| Marks available | 4 |
| Reference code | 15M.2.sl.TZ2.4 |
Question
State two features of a homologous series.
Ethane, a member of the homologous series of alkanes, can react with bromine.
Explain the free-radical mechanism of this reaction, including any necessary reaction conditions.
Markscheme
same functional group;
same general formula;
(successive members) differ by CH2;
similar chemical properties;
gradation in physical properties;
Do not accept “same” instead of “similar”, or vice versa.
Initiation:
\({\text{B}}{{\text{R}}_2}\xrightarrow{{{\text{UV/hf/hv}}}}2{\text{Br}} \bullet \);
Reference to UV light or high temperature must be included.
Propagation:
\({\text{Br}} \bullet + {{\text{C}}_2}{{\text{H}}_6} \to {{\text{C}}_2}{{\text{H}}_5} \bullet + {\text{HBr}}\);
\({{\text{C}}_2}{{\text{H}}_5} \bullet + {\text{B}}{{\text{r}}_2} \to {{\text{C}}_2}{{\text{H}}_5}{\text{Br}} + {\text{Br}} \bullet \);
Termination:
\({\text{Br}} \bullet + {\text{Br}} \bullet \to {\text{B}}{{\text{r}}_2}/{{\text{C}}_2}{{\text{H}}_5} \bullet + {\text{Br}} \bullet \to {{\text{C}}_2}{{\text{H}}_5}{\text{Br}}/{{\text{C}}_2}{{\text{H}}_5} \bullet + {{\text{C}}_2}{{\text{H}}_5} \bullet \to {{\text{C}}_4}{{\text{H}}_{10}}\);
Accept representation of radical without • (eg, Br, C2H5) if consistent throughout mechanism.
Penalize reference to heterolytic fission once only.
Award [0] to any mechanism involving ions.
Accept further bromination.
Award [3 max] if initiation, propagation and termination are not stated or are incorrectly labelled for equations.
Accept correct description of processes without equations.
Examiners report
This question tended to be well answered. The most common errors were that some candidates stated that homologous series have the same empirical formula, and that the difference between “the same” and “similar” were confused. The knowledge of the free radical substitution was very good, with the three processes of initiation, propagation and termination quoted often, although some only gave the first and last, the equations for propagation were most likely to be incorrect.
This question tended to be well answered. The most common errors were that some candidates stated that homologous series have the same empirical formula, and that the difference between “the same” and “similar” were confused. The knowledge of the free radical substitution was very good, with the three processes of initiation, propagation and termination quoted often, although some only gave the first and last, the equations for propagation were most likely to be incorrect.
| Marks available | 1 |
| Reference code | 15M.2.sl.TZ2.7 |
Question
Some reactions of but-2-ene are given below.
Deduce the full structural formula of compound A.
Apply IUPAC rules to name compound A.
Describe the colour change observed when excess but-2-ene reacts with bromine to form compound A.
State the names of the reagents D and E.
(i) Outline two reasons why the polymerization of alkenes is of economic importance.
(ii) Identify the structure of the repeating unit of poly(but-2-ene).
Compound C, \({{\text{C}}_{\text{4}}}{{\text{H}}_{\text{9}}}{\text{OH}}\), can also be formed directly from compound B, \({\text{C}}{{\text{H}}_{\text{3}}}{\text{CHBrC}}{{\text{H}}_{\text{2}}}{\text{C}}{{\text{H}}_{\text{3}}}\).
(i) State the reagent and the conditions required for this reaction.
(ii) State the name of the type of reaction occurring in this conversion.
Compound C can be oxidized by acidified potassium dichromate(VI) to form compound F.
(i) State the name of the functional group present in compound F.
(ii) Deduce the structural formula of an alcohol which is a structural isomer of compound C and cannot be oxidized by acidified potassium dichromate(VI).
Explain why but-2-ene is more volatile than compound C, \({{\text{C}}_{\text{4}}}{{\text{H}}_{\text{9}}}{\text{OH}}\).
Define the term average bond enthalpy.
Deduce the equation for the complete combustion of compound C.
Determine the enthalpy change, \(\Delta H\), in \({\text{kJ}}\,{\text{mo}}{{\text{l}}^{ – 1}}\), for the complete combustion of compound C when all reactants and products are in the gaseous state, using table 10 of the data booklet.
