Coaching Centre
Testing for functional groups
Chemical tests for carbon–carbon double and triple bonds, hydroxyl groups and carboxylic acids, how to tell primary, secondary and tertiary alcohols apart, and why chemical tests are now backed up by instruments.
Three bottles of clear liquid sit on a bench: one is a solvent, one is a hand-sanitiser alcohol and one is vinegar essence. Smell would probably sort them out, but smelling unknown chemicals is exactly what you shouldn't do. A few drops of three reagents will identify them safely in under a minute, by finding which functional group each one has.
Why test for functional groups?
An organic compound's reactions are set mainly by its functional group, not by the length of its carbon chain. So a test that works for ethanol also works for butan-1-ol, and a test for C=C works for every alkene. You met these reactions in Module 7; here you use them as evidence.
A good test gives an obvious change (colour, gas, heat or precipitate) for one functional group and no change for the others. Always run a control, such as hexane or water, so you can see what "no reaction" looks like.
Carbon–carbon double and triple bonds
Bromine water is orange-brown. Shaken with an alkene or alkyne, it decolourises immediately because bromine adds across the multiple bond:
$\text{CH}_2{=}\text{CH}_2 + \text{Br}_2 \rightarrow \text{CH}_2\text{Br}{-}\text{CH}_2\text{Br}$
An alkyne adds two molecules of Br₂. Alkanes don't react in the dark; they react only slowly by substitution in UV light, releasing HBr. So "decolourises immediately, in the dark" is the test for unsaturation. (Strictly, in bromine water the main product of an alkene is a bromo-alcohol, because water also takes part, but the colour change is the same.)
Acidified potassium permanganate (purple) also decolourises with alkenes, but it is less specific: it oxidises alcohols and aldehydes too.
The hydroxyl group
- Sodium metal: alcohols (and acids) release bubbles of hydrogen, $2\text{CH}_3\text{CH}_2\text{OH} + 2\text{Na} \rightarrow 2\text{CH}_3\text{CH}_2\text{O}^-\text{Na}^+ + \text{H}_2$. A dry sample must be used, because sodium reacts violently with water. This test is done only as a teacher demonstration.
- Esterification: warming with ethanoic acid and a few drops of concentrated H₂SO₄ gives a fruity-smelling ester.
- Acidified potassium dichromate, warmed, changes from orange to green as Cr₂O₇²⁻ is reduced to Cr³⁺ by primary and secondary alcohols. Tertiary alcohols do not react.
The dichromate test does not separate primary from secondary alcohols, because both turn it green. To tell them apart, test the product: a primary alcohol heated under reflux gives a carboxylic acid (acidic, fizzes with carbonate), while a secondary alcohol gives a ketone (neutral).
Carboxylic acids
- Sodium carbonate or hydrogen carbonate: fizzing, and the gas turns limewater milky: $2\text{CH}_3\text{COOH} + \text{CO}_3^{2-} \rightarrow 2\text{CH}_3\text{COO}^- + \text{CO}_2 + \text{H}_2\text{O}$. Alcohols are not acidic enough to do this, so this test separates acids from alcohols.
- Universal indicator or a pH meter: a pH of about 3 for a 0.1 mol/L solution (a weak acid).
- Esterification with an alcohol gives a sweet smell, as for alcohols but with the roles reversed.
- Identify five unknowns. Count your tests: can you always do it in three or fewer?
- Run bromine water on hexane and on cyclohexene. Which observation shows a C=C bond?
- Find two unknowns that give exactly the same result for every test. Which two are they, and what extra test (from this page) would separate them?
- Which single test separates ethanoic acid from all the other compounds?
Limits of chemical tests
Chemical tests tell you which functional groups are present, but not the full structure. Butan-1-ol and butan-2-ol both turn dichromate green; propanoic acid and butanoic acid both fizz with carbonate. They also use up some sample and can be ambiguous when a molecule has several groups. That is why chemists now combine a few quick tests with the instrumental methods on the next four pages: mass spectrometry for the molecular mass, infrared for the bonds present, and NMR for how the atoms are connected.
Test yourself
Which compound would immediately decolourise bromine water in the dark?
Only the alkene has a C=C bond for bromine to add across.
Warm acidified potassium dichromate stays orange when added to a liquid. The liquid could be
Tertiary alcohols have no H on the C–OH carbon, so they are not oxidised.
Which test would distinguish ethanoic acid from ethanol?
Only the acid releases CO₂. Both react with sodium, and both form esters.
Bromine water is added to an alkyne. The best description of what happens is
Like alkenes, alkynes undergo addition across the multiple bond.
Name each test reagent and what it detects.
Sort each compound by its main positive test.
Bromine adds across a C=C bond in an reaction. Acidified dichromate changes from orange to when it oxidises an alcohol. A alcohol is not oxidised. Carboxylic acids react with carbonates to release .
Four unlabelled bottles contain hex-1-ene, hexane, hexan-1-ol and hexanoic acid. Describe a sequence of chemical tests that would identify all four. Include expected observations and an equation for one test.
Add a little sodium hydrogen carbonate to a sample of each. Only hexanoic acid fizzes, and the gas turns limewater milky: $\text{C}_5\text{H}_{11}\text{COOH} + \text{HCO}_3^- \rightarrow \text{C}_5\text{H}_{11}\text{COO}^- + \text{CO}_2 + \text{H}_2\text{O}$.
Shake fresh samples of the other three with bromine water in the dark. Hex-1-ene turns it from orange-brown to colourless immediately (addition across the C=C); the other two leave it orange.
Warm the last two with acidified potassium dichromate. Hexan-1-ol, a primary alcohol, turns it from orange to green as it is oxidised; hexane gives no change. Hexane is the compound left that reacts in none of the tests.