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HL Paper 3

Liquid-crystal displays (LCDs) have many uses.

A molecule which acts as a chiral nematic thermotropic liquid-crystal is given.

Label with an asterisk, *, the chiral carbon atom.

[1]
a.

Explain the effects of very low and high temperatures on the liquid-crystal behaviour of this molecule.

 

Low temperature:

High temperature:

[2]
b.

Markscheme

  [✔]

a.

Low temperature:
intermolecular forces prevent molecules moving AND solid/«normal» crystal formation    [✔]

High temperature:
«above a critical temperature» disrupts alignment of molecules AND behaves as fluid/liquid    [✔]

 

Note: Accept “weak intermolecular forces break AND behaves as fluid/liquid”.

b.

Examiners report

Identifying a chiral carbon atom was answered reasonably well.

a.

Explaining effects of very low and very high temperatures on liquid-crystal behaviour proved difficult for most candidates. Responses lacked the required detail about intermolecular forces between molecules.

b.



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.

Markscheme

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]

Examiners report

[N/A]



Amino acids are the building blocks of proteins.

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

The pKa of glycine is 2.34.

[2]
c.

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.

pKa (carbonic acid) = 6.36

[1]
d.

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

[1]
e.

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

[1]
f.

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.

[1]
g.

Markscheme

M18/4/CHEMI/HP3/ENG/TZ2/08.c/M

 

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]

c.

ALTERNATIVE 1

«pH = 6.36 + log  ( 2.50 × 10 2 1.25 × 10 3 ) =»

7.66

ALTERNATIVE 2

«Ka = 4.4 × 10–7 = [H+] ( 2.50 × 10 2 1.25 × 10 3 ) , [H+] = 2.2 × 10–8 mol dm–3»

«pH =» 7.66

 

Do not accept “«pH =» 8”.

[1 mark]

d.

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

Wedges AND dashes must be used.

[1 mark]

e.

« 0.725 49650  d m 3 c m 1 mo l 1 × 1.00 cm = » 1.46 × 10−5 «mol dm−3»

[1 mark]

 

f.

0.65 «μg cm–3»

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

[1 mark]

g.

Examiners report

[N/A]
c.
[N/A]
d.
[N/A]
e.
[N/A]
f.
[N/A]
g.



Taxol is an anticancer drug.

State the feature of Taxol that is a major challenge in its synthesis. Use section 37 of the data booklet.

[1]
a.

Describe how the challenge in (a) was resolved by pharmaceutical companies.

[1]
b.

Markscheme

numerous stereoisomers/chiral carbons/chiral centres/stereocentres/optical isomers ✔

NOTE: Accept exact number of chiral carbons ie 11, but do not accept just “chiral”.

a.

chiral auxiliaries/molecule binds to reactant blocking one reaction site «by steric hindrance»
OR
asymmetric synthesis / enantioselective catalysis «producing a specific enantiomer»
OR
biosynthesis / genetically modified bacteria/microorganisms ✔

NOTE: Accept “use of a chiral auxiliary leading to «the synthesis of» the desired enantiomer”.

b.

Examiners report

[N/A]
a.
[N/A]
b.



A polarimeter can be used to determine the optical rotation of an optically active substance.

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

[1]
a.

A mixture of enantiomers shows optical rotation.

Suggest a conclusion you can draw from this data.

[1]
b.

Markscheme

plane of polarization is rotated

 

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

[1 mark]

a.

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]

b.

Examiners report

[N/A]
a.
[N/A]
b.