A-LevelChemistryHalogen derivatives9701/42 · May/June 2017 · Q218 Marks

Complete the table to show how both AgNO3(aq) and NH3(aq) could be used to distinguish between solutions of NaCl(aq) and Nal(aq). test performed observation with NaCl observation with NaI

(a)Complete the table to show how both AgNO3(aq) and NH3(aq) could be used to distinguish between solutions of NaCl(aq) and Nal(aq). test performed observation with NaCl observation with NaI[2]
(b)(i)Suggest how the electrical conductivity will change as the reaction proceeds. Explain your answer.[1]
(b)(ii)Describe a suitable method for studying the rate of this reaction at a temperature of 40 °C, using the following. : an electrical conductance meter which measures the electrical conductivity of solutions solutions of known concentrations of 2-chlorobutane in propanone and sodium iodide in propanone stopclock access to standard laboratory equipment[3]
(c)(i)Deduce the order of reaction with respect to each of [CH3CH2CHCICH3] and [I-]. Explain your reasoning. order with respect to [CH3CH2CHCICH3] order with respect to [I-][2]
(c)(ii)Write the rate equation for this reaction, stating the units of the rate constant, k. rate = ... mol dm-3 s-1 units of k = ...[1]
(c)(iii)Calculate the relative rate for experiment 4. relative rate for experiment 4 = ...[1]
(d)(i)Suggest the mechanism for the reaction of 2-chlorobutane with iodide ions. Draw out the steps involved, including the following. • all relevant lone pairs and dipoles • curly arrows to show the movement of electron pairs • the structure of any transition state or intermediate[3]
(d)(ii)This reaction was carried out using a single optical isomer of 2-chlorobutane. Use your mechanism in (i) to predict whether the product will be a single optical isomer or a mixture of two optical isomers. Explain your answer.[1]
(e)(i)State the number of peaks that would be seen in the carbon-13 NMR spectrum of CH3CH2CHCICH3.[1]
(e)(ii)There are two isomers of CH3CH2CHCICH₃ that have fewer peaks in their carbon-13 NMR spectra than CH3CH2CHCICH3. Draw the structures of the isomers and state the number of peaks for each isomer. isomer 1 number of peaks = isomer 2 number of peaks =[3]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

[1] Add AgNO3 Cl- gives a white ppt and I- gives a yellow ppt.
[1] Add NH3(aq); ppt dissolves and ppt is insoluble
[1] conductivity decreases during the reaction, AND number of Na⁺/ I⁻/ ions are decreased / used up (from solution)
[3] (Equilibrate) solutions at 40 °C / with a water bath (cannot be after mixing) mix known volumes and start the clock / timing clearly mentioned/implied measure conductance / conductivity at regular intervals / every measured time [method A] OR measure the time for conductance to go to zero / a specific value / to be constant [method B] prepare a curve of conductance vs. time [related to method A] OR prepare a curve of conductance vs. concentration [related to method A] OR repeating the experiment at different concentrations [related to method A and B] any 3 points
[1] [R-Cl]: rate increases by 5/3 when concentration increases by 10/6 (5/3), so order = 1
[1] [I⁻]: rate increases by 5/3 when concentration increases by 5/3, so order = 1
[1] rate = k[I⁻][CH3CH2CHCICH3] AND units of k = dm³ mol⁻¹ s⁻¹
[1] relative rate = 5 / 5.3
[1] C-Cl dipole AND C-Cl curly arrow
[1] intermediate cation OR 5-valent transition state (charge essential)
[1] I⁻ with lone pair AND other curly arrow
[1] If SN1 in 2(d)(i) mixture of / two optical isomers will be formed, AND the intermediate can be formed by the I⁻ approaching from top or bottom plane
[1] If SN2 in 2(d)(i) one optical isomer AND attack always from fixed direction / opposite side
[1] 4 peaks
[1+1] CH3-CH(Cl)-CH3-CH3 (2-chlorobutane isomer 1), number of peaks = 2; CH3-C(Cl)(CH3)-CH3 (2-chloro-2-methylpropane, not 2-chlorobutane isomer) is not correct. Re-reading: 'There are two isomers of CH3CH2CHCICH3 that have fewer peaks in their carbon-13 NMR spectra than CH3CH2CHCICH3.' This means isomers of 2-chlorobutane, not CH3CH2CHCICH3. Original 2-chlorobutane is CH3-CH2-CH(Cl)-CH3. Two isomers with fewer peaks refers to two constitutional isomers of 2-chlorobutane. No, it says 'two isomers of CH3CH2CHCICH3 that have fewer peaks in their carbon-13 NMR spectra than CH3CH2CHCICH3'. This implies comparing other isomers with the given structure. The mark scheme gives CH3-C(Cl)(CH3)-CH3 and CH3-CH2-CH2-CH2-Cl. Ah, the QP formula is CH3CH2CHCICH3, which is 2-chlorobutane. So isomers *of* CH3CH2CHCICH3 means C4H9Cl. The MS gives 2-chloro-2-methylpropane (2-chloro-2-methylpropane is 2-chloro-2-methylpropane or t-butyl chloride) and 1-chlorobutane. The question clearly refers to 'isomers of CH3CH2CHCICH3'. So it means C4H9Cl isomers. The MS shows: Isomer 1: CH3C(Cl)(CH3)CH3 (2-chloro-2-methylpropane, number of peaks = 2). Isomer 2: CH3CH2CH2CH2Cl (1-chlorobutane, number of peaks = 3).
[1] number of peaks = 2; number of peaks = 3