AQA A-Level Chemistry: Organic analysis (7405) – Revision Notes

These notes condense section 3.3.6 Organic analysis of the AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards: 3.3.6.1 test-tube reactions with Required practical 6, 3.3.6.2 mass spectrometry and 3.3.6.3 infrared spectroscopy. It is AS content, examined on Paper 2 at AS and A-level, and A-level Paper 3 can test any content. For full explanations and worked examples, read the organic analysis study guide.

Then test yourself with the practice questions. The course hub is AQA A-Level Chemistry, the printable checklist lists every specification point, and the free 10-minute diagnostics help you find weak spots.

3.3.6.1 Test-tube reactions (Required practical 6)

Required practical 6 is “Tests for alcohol, aldehyde, alkene and carboxylic acid”. The tests come from elsewhere in the specification.

Test for	Reagent	Positive	Negative
C=C (alkene)	Bromine water, shake	Orange → colourless	Stays orange
1° or 2° alcohol	Acidified K₂Cr₂O₇, warm	Orange → green	Stays orange (3° alcohol)
Aldehyde	Tollens’ reagent, warm (water bath)	Silver mirror	No change (ketone)
Aldehyde	Fehling’s solution, warm (water bath)	Blue → brick-red precipitate	Stays blue (ketone)
Carboxylic acid	Na₂CO₃ or NaHCO₃	Effervescence; CO₂ turns limewater cloudy	No fizzing

Label: the carbonate reaction of carboxylic acids is stated in 3.3.9.1, which is A-level only. Required practical 6 (AS) still includes a carboxylic acid test.

What gets reduced in each test

• Dichromate: Cr₂O₇²⁻ (orange) → Cr³⁺ (green).
• Tollens’: [Ag(NH₃)₂]⁺ → Ag (the mirror).
• Fehling’s: Cu²⁺ complex (blue) → Cu₂O (red precipitate).
• Bromine water is different: Br₂ adds across C=C by electrophilic addition; no oxidising agent is involved.

Method in steps: building a key

1. List the compounds and the groups each contains.
2. Start with a test only one group passes, such as carbonate for a carboxylic acid.
3. Test for an aldehyde (Tollens’ or Fehling’s) before using dichromate.
4. Use bromine water to pick out the alkene.
5. Use dichromate last, to split 1°/2° alcohols from 3° alcohols and ketones.
6. One observable feature per step. Give the colour before and after.

3.3.6.2 Mass spectrometry

Key idea: molecules with the same whole-number Mr have different precise masses. The m/z of the molecular ion, measured to four decimal places, fixes the molecular formula. The basics of the mass spectrometer are in the atomic structure revision notes.

Precise masses used in these notes (questions give you the values to use): ¹H = 1.0078, ¹²C = 12.0000, ¹⁴N = 14.0031, ¹⁶O = 15.9949.

Method in steps

1. Write each candidate formula.
2. Multiply out and add the precise masses for each one, to 4 decimal places.
3. Match to the measured m/z and state the formula.
4. Show the sum for every candidate, not only the winner.

Worked reminder: m/z = 58.0417

C3H6O  : 3(12.0000) + 6(1.0078) + 15.9949           = 58.0417  match
C4H10  : 4(12.0000) + 10(1.0078)                    = 58.0780
C2H2O2 : 2(12.0000) + 2(1.0078) + 2(15.9949)        = 58.0054
C2H6N2 : 2(12.0000) + 6(1.0078) + 2(14.0031)        = 58.0530

Molecular formula C₃H₆O. That could be propanal or propanone; MS cannot decide.

3.3.6.3 Infrared spectroscopy

Key idea: bonds absorb infrared at characteristic wavenumbers. Spectra show transmittance against wavenumber (cm⁻¹); absorptions are dips. You use the Chemistry Data Sheet or Booklet for the ranges.

Bond	Wavenumber / cm⁻¹	What it looks like
O–H (alcohols)	3230–3550	Broad
O–H (acids)	2500–3000	Very broad, overlaps C–H
N–H (amines)	3300–3500
C–H	2850–3300	In almost every organic spectrum
C≡N	2220–2260
C=O	1680–1750	Strong, sharp
C=C	1620–1680
C–O	1000–1300
C–C	750–1100

Fingerprinting

The low-wavenumber region (below about 1500 cm⁻¹) has a pattern unique to each molecule. Compare it with the spectrum of a known compound: an exact match identifies the compound.

Impurities

An absorption the pure product cannot give means something else is present. Common cases:

• O–H (alcohol) in a product with no O–H: unreacted alcohol (e.g. after making an alkene by elimination).
• O–H (acid) at 2500–3000 in an aldehyde: the aldehyde has been over-oxidised.
• A broad O–H in a dried liquid: water still present.

