Hello everyone, in this lesson we are going to be looking at isomers. Right, so the definition of isomers is the following. Isomers are molecules with the same molecular formula, but different structural formula.
Another way of thinking of molecular formula is general formula. So you know how in the previous videos we were looking at, for example, aldehydes and ketones. Well what we saw was that their general formulas were the same. For example this one over here, by the way this one on the left is that a ketone or an aldehyde? Well well done to you if you said ketone.
Why? Because it's got the carbon double bond oxygen but it's in the middle, it's not on the end. Whereas here we've got the carbon double bond oxygen on the side so it's at the very end of the chain. Okay so this is your aldehyde and then this is our your ketone.
Now, the general formula for the ketone was CnH2n, and then we had an O. Let's see if that makes sense. We've got 1, 2, 3 carbons, okay, so that's C3.
Then we've got 6 hydrogens, and then we have an oxygen. Let's look at the aldehyde. Well, we said that the aldehyde is also going to be CnH2n, and then O, because here we have C3H6 and O. So look at this guys, both of these molecules which are completely different because the ones in aldehyde, the ones are ketone, they both have the same molecular formula. So if you had to read this molecule in a textbook, you would think they're the same thing.
And so this is where I'm going to redo the definition of isomers. So let's bring that up once more. So we said that they are molecules. So remember we said that they are molecules with the same.
molecular formula. So this part here, that's the molecular formula. If I use it in terms of n, like variables, then that's called the general formula.
But the actual one that I've got here, that's called the molecular formula. Okay, so both of these have the same molecular formula, and then carrying on, but with different structural formula. What they mean by structural formula is the way that it looks when you draw it, the structure. And so of course we can see that the structure is different. The one on the right has three carbons and then the double bond oxygen at the end.
The one on the left has three carbons but the double bond oxygen in the middle. So they are structurally different. So that is what isomers are.
We found isomers between esters and carboxylic acids as well. So let's examine those quickly. So here if we look at this one, if we look at the left, well is that the ester or is that the carboxylic acid?
Well here it's got that Cu. part, you know, the C-O-O-H. Now that's a carboxylic acid. So I'm just going to say carboxylic. Then you've got a random oxygen in the main chain.
And so that's your ester. Let's quickly look at the molecular formula. Well, the first one has one, two, three carbons. I'm going to say C3. Hydrogens will be one, two, three, four, five, six.
And then oxygens will be one and two. And so we have C3H6O2, that's the molecular formula. If we look at the one on the right, well, that's got 1, 2, 3 carbons.
So that's C3, H is 1, 2, 3, 4, 5, 6, and then oxygen, 1 and 2. So once again, these two molecules have the exact same molecular formula, but if you look at their structure, they are completely different. The way that they are arranged is totally different. Okay, now something that we can't do is the following.
Here we've got another ester that I've just replaced, but this ester has one, two, three, four, five carbons. So now we can't compare these two. Obviously, we have to have the same number of carbons as our starting point.
Otherwise, it just won't work. So isomers must have the same molecular formula, but they'll have different structural formulas. Now things are going to start getting interesting. Why?
Because look at these two molecules. The one on the left, what is that? What homologous series is that? Well, it's an alcohol. The one on the right, what homologous series is that?
That's also an alcohol. But these two are going to be isomers. Kevin, how's this going to work? Have a look at this, guys.
So, of course, they're going to have the same molecular formula, because the one on the left has 1, 2, 3, 4 carbons. That's C3. The number of hydrogens is 1, 2, 3, 4, 5, 6, 7, 8, 9. Sorry, there were four carbons.
Okay, so that's, whoops, C4H9. And then there's the OH. Remember, with alcohol, you don't count this H.
over there, you'd rather write it as C4H9 and then you put the OH at the end. We did look at that in one of our previous videos. Now the one on the right hand side has 1, 2, 3, 4 carbons, so C4, 1, 2, 3, 4, 5, 6, 7, 8, 9 hydrogens, and then there's the OH part. So there we have it, the two chemicals have the same molecular formula.
However, the one on the left has its OH attached to a carbon that is somewhere in the middle whereas this one has an oh attached to the carbon on the end and so the structure of those two is completely different so once again these are isomers now here's our last two that we're going to compare and then i'm going to summarize so here we've got alkanes right we can see on the left and the right they are alkanes the molecule on the left has one two three four carbons but the h's there are ten of them you can double check me if you want On the one on the right-hand side has one, two, three, four carbons, and if you looked at the hydrogens, there would be ten. So these two molecules, once again, have the same molecular formula, but if you look at the way that they are structured, it's totally different. The one on the left has a four-chain carbon, where the continuous chain, or the longest chain, has four carbons.
The one on the right has a three-chain carbon, see like this. that it flows continuously and then on the middle carbon there's a carb there's a branch off that goes like that i know we haven't dealt with branches and things like that but we will get to that but what you must just do now is take my word that these two are also going to be isomers because they have a totally different structure and so guys the next minute or two is going to be vital so we looked at isomers but now there are three different kinds of isomers and they are functional positional and chain So we looked at all three of those in this video without you even realizing it. So have a look at the first one.
So I'm working in a vertical direction. So this was the aldehyde and the ketone comparison. So is the functional group, remember the functional group is the part of the molecule that makes it do what it does.
Is that the same for both of them? Well, no, because we said that for a ketone, the functional part is the double bond oxygen. bonded to a carbon that is between two carbons. So remember we said that it was something like this.
Whereas for an aldehyde, it's just this little piece here at the end. So it's a C double bond oxygen with a hydrogen. So the functional parts are totally different.
And so that is called functional isomerism. The next ones we're going to look at are going to be these two. So we looked at these and we saw that this one was an alcohol and so was this.
They have the same number of carbon, same number of hydrogen, everything is the same. However, this OH is on one of the carbons in the middle of the chain, whereas this OH is on one of the carbons at the end. So they are both going to be called alcohols, but they are still considered to be structurally different.
So they are both alcohols, so it's not going to be functional. Their functional part is the same, so they're not functionally different. Their main chain, if you look at their main chain, there is one, two, three, four carbons in that one, and one, two, three, four carbons in that one. So the main chain is the same, and so it's the position of the functional part, which is this part here, that position is different.
If we label this structure on the right from right to left, that would be carbon number one, and that would be carbon number two. So we can see that the OH is on carbon number two. If we look at the one at the bottom, the OH is on carbon number one.
So the position of the functional part has changed. And so that is called positional isomerism. And then with the last one, we know that both of these are alkanes. How do we know that?
There's just carbon, hydrogen, single bonds, wherever we look. So the functional part of both of those is the same. It's an alkane. The position, we can't really look at that because I mean the position the functional group for an alkane remember it's all the single bonds and so those single bonds are everywhere the chain however like the actual backbone in the top one is one two three carbons is the longest chain you could have you could have also gone this way you could have said one two three but nonetheless you won't be able to form a chain longer than three carbons if you look at the one at the bottom you can form a continuous chain of four and so the chain is different And so obviously we're going to call this one chain isomerism.
And so that's it guys, isomers. Same molecular formula, but totally different structure. And then we get three different types, namely functional. That's when the functional group is different, such as a ketone and aldehyde. Then you get position.
That's when you've got the same functional group, but it's in the different position. And then chain is when the backbone chain is different. So four carbons compared to three carbons. However, they both do still have the same amount of carbons. This one's just got its fourth one up here.
So it's not part of the main chain. We call that a branch. But we'll explore that in later videos.
So thanks for watching. And I'm sure you guys now understand isomers.