Analytical Techniques (OCR A Level Chemistry A): Exam Questions

You are provided with the following selection of organic compounds, to investigate using test-tube reactions.

State a simple test-tube reaction which could be done to distinguish between Compound B and Compound C, including the observations that could be made.

State the simple test-tube reactions that could be done to distinguish between Compound A and Compound D from part (a), including the observations that could be made.

Compound D can be formed from an alcohol if the correct reagents and conditions are used for the reaction. 

i) State the reagents, conditions and any observations that would be made during the above reaction.

ii) Give the IUPAC name of the alcohol which would need to be used and write an equation using structural formulae for the reaction.

A student claims to have fully oxidised a different primary alcohol than that in part (c). The student analysed their product and obtained the following IR spectrum. 

Use the spectrum to suggest whether full oxidation has taken place

A student was investigating the structure of three hydrocarbons, L, M and N.

L and M are isomers.

L has two stereoisomers, but neither of the other compounds do. 

Define the term stereoisomer and explain the key feature which must be present in L for it to show stereoisomerism.

The homologous series that L and M belong to have the general formula C_nH_2n and the homologous series of N has the general formula C_nH_2n+2.

State a simple test-tube reaction that could be done to distinguish between L and N, including the observations you would see.

Simple test-tube reactions like those in part (b) can also be used to distinguish between the following organic compounds. 

2-bromopropane and hexane. 

State the test-tube reaction which could be used and any observations that would be seen.

Simple test-tube reactions like those in part (b) and part (c) can also be used to distinguish between the following pair of organic compounds. 

Pentan-3-ol and 3-methyl-pentan-3-ol 

State the test-tube reaction which could be used and any observations that would be seen.

Column chromatography and gas chromatography work by the same principles of components travelling through the column at different rates. 

State the difference between the eluents involved in column chromatography and gas-liquid chromatography.

Gas- chromatography is performed on three compounds, A, B and C. The gas chromatogram trace is shown.

Identify if compound A, B or C has the greatest affinity for the solid phase. Explain your reasoning.

Use the gas chromatogram shown in part (b), to identify the most abundant compound in the sample. Explain your reasoning.

An oil tanker is travelling through the English Channel. The tanker has a slight leak which is not large enough to result in an oil slick but some oil is noticed on a beach.

Suggest how gas chromatography could be used to identify the tanker as the source of crude oil pollution on the beach.

A student is asked to identify a mixture using thin-layer chromatography. 

Draw the labelled experimental set up that the student should use to identify the number of compounds in the mixture.

The student runs their thin-layer chromatography experiment and plans to determine the compounds from their R_f values.

Describe the steps that the student needs to perform to determine the identity of the compounds.

The student’s results are shown.

For their measurements, the student locates the centre of each spot by estimating its rough position by eye.

Suggest an improved method to locate the centre of each spot.

The student runs another TLC practical on a mixture of benzaldehyde and benzyl alcohol is placed on a TLC plate using 7:3 pentane / diethyl ether as a solvent. 

The student's chromatogram is shown.

Calculate the R_f values for both compounds in the chromatogram.

Explain why the maximum R_f value of a compound cannot exceed 1.

During the production of an NMR spectrum, tetramethylsilane (TMS) is mixed with the sample.

i) Suggest the structural formula of the standard reference chemical used for ¹³C NMR spectroscopy. 

ii) State two reasons why this chemical is suitable to be used as the standard reference chemical.

Predict the number of peaks in the ¹³C NMR spectrum of 1,3-dichlorobenzene.

The structural formula of ethylbenzene is shown below.

i) Predict the number of peaks in the ¹³C NMR spectrum of ethylbenzene 

ii) One of the hydrogen atoms in the structure of ethylbenzene shown above is labelled with an asterisk (*). Suggest a range of δ values for the peak due to this carbon atom in the ¹³C NMR spectrum of ethylbenzene.

Compound A contains the elements carbon, hydrogen, oxygen and nitrogen only. Compound A contains a benzene ring.

Part of the mass spectrum of A is shown below.

i) State the molecular formula of the species that causes the peak at m / z = 76 in the mass spectrum of A.

ii) Draw the structures of the three possible dinitrobenzene isomers of A containing a benzene ring.

iii) The ¹³C NMR spectrum of compound A has four peaks. Identify the structure of A. Explain your reasoning by labelling the different carbon environments in all the structures drawn in part (ii).

Methyl cinnamate, C₁₀H₁₀O₂, is a white crystalline solid used in the perfume industry. A simplified high resolution mass spectrum of methyl cinnamate is shown below.

