Question
Steroids are lipids with a steroidal backbone. The structure of cholesterol is shown in section 34 of the data booklet.
a. Infrared (IR) spectroscopy is used to identify functional groups in organic compounds.
Deduce the wavenumber, in $\mathrm{cm}^{-1}$, of an absorption peak found in the IR spectrum of testosterone but not in that of cholesterol.
b. Describe a technique for the detection of steroids in blood and urine.
c. Explain how redox chemistry is used to measure the ethanol concentration in a breathalyser.
▶️Answer/Explanation
Markscheme
a. $1700-1750 \ll \mathrm{cm}^{-1} »$
NOTE: Accept a specific wavenumber value within range.
b. Any three of:
sample/liquids vaporized «in oven/at high temperature»
OR
sample injected into mobile phase/inert gas
OR
nitrogen/helium/inert gas acts as mobile phase
OR
sample carried by inert gas «through column»
NOTE: Award [1 max] for identifying suitable technique (eg GC-MS etc.).
Do not accept just “gas”.
Accept description of HPLC using liquid mobile phase.
stationary phase consists of a packed column
OR
packing/solid support acts as stationary phase
NOTE: Accept named stationary phase, such as «long-chain» hydrocarbon/polysiloxane/silica.
components separated by partition «between mobile phase and stationary phase» OR
gases/liquids/components have different retention times/ $R_{\mathrm{f}}$
OR
gases/liquids/components move through tube/column at different speeds/rates
detector/mass spectrometer/MS «at end of column»
$O R$
databases/library of known fragmentation patterns can be used
NOTE: Accept “area under peak proportional to quantity/amount/concentration of component present «in mixture»”.
c. ALTERNATIVE 1
oxidizing agent/«acidified» potassium dichromate $(\mathrm{VI})$ converts ethanol to ethanoic acid
colour change «from orange to green» is measured/analysed «using photocell»
ALTERNATIVE 2
ethanol is oxidized to ethanoic acid «at anode and oxygen is reduced to water at cathode» current/voltage/potential is measured «by computer»
OR
current/voltage/potential is proportional to ethanol concentration
NOTE: Accept names or formulas for reagents.
Accept ” “acidified» dichromate/ $\mathrm{Cr}_2 \mathrm{O}_7^{2-\text { ” }}$ for ” $\mathrm{K}_2 \mathrm{Cr}_2 \mathrm{O}_7$ “.
Award [1 max] for “Cr(VI) going to $\mathrm{Cr}(I I I)$ AND colour changing/colour changing from orange to green”.
Do not penalize incorrect oxidation state notation here.
Accept “EMF” for “voltage”.
Question
Propene is an important starting material for many products. The following shows some compounds which can be made from propene, $\mathrm{C}_3 \mathrm{H}_6$.
Propene $\left(\mathrm{C}_3 \mathrm{H}_6\right) \rightarrow \mathrm{C}_3 \mathrm{H}_7 \mathrm{Cl} \rightarrow \mathrm{C}_3 \mathrm{H}_8 \mathrm{O} \rightarrow \mathrm{C}_3 \mathrm{H}_6 \mathrm{O}$
Consider the conversion of propene to $\mathrm{C}_3 \mathrm{H}_7 \mathrm{Cl}$.
An experiment was carried out to determine the order of reaction between one of the isomers of $\mathrm{C}_3 \mathrm{H}_7 \mathrm{Cl}$ and aqueous sodium hydroxide. The following results were obtained.
$a$ (i)State the type of reaction.
a(ii) State the IUPAC name of the major product.
a(iiiputline why it is the major product.
$\mathrm{a}$ (iW)/rite an equation for the reaction of the major product with aqueous sodium hydroxide to produce $\mathrm{a}_3 \mathrm{H}_8 \mathrm{O}$ compound, showing structural formulas.
b(i)Determine the rate expression from the results, explaining your method.
$b$ (iiDeduce the type of mechanism for the reaction of this isomer of $\mathrm{C}_3 \mathrm{H}_7 \mathrm{Cl}$ with aqueous sodium hydroxide.
b(iiisketch the mechanism using curly arrows to represent the movement of electrons.
c(i)Write an equation for the complete combustion of the compound $\mathrm{C}_3 \mathrm{H}_8 \mathrm{O}$ formed in (a)(iv).
c(iiDetermine the enthalpy of combustion of this compound, in $\mathrm{kJ} \mathrm{mol}^{-1}$, using data from section 11 of the data booklet.
d(i)State the reagents for the conversion of the compound $\mathrm{C}_3 \mathrm{H}_8 \mathrm{O}$ formed in (a)(iv) into $\mathrm{C}_3 \mathrm{H}_6 \mathrm{O}$.
d(ii Explain why the compound $\mathrm{C}_3 \mathrm{H}_8 \mathrm{O}$, produced in (a)(iv), has a higher boiling point than compound $\mathrm{C}_3 \mathrm{H}_6 \mathrm{O}$, produced in d(i).
d(iifyxplain why the ${ }^1 \mathrm{H}$ NMR spectrum of $\mathrm{C}_3 \mathrm{H}_6 \mathrm{O}$, produced in (d)(i), shows only one signal.
e. Propene is often polymerized. Draw a section of the resulting polymer, showing two repeating units.
