| Date | May 2019 | Marks available | 1 | Reference code | 19M.3.hl.TZ1.5 |
| Level | HL | Paper | 3 | Time zone | TZ1 |
| Command term | Label | Question number | 5 | Adapted from | N/A |
Question
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.
Explain the effects of very low and high temperatures on the liquid-crystal behaviour of this molecule.
Low temperature:
High temperature:
Markscheme
[✔]
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”.
Examiners report
Identifying a chiral carbon atom was answered reasonably well.
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.
Syllabus sections
-
22M.2.hl.TZ1.5a(v):
Identify the isomer of Compound B that exists as optical isomers (enantiomers).
- 17N.2.hl.TZ0.8a.iv: Deduce, giving a reason, which of the two compounds can show optical activity.
-
17M.2.hl.TZ2.7d:
Below are two isomers, A and B, with the molecular formula C4H9Br.
Explain the mechanism of the nucleophilic substitution reaction with NaOH(aq) for the isomer that reacts almost exclusively by an SN2 mechanism using curly arrows to represent the movement of electron pairs.
- 18M.1.hl.TZ2.37: Which isomers exist as non-superimposable mirror images? A. cis-trans isomers B. ...
- 16N.2.hl.TZ0.6b: When one enantiomer undergoes substitution by alkaline hydrolysis approximately 75 % of the...
-
17M.3.hl.TZ1.28a:
Describe what happens to plane-polarized light when it passes through a solution of an optically active compound.
- 19M.1.hl.TZ2.36: Which compound exists as two configurational isomers? A. CBr2=CH2 B. CH2=CHBr C....
- 18M.1.hl.TZ1.35: What is name of this compound applying IUPAC rules? A. E...
-
18M.3.hl.TZ2.8e:
Sketch the wedge and dash (3-D) representations of alanine enantiomers.
-
19N.1.hl.TZ0.38:
Which can show optical activity?
A. CHBrCHCl
B. CH3CH2CHBrCH2CH3
C. (CH3)2CBrCl
D. CH3CH2CH(CH3)Br
-
22M.2.hl.TZ2.8d(i):
Draw the full structural formula of (Z)-but-2-ene.
-
18N.1.hl.TZ0.37:
How many chiral carbon atoms are present in one molecule of (CH3)2CHCHClCHBrCH3?
A. 0
B. 1
C. 2
D. 3
- 16N.2.hl.TZ0.6a: Draw the three-dimensional shape of each enantiomer of this isomer showing their spatial...
- 18N.2.hl.TZ0.8b.ii: State, giving a reason, whether methyloxirane can form cis-trans isomers.
-
19M.2.hl.TZ1.1e:
The organic product is not optically active. Discuss whether or not the organic product is a racemic mixture.
-
17M.3.hl.TZ1.28b:
A mixture of enantiomers shows optical rotation.
Suggest a conclusion you can draw from this data.
- 17M.1.hl.TZ2.35: Which pair of isomers always shows optical activity? A. Cis-trans B. ...
-
17M.3.hl.TZ1.17:
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.
- 17N.2.hl.TZ0.8a.v: Draw three-dimensional representations of the two enantiomers.
- 19M.1.hl.TZ1.37: Which compound can exist as cis- and trans-isomers?
- 17N.1.hl.TZ0.37: What is the number of optical isomers of isoleucine? A. 0 B. 2 C. 4 D. 8
- 17M.1.hl.TZ1.37: Which molecule is chiral? A. 2-chlorobutane B. 2,2-dichloropentane C. ...
- 16N.1.hl.TZ0.37: Which statement is correct for a pair of enantiomers under the same conditions? A. A...
- 19N.3.hl.TZ0.26a: State the feature of Taxol that is a major challenge in its synthesis. Use section 37 of the...
-
18M.1.hl.TZ1.36:
Which molecule contains a chiral carbon?
A. CH3CH2CHBrCH2CH3
B. CH3CH2CHBrCH3
C. CH2BrCH(CH3)CH2Br
D. CH3CH2CH2CH2CH2Br
- 21M.2.hl.TZ2.4h(ii): Outline how two enantiomers can be distinguished using a polarimeter.
-
18M.2.hl.TZ2.9b.iv:
Suggest, giving a reason, the percentage of each isomer from the SN1 reaction.
-
19M.2.hl.TZ2.6d(ii):
The minor product, C6H5–CH2–CH2Br, can exist in different conformational forms (isomers).
Outline what this means.
-
19M.2.hl.TZ2.6d(i):
Outline why the major product, C6H5–CHBr–CH3, can exist in two forms and state the relationship between these forms.
Two forms:
Relationship:
- 21M.1.hl.TZ1.37: How many chiral centres are there in the following molecule? A. 2 B. 3 C. 4 D. 6
- 21N.2.hl.TZ0.10b(i): State, giving a reason, if but-1-ene exhibits cis-trans isomerism.
- 21M.1.hl.TZ2.35: Which compound shows cis-trans isomerism? A. CH3CH=CCl2 B. CCl2=CH2 C. D.
-
21M.1.hl.TZ2.36:
Which compound rotates the plane of plane-polarized light?
A. CH3C(CH3)ClCH3
B. CH3CH2CHClCH3
C. CH3C(Cl)2CH3
D. CH3CClBrCH3
-
20N.1.hl.TZ0.37:
Which molecule has an enantiomer?
A.
B.
C.
D.
- 21M.2.hl.TZ2.4h(i): Draw the stereoisomers of butan-2-ol using wedge-dash type representations.
- 22M.1.hl.TZ1.37: What are the E/Z designations of these stereoisomers?
- 21N.2.hl.TZ0.10b(iv): State, giving a reason, if the product of this reaction exhibits stereoisomerism.
- 21N.1.hl.TZ0.35: Which statement is correct about configurational isomers? A. Configurational isomers can...
