Zadanie 6c
Wzorowane na: Chemistry HL Paper 2, May 2023, TZ2, pytanie 6(c). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
Darmowe konto pozwoli wrócić do niego później.
Polecenie
Propanoic acid reacts with methanol to produce an ester.
(i) Draw the full structural formula of the organic product.
(ii) Predict the number of signals, and their splitting patterns, in the 1H NMR spectrum of this ester.
(iii) The chemical shifts in section 21 of the data booklet are quoted relative to an internal reference standard. State two properties that make a compound suitable for this purpose.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) full structural formula of the ester CH3CH2COOCH3 with every atom and bond shown: ethyl group bonded to the carbonyl carbon, C=O double bond, the carbonyl carbon bonded to the oxygen atom, and the oxygen atom bonded to the methyl group «all C–H bonds drawn» ✔
(ii) number of signals: 3 ✔
splitting patterns: singlet AND triplet AND quartet ✔
(iii) Any two of:
all the hydrogen atoms are in the same environment, so it gives one strong, sharp signal «a singlet» ✔
signal at a chemical shift outside the range of common signals «so it does not overlap with them» ✔
volatile / easily removed from the sample ✔
inert / does not react with the sample ✔
dissolves in the organic solvents used for NMR samples ✔
Schemat punktowania
(i) [1]: all atoms and bonds must be shown, with the ester linkage the right way round (carbonyl carbon bonded to the ethyl group, oxygen bonded to the methyl group); do not accept a condensed or skeletal formula alone, because the question asks for the full structural formula; ignore bond angles; do not accept the isomer CH3COOCH2CH3. (ii) M1: three signals; M2: all three patterns, one singlet, one triplet and one quartet, in any order; ECF from an incorrect ester drawn in (i). (iii) Any two for [2]. Do not accept "its chemical shift is zero" on its own.
Komentarz
The guide does not name TMS; the data booklet quotes shifts relative to it, so the question asks about the properties of a reference standard in general, which a student can reason out without knowing the compound.