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*Revision Materials and Past Papers* 2.1.1 Atomic structure and isotopes 2.1.2 Compounds, formulae and equations 2.1.3 Amount of substance 2.1.4 Acids 2.1.5 Redox 2.2.1 Electron structure 2.2.2 Bonding and structure 3.1.1 Periodicity 3.1.2 Group 2 3.1.3 The halogens 3.1.4 Qualitative analysis 3.2.1 Enthalpy 3.2.2 Reaction Rates 3.2.3 Chemical equilibrium 4.1 Basic concepts and hydrocarbons 4.1.2 Alkanes 4.1.3 Alkenes 4.2.1 Alcohols 4.2.2 Haloalkanes 4.2.3 Organic synthesis 4.2.4 Analytical techniques 5.1.1 How fast? 5.1.2 How far? 5.1.3 Acids, bases and buffers 5.2.1 Lattice enthalpy 5.2.2 Enthalpy and entropy 5.2.3 Redox and electrode potentials 5.3.1 Transition elements 5.3.2 Qualitative analysis 6.1.1 Aromatic compounds 6.1.2 Carbonyl compounds 6.1.3 Carboxylic acids and esters 6.2.1 Amines 6.2.2 Amino acids, amides and chirality 6.2.3 Polyesters and polyamides 6.2.4 Carbon–carbon bond formation 6.2.5 Organic synthesis 6.3.1 Chromatography and qualitative analysis 6.3.2 Spectroscopy Required Practicals

6.1.2 Carbonyl compounds

Characteristic tests for carbonyl compoundsReactions of carbonyl compounds

Characteristic tests for carbonyl compounds

Specification Reference 6.1.2 (d)–(e)

Quick Notes

  • 2,4-DNPH Test
    • Detects C=O groups in aldehydes and ketones.
    • Orange/yellow precipitate forms.
    • Can recrystallise the precipitate and measure its melting point to identify the original carbonyl compound.
  • Tollens’ Reagent
    • [Ag(NH3)2]+ oxidises aldehydes to carboxylic acids.
    • Silver is reduced: solution turns from colourless to a silver mirror on test tube.
    • Ketones do not react.

Full Notes

2,4-DNPH Test

Carbonyl containing compounds (aldehydes and ketones) form an orange precipitate when added to a solution of 2,4-DNPH (Brady’s reagent)

Reagent: 2,4-dinitrophenylhydrazine

OCR (A) A-Level Chemistry Brady’s reagent colour changes used in the 2,4-DNPH test for carbonyls.

Observation: Bright orange or yellow precipitate

Explanation: Forms a hydrazone derivative with C=O compounds (both aldehydes and ketones).

OCR (A) A-Level Chemistry mechanism/scheme showing 2,4-DNPH reacting with a carbonyl to form a hydrazone derivative.

The precipitate’s melting point can be used to identify the compound. (Measure its melting point and compare with known values to identify the original carbonyl compound).

Tollens’ Reagent

Aldehydes react with Tollen’s reagent to form a silver mirror precipitate. Ketones don’t react.

OCR (A) A-Level Chemistry Tollens’ test showing colourless solution forming a silver mirror with aldehydes.

Conditions: Alkaline

Aldehydes are oxidised to carboxylate ions (RCOO−)

Observation: Colourless → Silver mirror (Ag metal)

Example:
CH3CHO + [O] → CH3COO− + H2O
Silver mirror forms due to reduction of Ag+ ions (Ag+ + e− → Ag(s))

Ketones: No visible change

Ketones do not oxidise easily under standard conditions meaning we can use the above reactions to distinguish between aldehydes and ketones.

Summary