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Chemicals tests on carbonyl compounds

Chemicals tests on carbonyl compounds. L.O.: Detect the presence of a carbonyl compound (DNPH). Identify a carbonyl compound Distinguish between an aldehyde or ketone. 2,4-DNP (2,4-Dinitrophenylhydrazine).

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Chemicals tests on carbonyl compounds

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  1. Chemicals tests on carbonyl compounds L.O.: Detect the presence of a carbonyl compound (DNPH). Identify a carbonyl compound Distinguish between an aldehyde or ketone

  2. 2,4-DNP(2,4-Dinitrophenylhydrazine) • A test (also known as Brady’s test) for the C=O group found in aldehydes and ketones. • A positive result produces a yellow to red precipitate.

  3. Test for the carbonyl group

  4. Identification of a carbonyl compound • The precipitate made from the 2,4-DNP test can be purified and its melting point can be determined • This can then be compared against known melting points of DNP derivatives to ID the carbonyl precursor molecule

  5. Tollen’s reagent Ball-and-stick model of the diamminesilver(I) cation, Ag(NH3)2+

  6. Activity 2, steps 1-2 (detecting a carbonyl group) • Activity 3, steps 1-3 (aldehyde or ketone?) • Activity 2, steps 5-9 (identifying a carbonyl compound)

  7. Chemicals tests on carbonyl compounds L.O.: Detect the presence of a carbonyl compound (DNPH). Identify a carbonyl compound Distinguish between an aldehyde or ketone

  8. Brady’s test - Protocol • To 2ml of 2,4-DNP solution add a few drops of the unknown sample • Test is positive if a orange precipitate forms • If no precipitate forms, then add acid to the solution, the test is only negative if this then produces no precipitate

  9. Reaction between Tollen’s reagent and an aldehyde The silver solid builds up as a silver film on the glassware and indicated the presence of aldehyde

  10. Tollen’s test • Reacts with C=O in aldehydes only, oxidising them to carboxylic acids • Positive result produces a silver mirror effect on inside of the test tube

  11. Tollen’s test - Protocol • To 2ml of silver nitrate in a test tube add a few drops of NaOH until a brown/black precipitate forms • Add ammonia solution to this drop by drop until the brown precipitate dissolves • Add 5ml of water – this is Tollen’s reagent • To this add a few drops of the unknown sample and leave in a hot water bath for a few minutes • Test is positive if a mirror finish is found on the inside of the test tube

  12. Exam style questions • Describe a simple chemical test that would show that Propanal is an aldehyde. (3 marks) • Explain why this test gives a different result with aldehydes than it does with ketones.

  13. Exam style questions • Describe a simple chemical test that would show that Propanal is an aldehyde. (3 marks) • Add Tollens’ reagent. • Heat reaction in a waterbath. • It gives a silver precipitate or silver mirror.

  14. Exam style questions • Explain why this test gives a different result with aldehydes than it does with ketones. Aldehydes can be oxidised but ketones cannot.

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