Markscheme
;
Accept bromine atoms cis to each other.
2,3-dibromobutane;
Do not penalize the incorrect use of spaces, comma or hyphen.
red/brown/orange/yellow to colourless/decolourized;
Do not accept clear.
Do not accept just “decolourized”.
water;
sulfuric acid / phosphoric acid;
Accept formulas instead of names.
(i) (synthesis of) plastics/polymers/organic materials not naturally available / synthetic materials;
wide range of uses/physical properties / versatile;
large industry / many tons of plastics consumed by society / OWTTE;
Do not accept “useful” for M2.
Award [1 max] if specific addition polymer and its use is given.
Penalize reference to condensation polymers once only.
(ii) 
;
Ignore n.
Brackets are not required for the mark, but continuation bonds are.
Do not penalize if methyl groups are trans to each other.
(i) aqueous sodium hydroxide/NaOH/potassium hydroxide/KOH and warm/heat/reflux;
(ii) (nucleophilic) substitution;
Accept (nucleophilic) displacement.
(i) carbonyl;
Accept ketone.
(ii) 
;
Accept condensed or full structural formula.
hydrogen bonding in compound C;
dipole-dipole forces in C / C is more polar;
C has greater molar mass/more dispersion/London/instantaneous induced dipole-induced dipole forces/van der Waal forces;
Accept converse argument.
Award [1 max] for stronger intermolecular forces.
energy required to break (1 mol of) a (covalent) bond in a gaseous molecule/state;
Accept energy released when (1 mol of) a (covalent) bond is formed in a gaseous molecule/state / energy change when (1 mol of) bonds are formed or broken in the gaseous molecule/state.
average value in similar compounds / OWTTE;
\({{\text{C}}_4}{{\text{H}}_9}{\text{OH(l)}} + {\text{6}}{{\text{O}}_2}{\text{(g)}} \to {\text{4C}}{{\text{O}}_{\text{2}}}{\text{(g)}} + {\text{5}}{{\text{H}}_{\text{2}}}{\text{O(l)}}\);
Ignore state symbols.
Bonds broken:
3C–C + 9C–H + 1C–O + 1O–H + 6O=O /
\(3 \times 347 + 9 \times 413 + 1 \times 358 + 1 \times 464 + 6 \times 498/8568{\text{ (kJ)}}\);
Bonds formed:
8C=O + 10O–H / \(8 \times 746 + 10 \times 464/10608{\text{ (kJ)}}\);
\(\Delta H = (8568 – 10608) = – 2040{\text{ (kJ}}\,{\text{mo}}{{\text{l}}^{ – 1}}{\text{)}}\);
Award [3] for correct final answer.
Award [2] for +2040 (kJ mol–1).
Examiners report
The few who opted for this option, showed a good knowledge. The drawing of structural formulae and naming was good. The reagent and conditions for the reaction was less well recalled. In 7ci, most students scored at least one mark, but lost the second. There was a lack of awareness of the importance of the system being aqueous in the conversion to the alcohol and a fully correct answer was very rare, as was the identification of the functional group. In the volatility question, most were aware of hydrogen bonding, but the fact that C also has greater other forces due to its greater mass was not present in most answers. The gaseous mark was often present, but the averaging over a range of compounds was not. With the calculation of enthalpy quite a few candidates benefitted from transferred error, from an incorrect equation.
The few who opted for this option, showed a good knowledge. The drawing of structural formulae and naming was good. The reagent and conditions for the reaction was less well recalled. In 7ci, most students scored at least one mark, but lost the second. There was a lack of awareness of the importance of the system being aqueous in the conversion to the alcohol and a fully correct answer was very rare, as was the identification of the functional group. In the volatility question, most were aware of hydrogen bonding, but the fact that C also has greater other forces due to its greater mass was not present in most answers. The gaseous mark was often present, but the averaging over a range of compounds was not. With the calculation of enthalpy quite a few candidates benefitted from transferred error, from an incorrect equation.
