2020 P1 Q22 - Deducing Identity of Carbonyl Compound V
Let's take a look at 2020 A Levels H2 Chemistry Paper 1 Question 22.
We are asked to deduce correct structures of V.
Let's consider the observations and deduce the functional groups present.
Observation 1 - V (C5H8O2) is reduced to W (C5H12O2).
By comparing molecular formula, we can deduce 4 hydrogen atoms are added on reduction.
This means there are 2 double bonds present in V.
Possible functional groups in V will be alkene C=C or carbonyl C=O groups.
Observation 2 - V reacts with Tollens' reagent.
In syllabus only aldehydes can be oxidised by weak oxidising agents like Tollens' reagent and Fehling's solution.
This means aldehyde functional group is present in V.
Observation 3 - W reacts with alkaline aqueous iodine.
Only 2 specific groups CH3CO- or CH3CH(OH)- will give yellow ppt on warming with alkaline aqueous iodine.
Since W is the product of reduction of V, it must be saturated and cannot have carbonyl group.
Therefore it should only consist of CH3CH(OH)- group.
We can now put everything together to deduce which compounds are possible.
Compound 1 has 2 double bonds (ketone and aldehyde), aldehyde functional group, and product W has CH3CH(OH)- group.
Hence compound 1 can be possible structure for V.
Compound 2 has 2 double bonds (alkene and aldehyde), aldehyde functional group, and product W has CH3CH(OH)- group.
Hence compound 2 can be possible structure for V.
Compound 3 has 2 double bonds (alkene and ketone), NO aldehyde functional group, and product W has CH3CH(OH)- group.
Hence compound 3 cannot be the answer since there is no aldehyde group to react with Tollens' reagent.
Therefore only compounds 1 and 2 are correct and the answer to this question will be option B.
Topic: Carbonyl Compounds, Organic Chemistry, A Level Chemistry, Singapore
Back to list of questions for 2020 A Level H2 Chemistry Paper 1.
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