▶️Answer/Explanation
Markscheme
a(i)«electrophilic» addition
NOTE: Do not accept “nucleophilic addition” or “free radical addition”.
Do not accept “halogenation”.
a(ii)-chloropropane
a(iiisecondary carbocation/carbonium «ion» is more stable
$O R$
carbocation/carbonium «ion» stabilized by two/more alkyl groups
$$
\text { a(iq. } \mathrm{H}_3 \mathrm{CHClCH}_3(\mathrm{l})+\mathrm{OH}^{-}(\mathrm{aq}) \rightarrow \mathrm{CH}_3 \mathrm{CH}(\mathrm{OH}) \mathrm{CH}_3(\mathrm{aq})+\mathrm{Cl}^{-} \text {(aq) }
$$
OR
$$
\mathrm{CH}_3 \mathrm{CHClCH}_3(\mathrm{l})+\mathrm{NaOH}(\mathrm{aq}) \rightarrow \mathrm{CH}_3 \mathrm{CH}(\mathrm{OH}) \mathrm{CH}_3(\mathrm{aq})+\mathrm{NaCl}(\mathrm{aq})
$$
$\mathrm{b}(\mathrm{i})$ Rate $=k\left[\mathrm{C}_3 \mathrm{H}_7 \mathrm{Cl}\right]\left[\mathrm{OH}^{-}\right]$
«[ $\left[\mathrm{OH}^{-}\right]$held constant and» $\left[\mathrm{C}_3 \mathrm{H}_7 \mathrm{Cl}\right]$ triples $A N D$ rate triples «so first order wrt $\mathrm{C}_3 \mathrm{H}_7 \mathrm{Cl}$ » $\left[\mathrm{C}_3 \mathrm{H}_7 \mathrm{Cl}\right]$ doubles AND $\left[\mathrm{OH}^{-}\right]$doubles $\boldsymbol{A N D}$ rate quadruples «so first order wrt $\mathrm{OH}^{-}$» b(ii)SN2
NOTE: Accept ‘bimolecular nucleophilic substitution.’
b(iii)
curly arrow going from lone pair on O/negative charge on $\mathrm{OH}^{-}$to $\mathrm{C} \checkmark$
curly arrow showing $\mathrm{C}-\mathrm{Cl}$ bond breaking
representation of transition state showing negative charge, square brackets and partial bonds
formation of $\mathrm{CH}_3 \mathrm{CH}(\mathrm{OH}) \mathrm{CH}_3$ AND $\mathrm{Cl}^{-}$
NOTE: Do not allow arrow originating on $\mathrm{H}$ in $\mathrm{OH}^{-}$.
Allow curly arrow going from bond between $\mathrm{C}$ and $\mathrm{Cl}$ to $\mathrm{Cl}$ in either reactant or transition state.
Do not award $\mathrm{M} 3$ if $\mathrm{OH}-\mathrm{C}$ bond is represented.
Accept formation of $\mathrm{NaCl}$ instead of $\mathrm{Cr}$.
$$
\text { c(i) } 2 \mathrm{C}_3 \mathrm{H}_8 \mathrm{O}(\mathrm{l})+9 \mathrm{O}_2(\mathrm{~g}) \rightarrow 6 \mathrm{CO}_2(\mathrm{~g})+8 \mathrm{H}_2 \mathrm{O}(\mathrm{g})
$$
OR
$$
\mathrm{C}_3 \mathrm{H}_8 \mathrm{O}(\mathrm{l})+4.5 \mathrm{O}_2(\mathrm{~g}) \rightarrow 3 \mathrm{CO}_2(\mathrm{~g})+4 \mathrm{H}_2 \mathrm{O}(\mathrm{g})
$$
c(iibonds broken:
$$
7(\mathrm{C}-\mathrm{H})+\mathrm{C}-\mathrm{O}+\mathrm{O}-\mathrm{H}+2(\mathrm{C}-\mathrm{C})+4.5(\mathrm{O}=\mathrm{O})
$$
OR
$7\left(414 \ll \mathrm{kJ} \mathrm{mol}^{-1} »\right)+358 \ll \mathrm{kJ} \mathrm{mol}^{-1} »+463 « \mathrm{~kJ} \mathrm{~mol}^{-1} »+2\left(346 \ll \mathrm{kJ} \mathrm{mol}^{-1} »\right)+4.5\left(498 « \mathrm{~kJ} \mathrm{~mol}^{-1} »\right) / 6652 « k J »$
bonds formed:
$6(\mathrm{C}=\mathrm{O})+8(\mathrm{O}-\mathrm{H})$
OR
$6\left(804\right.$ «kJ mol$\left.{ }^{-1} »\right)+8\left(463\right.$ «kJ mol$\left.{ }^{-1} »\right) / 8528$ «kJ»
$« \Delta H=$ bonds broken – bonds formed $=6652-8528=»-1876 « \mathrm{~kJ} \mathrm{~mol}^{-1} »$
NOTE: Award [3] for correct final answer.