The few who opted for this option, showed a good knowledge. The drawing of structural formulae and naming was good. The reagent and conditions for the reaction was less well recalled. In 7ci, most students scored at least one mark, but lost the second. There was a lack of awareness of the importance of the system being aqueous in the conversion to the alcohol and a fully correct answer was very rare, as was the identification of the functional group. In the volatility question, most were aware of hydrogen bonding, but the fact that C also has greater other forces due to its greater mass was not present in most answers. The gaseous mark was often present, but the averaging over a range of compounds was not. With the calculation of enthalpy quite a few candidates benefitted from transferred error, from an incorrect equation.
The few who opted for this option, showed a good knowledge. The drawing of structural formulae and naming was good. The reagent and conditions for the reaction was less well recalled. In 7ci, most students scored at least one mark, but lost the second. There was a lack of awareness of the importance of the system being aqueous in the conversion to the alcohol and a fully correct answer was very rare, as was the identification of the functional group. In the volatility question, most were aware of hydrogen bonding, but the fact that C also has greater other forces due to its greater mass was not present in most answers. The gaseous mark was often present, but the averaging over a range of compounds was not. With the calculation of enthalpy quite a few candidates benefitted from transferred error, from an incorrect equation.
The few who opted for this option, showed a good knowledge. The drawing of structural formulae and naming was good. The reagent and conditions for the reaction was less well recalled. In 7ci, most students scored at least one mark, but lost the second. There was a lack of awareness of the importance of the system being aqueous in the conversion to the alcohol and a fully correct answer was very rare, as was the identification of the functional group. In the volatility question, most were aware of hydrogen bonding, but the fact that C also has greater other forces due to its greater mass was not present in most answers. The gaseous mark was often present, but the averaging over a range of compounds was not. With the calculation of enthalpy quite a few candidates benefitted from transferred error, from an incorrect equation.
The few who opted for this option, showed a good knowledge. The drawing of structural formulae and naming was good. The reagent and conditions for the reaction was less well recalled. In 7ci, most students scored at least one mark, but lost the second. There was a lack of awareness of the importance of the system being aqueous in the conversion to the alcohol and a fully correct answer was very rare, as was the identification of the functional group. In the volatility question, most were aware of hydrogen bonding, but the fact that C also has greater other forces due to its greater mass was not present in most answers. The gaseous mark was often present, but the averaging over a range of compounds was not. With the calculation of enthalpy quite a few candidates benefitted from transferred error, from an incorrect equation.
The few who opted for this option, showed a good knowledge. The drawing of structural formulae and naming was good. The reagent and conditions for the reaction was less well recalled. In 7ci, most students scored at least one mark, but lost the second. There was a lack of awareness of the importance of the system being aqueous in the conversion to the alcohol and a fully correct answer was very rare, as was the identification of the functional group. In the volatility question, most were aware of hydrogen bonding, but the fact that C also has greater other forces due to its greater mass was not present in most answers. The gaseous mark was often present, but the averaging over a range of compounds was not. With the calculation of enthalpy quite a few candidates benefitted from transferred error, from an incorrect equation.
The few who opted for this option, showed a good knowledge. The drawing of structural formulae and naming was good. The reagent and conditions for the reaction was less well recalled. In 7ci, most students scored at least one mark, but lost the second. There was a lack of awareness of the importance of the system being aqueous in the conversion to the alcohol and a fully correct answer was very rare, as was the identification of the functional group. In the volatility question, most were aware of hydrogen bonding, but the fact that C also has greater other forces due to its greater mass was not present in most answers. The gaseous mark was often present, but the averaging over a range of compounds was not. With the calculation of enthalpy quite a few candidates benefitted from transferred error, from an incorrect equation.
The few who opted for this option, showed a good knowledge. The drawing of structural formulae and naming was good. The reagent and conditions for the reaction was less well recalled. In 7ci, most students scored at least one mark, but lost the second. There was a lack of awareness of the importance of the system being aqueous in the conversion to the alcohol and a fully correct answer was very rare, as was the identification of the functional group. In the volatility question, most were aware of hydrogen bonding, but the fact that C also has greater other forces due to its greater mass was not present in most answers. The gaseous mark was often present, but the averaging over a range of compounds was not. With the calculation of enthalpy quite a few candidates benefitted from transferred error, from an incorrect equation.
