A LevelChemistryEdexcelExam Questions7. Advanced Organic Chemistry (A Level Only)7.7 Modern Analytical Techniques II

Modern Analytical Techniques II (Edexcel A Level Chemistry)

Exam Questions

1 hour5 questions
Medium
Hard
1a
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Chromatography is a technique used to separate the components of a mixture.

A sample of a tripeptide was hydrolysed and then placed on a thin layer chromatography (TLC) plate. Samples of possible amino acids present were also placed on the TLC plate for reference.
A simplified diagram of the developed TLC plate is shown.

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A – Lysine B – Serine C – Histidine D – Tyrosine
E – Isoleucine F – Methionine G – Proline H – Hydrolysed tripeptide

i)
Calculate the Rf value for the amino acid lysine.
Give your answer to an appropriate number of significant figures.
(1)
ii)
Identify by name the two amino acids present in the tripeptide, giving a reason for the lack of a third spot.
(3)
iii)
Give two reasons why different amino acids have different Rf values.
(2)
iv)
In chromatography, a ‘locating’ reagent is often used when the components in a mixture are colourless.

Which reagent is used to locate the amino acid spots?

(1)

  ☐ A iodine
  ☐ B methyl orange
  ☐ C ninhydrin
  ☐ D phenolphthalein

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1b4 marks

Gas chromatography can be used both to separate the components in a mixture and to determine the amount of each present.

i)
State why argon and nitrogen are suitable carrier gases for gas chromatography.
(1)
ii)
A mixture containing one part substance X, two parts substance Y and one part substance Z was separated by gas chromatography.

Substance X has a retention time of 10 seconds, substance Y of 15 seconds and substance Z of 40 seconds.

Complete the sketch of this chromatogram.

(3)

q3bii-9cho-al-2-june-2018-qp-edexcel-a-level-chem

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2a
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Methyl cinnamate, C10H10O2, is a white crystalline solid used in the perfume industry. 

q4-9cho-al-2-june-2019-qp-edexcel-a-level-chem

Calculate the mass of carbon in 2.34 g of methyl cinnamate. 

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2b5 marks

A sample of methyl cinnamate was analysed by high resolution proton NMR spectroscopy.

A simplified spectrum is shown.  

q4b-9cho-al-2-june-2019-qp-edexcel-a-level-chem

 

i)
Name the compound responsible for the peak at a chemical shift of 0 ppm, stating its purpose. 
(2)

ii)
Identify the proton environment that causes the peak at a chemical shift of 3.8 ppm by circling it on the diagram shown. Fully justify your answer. 
(3)
q4bii-9cho-al-2-june-2019-qp-edexcel-a-level-chem

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2c6 marks

Methyl cinnamate undergoes an addition reaction in the dark with bromine. 

i)
Draw the mechanism for the reaction between methyl cinnamate and bromine, Br2.
Include curly arrows, and relevant lone pairs and dipoles.  
(4)

ii)
Deduce the number of optical isomers of the addition product that can exist. 
(1)
  ☐ A 2
  ☐ B 3
  ☐ C 4
  ☐ D 8

iii)
When plane-polarised light is passed through an optical isomer, the plane of polarisation is 
(1)
  ☐ A diffracted
  ☐ B reflected
  ☐ C refracted
  ☐ D rotated

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3a4 marks

This question is about the analysis of organic compounds.

X is an organic compound.

i)
The accurate relative atomic masses, Ar , of the four elements that could make up X are shown in the table.

Element Ar
hydrogen, H 1.0078
carbon, C 12.0000
nitrogen, N 14.0031
oxygen, O 15.9949

X gives a molecular ion peak at m/ z = 100.0522 on its mass spectrum.
Which is the molecular formula of X?
  ☐ A C7H16
  ☐ B C6H12O 
  ☐ C C6H14N 
  ☐ D C5H8O2 

ii)
The infrared spectrum of X contains major absorption wavenumber ranges at 3300–2500 cm−1, 1725–1700 cm−1 and 1669–1645 cm−1.

Identify the two functional groups in X.

(2)

iii)
X has an unbranched carbon chain and does not exhibit geometric isomerism.
Draw the skeletal formula of X.

(1)

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3b6 marks

There are similarities and differences in the 13C NMR spectra and the high resolution 1H NMR spectra of isomeric organic compounds.

Compare the NMR spectra of propan-1-ol with those of propan-2-ol.

Include the number of peaks, relative peak areas and splitting patterns, where appropriate.


Chemical shift values are not required.

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1a3 marks

This question is about the use of NMR spectroscopy to distinguish between isomers of  C6H12O2.

Tetramethylsilane (TMS) is a compound used as a standard when recording both straight H presuperscript 1 and  straight C presuperscript 13 NMR spectra.

i)
Give the structural formula of TMS.
(1)
ii)
TMS is an inert and non-toxic compound. State two other reasons why TMS is suitable for use as a standard when recording NMR spectra.
(2)

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1b7 marks
i)
Draw the structural formulae of the two esters with formula C6H12O2 that each have only two peaks, both singlets, in their high resolution proton NMR spectra. The relative peak areas are 3:1 for both esters.

(2)

ii)
The high resolution proton NMR spectrum of another isomer of C6H12O2 is shown.
q4bii-9cho-al-3-june-2018-qp-edexcel-a-level-chemThe ratios of the number of protons for the five sets peaks in the spectrum are given in the table.
  δ / ppm 3.8 3.5 2.6 2.2 1.2
  Ratio of the number of protons 2 2 2 3 3

Show that all these data are consistent with the displayed formula shown.
Refer to the five chemical shifts and explain two of the splitting patterns.
q4bii-1-9cho-al-3-june-2018-qp-edexcel-a-level-chem
(5)

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1c3 marks
i)
There are three other isomers of C6H12O2 which are carboxylic acids with five peaks in their carbon-13 NMR spectra. 

Draw the structural formula of two of these isomers.

(2)

ii)
Draw the skeletal formula of a cyclic diol isomer of C6H12O2 that has only two peaks in its carbon-13 NMR spectrum.

(1)

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2a2 marks

This is a question about the analysis of three aromatic substances with —OH groups.

q6-9cho-al-3-nov-2020-qp-edexcel-a-level-chem

Write the equation for the complete combustion of phenol. State symbols are not required.

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2b
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When burned in air, these aromatic compounds undergo incomplete combustion.

i)
Calculate the percentage composition by mass of carbon in both phenylmethanol and benzoic acid.

(3)

ii)
Give the expected observation when these aromatic compounds undergo incomplete combustion.

(1)

iii)
Identify another type of organic compound which will also produce the same observation as in (b)(ii).

(1)

iv)
These combustion experiments must be carried out in a fume cupboard.

q6biv-9cho-al-3-nov-2020-qp-edexcel-a-level-chem

Explain why the front window of the fume cupboard must be below the safety line even with the exhaust fan switched on.

(2)

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2c10 marks

Spectroscopy is an effective means of distinguishing between molecules.

i)
Compare and contrast the infrared spectra of phenol, phenylmethanol and benzoic acid.
Include relevant bonds and their wavenumber ranges using the Data Booklet.

(5)

ii)
Predict the number of peaks present, and their chemical shifts, in the 13C nuclear magnetic resonance (NMR) spectrum of phenylmethanol.
Use the information in the Data Booklet to help you.

(3)

q6cii-9cho-al-3-nov-2020-qp-edexcel-a-level-chem

iii)
Give the formula of a fragment ion, with its m/z value, that you would expect to be present in the mass spectrum of benzoic acid but not in the mass spectrum of phenol or the mass spectrum of phenylmethanol.

(2)

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