International A LevelChemistryEdexcelTopic Questions4: Rates, Equilibria and Further Organic Chemistry4.7 Organic Chemistry: Carbonyls

Organic Chemistry: Carbonyls (Edexcel International A Level Chemistry)

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1a
Sme Calculator6 marks

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.

(1)

ii)

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

(5)

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1b9 marks

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.

(6)

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.

(3)

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1c5 marks

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

q16c-paper-4-oct-2021-edexcel-ial-chemistry

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.

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2a8 marks

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.
q19a

(4)

ii)

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

[For HCN, Ka = 4.9×10–10 mol dm–3]

(2)

iii)

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

(2)

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2b2 marks

The organic products of this reaction are enantiomers.

Draw the three-dimensional structures of these enantiomers.

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3a11 marks

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

q19-paper-4-2021-june-edexcel-ial-chemistry

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

(4)

ii)
A student predicted that the product of Step 1 would rotate the plane of plane-polarised light.
Comment on this prediction.
(3)
iii)
Complete the table that summarises information about Step 2.
State symbols are not required for the equation.
(4)
Conversion of CH3CH(OH)CN to lactic acid
  Reaction type  
  Reagent  
  Conditions  
  Equation  

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3b1 mark

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.

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3c
Sme Calculator6 marks

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.
(3)

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+

Ka = 4.50 x 10–7 mol dm–3

(3)

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1a1 mark

This question is about carbonyl compounds.

The skeletal formula of menthone is shown.

q19a-paper-4-jan-2021-edexcel-ial-chemistry

Give the molecular formula of menthone.

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1b4 marks

Ethanal, CH3CHO, reacts with hydrogen cyanide in the presence of cyanide ions to form 2-hydroxypropanenitrile.

Draw the mechanism for this reaction.
Include curly arrows, and any relevant lone pairs and dipoles.

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1c5 marks

A carbonyl compound, F, has the molecular formula C6H12O.

i)
F reacts with iodine in an alkaline solution to give a pale yellow precipitate.

Give the name or formula of the group in F identified by this test.
(1)
ii)
Draw the skeletal formulae of the four possible structures of carbonyl compound F.
(2)
   
   

iii)
The carbon-13 (13C) NMR spectrum of F has four peaks.

Identify F by drawing its displayed formula.
Justify your answer by labelling the carbon atoms or groups of carbon atoms responsible for the four peaks in the spectrum.
(2)

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1d6 marks

Explain, in terms of all the intermolecular forces involved, why butanal has a higher boiling temperature than pentane but a lower boiling temperature than propanoic acid.

Substance Boiling temperature / °C
butanal 76
pentane 36
propanoic acid 141

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