The few who opted for this option, showed a good knowledge. The drawing of structural formulae and naming was good. The reagent and conditions for the reaction was less well recalled. In 7ci, most students scored at least one mark, but lost the second. There was a lack of awareness of the importance of the system being aqueous in the conversion to the alcohol and a fully correct answer was very rare, as was the identification of the functional group. In the volatility question, most were aware of hydrogen bonding, but the fact that C also has greater other forces due to its greater mass was not present in most answers. The gaseous mark was often present, but the averaging over a range of compounds was not. With the calculation of enthalpy quite a few candidates benefitted from transferred error, from an incorrect equation.
The few who opted for this option, showed a good knowledge. The drawing of structural formulae and naming was good. The reagent and conditions for the reaction was less well recalled. In 7ci, most students scored at least one mark, but lost the second. There was a lack of awareness of the importance of the system being aqueous in the conversion to the alcohol and a fully correct answer was very rare, as was the identification of the functional group. In the volatility question, most were aware of hydrogen bonding, but the fact that C also has greater other forces due to its greater mass was not present in most answers. The gaseous mark was often present, but the averaging over a range of compounds was not. With the calculation of enthalpy quite a few candidates benefitted from transferred error, from an incorrect equation.
| Marks available | 3 |
| Reference code | 16M.2.sl.TZ0.4 |
Question
Alkenes are widely used in the production of polymers. The compound A, shown below, is used in the manufacture of synthetic rubber.
(i) State the name, applying IUPAC rules, of compound A.
(ii) Draw a section, showing three repeating units, of the polymer that can be formed from compound A.
(iii) Compound A is flammable. Formulate the equation for its complete combustion.
Compound B is related to compound A.
(i) State the term that is used to describe molecules that are related to each other in the same way as compound A and compound B.
(ii) Suggest a chemical test to distinguish between compound A and compound B, giving the observation you would expect for each.
Test:
Observation with A:
Observation with B:
(iii) Spectroscopic methods could also be used to distinguish between compounds A and B.
Predict one difference in the IR spectra and one difference in the 1H NMR spectra of these compounds, using sections 26 and 27 of the data booklet.
IR spectra:
1H NMR spectra:
A sample of compound A was prepared in which the 12C in the CH2 group was replaced by 13C.
(i) State the main difference between the mass spectrum of this sample and that of normal compound A.
(ii) State the structure of the nucleus and the orbital diagram of 13C in its ground state.
Draw a 1s atomic orbital and a 2p atomic orbital.
Markscheme
(i)
methylpropene
(ii)
−CH2−C(CH3)2−CH2−C(CH3)2−CH2−C(CH3)2−
Must have continuation bonds at both ends.
Accept any orientation of the monomers, which could give methyl side-chains on neighbouring atoms etc.
(iii)
C4H8 (g) + 6O2 (g) → 4CO2 (g) + 4H2O(l)
(i)
«structural/functional group» isomer«s»
(ii)
Test:
«react with» bromine/Br2 «in the dark»
OR
«react with» bromine water/Br2 (aq) «in the dark»
IR: A would absorb at 1620–1680cm−1 AND B would not
1H NMR: A would have 2 signals AND B would have 1 signal
OR
A would have a signal at 4.5–6.0 ppm AND B would not
OR
A would have a signal at 0.9–1.0 ppm AND B would not
OR
B would have a signal at 1.3–1.4 ppm AND A would not
Accept “peak” for “signal”.
Award [1 max] if students have a correct assignation of a signal, but no comparison, for both IR and NMR.
Accept “B would have a signal at 2.0 ppm” as shown in its 1H NMR spectrum.
(i)
«molecular ion» peak at «m/z =» 57, «not 56»
OR
«molecular ion» peak at one «m/z» higher
OR
will not have a «large» peak at 56
Accept a peak at m/z one greater than the 12C one for any likely fragment.
(ii)
protons: 6 AND neutrons: 7
Accept full arrows.
Accept p orbitals aligned on y− and z−axes, or diagrams correctly showing all three p-orbitals.
Do not accept p-orbitals without a node.
Examiners report
| Marks available | 1 |
| Reference code | 16N.2.sl.TZ0.1 |
Question
Ethane-1,2-diol, HOCH2CH2OH, has a wide variety of uses including the removal of ice from aircraft and heat transfer in a solar cell.
Ethane-1,2-diol can be formed according to the following reaction.
2CO (g) + 3H2 (g) \( \rightleftharpoons \) HOCH2CH2OH (g)
(i) Deduce the equilibrium constant expression, Kc, for this reaction.
