I’ve made it really easy for you by building and filling out this chart. Eventually you should be able to fill in this chart yourself ;)

The percentages we yield correspond to the amount of each product we would expect if we were to run this halogenation ourselves.

3

Problem

Problem

What is the percentage yield of the major product?

A

64.43%

B

99.44%

C

99.93%

D

76.25%

4

Problem

Problem

What is the percentage yield of the major product?

A

68.2%

B

95.2%

C

90.8%

D

76.9%

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