Functional groups play a significant role in directing and controlling organic reactions,
so chemists need to be able to identify which functional groups are present in a molecule.
For example, material scientists may look to add functional groups to a molecule
for desired properties and effect. Whilst food scientists may look to replace saturated fats
with alternative unsaturated fats, molecules containing carbon-carbon double bonds, in their products.
In organic chemistry, the most common functional groups are alkenes;
carbonyls, which include aldehydes and ketones; alcohols; carboxylic acids and haloalkanes.
Each functional group behaves in a similar fashion by undergoing similar reactions no matter what compound it's in.
Here I have six unknown organic samples, each from one of these groups.
I'm going to carry out a series of qualitative tests to identify which functional group, or groups,
are present in each sample. In some cases, we will be able to identify the functional groups
present from just one test. However for others, we will need a second test to confirm the result.
When carrying out investigations of this type it is incredibly important to be systematic
and to make sure everything is clearly labelled and you record all of your results as you go along.
You will need to spend some time planning the order of these tests as this will save you time later.
Some tests will give a single positive result, so it makes sense to do those tests first.
Other tests will give multiple positive results, so we will save those 'til later.
It also makes sense to do the test with the least hazardous or harmful reagents first
and save the test with the most hazardous or harmful reagents until later when there are fewer samples.
I'm going to begin by doing a test where I expect to get one positive result.
The first test we are going to do is for the carboxylic acid functional group.
There are many tests for acids, some which you may already be familiar with.
However today, we are going to use the metal carbonate test.
Here I have labelled six test tubes, a b c d e and f, and added two centimetres-cubed of each sample into them.
I am now going to add a few drops of sodium hydrogen carbonate solution to each test tube.
Think about what we would expect to see for a positive result. Make sure you record your observations in a table.
There you go we can see effervescence in test tube f, showing the release of carbon dioxide gas.
Only one sample showed a positive result; effervescence when added to a carbonate.
This indicates that sample f is the carboxylic acid and we do not need to test it any further.
Next I'm going to perform a test using bromine water. Again you have probably done this before.
Here I am testing for an unsaturated hydrocarbon, or the presence of a carbon-carbon double bond.
And we would expect to see only one positive result. First I am adding a few drops of each
sample, a to e, to a labelled test tube. Now add one centimetre-cubed of bromine water to each sample.
And shake vigorously from side to side.
You can see the bromine water has become colourless - a positive result.
I'm going to continue testing the rest of the unknown samples.
Well that was unexpected ... a second positive result.
A negative result for d.
And another negative result for e. You can see we have a positive result for samples a and c.
You might be surprised by that result if you were expecting one positive result confirming
the presence of the double bond. At this stage, I can't identify the unsaturated hydrocarbon,
as clearly other functional groups also give a positive result.
This shows the importance of completing the tests on all of the unidentified samples.
When carrying out an investigation like this you may get unexpected results,
meaning you need to go back and adapt your original plan. Be prepared to be flexible.
Next I'm going to carry out a test to identify the haloalkane functional group. Again I expect to see only one positive result from our samples a to e.
Let's find out what happens. This test takes part in two steps: in the first step I'm going to add 10 drops of each of our samples to two centimetres-cubed of ethanol.
As always I have labelled our test tubes. Next I'm going to make two water baths, with half of the water coming from a freshly boiled kettle and the other half from a cold tap.
I'm now going to place our test tubes containing samples a to e into one of the water baths.
Into the other water bath, I'm going to place five test tubes containing 2.5 centimetres-cubed of silver nitrate solution.
I'm going to leave this for around five minutes. The ethanol will react with a haloalkane to release halide ions.
It is these halide ions that will react with silver nitrate in the second step of our test.
In the second step we are going to mix our heated silver nitrate with our heated samples. I am first going to take our silver nitrate out of the water bath.
I am now going to mix our samples with the silver nitrate to see the precipitate forms. Carefully observe and note down any changes that you see.
As you can see, a precipitate has only been produced in one of our samples, sample e.
The colour of the precipitate that has formed is cream. This indicates that sample e is actually a bromoalkane.
If you need any help with identifying ions, click the link in the description.
Now I am going to test for the presence of an alcohol or -OH functional group.