(ii) State how increasing the pressure of the reaction mixture at constant temperature will affect the position of equilibrium and the value of Kc.
Position of equilibrium:
Kc:
(iii) Calculate the enthalpy change, ΔHθ, in kJ, for this reaction using section 11 of the data booklet. The bond enthalpy of the carbon–oxygen bond in CO (g) is 1077kJmol-1.
(iv) The enthalpy change, ΔHθ, for the following similar reaction is –233.8 kJ.
2CO(g) + 3H2(g) \( \rightleftharpoons \) HOCH2CH2OH (l)
Deduce why this value differs from your answer to (a)(iii).
Determine the average oxidation state of carbon in ethene and in ethane-1,2-diol.
Ethene:
Ethane-1,2-diol:
Explain why the boiling point of ethane-1,2-diol is significantly greater than that of ethene.
Ethane-1,2-diol can be oxidized first to ethanedioic acid, (COOH)2, and then to carbon dioxide and water. Suggest the reagents to oxidize ethane-1,2-diol.
Markscheme
(i)
\(\ll {K_{\text{C}}} = \gg \frac{{\left[ {{\text{HOC}}{{\text{H}}_{\text{2}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{OH}}} \right]}}{{{{\left[ {{\text{CO}}} \right]}^{\text{2}}} \times {{\left[ {{{\text{H}}_{\text{2}}}} \right]}^{\text{3}}}}}\)
(ii)
Position of equilibrium: moves to right OR favours product
Kc: no change OR is a constant at constant temperature
(iii)
Bonds broken: 2C≡O + 3(H-H) / 2(1077kJmol-1) + 3(436kJmol-1) / 3462 «kJ»
Bonds formed: 2(C-O) + 2(O-H) + 4(C-H) + (C-C) / 2(358kJmol-1) + 2(463kJmol-1) + 4(414kJmol-1) + 346kJmol-1 / 3644 «kJ»
«Enthalpy change = bonds broken – bonds formed = 3462 kJ – 3644 kJ =» -182 «kJ»
Award [3] for correct final answer.
Award [2 max] for «+»182 «kJ».
(iv)
in (a)(iii) gas is formed and in (a)(iv) liquid is formed
OR
products are in different states
OR
conversion of gas to liquid is exothermic
OR
conversion of liquid to gas is endothermic
OR
enthalpy of vapourisation needs to be taken into account
Accept product is «now» a liquid.
Accept answers referring to bond enthalpies being means/averages.
Ethene: –2
Ethane-1,2-diol: –1
Do not accept 2–, 1– respectively.
ethane-1,2-diol can hydrogen bond to other molecules «and ethene cannot»
OR
ethane-1,2-diol has «significantly» greater van der Waals forces
Accept converse arguments.
Award [0] if answer implies covalent bonds are broken
hydrogen bonding is «significantly» stronger than other intermolecular forces
acidified «potassium» dichromate«(VI)»/H+ AND K2Cr2O7/H+ AND Cr2O72-
OR
«acidified potassium» manganate(VII)/ «H+» KMnO4 /«H+» MnO4–
Accept Accept H2SO4 or H3PO4 for H+.
Accept “permanganate” for “manganate(VII)”.
Examiners report
| Marks available | 4 |
| Reference code | 16N.2.sl.TZ0.5 |
Question
Propane and propene are members of different homologous series.
Draw the full structural formulas of propane and propene.
Both propane and propene react with bromine.
(i) State an equation and the condition required for the reaction of 1 mol of propane with 1 mol of bromine.
(ii) State an equation for the reaction of 1 mol of propene with 1 mol of bromine.
(iii) State the type of each reaction with bromine.
Propane:
Propene:
Markscheme
Propane:
AND
Propene:
i
C3H8 + Br2 → C3H7Br + HBr
«sun»light/UV/hv
OR
high temperature
Do not accept “reflux” for M2.
ii
C3H6 + Br2 → C3H6Br2
iii
Propane: «free radical» substitution / SR
AND
Propene: «electrophilic» addition / AE
Award mark even if incorrect type of substitution/ addition given.
Examiners report
| Marks available | 1 |
| Reference code | 17M.2.sl.TZ1.5 |
Question
This question is about carbon and chlorine compounds.
