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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

6.3.1 Chromatography and qualitative analysis

ChromatographyTests for organic functional groups

Tests for organic functional groups

Specification Reference 6.3.1 (c)

Quick Notes

  • Test-Tube Reactions for Functional Groups
    • Alkene: decolourises bromine water
    • Haloalkane: reacts with AgNO3 in ethanol, precipitate forms
    • Phenol: weakly acidic, no reaction with Na2CO3
    • Carbonyl: reacts with 2,4-DNP, giving an orange/yellow precipitate
    • Aldehyde: Tollens’ reagent, gives a silver mirror
    • Primary/Secondary Alcohol & Aldehyde: acidified K2Cr2O7 → orange to green
    • Carboxylic acid: reacts with Na2CO3, effervescence (CO2)

Full Notes

Functional Group Tests (Test-Tube Scale)

These qualitative tests help identify the presence of functional groups in organic molecules:

Alkenes

Test: Add bromine water

OCR (A) A-Level Chemistry test for alkene with bromine water showing colour change from orange to colourless.

Positive result: Orange → colourless (electrophilic addition to C=C)

Haloalkanes

Test: Add ethanolic AgNO3, warm

OCR (A) A-Level Chemistry test for haloalkane with AgNO3 in ethanol forming coloured silver halide precipitates.

Result: Precipitate forms

Phenols

Test 1: React with NaOH → dissolves (acid-base)

OCR (A) A-Level Chemistry test showing phenol reacting with NaOH to dissolve forming sodium phenoxide.

Test 2: React with Na2CO3 → no reaction (unlike carboxylic acids)

Carbonyl compounds (aldehydes or ketones)

Test: Add 2,4-DNP (Brady’s reagent)

OCR (A) A-Level Chemistry test for carbonyls using 2,4-DNP showing orange/yellow precipitate.

Result: Orange/yellow precipitate of a 2,4-dinitrophenylhydrazone derivative

Aldehydes

Test: Tollens’ reagent (ammoniacal silver nitrate), warm gently

OCR (A) A-Level Chemistry test for aldehyde using Tollens’ reagent giving silver mirror result.

Positive result: Silver mirror on test tube walls

Primary or Secondary Alcohols and Aldehydes

Test: Add acidified potassium dichromate (K2Cr2O7/H2SO4)

OCR (A) A-Level Chemistry oxidation test for alcohols and aldehydes using acidified dichromate with orange to green colour change.

Result: Orange → green (Cr6+ reduced to Cr3+)
Aldehydes and 1°/2° alcohols oxidised

Carboxylic acids

Test: Add Na2CO3 or NaHCO3

OCR (A) A-Level Chemistry test for carboxylic acids with sodium carbonate showing effervescence of CO2.

Result: Effervescence (CO2 gas evolved)
Confirm with limewater test: CO2 turns limewater cloudy

Summary