6.9 Analytical Techniques

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.

[1]

[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.

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

[1]

[2]

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

[1]

[1]

[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.

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.

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

The structural formula of ethylbenzene is shown below.

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.

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.

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.