Ethane, C2H6, reacts with chlorine in sunlight. State the type of this reaction and the name of the mechanism by which it occurs.
Formulate equations for the two propagation steps and one termination step in the formation of chloroethane from ethane.
One possible product, X, of the reaction of ethane with chlorine has the following composition by mass:
carbon: 24.27%, hydrogen: 4.08%, chlorine: 71.65%
Determine the empirical formula of the product.
The mass and 1H\(\,\)NMR spectra of product X are shown below. Deduce, giving your reasons, its structural formula and hence the name of the compound.
Chloroethene, C2H3Cl, can undergo polymerization. Draw a section of the polymer with three repeating units.
Markscheme
substitution AND «free-»radical
OR
substitution AND chain
Award [1] for “«free-»radical substitution” or “SR” written anywhere in the answer.
[1 mark]
Two propagation steps:
C2H6 + •Cl → C2H5• + HCl
C2H5• + Cl2 → C2H5Cl + •Cl
One termination step:
C2H5• + C2H5• → C4H10
OR
C2H5• + •Cl → C2H5Cl
OR
•Cl + •Cl → Cl2
Accept radical without • if consistent throughout.
Allow ECF from incorrect radicals produced in propagation step for M3.
[3 marks]
\({\text{C}} = \frac{{24.27}}{{12.01}}\) = 2.021 AND \({\text{H}} = \frac{{4.08}}{{1.01}}\) = 4.04 AND \({\text{Cl}} = \frac{{71.65}}{{35.45}} = 2.021\)
«hence» CH2Cl
Accept \(\frac{{24.27}}{{12.01}}\) : \(\frac{{4.08}}{{1.01}}\) : \(\frac{{71.65}}{{35.45}}.\)
Do not accept C2H4Cl2.
Award [2] for correct final answer.
[2 marks]
molecular ion peak(s) «about» m/z 100 AND «so» C2H4Cl2 «isotopes of Cl»
two signals «in 1H\(\,\)NMR spectrum» AND «so» CH3CHCl2
OR
«signals in» 3:1 ratio «in 1H\(\,\)NMR spectrum» AND «so» CH3CHCl2
OR
one doublet and one quartet «in 1H\(\,\)NMR spectrum» AND «so» CH3CHCl2
1,1-dichloroethane
Accept “peaks” for “signals”.
Allow ECF for a correct name for M3 if an incorrect chlorohydrocarbon is identified
[3 marks]
Continuation bonds must be shown.
Ignore square brackets and “n”.
Accept 
.
Accept other versions of the polymer, such as head to head and head to tail.
Accept condensed structure provided all C to C bonds are shown (as single).
[1 mark]
Examiners report
| Marks available | 2 |
| Reference code | 17M.2.sl.TZ1.6 |
Question
Benzene is an aromatic hydrocarbon.
Discuss the physical evidence for the structure of benzene.
State the typical reactions that benzene and cyclohexene undergo with bromine.
Markscheme
Any two of:
planar «X-ray»
C to C bond lengths all equal
OR
C to C bonds intermediate in length between C–C and C=C
all C–C–C bond angles equal
Accept all C to C bonds have same bond strength/bond energy.
[2 marks]
benzene: «electrophilic» substitution/SE
AND
cyclohexene: «electrophilic» addition/AE
Accept correct equations.
[1 mark]
Examiners report
| Marks available | 3 |
| Reference code | 17M.2.sl.TZ2.6 |
Question
The photochemical chlorination of methane can occur at low temperature.
Using relevant equations, show the initiation and the propagation steps for this reaction.
Bromine was added to hexane, hex-1-ene and benzene. Identify the compound(s) which will react with bromine in a well-lit laboratory.
Polyvinyl chloride (PVC) is a polymer with the following structure.
State the structural formula for the monomer of PVC.
Markscheme
Initiation:
Cl–Cl → Cl• + Cl•
Propagation:
Cl• + CH4 → Cl–H + •CH3
Cl–Cl + •CH3 → Cl–CH3 + Cl•
Do not penalize missing electron dot on radicals if consistent throughout.
Accept Cl2, HCl and CH3Cl without showing bonds.
Do not accept hydrogen radical, H• or H, but apply ECF to other propagation steps.
