Infrared Spectroscopy (Cambridge (CIE) AS Chemistry): Exam Questions

Compounds X, Y and Z are structural isomers, as shown below.

Explain why determining the exact mass using mass spectrometry would not help in distinguishing between 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

Use Table 1.1 and Fig. 1.1 to deduce the identity of the compound whose spectrum is shown. Explain your answer by identifying the bonds responsible for the relevant absorptions.

Table 1.1

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

Mass spectrometry can be used to distinguish between compounds X and Y.

Deduce the m/e value and formula of a fragment ion present in the mass spectrum of compound Y but absent from that of compound X.

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

A is unbranched and reacts with alkaline I₂ (aq) to produce a yellow precipitate.

B does not react with alkaline I₂ (aq). 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 I₂ (aq).

i) Give the structural formula of A and of B.

A ..........................................................................................................

B ..........................................................................................................

[2]

ii) Explain the meaning of the term chiral centre. 

[1]

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 absorptions above 1500 cm^-1 in each spectrum.

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

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

Draw the displayed formulae of possible structures for isomers C and D.

Deduce the formulae of the fragment ions responsible for the peaks at m/e = 73, 45 and 29 in the mass spectrum of compound C.

Explain how infrared spectroscopy can provide information about the bonds present in 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. 

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.

Table 1.1

Construct 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 of the reaction in part (a). Use one test for each ion.

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

Fig. 1.1

Use Table 1.2 to identify three differences between the infrared spectrum of C₂H₅CHBrCH₃ and that of alcohol A. In your answer, identify the bonds responsible for the relevant absorptions.

Table 1.2

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

Fig. 1.2

Deduce the formula of the fragment ion responsible for the peak with the greatest relative abundance.

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.

Table 2.1

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

Identify the isotope responsible for the M+1 peak.

Calculate the percentage by mass of each element in compound C.

Give your answers to 3 significant figures.

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

Fig. 2.3

State the reagents and conditions 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.