2021 P1 Q19 - Silver Nitrate Test for Halogenoalkanes
Let's take a look at 2021 A Levels H2 Chemistry Paper 1 Question 19.
We need to determine which of the students' comments are correct.
The reagents and conditions for silver nitrate test for halogenoalkanes are as follows:
A prior nucleophilic substitution reaction with NaOH(aq), reflux is required to release the free halide ions.
Dilute HNO3 is then added to acidify the solution, followed by AgNO3(aq) to precipitate the halide for observation.
Based on the colour of the precipitate, we can determine which halogen is present in the organic compound.
White ppt = AgCl = chloro compound
Cream ppt = AgBr = bromo compound
Yellow ppot = AgI = iodo compound
If we look at the test in the question, the unknown organic compound is added to warm silver nitrate solution.
There can be 2 possible explanations for lack of observation:
1. There is no halogenoalkane present in the compound.
2. There is halogenoalkane present but the reagents and conditions might not be drastic enough to break the C-X bond since only warming is involved.
Let us consider each student's comments:
Student P - This compound is definitely a halogenoarene, but it is not possible to state which halogen is present.
The C-X bond in halogenoarene is stabilised by resonance, hence will not be broken and give a positive silver nitrate test.
However it is also possible that the compound simply does not contain any halogen hence there is no observation.
Therefore student P is not correct.
Student Q - This compound is definitely not a halogenoalkane.
As mentioned previously the compound could be a halogenoalkane but the reagents and conditions might not be drastic enough to break the C-X bond.
Hence no halide ions are released for precipitation with silver nitrate.
Therefore student Q is also not correct.
We can now go through the options and determine the answer to this question is option D (both students are not correct).
Topic: Halogenoalkanes, Organic Chemistry, A Level Chemistry, Singapore
Back to list of questions for 2021 A Level H2 Chemistry Paper 1.
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