[3 marks]
hexane AND hex-1-ene
Accept “benzene AND hexane AND hex-1-ene”.
[1 mark]
OR
Accept “CH2CHCl” or “CHClCH2”.
Do not accept “C2H3Cl”.
[1 mark]
Examiners report
| Marks available | 1 |
| Reference code | 17N.2.sl.TZ0.6 |
Question
The reactivity of organic compounds depends on the nature and positions of their functional groups.
The structural formulas of two organic compounds are shown below.
Deduce the type of chemical reaction and the reagents used to distinguish between these compounds.
State the observation expected for each reaction giving your reasons.
Deduce the number of signals and the ratio of areas under the signals in the 1H NMR spectra of the two compounds.
Explain, with the help of equations, the mechanism of the free-radical substitution reaction of ethane with bromine in presence of sunlight.
Markscheme
oxidation/redox AND acidified «potassium» dichromate(VI)
OR
oxidation/redox AND «acidified potassium» manganate(VII)
Accept “acidified «potassium» dichromate” OR “«acidified potassium» permanganate”.
Accept name or formula of the reagent(s).
ALTERNATIVE 1 using K2Cr2O7:
Compound A: orange to green AND secondary hydroxyl
OR
Compound A: orange to green AND hydroxyl oxidized «by chromium(VI) ions»
Compound B: no change AND tertiary hydroxyl «not oxidized by chromium(VI) ions»
Award [1] for “A: orange to green AND B: no change”.
Award [1] for “A: secondary hydroxyl AND B: tertiary hydroxyl”.
ALTERNATIVE 2 using KMnO4:
Compound A: purple to colourless AND secondary hydroxyl
OR
Compound A: purple to colourless AND hydroxyl oxidized «by manganese(VII) ions»
Compound B: no change AND tertiary hydroxyl «not oxidized by manganese(VII) ions»
Accept “alcohol” for “hydroxyl”.
Award [1] for “A: purple to colourless AND B: no change”
Award [1] for “A: secondary hydroxyl AND B: tertiary hydroxyl”.
Accept “purple to brown” for A.
Accept ratio of areas in any order.
Do not apply ECF for ratios.
Initiation:
Br2 
2Br•
Propagation:
Br• + C2H6 → C2H5• + HBr
C2H5• + Br2 → C2H5Br + Br•
Termination:
Br• + Br• → Br2
OR
C2H5• + Br• → C2H5Br
OR
C2H5• + C2H5• → C4H10
Reference to UV/hν/heat not required.
Accept representation of radical without • (eg, Br, C2H5) if consistent throughout mechanism.
Accept further bromination.
Award [3 max] if initiation, propagation and termination are not stated or are incorrectly labelled for equations.
Award [3 max] if methane is used instead of ethane, and/or chlorine is used instead of bromine.
Examiners report
| Marks available | 2 |
| Reference code | 18M.2.sl.TZ1.3 |
Question
This question is about ethene, C2H4, and ethyne, C2H2.
Ethyne, like ethene, undergoes hydrogenation to form ethane. State the conditions required.
Outline the formation of polyethene from ethene by drawing three repeating units of the polymer.
Under certain conditions, ethyne can be converted to benzene.
Determine the standard enthalpy change, ΔHϴ, for the reaction stated, using section 11 of the data booklet.
3C2H2(g) → C6H6(g)
Determine the standard enthalpy change, ΔHΘ, for the following similar reaction, using ΔHf values in section 12 of the data booklet.
3C2H2(g) → C6H6(l)
Explain, giving two reasons, the difference in the values for (b)(i) and (ii). If you did not obtain answers, use −475 kJ for (i) and −600 kJ for (ii).
One possible Lewis structure for benzene is shown.
State one piece of physical evidence that this structure is incorrect.
State the characteristic reaction mechanism of benzene.
Markscheme
nickel/Ni «catalyst»
high pressure
OR
heat
Accept these other catalysts: Pt, Pd, Ir, Rh, Co, Ti.
Accept “high temperature” or a stated temperature such as “150 °C”.
[2 marks]
Ignore square brackets and “n”.
Connecting line at end of carbons must be shown.
[1 mark]
ΔHϴ = bonds broken – bonds formed
«ΔHϴ = 3(C≡C) – 6(C=C)benzene/3 × 839 – 6 × 507 / 2517 – 3042 =»
–525 «kJ»
Award [2] for correct final answer.
