22.1 Infrared Spectroscopy | CIE A Level Chemistry Topic Questions 2025

1a

2 marks

There are three possible isomers, A, B and C, with the molecular formula C₃H₈O.

Isomers A and B are positional isomers of each other.

Isomer C is a functional group isomer of A and B.

A student uses the following equation to calculate that the percentage by mass of carbon in all three isomers is 60.0%.

Percentage by mass = \frac{total mass of element}{overall mass of compound} \times 100

Calculate the percentage by mass of hydrogen and oxygen in all three isomers.

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1b2 marks

The infrared spectrum of isomer C shown in Fig. 1.1 does not have a peak in the region of 3200 - 3600 cm^-1.

Fig. 1.1

i)

Using the infrared spectrum in Fig. 1.1 and Table 1.1, suggest a structure for isomer C.

Table 1.1

Bond	Functional groups containing the bond	Characteristic infrared absorption range (in wavenumber) / cm^–1
C−O	hydroxy, ester	1040 – 1300
C=C	aromatic compound, alkene	1500 – 1680
C=O	amide carbonyl, carboxyl ester	1640 – 1690 1670 – 1740 1710 – 1750
C≡N	nitrile	2200 – 2250
C−H	alkane	2850 – 2950
N−H	amine, amide	3300 – 3500
O−H	carboxyl hydroxy	2500 – 3000 3200 – 3600

[1]

ii)

Suggest which bonds the highlighted peak at around 1130 cm^-1 could represent.

[1]

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1c1 mark

Isomers A and B both have a peak at 3200 - 3600 cm^-1 in their infrared spectra.

State two possible chemical names for isomers A and B.

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1d

2 marks

Isomer A has a major peak on its mass spectrum at m/e = 31.

Suggest, with a reason, which compound is isomer A.

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2a2 marks

This question is about some isomeric alcohols with the molecular formula C₅H₁₂O.

Some alcohols were heated with potassium dichromate(VI) and sulfuric acid. The organic compounds were separated from the reaction mixtures and purified.

The infrared spectra of two of these organic compounds are shown in Fig. 2.1.

Fig. 2.1

Using the data in Table 2.1, deduce the type of compound responsible for each spectrum.

Table 2.1

Bond	Functional groups containing the bond	Characteristic infrared absorption range (in wavenumber) / cm^–1
C−O	hydroxy, ester	1040 – 1300
C=C	aromatic compound, alkene	1500 – 1680
C=O	amide carbonyl, carboxyl ester	1640 – 1690 1670 – 1740 1710 – 1750
C≡N	nitrile	2200 – 2250
C−H	alkane	2850 – 2950
N−H	amine, amide	3300 – 3500
O−H	carboxyl hydroxy	2500 – 3000 3200 – 3600

Your answer should include references to wavenumbers and their corresponding bonds.

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2b1 mark

The infrared spectrum a student obtained of an organic molecule which contains carbon, hydrogen and oxygen atoms is shown in Fig. 2.2.

Fig. 2.2

Use Table 2.1 in part (a) and information from the infrared spectrum to explain how the student deduced that the spectrum shows the presence of a carbonyl group.

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2c2 marks

In an experiment to prepare a sample of ethanal, CH₃CHO, ethanol, C₂H₅OH, is reacted with acidified potassium dichromate(VI) and the reaction mixture is distilled. The infrared spectra for ethanol and ethanal are shown in Fig. 2.3.

ethanol-ir-spectrum

ethanal-ir-spectrum

Fig. 2.3

i)

State the bonds that give rise to the absorption in the ethanol spectrum at 3400 cm^-1and the absorption in the ethanal spectrum at 1720 cm^-1.

[1]

ii)

Explain why the absorption at 3400 cm^-1 in the ethanol spectrum does not appear in the spectrum for ethanal.

[1]

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1a2 marks

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.

Table 1.1

	melting point / K	boiling point / K
alcohol A	158	373
C₂H₅CHBrCH₃	161	364

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

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1b3 marks

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

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1c2 marks

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

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1d3 marks

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

Fig. 1.1

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

Table 1.2

Bond	Functional groups containing the bond	Characteristic infrared absorption range (in wavenumber) / cm^–1
C−O	hydroxy, ester	1040 – 1300
C=C	aromatic compound, alkene	1500 – 1680
C=O	amide carbonyl, carboxyl ester	1640 – 1690 1670 – 1740 1710 – 1750
C≡N	nitrile	2200 – 2250
C−H	alkane	2850 – 2950
N−H	amine, amide	3300 – 3500
O−H	carboxyl hydroxy	2500 – 3000 3200 – 3600

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1e1 mark

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

Fig. 1.2

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

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1a

2 marks

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

xyz

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

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1b2 marks

Compounds X, Y and Z were analysed using IR spectroscopy. The spectrum of one of the compounds is shown in Fig 1.1.

3c-ir-spec

Fig 1.1

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

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1c2 marks

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

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1d

3 marks

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

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2a1 mark

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.

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2b3 marks

i)

Give the structural formula of A and of B.

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

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

[2]

ii)

Explain the meaning of the term chiral centre.

[1]

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2c2 marks

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

Bond	Functional groups containing the bond	Characteristic infrared absorption range (in wavenumbers)/cm^–1
C−O	hydroxy, ester	1040 – 1300
C=C	aromatic compound, alkene	1500 – 1680
C=O	amide carbonyl, carboxyl ester	1640 – 1690 1670 – 1740 1710 – 1750
C≡N	nitrile	2200 – 2250
C−H	alkane	2850 – 2950
N−H	amine, amide	3300 – 3500
O−H	carboxyl hydroxy	2500 – 3000 3200 – 3600

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 ............................................................................................

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2d2 marks

Draw possible structures of isomers C and D


Isomer C	Isomer D

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2e3 marks

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

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Infrared Spectroscopy (CIE A Level Chemistry)

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