All right guys, let's go through this question.
It says draw all monochlorination products of butane and calculate the percentage yields
of each product.
First of all, I just want to show you this table that I made.
This table is going to 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.
We'll take this one as a worked example.
First of all, butane looks like that bolded molecule there.
And the first thing we have to do is you have to calculate or count up the different types
of protons that you have because remember that every type of proton has a different
relative rate.
So as we can see I split it up into primary protons and secondary protons.
What we can see is that after counting them up, you actually have six primary H's and
you have four secondary H's.
Does that make sense so far?
Pretty easy.
Now notice that there's no slot for tertiary, that is because there's no tertiaries here.
But if I did have tertiaries, I'd have to add another row.
Makes sense, right?
Awesome.
So now we're going to plug in the relative rate.
Where do I get that number?
From my table.
Yeah, relative rate of primary chlorination, see it says monochlorination.
That's where this becomes important.
I'm going to look at the numbers and it's 1 and 4, so I'm going to put that in here.
1 and 4.
Wait.
No, that's right.
That's right.
Perfect.
Awesome.
So I have 6 at a rate of 1.
4 at a rate of 4.
Now what I do is I multiply these numbers together.
So what I'm going to get is – I'm going to get 6 here, so 6 times 1 equals 6.
That's the relative reaction.
Then 4 times 4 is 16.
That means that literally, I'm getting that ratio – you see 6 over 16, that's going
to be the final answer, but I need to put this in a percentage yield way.
The way we do that is we add up these two numbers.
That means that the sum of these two numbers is 22.
That's going to be the number that I put in my denominator.
I'm going to put 22 as the bottom of my fraction for each one.
Then for the top, I'm just going to put the actual relative amount.
That means that 6 out of 22 went to the primary, so the fraction yield was 6 over 22.
The fraction yield of my secondaries was 16 over 22.
Now these ratios came up a little bit too difficult to calculate in our heads, so let's
use a calculator.
You could also use – this is pretty simple math, so you could also use your iPhone if
you wanted to.
I'm an iPhone guy, sorry.
So 6 divided by 22 gives me 27.2% or .3, let me round.
Then 16 divided by 22 gives me 72.7%.
Okay, that looks good.
So now I draw my products out.
What you can see is that for my primary product where I get a chlorine here, I'm going to
get 27.3% of that one.
And then for my secondary where I put my chlorine here, I'm going to 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're talking chemistry terms, this
is not good enough and this difference is enough for a professor to say non-selective
because no professor or no chemist wants to get 27% of their product as a waste.
That would be considered a low-yield reaction.
Even though it does seem kind of selective, it's actually not as selective as we need
it to be to make this reaction worthwhile.
Got it?
Awesome.
Hopefully, this little layout that I made helps it to make sense.
Eventually, I want you guys to be able 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, where in the first one
I'll help you along and then in the second one I'll take the training wheels off.
Let's go ahead and work on the first problem.