Infrared Spectroscopy — Structured Questions (Cambridge International A-Level Chemistry): Exam Questions

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.

The percentage by mass of carbon in all three isomers is 60.0%.

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

Extract:

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.

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

Fig. 1.1

i) Using the infrared spectrum in Fig. 1.1 and Table 1.1, deduce the fully displayed formula of isomer C.

Table 1.1

ii) Identify the bond responsible for the absorption at around 1130 cm^-1.

Extract:

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.

Isomers A and B both have an absorption at 3200–3600 cm^-1 in their infrared spectra.

State the systematic names of isomers A and B.

Extract:

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.

Isomers A and B both have an absorption at 3200–3600 cm^-1^ in their infrared spectra.

The systematic names of isomers A and B are:

• Propan-1-ol
• Propan-2-ol

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

Deduce which isomer is compound A. Explain your reasoning by identifying the fragment ion responsible for the peak at m/e = 31.

Some alcohols with the molecular formula C₅H₁₂O were oxidised using acidified potassium dichromate(VI). The organic products were separated and purified.

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

Fig. 2.1

Using the data in Table 2.1, identify the functional group present in each compound. Explain your answer, referring only to absorptions at frequencies greater than 1500 cm^-1.

Table 2.1

Extract:

Table 2.1

The infrared spectrum of an organic compound containing carbon, hydrogen and oxygen is shown in Fig. 2.2.

Fig. 2.2

Use Table 2.1 and the infrared spectrum in Fig. 2.2 to explain how the presence of a carbonyl group can be identified.

Extract:

Table 2.1

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.

Fig. 2.3

i) Identify the bond responsible for the absorption at 3400 cm^-1 in the ethanol spectrum and the bond responsible for the absorption at 1720 cm^-1 in the ethanal spectrum.

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

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.

Extract:

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

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

Extract:

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

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

Extract:

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

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).

Extract:

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.

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

A

B

ii) Explain what is meant by the term chiral centre.

C and D are isomers with the molecular formula C₃H₆O₂. The infrared 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 ........................................................................

Extract:

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

The bonds responsible for the principal absorptions above 1500 cm^-1^ are:

Compound C:

• C=O (carboxyl / carbonyl) AND O−H (carboxyl)

Compound D:

• C=O (carbonyl / ester) AND C−H (alkane)

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

Extract:

C and D are isomers with the molecular formula C3H6O2.

The displayed formulae of isomers C and D are:

1 mark for the correct displayed formula of propanoic acid (CH3CH2COOH)

1 mark for the correct displayed formula of methyl ethanoate (HCOOCH2CH3) OR ethyl methanoate (CH3COOCH3)

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.

Infrared 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.

Extract:

Table 3.1

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

Use Table 3.1 to explain how infrared 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.

Extract:

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

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

Extract:

Alcohol A can be prepared by hydrolysing the halogenoalkane C2H5CHBrCH3 with aqueous sodium hydroxide.

The equation for the hydrolysis of C2H5CHBrCH3 with aqueous sodium hydroxide is:

C2H5CHBrCH3 (l) + NaOH (aq) → C2H5CHOHCH3 (l) + NaBr (aq)

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.

Extract:

Alcohol A can be prepared by hydrolysing the halogenoalkane C2H5CHBrCH3 with aqueous sodium hydroxide.

The equation for the hydrolysis of C2H5CHBrCH3 with aqueous sodium hydroxide is:

C2H5CHBrCH3 (l) + NaOH (aq) → C2H5CHOHCH3 (l) + NaBr (aq)

The infrared 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 infrared spectroscopy. The mass spectrum is shown in Fig. 2.1 and the infrared 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 infrared 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.

Extract:

Unknown organic compound C is analysed by mass spectrometry and infrared spectroscopy. The mass spectrum is shown in Fig. 2.1 and the infrared spectrum is shown in 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 infrared absorptions in the region above 1500 cm^-1^ and Table 2.1.

One possible identity of compound C is propanenitrile / propionitrile / C2H5CN (molecular formula C3H5N, Mr = 55).

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

Give your answers to 3 significant figures.

Extract:

Unknown organic compound C is analysed by mass spectrometry and infrared spectroscopy. The mass spectrum is shown in Fig. 2.1 and the infrared spectrum is shown in 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 infrared absorptions in the region above 1500 cm^-1^ and Table 2.1.

Table 2.1

One possible identity of compound C is: Propanenitrile / propionitrile / C2H5CN. The infrared spectrum shows an absorption at around 2250 cm^-1^ which corresponds to a C≡N (nitrile) group.

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 infrared spectrum to justify your answer.