Global warming link

CO₂, methane and water vapour absorb infrared radiation emitted by the Earth’s surface because their bonds (C=O, C–H, O–H) absorb at infrared wavenumbers. They re-emit energy, some back towards the surface, which keeps energy in the atmosphere. More of these gases means more absorption, which contributes to global warming.

Combining the techniques

Structure questions often give you all three kinds of evidence. Use them in this order.

1. Mass spectrum: use the precise m/z of the molecular ion to fix the molecular formula.
2. Candidates: draw every reasonable structure with that formula from the groups you know.
3. IR spectrum: for each candidate, list which absorptions it must show and which it cannot show. Rule out any candidate that does not fit. Quote the wavenumber range and the bond each time.
4. Test-tube reaction: use a test to split any candidates IR cannot separate (for example, Tollens’ reagent for an aldehyde against a ketone). At A-level, NMR spectroscopy (A-level only) adds further evidence.
5. Conclusion: name the compound and draw its structure.

Worked reminder: reading a spectrum in steps

A compound C₄H₈O shows a strong absorption at 1715 cm⁻¹ and nothing between 3230 and 3550 cm⁻¹. It does not change Fehling’s solution.

• 1715 cm⁻¹ is C=O (1680–1750), so it is an aldehyde or a ketone.
• No broad absorption at 3230–3550 cm⁻¹, so there is no alcohol O–H.
• Fehling’s stays blue, so it is not an aldehyde.
• It is a ketone with four carbons: butanone, CH₃COCH₂CH₃.

Must-know distinctions

Easily confused	The difference
Alcohol O–H vs acid O–H	3230–3550 broad vs 2500–3000 very broad
Tollens’ vs Fehling’s	Silver mirror vs blue → brick-red precipitate
1°/2° vs 3° alcohol	Orange → green vs stays orange with acidified dichromate
Mass spectrometry vs IR	MS gives the molecular formula; IR gives the bonds present
Functional group region vs fingerprint region	Group region identifies bonds; fingerprint identifies the whole molecule by comparison
Colourless vs clear	Bromine water goes colourless; “clear” is not a colour

Quick self-test

1. Give the reagent and full observation for a test for C=C.
2. 2-methylpropan-2-ol is warmed with acidified potassium dichromate(VI). What do you see?
3. Which reagent gives a brick-red precipitate with an aldehyde, and what is the precipitate?
4. Why should you test for an aldehyde before using acidified dichromate(VI) to look for an alcohol?
5. A molecular ion peak is at m/z = 46.0054. Is it CH₂O₂ or C₂H₆O? Show working.
6. Give the wavenumber range for the C=O bond.
7. A spectrum has a broad absorption at 3350 cm⁻¹. Which bond, in which type of compound?
8. A spectrum shows an absorption at 1650 cm⁻¹ and no absorption above 3000 cm⁻¹ except C–H. Which group?
9. Why can IR functional-group absorptions alone not tell butanal from butanone?
10. Name the three gases the specification links to global warming through IR absorption.
11. What is the gas given off when a carboxylic acid reacts with sodium hydrogencarbonate, and how is it tested?

Answers

1. Bromine water; orange to colourless.
2. The solution stays orange (tertiary alcohols are not easily oxidised).
3. Fehling’s solution; copper(I) oxide, Cu₂O.
4. Aldehydes also turn acidified dichromate(VI) from orange to green, so the result would not tell them apart.
5. CH₂O₂: 12.0000 + 2(1.0078) + 2(15.9949) = 46.0054. C₂H₆O: 2(12.0000) + 6(1.0078) + 15.9949 = 46.0417. So CH₂O₂.
6. 1680–1750 cm⁻¹.
7. O–H in an alcohol (3230–3550 cm⁻¹, broad from hydrogen bonding).
8. C=C, an alkene (1620–1680 cm⁻¹).
9. Both contain C=O and C–H. Use the fingerprint region (compare with known spectra) or Tollens’ reagent.
10. Carbon dioxide, methane and water vapour.
11. Carbon dioxide; it turns limewater cloudy.

Where marks are usually lost

• Writing “clear” instead of “colourless” for bromine water.
• Giving only the final colour (“turns green”) instead of orange to green.
• Mixing up the observations: silver mirror is Tollens’, brick-red precipitate is Fehling’s.
• Using dichromate to identify an alcohol while an aldehyde is still a possibility.
• Choosing a molecular formula without showing the precise-mass sum for each candidate.
• Rounding precise masses to 1 or 2 decimal places, which hides the difference.
• Calling a broad 2500–3000 cm⁻¹ absorption an alcohol O–H.
• Treating a C–H absorption as evidence for a particular functional group.
• Explaining fingerprinting without saying the spectrum is compared with a known one.
• Describing global warming without saying the bonds in the gases absorb infrared.

Official syllabus

AQA, AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards: section 3.3.6 Organic analysis (3.3.6.1 to 3.3.6.3), with Required practical 6.