Zadanie 16a
Wzorowane na: Chemistry HL Paper 3, November 2024, pytanie 16(a). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
Darmowe konto pozwoli wrócić do niego później.
Polecenie
Ethyl 4-hydroxybenzoate is a preservative used in some oral medicines. It was synthesized by heating 0.745 g of 4-hydroxybenzoic acid with excess ethanol and a few drops of concentrated sulfuric acid as catalyst.
C7H6O3 + C2H6O ⇌ C9H10O3 + H2O
Mr (4-hydroxybenzoic acid) = 138.13. Mr (ethyl 4-hydroxybenzoate) = 166.19.
(i) Calculate the theoretical yield of ethyl 4-hydroxybenzoate, in g.
(ii) Water was added to the cooled mixture and 0.99 g of solid product was isolated. Suggest, giving a reason, the identity of one possible impurity in the solid.
(iii) Outline how the solid could be purified by recrystallization.
(iv) Deduce one absorption in the infrared spectra of both 4-hydroxybenzoic acid and ethyl 4-hydroxybenzoate, and one in the infrared spectrum of 4-hydroxybenzoic acid only. Use section 20 of the data booklet.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) «n = 0.745/138.13 = 5.394 × 10−3 mol»
«5.394 × 10−3 × 166.19 =» 0.896 «g» ✔
(ii) water AND not completely dried
OR
ethanol AND not all evaporated/removed
OR
4-hydroxybenzoic acid AND the reaction did not go to completion/it is a reversible reaction ✔ «the isolated mass exceeds the theoretical yield, so the solid cannot be pure»
(iii) any three of:
dissolve the solid in the minimum volume of hot solvent ✔
filter the hot solution to remove insoluble impurities ✔
cool the solution so that crystals form «soluble impurities stay in solution» ✔
filter off the crystals ✔
wash the crystals with a little cold solvent ✔
dry the crystals ✔
(iv) in both: 1050–1410 «C–O» OR 1700–1750 «C=O» OR 2850–3090 «C–H» OR 3200–3600 «O–H of the phenol group» ✔
in the acid only: 2500–3000 «O–H of the carboxylic acid group» ✔
Schemat punktowania
(i) Accept 0.90. (ii) The reason must match the impurity named. (iii) Steps must belong to recrystallization; do not award 'filter' twice unless the two functions (removing insoluble impurities, collecting the crystals) are clear. (iv) Accept the bond or the wavenumber range, but a range alone must be one of the listed ones. The ester still contains the phenolic O–H, so a band at 3200–3600 is common to both and cannot be given as 'acid only'.
Komentarz
The isolated mass (0.99 g) is above the theoretical yield (0.896 g), which is the clue for (ii).