Words you need first
A functional group is the part of an organic molecule that decides how it reacts. For example, C=C (alkene), –OH (alcohol), –CHO (aldehyde), C=O inside a chain (ketone) and –COOH (carboxylic acid).
A positive test is a clear change you can see: a colour change, a solid forming (precipitate), a mirror, or gas bubbles. A reagent is the chemical you add to test.
Primary, secondary and tertiary alcohols differ in how many carbons are joined to the carbon carrying –OH: one, two or three.
Test-tube tests for functional groups
Alkene (C=C): bromine water
Shake with orange bromine water. Bromine adds across the double bond, so the colour goes from orange to colourless. Alkanes do not react without UV light.
Alcohols: acidified potassium dichromate(VI)
Warm with K₂Cr₂O₇ and dilute sulfuric acid. Primary and secondary alcohols are oxidised, and the dichromate changes from orange to green (Cr₂O₇²⁻ → Cr³⁺). Tertiary alcohols are not oxidised, so the colour stays orange.
Aldehyde vs ketone: Tollens' and Fehling's
Tollens' reagent (silver ions in ammonia solution), warmed in a water bath: an aldehyde gives a silver mirror on the glass because Ag⁺ is reduced to Ag. Fehling's solution (blue copper(II)) gives a brick-red precipitate of copper(I) oxide with an aldehyde. Ketones give no change in both tests. Aldehydes also turn dichromate green.
Carboxylic acid: sodium carbonate or hydrogencarbonate
Acids react to give carbon dioxide, so you see fizzing. The gas turns limewater cloudy.
Haloalkanes (extra)
Warm with aqueous sodium hydroxide, add dilute nitric acid, then silver nitrate: white (Cl), cream (Br) or yellow (I) precipitate.
Safety: use a water bath, not a flame, because many organic liquids catch fire easily. Dichromate is toxic; wear gloves and goggles.
Mass spectrometry
A mass spectrometer turns molecules into positive ions, separates them by mass-to-charge ratio (m/z) and counts them. The result is a graph of peaks.
- The molecular ion peak, M⁺, is the peak with the highest m/z (ignoring a tiny M+1 peak from carbon-13). Its m/z equals the relative molecular mass, Mr.
- Smaller peaks come from fragments when the molecule breaks up. The tallest peak is the base peak.
- High-resolution mass spectrometry gives Mr to 4 decimal places. Using exact masses H = 1.0078, C = 12.0000, O = 15.9949, N = 14.0031, molecules with the same whole-number Mr can be told apart: C₃H₈ = 44.0624, C₂H₄O = 44.0261, CO₂ = 43.9898.
Infrared (IR) spectroscopy
Bonds vibrate (stretch and bend). Each kind of bond absorbs infrared radiation at its own wavenumber (in cm⁻¹). An IR spectrum shows dips (or peaks) where the sample absorbs.
- O–H in alcohols: broad, 3230–3550 cm⁻¹.
- O–H in carboxylic acids: very broad, 2500–3000 cm⁻¹.
- N–H: 3300–3500 cm⁻¹.
- C–H: 2850–3300 cm⁻¹ (in almost every organic compound).
- C=O (aldehydes, ketones, acids, esters): sharp and strong, 1680–1750 cm⁻¹.
- C=C: 1620–1680 cm⁻¹.
- C–O: 1000–1300 cm⁻¹.
The region below about 1500 cm⁻¹ is the fingerprint region. Its pattern is unique to each compound, so it can be matched against a database. The same physics explains the greenhouse effect: CO₂, H₂O and CH₄ absorb infrared radiation given off by the Earth.
Other methods of studying substances
- UV–visible spectroscopy: measures how much ultraviolet or visible light a substance absorbs. Coloured compounds and molecules with many alternating double bonds absorb strongly. Used to find concentrations.
- NMR spectroscopy: places the sample in a strong magnet and uses radio waves. Hydrogen and carbon-13 nuclei in different surroundings give separate signals, showing how the atoms are joined.
