2020 P1 Q23 Deducing Functional Group in Compound T

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Let's take a look at 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, CxHyOz can be reduced by NaBH4 to form product with molecular formula CxHy+2Oz • 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 CxHyOz+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 • Please SUBSCRIBE to my channel and SHARE this video with your friends if you find it helpful! • View the full video with description at my website https://chemistryguru.com.sg/2020-a-l... • Looking for more FREE A Level Chemistry videos? Check out our huge collection of video lessons at https://chemistryguru.com.sg/a-level-...

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