AQA Chemistry A-level - 3.3.6 Organic Analysis - Detailed Notes - 3.3.6.1 Tests for Functional Groups

Oct 14, 2026

Organic Analysis notes for AQA Chemistry A-level cover identification of key functional groups and the analytical techniques used. Tests explain how alcohols are oxidised by acidified potassium dichromate, how aldehydes are detected using Tollens’ reagent or Fehling’s solution, how alkenes are identified with bromine water, and how carboxylic acids generate CO2 tested with limewater. The document also summarizes mass spectrometry (including high-resolution molecular formula determination) and infrared spectroscopy for functional-group assignment using characteristic absorption ranges and fingerprint regions, including a brief link to atmospheric IR absorption and global warming.

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3.3.6 Organic Analysis Detailed Notes

3.3.6.1 - Tests for Functional Groups

3.3.6.2 - Mass Spectrometry

3.3.6.3 - Infrared Spectroscopy

AQA Chemistry A-level # 3.3.6: Organic AnalysisDetailed Notes ## 3.3.6.1 - Tests for Functional Groups Alcohols 1o and 2o alcohols are identified using acidified potassium dichromate. It turns from orangeto green if they are present. 3o alcohols are not oxidised andthe potassium dichromate remains orange. Example: Aldehydes Tollen’s reagent can be used to identify aldehydes. Itis added to the solution being tested andwarmed gently. If an aldehyde is present, a silver mirror will form in the test tube. If not, thesolution will remain colourless. Example: Fehling's solution is another way of testing for aldehydes. Itis added to the solution beingtested and heated. A brick red precipitate will form if an aldehyde is present and if not, thesolution remains blue and thereis no observed change. Example: Alkenes Bromine water is used to test for unsaturated hydrocarbons. It changes from orange-brownto colourless if a carbon-carbon double bond is present. If not, no change is observed. Example: Carboxylic Acid These compounds react with sodium carbonate as acids, producing CO2 . This gas can becollected and tested using limewater. This solution will turn cloudy if the gas is CO2 . Example: ## 3.3.6.2 - Mass Spectrometry This analytical technique is used to identify compounds and determine their molecular formula(see 3.1.1) . High Resolution Mass Spectrometry This is a much more sensitive form of mass spectrometry which allows the Mr of a substanceto be determined to several decimal places. Precise atomic masses are always given and canthen be used to calculate the molecular formula of the compound being tested. ## 3.3.6.3 - Infrared Spectroscopy This analytical technique uses infrared (IR) radiation to determine the functional groupspresent in organic compounds. The IR radiation is passed through a sample where the differenttypes of bonds absorb the radiation in different amounts. These varying amounts of absorbanceare measured and recorded allowing certain bonds and therefore functional groups to beidentified. A spectrum is produced from the measurements which has characteristic curves forthedifferent functional groups: | -OH Alcohol Group | | | --- | --- | | | | The characteristic peak is in the range 3230 - 3550 cm-1. -OH Acid Group The characteristic peak is in the range 2500 - 3000 cm-1. C=C Unsaturated Group The characteristic peak is in the range 1620 - 1680 cm-1. C=O Carbonyl Group The characteristic peak is in the range 1680 - 1750 cm-1. Fingerprint Region Each IR spectrum has a fingerprint region to the right-hand side. This contains tinydifferences from species to species which act as a molecules‘fingerprint’, allowing it to beidentified. Global Warming Infrared absorption also occurs in the atmosphere with molecules such as ozone. This causesheat to be trapped within the Earth’s atmosphere, an important factor forthe existence of lifehere. However when chemicals such as CFCs are released into the atmosphere from humanactivity, this heating effect is enhanced , leading to global warming.

FAQs

How are 1o and 2o alcohols identified in the functional group tests?

Acidified potassium dichromate is used. It turns from orange to green if 1o and 2o alcohols are present.

What reagents and observations identify aldehydes?

Tollens’ reagent forms a silver mirror if an aldehyde is present. Fehling’s solution produces a brick red precipitate with aldehydes; otherwise it remains blue with no observed change.

How does infrared spectroscopy help identify functional groups?

IR radiation is absorbed differently by various bonds in a sample. The resulting spectrum shows characteristic peak ranges for functional groups (and a fingerprint region for fine distinctions between species).

What is the purpose of high-resolution mass spectrometry?

It determines the relative molecular mass (Mr) to several decimal places and provides precise atomic masses, which can then be used to calculate the molecular formula of the compound being tested.

Oct 14, 2026

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