Name the compound responsible for the peak at a chemical shift of 0 ppm. State its purpose.

Identify the proton environment that causes the peak at a chemical shift of 3.8 ppm by circling it on the diagram of shown. Explain your reasoning.

This question is about the use of ¹H NMR spectroscopy to distinguish between isomers of C₆H₁₂O₂.

Draw the two esters with formula C₆H₁₂O₂ that each have only two peaks, both singlets, in their ¹H NMR spectra. The relative peak areas are 3:1 for both esters.

The high resolution ¹H NMR spectrum of another isomer of C₆H₁₂O₂ is shown.

The integration values for the peaks in the ¹H NMR spectrum of this isomer are given in the table below.

i) Deduce the simplest ratio of the relative numbers of protons in each environment in the isomer.

ii) Use the Data sheet to deduce the part of the isomer that causes the signal at δ= 3.5 and the part of the structure at the isomer that causes the signal at δ=1.2. Explain why the splitting patterns of these peaks are produced.

¹³C NMR spectrometry can be used to analyse organic compounds. Four isomers of C₆H₁₂O₂, A, B, C and D, were analysed by ¹³C NMR spectrometry.

The ¹³C NMR spectrum of one of the four isomers of C₆H₁₂O₂ is shown.

Use your data sheet to determine which of the four isomers, A, B, C or D of C₆H₁₂O₂ produced the ¹³C NMR spectra shown.

Chemical analysis of an unknown compound showed it to contain 62.1% carbon, 10.3% hydrogen and the rest oxygen.

The unknown compound gives a molecular ion peak at m/z = 116.

Determine the molecular formula of the unknown compound.

The infrared and ¹³C NMR spectra of the unknown compound are shown.

Analyse the spectra and suggest two possible skeletal structures for the unknown compound. 

One of the two possible isomers of the unknown compound produces only three peaks in its ¹H NMR spectrum.

i) Name the isomer responsible for this spectrum.

ii) State the splitting patterns and chemical shifts of the protons for this isomer.

iii) State the relative peak heights for protons of this isomer.

Compound A is a liquid used as a solvent in organic synthesis. Compound A contains carbon, hydrogen and oxygen only.

Analysis of compound A provides the following percentage composition by mass:

C: 54.54% H: 9.10% O: 36.36%

Calculate the empirical formula of compound A. Show your working.

The mass spectrum of compound A is shown below.

i) State the m/z value of the molecular ion peak.

Use your answer and the empirical formula from (a) to determine the molecular formula of compound A.

[2]

ii) Write the formulae of the ions responsible for the peaks at m/z = 29 and m/z = 57.

[2]

The infrared (IR) spectrum of compound A is shown below.

i) A student suggests that compound A contains an alcohol functional group.

State two pieces of evidence from the IR spectrum that prove the student is incorrect.

[2]

ii) Use the IR spectrum to identify the functional group present in compound A.

[1]

Compound A is methyl propanoate.

i) Draw the full displayed formula of compound A.

[1]

ii) Using your answer to (b)(ii), explain how the fragmentation pattern in the mass spectrum confirms this structure.

[2]

A chemist analyses a naturally occurring compound, P, which is found in strawberries, apples and Parmesan cheese. The percentage composition and mass spectrum of the compound are shown below.

Percentage composition by mass: C, 58.82%; H, 9.80%; O, 31.38%.

Determine the molecular formula of the compound. Show your working.

Qualitative tests are carried out on compound P. The results are shown below.

Use these observations to analyse the potential functional groups in compound P. Explain your reasoning.

The ¹³C NMR spectrum of compound P is shown below.

i) Identify the structural features responsible for the peaks at 162 ppm and 63 ppm.

[2]

ii) Use the information in parts (a) and (b) to determine the functional group(s) present in compound P.

[1]

The high-resolution proton NMR spectrum of compound P is shown below.

Use the spectrum and the information from parts (a), (b) and (c) to deduce the structure of compound P. 

Explain your reasoning.

The following table shows information about the C-13 NMR spectrum of three isomers with molecular formula C₈H₁₀.

Deduce the structures of L, M and N

Complete the table of information about the proton NMR spectra for isomers L, M and N.

Deduce whether peak splitting would be seen in the high-resolution proton NMR spectra of each of L, M and N.

Explain your reasoning in each case.

Suggest, with a reason, which of the three isomers, L, M or N would have the highest melting point.

Methamphetamine, or crystal meth as it is commonly known, is synthesised from phenyl acetone and N-methylformamide. The structure of the reactants and product are shown below.

Suggest the systematic names for the reactants.

