Zadanie 8c, Paper 2, maj 2025, TZ1, Chemia IB HL
Wzorowane na: Chemistry HL Paper 2, May 2025, TZ1, pytanie 8(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
Many everyday materials, flavourings and fuels are organic compounds.
Polecenie
But-2-ene and other alkenes serve as starting materials for a wide range of compounds.
(i) Predict the product formed when but-2-ene reacts with bromine.
(ii) Describe how this reaction proceeds at the mechanistic level, drawing curly arrows for each electron-pair movement.
(iii) Outline why but-2-ene, unlike butane, readily undergoes this type of reaction.
(iv) State the general molecular formula shared by every member of the alkene homologous series.
(v) Alkenes can be converted into alkanes by the addition of hydrogen. Explain the trend in the boiling points of the first four straight-chain alkanes, shown in the table, in terms of the intermolecular forces present.
| Alkane | Boiling point / K |
|---|---|
| methane | 112 |
| ethane | 185 |
| propane | 231 |
| butane | 273 |
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Pokaż rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) 2,3-dibromobutane / CH3CHBrCHBrCH3 ✔
(ii) curly arrow from the C=C bond to the nearer Br atom of Br2 AND curly arrow from the Br-Br bond to the far Br atom «heterolytic fission of the polarized Brδ+−Brδ−» ✔
carbocation CH3−CHBr−C+H−CH3 with the positive charge on the carbon next to the carbon bonded to Br «and Br− shown» ✔
curly arrow from a lone pair / the negative charge on Br− to the C+ ✔
(iii) the C=C double bond has a high electron density «π electrons» which attracts electrophiles «and polarizes Br2» / the π bond can break so that two new bonds form ✔
(iv) CnH2n ✔
(v) London (dispersion) forces «only» act between the alkane molecules ✔
these forces become stronger as the chain length / number of electrons / size of the electron cloud / molar mass increases, so more energy is needed to separate the molecules ✔
Schemat punktowania
(i) [1] for the name or the structure. (ii) [1] per marking point; the arrows must start from the π bond, from the Br-Br bond and from the bromide lone pair respectively. (iii) [1]; accept «susceptible to electrophilic attack». (iv) [1]; subscripts not essential. (v) M1: London (dispersion) forces, not «van der Waals forces» alone; M2: strength increasing with chain length, number of electrons or surface area.