A-LevelChemistryCarboxylic acids and derivatives9701/41 · May/June 2021 · Q618 Marks

Compare and explain the relative acidities of butanoic acid, ethanol, ethanoic acid and water. > most acidic > least acidic

(a)Compare and explain the relative acidities of butanoic acid, ethanol, ethanoic acid and water. > most acidic > least acidic[4]
(b)(i)Three carboxylic acids, methanoic acid, HCO₂H, ethanedioic acid, HO₂CCO₂H, and butanedioic acid, HO₂CCH₂CH₂CO₂H, are compared. Two tests were carried out on separate samples of each organic acid, as shown. The following results were obtained. ✓ = observed change X = no observed reaction test | reagents and conditions | HCO₂H | HO₂CCO₂H | HO₂CCH₂CH₂CO₂H | observed change -----|-------------------------|-------|-----------|-----------------|---------------- 1 | | ✓ | X | X | 2 | | ✓ | ✓ | X | Complete the table with the reagents and conditions and the observed change for a positive test. Assume these organic acids all have a similar acid strength.[3]
(b)(ii)Each compound, HCO₂H, HO₂CCO₂H and HO₂CCH₂CH₂CO₂H, is dissolved seperately in CDCl₃. Proton (¹H) NMR and carbon-13 (¹³C) NMR spectra are then obtained. Complete the table. compound | number of peaks in proton NMR | number of peaks in carbon-13 NMR ----------------|-------------------------------|---------------------------------- HCO₂H | HO₂CCO₂H | HO₂CCH₂CH₂CO₂H |[2]
(b)(iii)The proton NMR spectrum of HCO₂H in D₂O is obtained. Describe and explain the difference observed between this spectrum and the proton NMR spectrum of HCO₂H in (b)(ii).[1]
(c)(i)1,4-dibromobutane, Br(CH₂)₄Br, is used in the synthesis of the dicarboxylic acid J and diamine K as shown. [Figure showing reaction scheme from Br(CH₂)₄Br to G (step 1), G to J (step 2), Br(CH₂)₄Br to H (step 3), H to K (step 4). Structures for J and K are given: J is HO₂CCH₂CH₂CO₂H, K is H₂NCH₂CH₂CH₂CH₂CH₂CH₂NH₂.] Draw the structures of G and H in the boxes.[2]
(c)(ii)Suggest reagents and conditions for each of steps 1 to 4. step 1 step 2 step 3 step 4[4]
(d)Polyamide L can be synthesised from dicarboxylic acid J, HO₂C(CH₂)₂CO₂H, and diamine K, H₂N(CH₂)₆NH₂. Draw the repeat unit of the polymer formed in the box. Any functional groups should be shown displayed.[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.

[1] M1: ethanoic acid > butanoic acid > water > ethanol
[1] M2: a reason given in terms of an electron donating or an electron withdrawing group for one of: strengthening of O–H bond OR weakening of O–H bond OR stability of anion
[1] M3: ethanol: positive inductive effect / electron donating effect of ethyl / alkyl / R group
[1] M4: butanoic acid: positive inductive effect / electron donating effect of propyl / alkyl / R group
[1] M5: (either ethanoic or butanoic) acid: negative inductive effect of either C=O or carbonyl OR negative charge delocalised over COO-
[1] M1/M2: reagents and conditions × 2
[1] M3: observations both correct test 1: Tollen's reagent, warm OR Fehling's solution, warm ; silver mirror OR (brick) red ppt / solid test 2: acidified MnO4, warm ; decolourises OR bubbles
[2] HCO2H: proton NMR 2, carbon-13 NMR 1 HO2CCO2H: proton NMR 1, carbon-13 NMR 1 HO2CCH2CH2CO2H: proton NMR 2, carbon-13 NMR 2
[1] OH peak disappears AND proton / H exchanges with deuterium
[1] G = HOCH2CH2CH2CH2OH
[1] H = NCCH2CH2CH2CH2CN
[1] M1: step 1 NaOH(aq) + heat
[1] M2: step 2 acidified KMnO4 + heat / acidified K2Cr2O7 + heat
[1] M3: step 3 CN-/KCN / NaCN + heat
[1] M4: step 4 LiAlH4 ALLOW Na in ethanol or H2 + Ni / Pd / Pt
[1] M1: correct displayed amide linkage
[1] M2: the rest of the repeat unit correct including trailing bonds [Figure showing repeat unit: -OC-(CH2)2-CO-NH-(CH2)6-NH-]