A sample of methamphetamine was added to a test tube containing some 2,4-DNP.

The result was the formation of a slight orange precipitate. Explain these observations

The purity of a sample of methamphetamine could be determined by IR spectroscopy.

Describe two pieces of evidence to support this statement.

Use the Data Sheet to support your answer.

The proton NMR spectrum of one of the substances in part a) is shown below.

Determine, with a reason, which substance the spectrum belongs to.

Gas chromatography is often connected to mass spectroscopy in order to analyse each compound as it exits the gas chromatography column. This combined technique is called GC-MS.

The gas chromatogram of an organic mixture is shown below. The stationary phase is a polar, high-boiling point liquid on a solid support.

i) Name an appropriate mobile phase that could be used to form this gas chromatogram.

[1]

ii) Identify the number of compounds present in this mixture and how to identify the most polar molecule present.

[2]

Two of the substances have very similar retention times. 

The mass spectra of the compounds revealed that they have the same molecular formulae as each other, with subtle differences in the spectra indicating structural isomerism is present. When mixed with bromine water the solutions remained orange.

i) Identify the molar mass and hence the molecular formula of the isomers.

[2]

ii) Identify the fragments responsible for the signals at m/z = 29, m/z = 43 and m/z 57.

[3]

iii) Suggest the most likely structure of the isomer responsible for this spectrum

[1]

There is a small, additional peak on the far right of the mass spectrum. 

i) Explain the presence of the small peak at m/z = 87.

[1]

ii) Suggest the type of molecules for which this would become more pronounced. Explain your answer.

[2]

The other isomer present in the original mixture was 2,3-dimethylbutane. 

i) Suggest one way in which the mass spectrum of 2,3-dimethylbutane will differ from that shown in part b).

[1]

ii) Suggest an alternative spectroscopic technique for distinguishing between the two isomers.

[1]

iii) Predict which of the two isomers would have the longest retention time and explain your reasoning.

[2]

Esters can be analysed by a number of techniques.

An ester with the molecular formula C₆H₁₂O₂ has the following high-resolution proton NMR data:

Draw the part of the ester responsible for the signal in proton environment 4.

Deduce the full structure of the ester in part a).

The ester can undergo acid hydrolysis to form two products. 

Describe laboratory tests and expected results for the presence of the functional groups in each of the hydrolysis products.

A structural isomer of the ester in part a), ester B, has a proton NMR with only two peaks, both being singlets. 

Draw the skeletal formula of ester B.

This question is about carbonyl chemistry and qualitative analysis.

Describe a sequence of simple test-tube reactions, stating the reagents, conditions, and visible observations, that could be used to distinguish between the following three compounds:

• Pentanal

• Pentan-2-one

• Pentan-3-ol

Benzaldehyde reacts with 2,4-dinitrophenylhydrazine (2,4-DNPH) to form a crystalline solid.

i) Draw the structure of the organic product formed in this reaction.

[2]

ii) State the purpose of purifying this product by recrystallisation and how the purified product is used to identify the specific aldehyde.

[2]

This question is about the determination of the structure of unknown compound J. Compound J is a liquid with a fruity smell, used as a food flavouring.

Analysis of compound J provides the following percentage composition by mass:

C: 62.07% H: 10.34% O: 27.59%

The mass spectrum of compound J has a molecular ion peak (M⁺) at m/z = 116.

Calculate the molecular formula of compound J.

The infrared (IR) and proton (¹H) NMR spectra of compound J were analysed to determine its structure.

IR Spectrum Data:

• Strong absorption peak at 1735 – 1750 cm^-1.

• Strong absorption peak at 1050 – 1300 cm^-1.

• No broad absorption peak above 3000 cm^-1.

Proton (¹H) NMR Data:

The spectrum contains five peaks. The data is summarised in the table below.

Using the data provided, deduce the full structural formula of compound J.

In your answer, you should:

• Identify the functional group present using the IR data.

• Analyse the splitting patterns and chemical shifts in the NMR data to identify specific structural fragments and their connectivity.

• Explain how the NMR data allows you to distinguish between isomers.

State the systematic IUPAC name of compound J.

The mass spectrum of compound J shows a major fragment ion at m/z = 71.

i) Using the structure you determined in (b), identify the ion responsible for this peak.

[1]

ii) State how this ion is formed from the molecular ion.

[1]

Predict the number of peaks that would be observed in the Carbon-13 (¹³C) NMR spectrum of compound J.

Compound J can be synthesised from an alcohol and a carboxylic acid.

State the reagents and conditions required for this synthesis.