Question 22N.2.hl.TZ0.6
| Date | November 2022 | Marks available | [Maximum mark: 13] | Reference code | 22N.2.hl.TZ0.6 |
| Level | hl | Paper | 2 | Time zone | TZ0 |
| Command term | Deduce, Identify, Justify, Sketch, State, Suggest | Question number | 6 | Adapted from | N/A |
2-Bromobutane can react with cyanide, CN−, in a nucleophilic substitution reaction.
This reaction could proceed through either SN1 or SN2 mechanisms depending on the reaction conditions. Sketch a graph of the rate versus nucleophile concentration, [CN−], for each of the mechanisms.
[2]
SN1 mechanism:
✔
SN2 mechanism:
✔
The average mark was <50% which probably arose from many candidates having difficulty in translating their rate equation knowledge of SN1 and SN2 reactions into a graphical context.
Suggest, with a reason, whether the reaction follows an SN1 or SN2 mechanism if only one stereoisomer was obtained as a product.
[1]
SN2 AND SN2 «mechanism» occurs with inversion of configuration
OR
SN2 AND SN1 «mechanism» would create a racemic mixture ✔
Accept appropriate diagrams.
Again the average mark was <50%, which resulted, probably equally, from students being unaware of the stereochemical implications of the SN1 and SN2 mechanisms and from an inability to express this understanding clearly.
State an instrument that could be used to determine whether the product was a single enantiomer or a racemic mixture.
[1]
polarimeter ✔
Well under half the cohort seemed aware that a polarimeter is the instrument used to detect optical activity and this seemed largely unrelated to performance on the rest of the paper.
SN1 and SN2 reactions are better conducted using different types of solvents. Identify two properties of a solvent most suited for the mechanism proposed in (b).
[1]
aprotic AND polar ✔
Only about half the students were aware of the protic and polar nature of preferred solvents correlating to the mechanism identified in 6(b).
State, with a reason, how the rate of reaction of cyanide with 2-chlorobutane differs from its rate of reaction with 2-bromobutane under the same conditions.
[1]
slower AND C-Cl bond is stronger «than C-Br»
OR
slower AND Br/Br− is a better leaving group ✔
About half the candidates were aware that it is the strength of the carbon-halogen bond, rather than its polarity, which determines the relative rate of the nucleophilic substitution reactions of different halogenoalkanes.
2-Bromobutane reacts with hydroxide via the same mechanism identified in (b). Explain this mechanism using curly arrows to represent the movement of electron pairs.
[3]
arrow from – charge/lone pair to carbon attached to Br ✔
arrow from C-Br bond to Br ✔
transition state representing the partially formed and partially broken bonds ✔
If SN1 was selected in 6 (b):
arrow from C-Br bond to Br ✔
carbocation intermediate ✔
arrow from – charge/lone pair to carbocation ✔
Candidates gained on average just under 2 of the 3 marks for this rather routine mechanism question and the mark achieved seemed to correlate strongly to performance on the rest of the paper. Students need to check the mechanism drawn corresponds to the one previously identified and to give greater care to the start and end points of curly arrows.
Deduce the number of signals and the ratio of areas under the signals in the 1H NMR spectrum of 2-bromobutane.
[2]
Number of signals:
4 ✔
Ratio of areas:
3:1:2:3 ✔
Accept ratio of areas in any order.
Usually if candidates could deduce the number of peaks, as about three quarters could, they also gained the second mark for their relative areas.
Identify the splitting pattern of the signal of the hydrogen atoms on the circled carbon atoms in 2-bromobutane.
[2]
Splitting pattern of the signal of the hydrogen atoms in circle A:
doublet ✔
Splitting pattern of the signal of the hydrogen atoms in circle B:
triplet ✔
About two thirds of the cohort could deduce the multiplicity of peaks and, if they identified it for one, they usually also identified it for the second.