Zadanie 3b
Wzorowane na: Chemistry HL Paper 2, May 2026, TZ1, pytanie 3(b). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
Darmowe konto pozwoli wrócić do niego później.
Informacja do zadań 3a-3d.
Pentane, CH3CH2CH2CH2CH3, is a common fuel.
Polecenie
Pentane reacts with bromine, Br2, to form 2-bromopentane.
(i) Explain, using equations, the initiation step and two propagation steps for this reaction.
(ii) 2-Bromopentane reacts with aqueous sodium hydroxide, NaOH(aq), to form pentan-2-ol, CH3CH(OH)CH2CH2CH3. Suggest the mechanism for this reaction, giving your reason, based on the structure of 2-bromopentane.
(iii) The product was analysed by NMR. Predict the number and the relative areas of the signals for pentan-2-ol.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz CKE i punktacja
Odpowiedź z klucza
(i) Initiation: Br2 → 2 Br ∙ ✔ «UV light or heat may be stated but is not required»
Propagation 1: Br ∙ + CH3CH2CH2CH2CH3 → CH3CH ∙ CH2CH2CH3 + HBr ✔
Propagation 2: CH3CH ∙ CH2CH2CH3 + Br2 → CH3CHBrCH2CH2CH3 + Br ∙ ✔
(ii) Mechanism: both SN1 AND SN2 «a mixture of the two» ✔
Reason: 2-bromopentane is a secondary halogenoalkane «the carbon bonded to Br carries two alkyl groups» ✔
(iii) Number of signals: 6 ✔ «CH3 at C1, CH, OH, CH2 at C3, CH2 at C4, CH3 at C5»
Relative areas: 3 : 1 : 1 : 2 : 2 : 3 «in any order» ✔
Schemat punktowania
(i) [3] M1: initiation, homolytic fission of Br2 into two bromine radicals; M2 and M3: the two propagation steps, balanced, each carrying a radical, and leading to 2-bromopentane (the radical is on C2). A radical electron is required in each radical species; penalize a missing radical electron once only. Penalize once only the use of an unspecified radical such as ·C5H11 or of an incorrect hydrocarbon. (ii) [2] M1: both SN1 and SN2 (a mixture); M2: secondary halogenoalkane, tied to the structure of 2-bromopentane. (iii) [2] M1: 6 signals; M2: relative areas 3 : 1 : 1 : 2 : 2 : 3, accepted in any order. Award [1 max] for 5 signals AND 3 : 1 : 2 : 2 : 3 (OH ignored). Award [1] if the number of signals and the relative areas are each correct but are given the wrong way round.
Komentarz
Name the mechanism(s) and tie the reason to the substrate: a primary halogenoalkane would be SN2 and a tertiary one SN1.