Award [1 max] for +525 «kJ»
Award [1 max] for:
«ΔHϴ = 3(C≡C) – 3(C–C) – 3(C=C) / 3 × 839 – 3 × 346 – 3 × 614 / 2517 – 2880 =» –363 «kJ».
[2 marks]
ΔHΘ = ΣΔHf(products) – ΣΔHf(reactants)
«ΔHΘ = 49 kJ – 3 × 228 kJ =» –635 «kJ»
Award [2] for correct final answer.
Award [1 max] for “+635 «kJ»”.
[2 marks]
ΔHf values are specific to the compound
OR
bond enthalpy values are averages «from many different compounds»
condensation from gas to liquid is exothermic
Accept “benzene is in two different states «one liquid the other gas»“ for M2.
[2 marks]
equal C–C bond «lengths/strengths»
OR
regular hexagon
OR
«all» C–C have» bond order of 1.5
OR
«all» C–C intermediate between single and double bonds
Accept “all C–C–C bond angles are equal”.
[1 mark]
electrophilic substitution
OR
SE
[1 mark]
Examiners report
| Marks available | 2 |
| Reference code | 18M.2.sl.TZ2.7 |
Question
The structure of an organic molecule can help predict the type of reaction it can undergo.
Improvements in instrumentation have made identification of organic compounds routine.
The empirical formula of an unknown compound containing a phenyl group was found to be C4H4O. The molecular ion peak in its mass spectrum appears at m/z = 136.
The Kekulé structure of benzene suggests it should readily undergo addition reactions.
Discuss two pieces of evidence, one physical and one chemical, which suggest this is not the structure of benzene.
Formulate the ionic equation for the oxidation of propan-1-ol to the corresponding aldehyde by acidified dichromate(VI) ions. Use section 24 of the data booklet.
The aldehyde can be further oxidized to a carboxylic acid.
Outline how the experimental procedures differ for the synthesis of the aldehyde and the carboxylic acid.
Deduce the molecular formula of the compound.
Identify the bonds causing peaks A and B in the IR spectrum of the unknown compound using section 26 of the data booklet.
Deduce full structural formulas of two possible isomers of the unknown compound, both of which are esters.
Deduce the formula of the unknown compound based on its 1H NMR spectrum using section 27 of the data booklet.
Markscheme
Physical evidence:
equal C–C bond «lengths/strengths»
OR
regular hexagon
OR
«all» C–C have bond order of 1.5
OR
«all» C–C intermediate between single and double bonds
Chemical evidence:
undergoes substitution reaction «more readily than addition»
OR
does not discolour/react with bromine water
OR
substitution forms only one isomer for 1,2-disubstitution «presence of alternate double bonds would form two isomers»
OR
more stable than expected «compared to hypothetical molecule cyclohexa-1,3,5-triene»
OR
enthalpy change of hydrogenation/combustion is less exothermic than predicted «for cyclohexa-1,3,5-triene»
M1:
Accept “all C–C–C bond angles are equal”.
[2 marks]
3CH3CH2CH2OH(l) + Cr2O72–(aq) + 8H+(aq) → 3CH3CH2CHO(aq) + 2Cr3+(aq) + 7H2O(l)
correct reactants and products
balanced equation
[2 marks]
Aldehyde:
by distillation «removed from reaction mixture as soon as formed»
Carboxylic acid:
«heat mixture under» reflux «to achieve complete oxidation to –COOH»
Accept clear diagrams or descriptions of the processes.
[2 marks]
«\(\frac{{136}}{{48 + 4 + 16}} = 2\)»
C8H8O2
[1 mark]
A: C–H «in alkanes, alkenes, arenes»
AND
B: C=O «in aldehydes, ketones, carboxylic acids and esters»
[1 mark]
Any two of:
OR C6H5COOCH3
OR CH3COOC6H5
OR HCOOCH2C6H5
Do not penalize use of Kekule structures for the phenyl group.
Accept the following structures:
Award [1 max] for two correct aliphatic/linear esters with the molecular formula C8H8O2.
[2 marks]
C6H5COOCH3 «signal at 4 ppm (3.7 – 4.8 range in data table) due to alkyl group on ester
[1 mark]