2020 P1 Q22 - Deducing Identity of Carbonyl Compound V

Let's take a look at 2020 A Levels H2 Chemistry Paper 1 Question 22.

2020 p1 q22 question

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).

2020 p1 q22 observation 1 V reduced to W

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.

2020 p1 q22 observation 2 V reacts with tollens

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.

2020 p1 q22 observation 3 W reacts with alkaline aq 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.

2020 p1 q22 compare compounds 1 to 3

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.

2020 p1 q22 answer

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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