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Module 1: Practical chemistry for exams›PAG 7.2 - Identifying organic unknowns 2

PAG 7.2 - Identifying organic unknowns 2

An overview of PAG 7.2 - Identifying organic unknowns 2
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In OCR Chemistry Required Practical 7 (PAG 7.2), a series of qualitative tests for organic functional groups are performed. Alcohols, aldehydes, and carboxylic acids are tested.

Samples are tested with potassium dichromate VI, 2,4-DNP, Tollens’ reagent, and a carbonate.

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In OCR PAG 7.2: Identifying organic unknowns 2, there are several important safety requirements to be aware of (beyond normal lab practice such as wearing goggles).

  • Take care when handling hot apparatus.
  • Flammable reagents are used in this practical – ensure that there are no naked flames/sources of ignition.
  • Chemical vapours can be harmful – ensure that the laboratory is well ventilated.
  • Dispose of chemicals as directed by your teacher – take particular care with disposing of organic chemicals.
  • You may need to wear gloves if you have damaged skin on your hands.
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Experimental method for OCR Chemistry Required Practical 7 (PAG 7.2)

Part 1: Testing for primary and secondary alcohols, and aldehydes with potassium dichromate VI

EXPERIMENTAL METHOD OCR PAG 7.2 PART 1. 1. Prepare five test tubes with acidified potassium dichromate VI. Five labelled test tubes are filled with 2 cm³ of a premade oxidising mixture containing 0.1 mol dm⁻³ potassium dichromate VI and dilute sulfuric acid. 2. Add the test chemicals to the tubes. Five drops of the chemicals to be tested are added to each of the five test tubes. The chemicals to be tested are: A: propan-1-ol – a primary alcohol B: propan-2-ol – a secondary alcohol C: 2-methylpropan-2-ol – a tertiary alcohol D: propanal – an aldehyde E: propanone – a ketone. 3. Warm at 60 °C using a water bath. Heat for five minutes. 4. Look for a change in colour. Record your observations. A colour change from orange to green will occur if the test species can be oxidised by the oxidising mixture. Colour change is expected for primary and secondary alcohols, and aldehydes. Ketones, tertiary alcohols, and carboxylic acids do not produce a colour change.
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Experimental method for OCR Chemistry Required Practical 7 (PAG 7.2)

Part 2: Identifying aldehydes and ketones with 2,4-DNP

EXPERIMENTAL METHOD OCR PAG 7.2 PART 2. 1. Add 2,4-DNP reagent to clean, dry test tubes. Add five drops of 2,4-dinitrophenylhydrazine reagent (2,4-DNP), also known as Brady’s reagent, to each of four test tubes. Gloves and goggles must be worn. The chemical is very corrosive, toxic, and explosive, if dry. 2. Add the test chemicals to the tubes. Five drops of the chemicals to be tested are added to each of the test tubes. The chemicals to be tested are: Propan-1-ol, Propanal, Propanone, Ethanoic acid. 3. Record observations. Observe the test tubes for any changes and record what you see. 2,4-DNP produces a red/orange precipitate when an aldehyde or ketone is present.
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Experimental method for OCR Chemistry Required Practical 7 (PAG 7.2)

Part 3: Distinguishing between aldehydes and ketones with Tollens’ reagent

EXPERIMENTAL METHOD OCR PAG 7.2 PART 3. 1 Prepare the Tollens’ reagent. Tollens’ reagent must be prepared just before the test. Add 2 cm³ of 0.05 mol dm⁻³ silver nitrate solution to a test tube. Add one drop of 2.0 mol dm⁻³ sodium hydroxide solution to give a brown precipitate of AgO. Add concentrated ammonia solution dropwise to just dissolve the precipitate with shaking. Divide the colourless solution of Tollens’ reagent into equal portions in two test tubes. 2 Add the test solutions and warm. Add five drops of propanal (or any aldehyde) to one tube and five drops of propanone (or any ketone) to the other. Place the tube in a hot water bath for five minutes. 3 Record observations. Observe the test tubes for any changes and record what you see. A ‘silver mirror’ will form with an aldehyde, but not with a ketone. © Medify
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Experimental method for OCR Chemistry Required Practical 7 (PAG 7.2)

Part 4: Identification of carboxylic acid with a carbonate

EXPERIMENTAL METHOD OCR PAG 7.2 PART 4. 1. Add the samples to be tested to clean, dry test tubes. Add 1 cm³ of 2.0 mol dm⁻³ ethanoic acid solution to a test tube. Add 1 cm³ of ethanol or water as a control to another test tube. Ethanoic acid, Ethanol. 2. Add sodium carbonate solution. Add 1 cm³ of 5% sodium carbonate solution to both test tubes, and observe any changes. 3. Record observations. Observe the test tubes for any changes and record what you see. The acidic nature of carboxylic acids will cause the release of carbon dioxide from the sodium carbonate. Effervescence will be observed.
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Wearing the appropriate personal protective equipment is essential to avoid direct contact with harmful chemicals and to ensure safety throughout the experiment.

A simple illustration of a pair of goggles and gloves.
Do

Wear goggles and gloves when handling any chemicals, particularly concentrated acids, ammonia, and metal salts, as they can be corrosive or irritating to the skin and eyes.

A person wearing no goggles or gloves whilst handling chemicals.
Don't

Skip protective equipment just because ‘it’s only a few drops’.

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It is crucial to use clean and dry glassware when performing analytical tests.

Five test tubes arranged in a row, with varying heights and shapes.
Do

Ensure all glassware is clean and dry before use to avoid contamination and ensure accurate results.

A test tube with a white body and brownish stains or markings on its surface.
Don't

Reuse glassware that has not been properly cleaned and dried, as this could lead to inaccurate readings or unintended chemical reactions.

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Make sure to handle the bromine water safely.

A window with a blue sky reflection, accompanied by a pair of blue gloves and a protective goggles.
Do

Ensure good ventilation, and use gloves and eye protection when working with bromine water. Bromine is toxic and corrosive, and its fumes can be harmful if inhaled. Proper ventilation reduces the risk of exposure.

A hand is shown with a wound on the wrist, and a bottle labeled with a skull and crossbones is pouring a brown liquid onto the wound.
Don't

Let bromine water come into contact with skin or eyes, or use in enclosed spaces. Bromine can cause severe irritation and burns. Its fumes are harmful if inhaled.

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Below is a summary of results from OCR Chemistry Required Practical 7 (PAG 7.2):

A table with three columns labeled 'Functional group', 'Test', and 'Positive observation'. The rows list: 1. Primary alcohol, test: Add acidified potassium dichromate(VI), observation: Orange to blue-green. 2. Secondary alcohol, test: Add acidified potassium dichromate(VI), observation: Orange to blue-green. 3. Tertiary alcohol, test: Add acidified potassium dichromate(VI), observation: No colour change. 4. Aldehyde, test: Warm with Tollens’ reagent, observation: A silver mirror forms. 5. Carboxylic acid, test: Add sodium carbonate, observation: Effervescence occurs.
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