Infrared Spectroscopy (CIE A Level Chemistry)

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 in Fig 1.1.

Fig 1.1

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.

The isomers A and B, C₅H₁₀O, both form an orange precipitate when reacted with 2,4-DNPH.

A is unbranched and reacts with alkaline aqueous iodine to produce a yellow precipitate.

B does not react with alkaline aqueous iodine. It contains a chiral centre and produces a silver mirror when warmed with Tollens’ reagent.

Draw the structure of the yellow precipitate produced by the reaction between A and alkaline aqueous iodine.

C and D are isomers with the molecular formula C₃H₆O₂ . The infra-red spectra of isomers C and D are shown in Fig. 2.1 and Fig. 2.2.

Fig. 2.1

Fig. 2.2

Table 2.1

Using Table 2.1, identify the bonds responsible for the principal peaks above 1500 cm^–1 in each spectrum.

spectrum of C ............................................................................................

spectrum of D ............................................................................................

Draw possible structures of isomers C and D

Identify the ions responsible for peaks at 73, 45 and 29 in the mass spectrum for compound C.

Scientists use analytical techniques such as infrared spectroscopy to determine if a desired reaction has taken place.

Explain how infrared spectroscopy generates useful information about an organic molecule.

Infra-red spectroscopy is used to distinguish between a primary alcohol and an aldehyde. 

Table 3.1

Use Table 3.1 to explain how the spectra produced could distinguish between these two molecules.

The alcohol from part (b) is fully oxidised by refluxing with acidified potassium dichromate solution.

Use Table 3.1 to explain how infra-red spectroscopy could prove that the reaction has taken place. 

This question is about the analysis of alcohols.

Alcohol A can be prepared by hydrolysing the halogenoalkane C₂H₅CHBrCH₃ with aqueous sodium hydroxide. The melting and boiling point information of alcohol A and C₂H₅CHBrCH₃ are given in Table 1.1.

Write an equation for the hydrolysis of C₂H₅CHBrCH₃ with aqueous sodium hydroxide. Include state symbols in your answer

Using the information in Table 1.1, explain how alcohol A can be separated from the products identified in part (a). 

Describe two chemical tests and their expected observations to confirm the identity of the inorganic product.

The infra-red spectrum of C₂H₅CHBrCH₃ is shown in Fig. 1.1 with the C–Br bond absorption labelled.

Using Fig. 1.1 and Table 1.2, predict three main differences that would be seen between the spectra of C₂H₅CHBrCH₃ and alcohol A.

The mass spectrum of (CH₃)₂CHCH₂OH is shown in Fig. 1.2.

Identify the ion that is responsible for the peak with the greatest relative intensity.

Unknown organic compound C is analysed by mass spectrometry and infra-red spectroscopy. The mass spectrum is shown in Fig. 2.1 and the infra-red spectrum is shown in Fig. 2.2.

Fig 2.1

Fig 2.2

Deduce one possible identity for compound C. Explain your answer.

In your answer, use the molecular ion peak at m / e = 55, relevant infra-red absorptions in the region above 1500 cm^-1 and Table 2.1.

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

Suggest two ways that this peak could be formed.

Calculate the expected elemental percentage composition by mass of an 8.0 g sample of compound C. Give your answers to significant figures. 

Compound C undergoes a reaction to form compound D. The IR spectrum of compound D is shown in Fig. 2.3.

State the reagents required for this reaction. In your answer, use Table 2.1 and identify any relevant absorptions in the infra-red spectrum to justify your answer.