Alcohols can exist as several different isomers depending on where the -OH group lies on the chain of carbon atoms.
They are classified as either primary, secondary or tertiary alcohols.
In a primary alcohol, the carbon with the -OH group has either two or three hydrogen atoms attached.
In a secondary alcohol, the carbon atom with the -OH group has one hydrogen atom attached.
And in a tertiary alcohol, there are no hydrogen atoms attached to the carbon with the -OH group.
For this test, I'm going to add a few drops of acidified potassium dichromate to four wells in a spotting tile.
Potassium dichromate is an oxidizing agent. Here we'll expect to see a colour change from orange to green
as the chromium-6 ions are reduced to chromium-3 ions. This was in fact the basis for old-fashioned breathalysers.
A positive result here indicates the presence of either a primary alcohol, secondary alcohol or aldehyde.
Now I'm going to add our samples; a, b, c and d to each of the wells in turn.
We can see sample b is already turning green.
We can see sample c is also slowly turning from orange to green.
Sample a turns to green, albeit very slowly. Alkenes can be oxidized by acidified potassium dichromate.
However the reaction is very slow. This indicates that sample a is our alkene. We now see sample b is turning blue.
This is because the chromium (III) ions are being further reduced to chromium (II) ions.
Sample d remains unchanged. As you can see we have a positive result for samples b and c.
A partial positive result for sample a. And a negative result for sample d.
This indicates that samples b and c are either primary alcohols, secondary alcohols or aldehydes.
In this next test I am going to use Brady's reagent, a solution of 2,4-dinitrophenylhydrazine or DNPH, to test for the presence of carbonyl groups.
Like our last test, I'm going to perform this one in a spotting tile.
I'm going to start by adding a few drops of Brady's reagent to each well.
As usual I'm going to add each of our samples, a through d, in sequence.
A positive result here is the formation of an orange precipitate.
So as we can see, sample c and d have given us a positive result. And samples a and b are negative.
So if we take a look at our results we've had so far, we can conclude the following:
the metal carbonate test gave only one positive result, sample f, so that must have been the carboxylic acid.
The silver nitrate test gave only one positive result, sample e, so that must have been the haloalkane .
The bromine water test gave two positive results, sample a and sample c. However, now we have conducted the test with Brady's reagent
we can see that sample c has given a positive result where a sample a has given a negative result. Therefore, sample c is our aldehyde.
And sample a is our alkene. Sample d has given us a positive result with Brady's reagent and
gave us a negative result with acidified potassium dichromate. So sample d must be the ketone.
Sample b gave us a negative result with Brady's reagent and a positive result with acidified potassium dichromate, so sample b must be our alcohol.
Now we have identified the functional groups in all of our compounds, a through f. Though we are not done here. I have one final test to show you.
I am now going to show you another test for aldehydes. This test uses Tollens' reagent, which contains silver ions.
When added to aldehydes, the silver ions in Tollens' reagent are reduced to silver atoms.
This test is sometimes called the silver mirror test and you will see why. I'm going to make and use Tollens' reagent in situ.
This is because, if left to stand, explosive silver compounds could form. As you can imagine I'm also going to be doing this experiment on a small scale.
I'm first going to add 2.5 centimetres-cubed of silver nitrate solution to a clean dry test tube.
And I'm going to add a single drop of sodium hydroxide. You can see a precipitate of silver (I) oxide has formed.
I am now going to add ammonia dropwise until the precipitate just redissolves.
This is our Tollens' reagent. I'm now going to heat this using a water bath at around 60 to 70 degrees celsius.
I will now add our aldehyde. As we know it is an aldehyde, I am using sample c.
I'm going to add around 2 centimetres-cubed of sample c.
Give it a little shake and then leave it in the water bath for around 15 minutes.
It's now been 15 minutes. As you can see a silver mirror has formed on the inside of the test tube, where a reaction has taken place.
Don't forget to dispose of your reaction mixture immediately down a foul water drain.
That concludes our tests for functional groups. By working through the tests systematically,
I have been able to identify which functional groups are present in each of my organic samples.
Use the additional resources with this video to explore the underlying chemistry in greater detail.
Or to plan another investigation to identify the functional groups present in an unknown organic sample.
Thank you for watching and have fun with your experiments.
Inspiring your teaching and learning.