OR
«acidified potassium» manganate $(\mathrm{VII}) /$ « $\mathrm{H}^{+}$and» $\mathrm{KMnO}_4 /$ « $\mathrm{H}^{+}$and» $\mathrm{MnO}_4-\checkmark$
NOTE: Accept ” $\mathrm{H}_2 \mathrm{SO}_4$ ” or ” $\mathrm{H}_3 \mathrm{PO}_4$ ” for ” $\mathrm{H}^{+}$”.
Do not accept $\mathrm{HCl}$.
Accept “permanganate” for “manganate(VII)”.
$\mathrm{d}(\mathrm{i}) \mathrm{K}_2 \mathrm{Cr}_2 \mathrm{O}_7 / \mathrm{Cr}_2 \mathrm{O}_7^{2-/} /$ potassium» dichromate «(VI)» $A N D$ acidified/ $\mathrm{H}^{+}$
OR
«acidified potassium» manganate $(\mathrm{VII}) /$ « $\mathrm{H}^{+}$and» $\mathrm{KMnO}_4 /$ « $\mathrm{H}^{+}$and» $\mathrm{MnO}_4-\checkmark$
NOTE: Accept ” $\mathrm{H}_2 \mathrm{SO}_4$ ” or ” $\mathrm{H}_3 \mathrm{PO}_4$ ” for ” $\mathrm{H}^{+}$”.
Do not accept $\mathrm{HCl}$.
Accept “permanganate” for “manganate(VII)”.
d(ii $\mathrm{C}_3 \mathrm{H}_8 \mathrm{O} /$ propan-2-ol: hydrogen-bonding $A N D \mathrm{C}_3 \mathrm{H}_6 \mathrm{O} /$ propanone: no hydrogen bonding/«only» dipole-dipole/dispersion forces
hydrogen bonding stronger «than dipole-dipole»
$\mathrm{d}$ (iiignly one hydrogen environment
OR
methyl groups symmetrical «around carbonyl group»
NOTE: Accept “all hydrogens belong to methyl groups “which are in identical positions»”.
e.
NOTE: Continuation bonds must be shown.
Methyl groups may be drawn on opposite sides of the chain or head to tail. Ignore square brackets and ” $n$ “.
Question
Stearic acid $\left(M_{\mathrm{r}}=284.47\right)$ and oleic acid $\left(M_{\mathrm{r}}=282.46\right)$ have the same number of carbon atoms. The structures of both lipids are shown in section 34 of the data booklet.
a. The iodine number is the number of grams of iodine which reacts with $100 \mathrm{~g}$ of fat. Calculate the iodine number of oleic acid.
[1]
b. The chemical change in stored fats causes rancidity characterized by an unpleasant smell or taste.
[2]
Compare hydrolytic and oxidative rancidity.
c. State one similarity and one difference in composition between phospholipids and triglycerides.
Similarity:
Difference:
▶️Answer/Explanation
Markscheme
a. «one $\mathrm{C}=\mathrm{C}$ bond»
«1 mole iodine : 1 mole oleic acid»
« $\frac{100 \times 253.80}{282.46}=89.85$ «g of $\mathrm{I}_2 » \checkmark$
NOTE: Accept “90 «g of $\mathrm{I}_2$ “.
NOTE: Award [1] for any two sites or conditions from any of the four listed.
Accept “high temperature” for “heat”. Accept “lipase” for “enzyme”.
Do not accept just “double bond”.
Accept “air” for “oxygen” and “UV/sun” for “light”.
Ignore any reference to heat/high temperature as a condition for oxidative.
c. Similarity:
«derived from» propane-1,2,3-triol/glycerol/glycerin/glycerine
OR
«derived from» at least two fatty acids
OR
contains ester linkages
OR
long carbon chains
NOTE: Do not accept “two fatty acids as both a similarity and a difference”.
Do not accept just “hydrocarbon/carbon chains”.
Difference:
phospholipids contain two fatty acids «condensed onto glycerol» AND triglycerides three
OR
phospholipids contain phosphate/phosphato «group»/residue of phosphoric acid $A N D$ triglycerides do not
NOTE: Accept “phospholipids contain phosphorus AND triglycerides do not”.
Accept “phospholipids are amphiphilic AND triglycerides are not” OR “phospholipids have hydrophobic tails and hydrophilic heads AND triglycerides do not”.