Video: EC17-18-S2-Q29

Write a single practical procedure which would allow you to distinguish between ethanol and dimethyl ether.


Video Transcript

Write a single practical procedure which would allow you to distinguish between ethanol and dimethyl ether.

Let’s start by looking at the structures of both ethanol and dimethyl ether. We know from the name of ethanol, from the “eth-” prefix, that there are two carbons. And we know from the suffix “-ol” that it’s an alcohol. So it has a hydroxyl group. So this is the structure of ethanol.

Now let’s think about dimethyl ether. Again, we can work out the structure from the name. We know there are two methyl groups from dimethyl. And these are connected by an ether group. So here we have the structure of dimethyl ether.

We’ve been asked to provide a single practical procedure which will allow us to tell the difference between ethanol and dimethyl ether. Unfortunately, both ethanol and dimethyl ether are colorless liquids. So you can’t tell simply by looking at them. In the laboratory, both liquids have quite distinctive smells. But it’s not really a safe idea to go sniffing random liquids. So that’s not an appropriate procedure.

So what we’re looking for is a practical procedure which, when we add something to ethanol and dimethyl ether, it has a different reaction. And we’re looking for something like a color change, the production of a precipitate, or maybe bubbles of gas. So let’s compare the two molecular structures.

We can see that the structures are actually quite similar. Both molecules have two carbon atoms, six hydrogen atoms, and one oxygen atom. Both molecules also have a methyl group. This tells us that we can’t use any kind of technique which works out how many atoms of each element are in each molecule, nor a technique which looks for methyl groups.

The main differences between the two molecules are that ethanol has an OH, hydroxyl group, as well as a CH₂, while dimethyl ether has an ether oxygen. So what we need is a reactant which will react with one of these groups. Choosing something that reacts with a hydroxyl group is a good option. There are lots of things which react with a hydroxyl group. We just need to pick something that reacts with it in a visible way, so producing a color change or a precipitate or bubbles.

You might remember that sodium metal reacts with hydroxyl groups. You’ve probably seen sodium metal react with water, for example, in a really violent way. In that reaction, it’s also interacting with an OH group. Luckily for us, the reaction of sodium metal with an alcohol OH is less fierce. The equation for the reaction of sodium metal with ethanol is two molecules of ethanol plus two sodium metal atoms creates two molecules of sodium ethoxide and hydrogen gas. So does this reaction meet our requirements as a practical procedure to distinguish between ethanol and dimethyl ether?

To answer this, let’s see how sodium interacts with dimethyl ether. To work out whether sodium does react with dimethyl ether, let’s have another quick look at the structure. In the case of ethanol, the sodium reacts with the H on the hydroxyl group. In the ether molecule, we don’t have any Hs attached to oxygens. So there’s nothing for it to interact with. So there’s no reaction between sodium and dimethyl ether.

Because sodium only reacts with one of our two compounds, it’s a good option for the answer to this question. All we need to do now is make sure that we can visually determine that a reaction is taking place. So let’s look again at the product of our reaction. We can see that one of our products is hydrogen gas. This means that, in the laboratory, what we’re going to see are bubbles being produced in our colorless liquid. So from this, what we can say is that if we add a piece of sodium metal to ethanol, we’ll see bubbles form. But if we add a piece of sodium metal to dimethyl ether, nothing will happen. So this is our practical procedure. We just need to put it into words.

So the single practical procedure that we’ve chosen is treatment with sodium. To make this a really good answer, let’s be clear about what will happen. So our answer could be treatment with sodium. Sodium reacts with the hydroxyl group of ethanol, producing hydrogen gas, but does not react with dimethyl ether. So here is our answer.

As an aside, sodium ethoxide, an alkoxide, is actually a very strong base. So if you were to put a few drops of an indicator, for example, phenolphthalein, into the ethanol before you add the sodium, what you’ll see are not only hydrogen gas bubbles, but a color change in the indicator. But this makes our procedure far more complicated than it needs to be. Simply producing the gas bubbles is enough.

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