Zadanie 6c, Paper 2, specimen 2025, Chemia IB HL
Wzorowane na: Chemistry HL Paper 2, specimen 2025, pytanie 6(c). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
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Zadanie
Wspólne informacje do wiązki zadań
Halogens and halogenoalkanes are widely used as reactants in the laboratory and in industry.
Polecenie
(i) Draw the structural formula of 2-chloropentane and that of the structural isomer in which the chlorine atom is attached to a tertiary carbon atom.
(ii) Only one of the two isomers drawn in (c)(i) is chiral. Draw stereochemical formulas showing its two enantiomers.
(iii) The IR spectrum and the NMR spectrum of 2-chloropentane are shown below. Using sections 20 and 21 of the data booklet, identify in each spectrum the feature that shows the presence of the chlorine atom, quoting its approximate wavenumber or chemical shift.
Powiększ
Powiększ(iv) For each of the two techniques, explain whether its spectrum would allow the tertiary isomer to be told apart from 2-chloropentane.
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Odpowiedź z klucza
(i) CH3CHClCH2CH2CH3 «2-chloropentane» drawn as a full or condensed structural formula ✔
(CH3)2CClCH2CH3 «2-chloro-2-methylbutane» ✔
(ii) two tetrahedral representations of 2-chloropentane that are mirror images of each other, with H, Cl, CH3 and CH2CH2CH3 around the chiral carbon shown with wedge and dashed bonds ✔
(iii) IR: the strong absorption in the region 600–800 cm−1 «near 690 cm−1, C–Cl stretch» ✔
NMR: the signal near 4.0 ppm «3.5–4.4 ppm, the H on the carbon bonded to Cl» ✔
(iv) IR: no AND both isomers contain the same bonds «C–H, C–C and C–Cl» ✔
NMR: yes AND the tertiary isomer gives three signals «2-chloropentane gives five» / has no signal near 4 ppm «no H on the carbon bonded to Cl» ✔
Schemat punktowania
(i) [2]: one mark per correct structure. (ii) [1]: both enantiomers as mirror images with a correct three-dimensional representation; the same configuration drawn twice scores zero. (iii) [2]: M1 IR region 600–800 cm⁻¹ (accept the band near 690 cm⁻¹); M2 NMR signal at about 4 ppm (accept 3.5–4.4 ppm). (iv) [2]: M1 IR no with reason (accept yes with reference to differences in the fingerprint region below 1500 cm⁻¹); M2 NMR yes with a reason based on the number of signals or the absence of the signal near 4 ppm.
Komentarz. The chiral centre is C2 of 2-chloropentane (four different groups); 2-chloro-2-methylbutane has no hydrogen on the carbon bearing chlorine, so its spectrum has nothing between 2 and 4 ppm.