4.7 Analytical Techniques

This question is about the determination of the structure of an unknown compound, B.

Compound B is a trans stereoisomer that is a useful intermediate in organic synthesis. 

Analysis of compound B shows it to contain 68.6 % carbon, 8.6 % hydrogen and 22.8 % oxygen.

The infrared spectrum of the compound B is shown in Figure 1.

Figure 1

In the mass spectrum, shown in Figure 2, the peak with the greatest relative intensity is caused by the loss of a functional group from the molecular ion of the compound B.

Figure 2

Determine the structure of compound B. Explain your reasoning and show your working.

An unknown organic compound, C, is analysed by mass spectrometry and IR spectroscopy and the following data is collected.

Mass spectrum:

IR spectrum:

Using the molecular ion peak at m/z = 55 and the IR spectrum, suggest one possible identity of the unknown organic compound C. Explain your answer.

The mass spectrum of compound C shows an M+1 peak at m/z = 56. 

Suggest two ways that this peak could be formed.

Calculate, to three significant figures, the expected elemental composition of an 8.0 g sample of compound C. 

Compound C undergoes a reaction to form compound D. The IR spectrum of compound D is shown: 

 

State the reagents required for this reaction. Use the IR spectrum of compound D to justify your answer.

Isomers D, E and F, are analysed, using a combination of techniques. Some of the analytical data and other known information is shown below.

Elemental analysis by mass

• Carbon, hydrogen and oxygen in the ratio of 6 : 1 : 2

Mass spectrum

• Molecular ion peak at m/z = 72.0

IR spectrum

• The IR spectra of all three isomers shows a peak at 3500 cm^-1 but only D and E contain a peak at 1650 cm^-1 

Other known information

• D is the only branched carbon chain
• E is the E-isomer of a pair of stereoisomers

Determine the displayed formulae of isomers D, E and F, showing all your reasoning. 

2-Bromopropane was heated under reflux with concentrated potassium hydroxide dissolved in a mixture of water and ethanol.

The organic products, G and H, formed in this reaction belong to different homologous series.

The mass spectrum of G shows peaks at m/z = 45 and at m/z = 60.

Identify compound G, showing your reasoning.

The infrared spectra of products G and H are shown below. 

Explain how each spectrum shows the homologous series to which each product belongs.

This question is about analytical techniques.

An alcohol can be prepared by hydrolysing the haloalkane C₂H₅CH(Br)CH₃ with aqueous sodium hydroxide. The C-Br bond is labelled on the IR spectrum below.

Outline how IR spectroscopy will change as a result of the above reaction.

The mass spectrum of 2-methylpropan-1-ol is shown below. 

Determine which ion is responsible for the peak with the greatest relative intensity.

The IR spectrum of butanal is shown below. 

Explain why this spectrum cannot be butanoic acid, but-1-ene or butan-1-ol. 

Clenbuterol, shown below, is a performance-enhancing drug that is believed to increase short-term work rate and cardiovascular output. 

Determine the m/z value of the molecular ion, M⁺, of clenbuterol, giving your reasoning.

An alcohol I is analysed, using a combination of techniques. The analytical data is shown below.

Elemental analysis by mass

• C = 68.2%, H = 13.6%, O = 18.2%

Mass spectrum

• Molecular ion peak at m/z = 88.0

Determine the molecular formula of alcohol I.

Alcohol I oxidises to form a carboxylic acid. State what information this gives about alcohol I.

Draw the displayed formulae and give the systematic names of four possible structural isomers that could be alcohol I.

The mass spectrum of alcohol I has a major peak at m/z = 45. 

Draw the structure of the species that could give this peak.

Alcohol I has a branched chain. Identify alcohol I, showing your reasoning.

A student wanted to distinguish between the following three compounds using mass spectrometry. 

Explain why determining the exact mass using mass spectrometry would not help in distinguishing between the samples of X, Y and Z.

Compounds X, Y and Z were analysed using IR spectroscopy. The spectrum of one of the compounds is shown below.

Identify which of the three compounds X, Y or Z this spectrum belongs to. Explain your reasoning.

Explain why infrared spectroscopy alone could not be used to distinguish between compounds X and Y.

Explain how mass spectrometry can distinguish between compounds X and Y.

Infrared spectroscopy is a common analytical tool used in chemistry, to identify or distinguish between different organic compounds. 

State how an IR spectrum could be used to prove that a ketone was butan-2-one. 

Breathalysers are used to test whether someone has been drinking alcohol while driving. Early models of breathalysers used chemicals which changed colour during an oxidation reaction. 

Inside the breathalyser, ethanol was oxidised to ethanoic acid using the same reagents and the same oxidation reaction which would be completed in a lab.  

State the reagents which are used to cause this oxidation and state the colour change which would be observed.

The mass spectrum of the ketone, hexan-2-one has a large fragment ion at m/z = 43.

An organic molecule, J, has the following composition by mass. 

• 62.1% carbon
• 10.3% hydrogen 
• 27.6% oxygen

In the mass spectrum of J, the molecular ion peak has a value of m/z = 116.

Infrared spectroscopy and masss spectrometry are used in the search for extra-terrestrial organic compounds.

Compound X was analysed using both techniques. The infrared spectrum and mass spectrum are shown below.

What evidence is there to support that compound X is a hydrocarbon?

How does the mass spectrum support that compound X has a molecular formula of C₄H₁₀?

Draw the displayed formulae of two possible isomers for compound X.

Use the fragment ions at peaks at m/z 15, m/z 29 and m/z 43 in the mass spectrum to name compound X. Explain your reasoning.

An unknown saturated organic compound contains carbon, hydrogen and oxygen only.

Analysis of 1.00 g of the unknown compound showed it to contain 0.133 g of hydrogen and  0.600 g of carbon.

The infrared spectrum and mass spectrum of the unknown compound are shown below.

Calculate the empirical and molecular formulae for the unknown compound.

Using the infrared spectrum, suggest two structural formulae for the unknown compound. Explain your reasoning.

Using the mass spectrum, suggest a structure for the unknown compound. Explain your reasoning.