Describe what happens to plane-polarized light when it passes through a solution of an optically active compound.

A mixture of enantiomers shows optical rotation.

Suggest a conclusion you can draw from this data.

plane of polarization is rotated

Award zero if answer refers to plane-polarized light being bent.

[1 mark]

not a racemic mixture
OR
two enantiomers not equimolar
OR
mixture contains optically active impurity
OR
relative proportions of enantiomers in mixture can be determined

[1 mark]

(i)     Identify the two reagents used to form the electrophile in the nitration of benzene.

(ii)     Explain, using curly arrows to represent the movement of electrons, the mechanism for this reaction.

(i)     \({{\text{H}}_2}{\text{S}}{{\text{O}}_4}\)/sulfuric acid and \({\text{HN}}{{\text{O}}_3}\)/nitric acid;

(ii)     

curly arrow going from delocalized electrons in benzene to \(^ + {\text{N}}{{\text{O}}_2}\);

Do not penalize if NO₂⁺ is written.

representation of carbocation with correct formula and positive charge on ring;

curly arrow going from CH bond to benzene ring cation;

formation of organic product nitrobenzene and \({{\text{H}}^ + }\);

Allow mechanism with corresponding Kekulé structures.

Q27 was well answered by most candidates although some need to take care with the placement of the delocalized positive charge in the intermediate.

Using an asterisk (*), identify the chiral carbon atom in the structure of thalidomide.

Describe the composition of a racemic mixture.

 ;

equimolar/50:50 mixture (of a pair) of enantiomers/both isomers;

Identification of the chiral carbon atom was generally well done.

Some were not able to describe the composition of a racemic mixture correctly by not mentioning an equimolar (50:50) mixture.

Identify the organic product in the following reaction.

\[{{\text{C}}_6}{{\text{H}}_6} + {\text{HN}}{{\text{O}}_3}\xrightarrow{{{{\text{H}}_2}{\text{S}}{{\text{O}}_4}}}\]

nitrobenzene / \({{\text{C}}_6}{{\text{H}}_5}{\text{N}}{{\text{O}}_2}\) / ;

Accept multiple substitution products.

Nitrobenzene was usually easily identified in (d).

Vision is dependent on retinol (vitamin A) present in retina cells. Retinol is oxidized to the photosensitive chemical 11-cis-retinal and isomerizes to 11-trans-retinal on absorption of light.

Outline how the formation of 11-trans-retinal results in the generation of nerve signals to the brain.

11-trans retinal no longer fits into the rhodopsin/protein
OR
11-trans retinal is ejected from the rhodopsin/protein

leads to conformational change in rhodopsin/protein «to opsin generating signals»

[2 marks]

The action of a drug molecule often depends on its shape. Discuss a specific example of a drug where one stereoisomer has a different physiological activity to the other.

one enantiomer/isomer of thalidomide relieves nausea/morning sickness;

other enantiomer/isomer of thalidomide causes foetal deformities;

OR

one enantiomer/isomer of taxol is an anti-cancer drug;

other enantiomer/isomer of taxol is ineffective;

OR

cis-isomer of diamminedichloroplatinum(II)/cisplatin is an anti-cancer drug;

trans-isomer is ineffective;

Whilst a drug in which stereochemistry was important was well known (usually Thalidomide), the other two parts of the question were poorly answered in very vague terms. Few seemed to realize the importance of the 3D modelling of the drug and its active site or to have grasped the concept that combinatorial chemistry involved the random ordering of specific chemical building blocks.

Identify the chiral centre in this enantiomer with an asterisk, *.

The \(( + )d\)-enantiomer has the characteristic smell of oranges and the \(( - )l\)-enantiomer has the characteristic smell of lemons. Explain the meaning of the \(( + )d\) and \(( - )l\) symbols used in this notation.

The R, S notation is also used. The \(( + )d\)-enantiomer is often described as R-limonene and the \(( - )l\)-enantiomer as S-limonene. Explain what is meant by the R, S notation and state whether the structure shown is R or S.

dextro/\(d\) and levo/\(l\) refer to right and left-handed / clockwise and anti-clockwise rotation of plane polarized light;

clockwise and anti-clockwise sequence of prioritized atoms (working from high to low atomic numbers) / OWTTE;

Allow absolute configuration of enantiomers.

Allow convention for labelling chiral carbon atoms using the Cahn-Ingold-Prelog notation.

S;

In (a), surprisingly a number of students were not able to identify the chiral centre.

Although some knew what \(d\) and \(l\) referred to, a number did not refer to plane-polarized light.

R and S notation was not understood and only the best candidates were able to determine the structure as S. Some simple class exercises on chiral centres, \(d\) and \(d\) notation and R and S notation using a number of simple molecules would greatly improve student understanding of this topic.

State its name and, by means of a diagram, predict its structure.

