Organic Chemistry: Carbonyls (Edexcel International A Level Chemistry): Exam Questions

Compound X is used by mammals as an alternative energy source to sugars. X is a compound of carbon, hydrogen and oxygen only.

Complete combustion of a 2.50 g sample of X in dry oxygen produced 4.31 g of carbon dioxide and 1.32 g of water as the only products.

i) Give a reason why the oxygen used must be dry.

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ii) Show that the empirical formula of X is C₄H₆O₃ . You must show your working.

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Compound X gave an orange precipitate with Brady’s reagent (2,4-dinitrophenylhydrazine) but no reaction with Tollens’ reagent.
When X was added to a solution of sodium hydrogencarbonate, effervescence occurred and the gas evolved turned limewater cloudy.

The carbon-13 NMR spectrum of X had only four peaks.

i) Deduce the two possible structures of X, showing how this information supports your answer.

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ii) Give a chemical test which would allow you to distinguish between the two compounds you have given in (b)(i). Include the reagents required and the result for each of the compounds.

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A simplified high resolution proton (¹H) NMR spectrum of compound X is shown.

Explain how the number of peaks in the ¹H NMR spectrum, together with their relative heights, their chemical shifts and their splitting patterns, may be used to confirm the structure of X. Use the chemical shifts given in your Data Booklet.

Propanal reacts very slowly with HCN at 298 K. To increase the rate of reaction potassium cyanide, KCN, is added.

i) Complete the mechanism for this reaction by adding curly arrows, and relevant lone pairs and dipoles to Step 1 and Step 2.

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ii) Explain why the reaction between propanal and HCN in the absence of KCN is very slow, referring to the value of K_a. No calculation is required.

[For HCN, K_a = 4.9×10^–10 mol dm^–3]

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iii) KCN is a homogeneous catalyst in this reaction. Justify this description by referring to the mechanism.

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The organic products of this reaction are enantiomers.

Draw the three-dimensional structures of these enantiomers.

The compound lactic acid can be synthesised from ethanal in two steps.

i) Give the mechanism for Step 1. Include curly arrows, and any relevant lone pairs and dipoles.

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ii) A student predicted that the product of Step 1 would rotate the plane of plane-polarised light. Comment on this prediction.

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iii) Complete the table that summarises information about Step 2.
State symbols are not required for the equation.

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Sodium hydrogencarbonate, NaHCO₃ , has been used by some athletes to help prevent lactic acid causing muscle pain during exercise.

Write an equation for the reaction between sodium hydrogencarbonate and lactic acid.

Sodium hydrogencarbonate is part of a buffer in the body that controls the pH of blood. Two of the equilibria involved in this process are shown.

Equilibrium 1       HCO₃^- + H₃O⁺ ⇌ H₂CO₃ + H₂O

Equilibrium 2       H₂CO₃ ⇌ CO₂ + H₂O

i) Use the equilibria to explain how the buffer keeps the pH of blood nearly constant when a small increase in the concentration of hydrogen ions occurs.

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ii) The pH of a blood sample was found to be 7.41.  Calculate the ratio of the concentration of HCO₃^- to H₂CO₃ in the blood sample.

H₂CO₃ + H₂O  ⇌  HCO₃^- + H₃O⁺

K_a = 4.50 x 10^–7 mol dm^–3

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