Topic 15 · page 11 of 28

Practical, identifying carbonyl compounds

A sequence of test-tube tests with 2,4-DNPH, Tollens' reagent and iodine and alkali separates aldehydes, methyl ketones, other ketones and alcohols. The melting temperature of the derivative then names the compound.

15 min read15.8(i)15.8(iv)15.8(v)

Read first: Oxidation tests for aldehydes, 2,4-DNPH and the iodoform test

The further suggested practical 15.8(iv)

The specification suggests reactions of aldehydes and ketones with the reagents on the last two pages. This page gathers them into one plan for identifying an unknown liquid.

PracticalTests on a carbonyl compound

Wear eye protection. Work with small quantities of each sample, and use a fume cupboard for the reagents that your teacher says need one.

  1. 2,4-DNPH test. Add 2–3 drops of the sample to about 2 cm³ of 2,4-dinitrophenylhydrazine solution in a test tube. Look for an orange precipitate.
  2. Tollens' test. Put about 2 cm³ of freshly made Tollens' reagent in a clean test tube, add 2–3 drops of the sample, and stand the tube in a beaker of hot water for a few minutes. Look for a silver mirror.
  3. Fehling's or Benedict's test. Add 2–3 drops of the sample to about 2 cm³ of the solution and warm in a hot water bath. Look for a brick-red precipitate.
  4. Iodoform test. Add 2–3 drops of the sample to about 2 cm³ of iodine solution, then add dilute sodium hydroxide dropwise until the colour fades, and warm gently. Look for a pale yellow precipitate and an antiseptic smell.
  5. Derivative. From a positive 2,4-DNPH test, filter the crystals, recrystallise, dry and measure the melting temperature.

The quantities are typical test-tube amounts. Follow your own worksheet for exact volumes. Tollens' reagent must be made fresh and any left-over discarded straight away with plenty of water, because it can form explosive compounds if it is stored.

Expected observations 15.8(i)

Sample 2,4-DNPH Tollens' Iodine and alkali
ethanal orange precipitate silver mirror pale yellow precipitate
propanal orange precipitate silver mirror no precipitate
propanone orange precipitate no change pale yellow precipitate
pentan-3-one orange precipitate no change no precipitate
propan-1-ol no precipitate no change no precipitate

Fehling's and Benedict's solutions give the same pattern as Tollens' reagent: a brick-red precipitate for aldehydes and no change for ketones and alcohols.

Following a logical order 15.8(v)

A flow chart for identifying an unknown liquid. First add 2,4-dinitrophenylhydrazine: no precipitate means no C=O group, an orange precipitate means an aldehyde or ketone. Next warm with Tollens' reagent: a silver mirror means an aldehyde and no change means a ketone. Then warm a ketone with iodine and alkali: a yellow precipitate means it has a CH3CO group, and no precipitate means it does not. Finally the melting temperature of the derivative names the compound.
Each test removes some possibilities. The derivative's melting temperature names the compound.

Each test answers one question, in this order:

  1. Is there a C=O group? 2,4-DNPH.
  2. Aldehyde or ketone? Tollens' or Fehling's.
  3. Is there a CH₃CO– group? Iodoform test.
  4. Which compound? Melting temperature of the derivative.

Worked exampleIdentify an unknown

A liquid Y gives an orange precipitate with 2,4-DNPH, no silver mirror and no pale yellow precipitate with iodine and alkali. The derivative melts at 156 °C. Identify Y using the table on the 2,4-DNPH page.

  1. Orange precipitate: a carbonyl compound.
  2. No silver mirror: a ketone, not an aldehyde.
  3. No iodoform precipitate: no CH₃CO– group, so not propanone, butanone or pentan-2-one.
  4. A melting temperature of 156 °C matches pentan-3-one.
  5. Y is pentan-3-one, CH₃CH₂COCH₂CH₃.

Check yourself

  1. List the expected observations when propanal is tested with (a) 2,4-DNPH, (b) Tollens' reagent and (c) iodine and alkali.
  2. Describe a sequence of tests that would tell apart propanal, propanone and pentan-3-one, and give the result for each.
  3. Why must a precipitate from the 2,4-DNPH test be recrystallised before its melting temperature is measured?
  4. Why is Tollens' reagent made fresh and not stored?

Answers cover these until you have tried the questions

  1. (a) An orange precipitate. (b) A silver mirror when warmed. (c) No pale yellow precipitate, because propanal has no CH₃CO– group.
  2. Warm each with Tollens' reagent: only propanal gives a silver mirror. Test the other two with iodine and alkali: propanone gives a pale yellow precipitate and pentan-3-one does not. (Using 2,4-DNPH first would be positive for all three and would not separate them.)
  3. To remove impurities. Impure crystals melt below the true value and over a range, which could lead to the wrong compound being named.
  4. On standing it can form explosive silver compounds, so only a small amount is prepared, used straight away and washed away afterwards.