This video is looking at esters. In this video we'll be looking at the reaction to form esters, we'll be looking at how to name esters, how to draw esters and how to hydrolyse esters. So the reaction to form esters, we need two molecules with different functional groups. So the first one is the alcohol group and the second is the carboxylic acid group. When these two come together, they join to form the carboxylate or ester link.
In the process, you also lose a molecule of water. That makes this process a condensation reaction. The reaction is actually a reversible reaction and we'll come on to those more in Unit 3 where we'll be looking at chemical equilibria. What happens in this reaction is we have a condensation reaction to go from reactants to products and during a condensation reaction we lose a small molecule whilst we join the other two together. Now in this case we're losing water but the small molecule could be something like hydrogen chloride.
It doesn't have to be water. The reverse reaction where water is used to split up your ester is a hydrolysis reaction. Now we did look at this briefly in National 5, but we're going to have a look in more detail here as to what is going on in the reaction.
Now when we join these two together you can see that we form this link here and through experiments that have been carried out it's been found that the oxygen from the alcohol remains attached to the alcohol group and this oxygen here comes from the carboxylic acid group so we do not leave this part attached okay so this is the OH that comes off to join with this H here to become the water. So our H here and this OH become the water over here on the right hand side. Having a look now at naming esters. The ester name comes in two parts. The first part comes from the parent alcohol and the second part comes from the parent carboxylic acid.
Now sometimes you'll have the equation written the other way round, so you need to be able to pick out which is which from the names. So if we've got a parent alcohol of ethanol, you're going to take off this end part of the name, and we're just going to change it so that it's ethyl. Our parent carboxylic acid for the first example is methanoic acid, so the end of the name becomes oate. So we're going to have ethyl, methan, oate. If we have a look at the second example, where we have the parent alcohol of methanol, this will become methyl.
And our parent carboxylic acid is propanoic acid, so that becomes propanoate. Okay, so the names are in two parts coming from the parents that made them. Now let's have a look at the structures of the esters.
So earlier I said that an oxygen is left on each of the reactant molecules. So that means that we have the H here and this O. OH will leave to join to produce water in our condensation reaction. That means that the rest of the molecule will be made up from these two parts here. So we need to join this oxygen to this carbon to form our...
ester link. So if we start from the left hand side we have CH3, we have CH2 and then we have the oxygen. That oxygen is then attached directly to this carbon.
which also has a double bond O and then it has a methyl group on the other side. If we were to try and name this ester, we would start by having a look at this molecule here. We have two carbons on our alcohol group, so that will be ethyl. And we also have two carbons from our carboxylic acid group, so that will be ethanoate.
Here we named these esters. Now we're going to have a look at what their structures will be. So one of the easier ways to start would be to draw out the structures of the reactants. So for ethanol, we're going to have CH3, CH2, OH.
And for methanoic acid, we have one carbon with a carboxylic acid group. When we're going to join these together, we're going to have a group of two. Together we're going to remove water so we're removing this H and this OH. So you'll notice that I've faced the two functional groups towards each other for ease of drawing these together.
Which means that we'll form our ethyl methanoate in this way. If we have a look at our second example, we've got methanol and propanoic acid forming methylpropanoate. So for methanol, we have CH3OH. And for propanoic acid, again, I'm going to face the functional group towards the alcohol to make it easier to draw it.
So we have three carbons. When we join the two together, we'll lose the H from the alcohol and the OH from the carboxylic acid to form water, and we'll join up the other two parts. So we've got CH3 with the oxygen from the alcohol attaching to the C double bond O of the carboxylic acid, and then finally we have the rest of the chain.
So here we have methylpropanoate. You should be able to draw esters just from their name. So here we have ethyl pentanoate. So from the name you can see that our alcohol group must have had two carbons and the carboxylic acid group had five.
So if we just do these in two different colours to show which part is which. So the alcohol parent had two carbons, so if we draw that out we've got one, two, and then an oxygen. You know then that the ester link is to the C11O of what would have been your carboxylic acid.
and this one had 5 carbons so you've already got one there. So there's 2, 3, 4 and 5. You would then go in and insert all of the hydrogens making sure that you notice that this carbon already has 5. four bonds. I'm going to leave it without the hydrogens just for the ease of being able to see what's going on. If we have a look at the second example, we have propyl butanoate. So our parent alcohol had three carbons, one, two, three, and then the oxygen that was left behind.
And then our parent carboxylic acid had four carbons. So the oxygen is attached to what was the carboxyl group. and then you fill in the rest of the carbons to make up to four. When we're hydrolyzing esters, we're using water to split them back into the parent alcohol and the parent carboxylic acid. So for this example, we have methyl butanoate.
So this is going to be split back up into methanol as our parent alcohol and butanoic acid. If we were to try and draw this out, first of all we're going to draw out the ester. So we're going to have a C attached to an O. And we know that that's attached to a C with the double bond O, and that parent carboxylic acid had four carbons attached to it.
We're then going to split this up. So to split it up into the methanol and the butanoic acid, we need to leave an oxygen behind on each of the molecules. So you go along, you find the oxygen and the carbon to the double bond oxygen and you split down there. This is where the water will rejoin on.
If we draw water underneath, the water will split here so that the hydrogen will join back to the alcohol and the OH group will join back onto the carboxylic acid. And that should leave you behind then with what will be CH3OH. and then C double bond O to the OH with the three other carbons attached.
Again, just messing out the hydrogens here for clarity. Let's have a look at a final hydrolyzing example. In this final example we have our ester and we're going to introduce the water to hydrolyse it.
I'm drawing out the full structure of the water here. So the water will split between here and here to give you an H and an OH and for your ester you need to move along until you find the O and then the C double bond O and we're splitting here. This H will join on here and this OH will join on here. So if we draw that out we're going to have CH3, CH2, CH2OH so if we count that up we've got 1, 2, 3 so that will be propanol.
And then here we've got one, two, three, four, so we would have started with butanoic acid. So our original ester was propyl butanoate. I hope that you've enjoyed this video on esters and I hope you found it helpful.
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