- MRI (magnetic resonance imaging) is NMR used on the human body. It maps hydrogen in water and fat to make images of soft tissue without X-rays.
- Chromatography separates a mixture so each part can be identified, for example the several dyes inside one plant colour such as turmeric, beetroot or spinach. See the chromatography lesson.
- Chemical mutagenesis: some chemicals (for example certain dyes, nitrites turned into nitrosamines, and chemicals in tobacco smoke) can change DNA. Analysis methods like these help scientists find and limit such chemicals in food and the environment.
Try it: be the detective
In the 3D free-play step, pick Sample A to F. Before looking, predict which tube will change. Then use the three clues together: the test-tube result tells you the functional group, M⁺ tells you the Mr, and the IR peak confirms the bond. At home, you can see an acid–carbonate test safely: add a spoon of baking soda (sodium hydrogencarbonate) to vinegar (ethanoic acid) and watch it fizz.
Key formulas and definitions
- Alkene + bromine water: orange → colourless
- 1° / 2° alcohol or aldehyde + acidified K₂Cr₂O₇: orange → green; 3° alcohol: no change
- Aldehyde + Tollens': silver mirror; + Fehling's: blue → brick-red; ketone: no change
- Carboxylic acid + Na₂CO₃ or NaHCO₃: fizzing (CO₂)
- Mass spectrum: m/z of molecular ion M⁺ = Mr
- IR: O–H alcohol 3230–3550 (broad); O–H acid 2500–3000 (very broad); C=O 1680–1750; C=C 1620–1680; C–O 1000–1300 cm⁻¹
- Fingerprint region: below 1500 cm⁻¹
Worked examples
1. Liquid X decolourises bromine water. What does this tell you?
X contains a C=C double bond, so it is an alkene (or contains an alkene group).
2. Three alcohols are warmed with acidified potassium dichromate. A and B turn green; C stays orange. What can you say about C?
C is not oxidised, so it is a tertiary alcohol. A and B are primary or secondary.
3. Two compounds both have formula C₃H₆O. One gives a silver mirror with Tollens' reagent. Name both.
The one with the silver mirror is the aldehyde, propanal (CH₃CH₂CHO). The other is the ketone, propanone (CH₃COCH₃).
4. The mass spectrum of a compound has its molecular ion peak at m/z = 60. Its IR spectrum has a very broad peak at 2500–3000 cm⁻¹ and a sharp peak at 1715 cm⁻¹. Identify it.
Mr = 60. Very broad O–H (2500–3000) plus C=O (1715) means a carboxylic acid. CH₃COOH has Mr = 2×12 + 4×1 + 2×16 = 60, so it is ethanoic acid. Check: it would fizz with sodium carbonate.
5. High-resolution mass spectrometry gives M⁺ = 44.0262. Which is it: C₃H₈, CO₂ or C₂H₄O? (H = 1.0078, C = 12.0000, O = 15.9949)
C₃H₈ = 36 + 8.0624 = 44.0624. CO₂ = 12 + 31.9898 = 43.9898. C₂H₄O = 24 + 4.0312 + 15.9949 = 44.0261. The closest is C₂H₄O (ethanal).
6. An IR spectrum shows a strong sharp peak at 1720 cm⁻¹ and no broad peak above 2500 cm⁻¹. It gives no change with Tollens' reagent. What type of compound is it?
C=O is present (1720) but no O–H, so it is not an acid or alcohol. No silver mirror means not an aldehyde. So it is a ketone (or an ester; a smell test or other data would decide).
Common mistakes
- Saying bromine water turns "clear" in a way that sounds like "no colour change". Write "decolourised" or "orange to colourless".
- Thinking tertiary alcohols turn dichromate green. They do not oxidise, so the colour stays orange.
- Taking the tallest peak in a mass spectrum as Mr. The tallest is the base peak; Mr comes from the molecular ion, the last main peak on the right.
- Mixing up the two O–H ranges in IR: alcohols 3230–3550 cm⁻¹, carboxylic acids a very broad 2500–3000 cm⁻¹.