Tests for the functional groups

Test one group at a time

Once we know the elements, we look for the functional group. A functional group is the part of the molecule that decides how it reacts. Each group has a test that is quick, cheap and easy to see. The chart gives the order of the tests for several common groups.

Work from a small sample. Note the colour, the smell and whether the sample dissolves in water before you add any reagent.

Unsaturation: a double or triple bond

• Bromine water. Add bromine water drop by drop to a solution of the compound. The orange-brown colour is lost at once. The bromine has added across the double bond.
• Baeyer's reagent. This is cold, dilute, alkaline potassium permanganate. The purple colour is lost, and a brown precipitate of manganese(IV) oxide forms. Alkenes and alkynes give it. So do some other easily oxidised compounds, such as aldehydes.

Phenols and aromatic amines also decolourise bromine water. They give a white precipitate instead, because bromine substitutes on the ring.

Alcohol, $\text{-}\text{OH}$

• Sodium metal. Bubbles of hydrogen come off. (Phenols and acids also do this.)
• Ceric ammonium nitrate. The yellow solution turns red.
• Esterification. Warm the compound with ethanoic acid and a few drops of concentrated sulphuric acid. Pour the mixture into water. A fruity smell shows an ester.
• Lucas test. Shake with zinc chloride in concentrated hydrochloric acid. A tertiary alcohol turns the solution cloudy at once. A secondary alcohol takes about five minutes. A primary alcohol gives no cloudiness at room temperature.
• Iodoform test. An alcohol with the group $\text{CH}_{3}\text{CH}(\text{OH})\text{-}$ (ethanol is the only primary one) gives a yellow precipitate. See the carbonyl tests.

Phenol, $\text{C}_{6}\text{H}_{5}\text{OH}$

• Neutral iron(III) chloride. Neutralise iron(III) chloride solution with a little dilute sodium hydroxide until a faint brown precipitate stays, and filter. Add a few drops of the filtrate to a solution of the compound in water or ethanol. A violet (blue-violet) colour shows a phenol.
• Bromine water. A white precipitate of 2,4,6-tribromophenol.
• A phenol dissolves in sodium hydroxide, but it does not dissolve in sodium hydrogencarbonate. It is too weak an acid to release carbon dioxide.

Aldehyde and ketone, the carbonyl group

• 2,4-Dinitrophenylhydrazine (2,4-DNP). Add a few drops of the compound to the reagent, which is 2,4-DNP dissolved in ethanol and sulphuric acid. An orange or yellow precipitate shows an aldehyde or a ketone.
• Tollens' reagent. Add ammonia solution drop by drop to silver nitrate solution until the brown precipitate just dissolves. Warm the compound with this reagent in a clean tube. A bright silver mirror on the glass wall shows an aldehyde. A ketone gives none.
• Fehling's solution. Mix equal volumes of Fehling's A (copper(II) sulphate solution) and Fehling's B (alkaline sodium potassium tartrate solution). Warm the compound with the blue mixture. A red precipitate of copper(I) oxide shows an aliphatic aldehyde. Aromatic aldehydes such as benzaldehyde and all ketones give none.
• Schiff's reagent. The colourless reagent turns pink with an aldehyde.
• Iodoform test. Add iodine in potassium iodide solution and then sodium hydroxide solution, drop by drop, until the colour of iodine fades. Warm gently, to about 333 K. A pale yellow precipitate of iodoform, $\text{CHI}_{3}$, with a strong, medicinal smell, shows the group $\text{CH}_{3}\text{CO}\text{-}$ or $\text{CH}_{3}\text{CH}(\text{OH})\text{-}$. Ethanal, propanone, butan-2-one, ethanol, propan-2-ol and acetophenone give it. Methanal, propanal, butan-1-ol and benzaldehyde do not.

Sugars such as glucose and fructose also reduce Tollens' and Fehling's reagents, although they are not simple aldehydes.

Carboxylic acid, $\text{-}\text{COOH}$

• Sodium hydrogencarbonate. A brisk effervescence of carbon dioxide, which turns lime water milky, shows a carboxylic acid. $\text{RCOOH}\ +\ \text{NaHCO}_{3}\ \rightarrow \ \text{RCOONa}\ +\ \text{H}_{2}\text{O}\ +\ \text{CO}_{2}$
• Litmus. Blue litmus turns red.
• Esterification. A fruity smell with ethanol and concentrated sulphuric acid.
• Neutral iron(III) chloride. Benzoic acid gives a buff-coloured precipitate. Ethanoic acid gives a deep red colour.

Amines, $\text{-}\text{NH}_{2}$

• Carbylamine test. Warm with chloroform and alcoholic potassium hydroxide. A very unpleasant smell of an isocyanide shows a primary amine. $\text{RNH}_{2}\ +\ \text{CHCl}_{3}\ +\ 3\text{KOH}\ \rightarrow \ \text{RNC}\ +\ 3\text{KCl}\ +\ 3\text{H}_{2}\text{O}$
• Azo dye test for a primary aromatic amine. Dissolve in dilute hydrochloric acid and cool to 273 K. Add sodium nitrite. Pour the product into an alkaline solution of 2-naphthol. A bright orange-red dye forms.
• Hinsberg test. Benzenesulphonyl chloride in alkali. A primary amine gives a clear solution, and acid then gives a precipitate. A secondary amine gives an insoluble solid. A tertiary amine gives no reaction.
• Ninhydrin. An amino acid, heated with ninhydrin, gives a blue-violet colour.

Choose buta-1,3-diene and 1 g. Then choose phenol. Phenol has no double bond, but it uses three bromine molecules because the ring takes three bromine atoms.

Practise with new numbers

Try it in Python

The program holds a table of six compounds and six tests. It lists the positive tests and sorts the carbonyl compounds into aldehydes and ketones.

Common mistakes

• Using Fehling's solution for every aldehyde. Benzaldehyde does not respond.
• Thinking every alcohol gives the iodoform test. Only ethanol and alcohols with $\text{CH}_{3}\text{CH}(\text{OH})\text{-}$ do.
• Confusing phenol with a carboxylic acid. Sodium hydrogencarbonate separates them. The acid effervesces, and the phenol does not.
• Calling bromine water a test for a double bond alone. Phenols and anilines react too, but with a white precipitate.
• Reading a Lucas test in minutes for a primary alcohol. It gives nothing at room temperature.

Remember these

• Double bond: bromine water and Baeyer's reagent lose their colour.
• Phenol: violet with neutral iron(III) chloride. Acid: effervescence with sodium hydrogencarbonate.
• Carbonyl: 2,4-DNP. Aldehyde: Tollens' silver mirror, Schiff pink, Fehling red for aliphatic aldehydes.
• Methyl ketone or $\text{CH}_{3}\text{CH}(\text{OH})\text{-}$: yellow iodoform.
• Primary amine: carbylamine smell. Primary aromatic amine: orange-red azo dye.

References

• NCERT Chemistry Class 12, the chapters Aldehydes, Ketones and Carboxylic Acids, and Amines.
• Vogel's Textbook of Practical Organic Chemistry, the chapters on the identification of functional groups.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. A compound gives a silver mirror with Tollens' reagent but no red precipitate with Fehling's solution. What is it likely to be?

2. Phenol and benzoic acid are both acidic. How can sodium hydrogencarbonate tell them apart?

3. Why does a primary alcohol give no cloudiness in the Lucas test at room temperature?

4. Which two of ethanol, propan-1-ol and propan-2-ol give a yellow precipitate with iodine and sodium hydroxide?