2020 P1 Q23 - Deducing Functional Group in Compound T

Let Chemistry Guru, Singapore's top choice for JC Chemistry tuition, guide you through 2020 A Levels H2 Chemistry Paper 1 Question 23.

We want to deduce the functional group present in compound T based on the following 4 results.

Result 1 - T, C_xH_yO_z can be reduced by NaBH₄ to form product with molecular formula C_xH_y+2O_z

By comparing molecular formula, we can deduce that there is an increase in 2 hydrogen.

NaBH₄ is a mild reducing agent that can reduce carbonyl compounds.

Other functional groups such as alkenes, nitriles, acids and derivatives can only be reduced by stronger reducing agents.

Hence carbonyl functional group must be present in T.

Result 2 - T can be oxidised to form a compound with molecular formula C_xH_yO_z+1

By comparing molecular formula, we can deduce there is an increase in 1 oxygen.

This means an aldehyde functional group is present in T.

Oxidation of various functional groups in syllabus will have different changes in number of oxygen and hydrogen:

Aldehyde oxidised to acid - gain 1 O
Primary alcohol oxidised to aldehyde - lose 2 H
Primary alcohol oxidised to acid - lose 2 H and gain 1 O
Secondary alcohol oxidised to ketone - lose 2 H

This means we can compare molecular formula between reactant and product to deduce the functional group present in the reactant.

Result 3 - T gives orange ppt with 2,4-DNPH

This is a distinguishing test for carbonyl compounds, which includes aldehydes and ketones.

Hence carbonyl functional group must be present in T.

Result 4 - T does not react with sodium metal

This is a distinguishing test for functional groups with -OH bonds (alcohol, phenol and acid).

Hence no -OH group is present in T.

We can now compare the options and determine the best answer.

Notice option B and D are possible since compound T can contain more than 1 oxygen, but there is no reaction to prove that ketone functional group must be present.

Therefore the best answer to this question will be option A.

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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