A-LevelChemistryIntroduction to organic chemistry9701/41 · Oct/Nov 2023 · Q814 Marks

Capsaicin is found in chilli peppers. [Figure with structure of capsaicin] You should assume the CH3O group is unreactive in the reactions involved in this question.

(a)Name all the functional groups in capsaicin in addition to the CH3O group.[1]
(b)Complete the equation for the reaction of capsaicin with an excess of Br2(aq) in the dark. Draw the structure of the organic product in the labelled box. [Figure with capsaicin + Br2(aq) --> organic product box + ...][3]
(c)Capsaicin is heated with an excess of hydrogen gas in the presence of platinum metal. The six-membered ring reacts in the same way as benzene under these conditions. Draw the structure of the organic product formed.[2]
(d)When capsaicin is treated with reagent J under suitable conditions one of the products is methylpropanoic acid, CH3CH(CH3)COOH.
(d)(i)Identify reagent J and any necessary conditions.[1]
(d)(ii)There are three different peaks in the proton (¹H) NMR spectrum of CH3CH(CH3)COOH in CDC13. Table 8.1 environment of proton example chemical shift range δ/ppm alkane -CH3, -CH2-, >CH– 0.9-1.7 alkyl next to C=O CH3-C=O, –CH2–C=O, >CH–C=O 2.2-3.0 alkyl next to aromatic ring CH3-Ar, -CH2-Ar, >CH-Ar 2.3-3.0 alkyl next to electronegative atom CH3-O, -CH2-O, –CH2–Cl 3.2-4.0 attached to alkene =CHR 4.5-6.0 attached to aromatic ring H-Ar 6.0-9.0 aldehyde HCOR 9.3-10.5 alcohol ROH 0.5-6.0 phenol Ar-OH 4.5-7.0 carboxylic acid RCOOH 9.0-13.0 Use Table 8.1 to complete Table 8.2 and state: • the typical proton (¹H) chemical shift values (δ) for the protons • the splitting pattern (singlet, doublet, triplet, quartet or multiplet) shown by each peak • the explanation for the splitting patterns of the CH3 protons and the CH proton. Table 8.2 environment δ/ppm splitting pattern explanation for splitting pattern CH3 CH COOH[3]
(e)(i)Capsaicin is heated with an excess of hot aqueous NaOH. Draw the structures of the two organic products H and K. [Figure showing capsaicin reacting to form H (C8H10NO2Na) and K (C10H17O2Na)][2]
(e)(ii)Name the two types of reaction occurring in (e)(i).[1]
(f)Draw the structure of the organic product L formed when capsaicin is treated with LiAlH4 in dry ether. [Figure with empty box for L][1]

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 (Oct/Nov 2023). The mark or mark type is shown in brackets.

[1] phenol, amide AND alkene / C=C all three needed
M1: HBr [1] u / c
M2: structure of the organic product[1]
M3: correct balanced equation [1] ecf
[1] M1: ring reduced
[1] M2: alkene reduced
[1] hot AND concentrated acidified AND MnO4¯ / KMnO4 all
[1] for δ values [1] for splitting [1] for explanations
environment CH3: δ 0.9-1.7, splitting doublet, explanation 1H on neighbouring C / next to CH / one vicinal proton
environment CH: δ 2.2-3.0, splitting multiplet / heptet / septet, explanation 6H on neighbouring C / next to 2 x (CH3) / six vicinal protons
environment COOH: δ 9.0-13.0, splitting singlet, explanation (no neighbouring protons)
[1] (structure H) CH3O-C6H3(O⁻Na⁺)-CH2CH2NH2
[1] (structure K) (CH3)2CH(CH2)4CH(CH3)CH2CH2(O⁻Na⁺)
[1] hydrolysis AND neutralisation / acid-base
[1] (structure L) CH3O-C6H3(OH)-CH2CH2NH-CH2CH2CH(CH3)CH2CH=CH2