Testing for Functional Groups | A* Exam Question Walkthrough | Revision for Chemistry A-Level

[00:00:01][music] A student is given four unlabeled test tubes with samples in. Describe how they could differentiate between these samples using the fewest number of chemical tests possible. We know that in there there is two bromo 2 methropanel two bromo two methyl propanol two methyl propinoic acid to hydroxypropeninoic acid. So step one is to draw everything out. Here we have our two bromo two methile propanel. We can see the aldahhide group on the end. The bromine group and the methile group for our two bromo two methile prop and one. The aldahhide group is replaced by an alcohol group. We still have our bromo group and we still have our methile group in the middle there.

[00:00:59]For two methyl propinoic acid, we are going to have a caroxyic acid group over on the right and a methile group in the middle.

[00:01:11]For two hydroxypropaninoic acid, we're going to have our caroxyic acid group over on the right and then we are going to have a hydroxy and alcohol group in the middle there. If we were doing this in the exam and you didn't have time to draw all of these out, you could just do the following step where I highlight all the functional groups from the names if you're confident enough with your organic chemistry. However, I much prefer to draw things out to have them in front of me when I am looking at things.

[00:01:46]Now that I have all of my structures, I'm going to go through and identify all of the functional groups. So, I'm going to highlight these and use functional groups to to pick out all the different bits. And you'll see that the same functional group comes up in multiple different compounds. And so, we have our aldahhide in blue. Then, we have our bromine, our halogen group in green, our alcohol group in purple, and our caroxyic acid in lilac.

[00:02:17]Now, we can think about how we are going to differentiate these.

[00:02:22]The first one we're going to look at is identifying the aldahhide group. And there are two different tests that you can use to identify the aldahhide group.

[00:02:30]You can use failings or you can use tolerance. Both are acceptable in an exam.

[00:02:36]When we doing the failings test, we will get our blue to red color change. As that color change happens, you'll see a precipitate forming. You need to heat it for this to happen. The blue copper 2 plus ions when added to any aldahhide are going to give us a caroxyic acid and we are going to have copper oxide as a precipitate. For tolins we are doing the silver mirror test.

[00:03:08]When we have an aldahhide and we add it to silver and water, you will get a caroxyic acid and silver ions out. These silver ions will then coat the side of the test tube or conical flask or whatever it is you look at and will create a silver mirror effect.

[00:03:27]Tlins's reagent is ammonia plus silver nitrate which is going to give us a complex ion.

[00:03:35]Next, we can work out how to find our H hallogens. Now, in the lab, I always suggest having reference samples so that you can easily differentiate these. For the test for halogens, you need to add nitric acid, add silver nitrate, and a positive test for bromine is a cream precipitate.

[00:03:57]It's quite hard to differentiate between the white, cream, and yellow. So, I strongly suggest having something you compare it to if you're actually doing this in the lab. For an alcohol, you'll notice we have a primary alcohol in one situation and a secondary alcohol in a second situation. There are no tertiary alcohols here cuz that would involve a different type of test. You can add a named caroxilic acid. And I'm saying named because this is what they're looking for in the exam. The name of the caroxilic acid you would be adding to the named alcohols. This is going to give us an esther which you would also be expected to name. And you would notice the positive sweet smell from the esther.

[00:04:42]An alternative for an alcohol would be the addition of acidified potassium dromate.

[00:04:49]This would not only react with the alcohols, but it would react with the aldahhide as well. So you can't use this as a purely descriptive test for an alcohol. You can use it different between an alcohol and something else.

[00:05:03]It will change color from orange to green. In some of the practical books, it suggests the addition of metallic sodium as a test to identify an alcohol. This would not be a suitable test in this situation because metallic sodium would react with everything. It would not only react with the alcohols, it would react with the caroxilic acids as well.

[00:05:29]For caroxilic acids, there are lots of different tests we can do here. And there is no right or wrong answer. Just pick one and describe it carefully.

[00:05:40]You can add sodium hydrogen carbonate or sodium carbonate and a gas will be produced.

[00:05:47]The gas release will be carbon dioxide.

[00:05:49]You can collect that via delivery tube and it should turn lime water cloudy.

[00:05:55]Because it is an acid, you could use an indicator to show the acidic properties.

[00:06:00]You would have to name the indicator and the color change. Or like with our alcohol here, we're also going to make an esther. you would had to add a named alcohol to give the esester and the positive result would be a sweet smell.

[00:06:15]So, we've covered lots of different test to identify the four functional groups that we have in here. But the question says using the fewest number of tests possible, which means we can't do every single test on every single samples. We have to separate them into groups. So we go from having our first sample, we do one test to separate it into two groups and then we can do a further two tests.

[00:06:40]So test one on the first sample and another test on the second set of samples to work out our positive and our negative so we can positively identify everything. You could start with a test for aldahhide and I'm going to go through lots of different situations here. All of them are correct in the exam as long as you answer carefully and slowly. The exam is not going to persuively say you have to do the test for aldahhide first or you have to do the test for alcohol first. Doing the test for aldahhide first would give us two groups. One of one. So we've positively identified the two broine methile propanel. And then the second group would have three in it.

[00:07:24]The second test we could do would be then to identify for a caroxyic acid.

[00:07:29]Further separating out one in one group and two in another group and then to the two caroxyic acids we have identified. We could then do the test for an alcohol.

[00:07:45]This would allow us to separate out all four compounds doing three tests.

[00:07:53]And if you get a question like this in the exam, you have to work through it slowly. You have to work through it carefully and logically so the examiner can follow your line of thinking and follow which test you're doing when and what the positive and negative results are. It is important to include all the reagents and the color changes. An alternative pathway will be to do the test for halogen first.

[00:08:20]This would give us two groups of two.

[00:08:22]And then to the first group, you could either do the test for an aldahhide or you could do the test for an alcohol, which would differentiate those two from each other.

[00:08:35]For the second group, because they are both caroxyic acids, we would have to do the test for an alcohol to find the hydroxy group to differentiate them from each other. You could start with a test for an alcohol first, which would give us two groups of two and then onto the first group. You could either test for a halogen or you could test for a caroxilic acid.

[00:09:05]On the second group, again, you could test for a halogen here, and that would tell you which was which. You could start this by testing for a caroxyic acid, giving us two groups of two.

[00:09:23]For the first group, you could then go on to test for an alcohol, which would differentiate the two of them. And then for the second group, you could either go on to test for an aldahhide or test for an alcohol, which would give us the positive or a negative.

[00:09:38]Ouch.

[00:09:44]This is why in some videos I have unexplained scratches.

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