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Identifying Functional Groups
2017 Paper Two
Looking at past AQA A-level Chemistry questions, I wanted to go beyond the answers and focus on thought processes, ideas and tips that will help in examinations.
The series will help you to spot weaknesses and help with revision, or it can serve as addenda to your notes.If you haven't gone through the paper, please look up the questions at www.aqa.org.uk, or click the direct link below, and have a go...
A 6-mark question assessing your knowledge about tests used to identify organic functional groups.
The first thing... if you want all the marks available, you will need to be systematic and coherent in your approach. Make sure you include both the reagent used AND the observation you would make.
The best way to approach this is to initially test with an aim to divide them into groups - you can then perform additional tests without repeats. A table may help to keep track as you move on through the tests.
We have two molecules that have a -COOH group, K and M. If you add sodium carbonate to a sample of each, only K and M would show a positive test - effervesce.
For the next test, you could add potassium dichromate (acidified) to a sample of each. If you recall, this is a test for alcohols and aldehydes. The orange colour will change to green as the alcohol/aldehyde is oxidised. This test will prove positive for K, L and N.
Next up, it's time we tested only for aldehydes to try to differentiate K, L and N. There are two ways to go here: use Tollen's Reagent (silver mirror is a positive test for aldehydes) or Fehling's solution. With Fehling's, a positive test for an aldehyde would be a red precipitate.
You would see this for N only.
A final test would be a test for halides with acidified silver nitrate solution (also containing NaOH, and warmed) as the reagent. A positive result gives a precipitate - white if Cl, pale cream if Br and pale yellow if I. You would see a pale cream precipitate for L and N.K contains carboxylic and alcohol groupsL contains alcohol and halide (Br) groupsM contains a carboxylic groupN has an aldehyde and halide (Br) group
Positive tests for functional groups
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