Question 19M.2.sl.TZ2.c(v)
| Date | May 2019 | Marks available | [Maximum mark: 2] | Reference code | 19M.2.sl.TZ2.c(v) |
| Level | sl | Paper | 2 | Time zone | TZ2 |
| Command term | Deduce | Question number | c(v) | Adapted from | N/A |
Ethyne, C2H2, reacts with oxygen in welding torches.
Ethyne reacts with steam.
C2H2 (g) + H2O (g) → C2H4O (g)
Two possible products are:
The IR spectrum and low resolution 1H NMR spectrum of the actual product formed are shown.
Deduce whether the product is A or B, using evidence from these spectra together with sections 26 and 27 of the data booklet.
Identity of product:
One piece of evidence from IR:
One piece of evidence from 1H NMR:
[2]
Identity of product: «B»
IR spectrum:
1700–1750 «cm–1 band» AND carbonyl/CO group present
OR
no «band at» 1620–1680 «cm–1» AND absence of double bond/C=C
OR
no «broad band at» 3200–3600 «cm–1» AND absence of hydroxyl/OH group [✔]
Note: Accept a specific value or range of wavenumbers and chemical shifts.
1H NMR spectrum:
«only» two signals AND A would have three
OR
«signal at» 9.4–10.0 «ppm» AND «H atom/proton of» aldehyde/–CHO present
OR
«signal at» 2.2–2.7 «ppm» AND «H atom/proton of alkyl/CH next to» aldehyde/CHO present
OR
«signal at» 2.2–2.7 «ppm» AND «H atom/proton of» RCOCH2- present
OR
no «signal at» 4.5–6.0 «ppm» AND absence of «H-atom/proton next to» double bond/C=C [✔]
Note: Accept “two signals with areas 1:3”.
The majority of candidates handled this question competently and nearly half of the candidates obtained both marks. They obtained the value of the absorption from the spectra provided and compared it to the values in the data booklet to deduce the identity of the product. Common mistakes included not identifying the peaks and signals precisely (for example C=O instead of CHO for 1H NMR signal at 9.4-10.0 ppm). Some teachers commented that the TMS signal should not have been included as the SL do not know about it. Other teachers commented that using the 'actual' rather than an ‘idealized’ IR spectrum may have caused confusion due to the peak at around 3400 cm-1 which could be confused for O-H in alcohols. Thankfully both of these answers were hardly seen in the scripts. The peak at 3400 cm-1 was not at all broad and did not confuse the majority of students. Please note that real spectra are usually used in examination papers, and it is worth encouraging students to check more than one peak to confirm their deductions.