Describe the structural feature of the carvone molecule that allows it to exist as optical isomers.

Name:

+(d)-carvone/s-carvone/(+)-(S)-carvone / dextro-carvone;

Structure:

 ;

asymmetric/chiral carbon / OWTTE;

This question was reasonably well answered.

This question was reasonably well answered

(i) Serine is a chiral amino acid. Draw both enantiomers of serine.

(ii) State the enantiomeric form of serine found in proteins.

(i)

(ii)
L

Methylbenzene reacts when heated with a mixture of concentrated sulfuric acid and concentrated nitric acid.

(i)     Name the major organic products formed.

(ii)     Identify the electrophile in this reaction and explain how it is formed.

(i)     1-methyl-2-nitrobenzene and 1-methyl-4-nitrobenzene;

Accept 2-methylnitrobenzene and 4-methylnitrobenzene.

Accept 2-nitromethylbenzene and 4-nitromethylbenzene.

Accept o-methylnitrobenzene and p-methylnitrobenzene.

(ii)     \({\text{NO}}_2^ + \) / nitronium ion;

the (concentrated) sulfuric acid protonates the nitric acid /

\({\text{HN}}{{\text{O}}_3} + {{\text{H}}_2}{\text{S}}{{\text{O}}_4} \to {{\text{H}}_2}{\text{NO}}_3^ +  + {\text{HSO}}_4^ - \);

the \({{\text{H}}_{\text{2}}}{\text{NO}}_3^ + \) formed loses water / \({{\text{H}}_2}{\text{NO}}_2^ +  \to {{\text{H}}_2}{\text{O}} + {\text{NO}}_2^ + \);

Accept HNO₃ + H₂SO₄ \( \to \) NO₂⁺H₂O + H₂SO₄^– for the second and third points.

Candidates often named the products but had difficulty in the formation of the electrophile. The charge for \({\text{NO}}_2^ + \) was often omitted.

Draw the structures of the main form of glycine in buffer solutions of pH 1.0 and 6.0.

The pK_a of glycine is 2.34.

Calculate the pH of a buffer system with a concentration of 1.25 × 10⁻³ mol dm⁻³ carbonic acid and 2.50 × 10⁻² mol dm⁻³ sodium hydrogen carbonate. Use section 1 of the data booklet.

pK_a (carbonic acid) = 6.36

Sketch the wedge and dash (3-D) representations of alanine enantiomers.

UV-Vis spectroscopy can be used to determine the unknown concentration of a substance in a solution.

Calculate the concentration of an unknown sample of pepsin with an absorbance of 0.725 using section 1 of the data booklet.

Cell length = 1.00 cm

Molar absorptivity (extinction coefficient) of the sample = 49650 dm³ cm⁻¹ mol⁻¹

A different series of pepsin samples is used to develop a calibration curve.

Estimate the concentration of an unknown sample of pepsin with an absorbance of 0.30 from the graph.

Penalize charge on incorrect atom once only.

Penalize missing hydrogens or incorrect bond connectivities once only in Option B.

Accept condensed structural formulas.

Accept skeletal structures.

[2 marks]

ALTERNATIVE 1

«pH = 6.36 + log \(\left( {\frac{{2.50 \times {{10}^{ - 2}}}}{{1.25 \times {{10}^{ - 3}}}}} \right)\) =»

7.66

ALTERNATIVE 2

«K_a = 4.4 × 10^–7 = [H⁺] \(\left( {\frac{{2.50 \times {{10}^{ - 2}}}}{{1.25 \times {{10}^{ - 3}}}}} \right)\), [H⁺] = 2.2 × 10^–8 mol dm^–3»

«pH =» 7.66

Do not accept “«pH =» 8”.

[1 mark]

Penalize missing hydrogens or incorrect bond connectivities once only in Option B.

Wedges AND dashes must be used.

[1 mark]

«\(\frac{{0.725}}{{49650{\text{ d}}{{\text{m}}^3}\;{\text{c}}{{\text{m}}^{ - 1}}\;{\text{mo}}{{\text{l}}^{ - 1}} \times 1.00\,{\text{cm}}}} = \)» 1.46 × 10⁻⁵ «mol dm⁻³»

[1 mark]

0.65 «μg cm^–3»

Accept any value in the range 0.60–0.70 «μg cm^–3»

[1 mark]

Identify the functional group circled in the structure of oseltamivir.

Predict the number of signals and relative integration you would expect to see in the nuclear magnetic resonance spectroscopy (¹H NMR) spectrum for the circled portion in the structure.

Number of signals:

Relative integration:

Oseltamivir is a chiral compound.

(i) Identify an apparatus that can be used to distinguish between its enantiomers.

(ii) Explain how the differentiation between the enantiomers is obtained using this apparatus.

ether

(i)
polarimeter