Proton (1H) NMR Spectroscopy (Cambridge (CIE) A Level Chemistry): Exam Questions

The proton (¹H) NMR spectrum of methyl cinnamate, C₁₀H₁₀O₂, recorded using CDCl₃ as a solvent is shown in Fig. 1.1.

Fig. 1.1

i) Explain why CDCl₃ is used as the solvent instead of CHCl₃.

[1]

ii) State the purpose of adding tetramethylsilane (TMS) to the sample.

[1]

The skeletal formula of methyl cinnamate is shown in Fig. 1.2.

Fig. 1.2

Use Table 1.1 to deduce the proton environment that gives rise to the peak at δ = 3.8 ppm in Fig. 1.1. Explain your reasoning using the chemical shift, integration, and splitting pattern.

Table 1.1

Draw the skeletal formulae of the two ester isomers of C₆H₁₂O₂ that each produce exactly two peaks, both singlets, in their ¹H NMR spectra. The relative peak areas are 3:1 for both esters.

The proton (¹H) NMR spectrum of another isomer of C₆H₁₂O₂ is shown in Fig. 1.3.

Fig. 1.3

The integration values for the peaks are given in Table 1.2.

Table 1.2

i) Deduce the simplest whole-number ratio of the number of protons in each environment.

[1]

ii) Use Table 1.1 to explain the chemical shift and splitting pattern of the peaks at δ = 3.5 and δ = 1.2.

[4]

Four isomers of C₆H₁₂O₂, A, B, C and D, are shown in Fig. 1.4.

Fig. 1.4

The ¹³C NMR spectrum of one of these four isomers is shown in Fig. 1.5.

Fig. 1.5

Table 1.3

Use Table 1.3 to deduce which of the four isomers A, B, C or D produced the ¹³C NMR spectrum shown in Fig. 1.5.

Ethane-1,2-diol, C₂H₆O₂, and ethanedioic acid, C₂H₂O₄, can be distinguished using mass spectrometry, infrared spectroscopy and proton NMR spectroscopy.

Fig. 2.1 (spectrum A) and Fig. 2.2 (spectrum B) show the mass spectra of ethane-1,2-diol and ethanedioic acid.

Fig. 2.1 (spectrum A)

Fig. 2.2 (spectrum B)

Complete Table 2.1. Deduce which compound is responsible for each spectrum and explain your answer.

Table 2.1

The IR spectra of ethane-1,2-diol, C₂H₆O₂, and ethanedioic acid dihydrate, C₂H₂O₄.2H₂O, are shown in Fig. 2.3 (spectrum C) and Fig. 2.4 (spectrum D).

Fig. 2.3 (spectrum C)

Fig. 2.4 (spectrum D)

Use the Data Booklet to complete Table 2.2. Deduce which compound is responsible for each spectrum and explain your answer.

Table 2.2

The proton (¹H) NMR spectrum of ethane-1,2-diol is shown in Fig. 2.5.

Fig. 2.5

Use the spectrum to deduce the two proton environments in ethane-1,2-diol. State the number of peaks, their integration ratio and identify each environment.

State the number of peaks and the splitting pattern in the proton (¹H) NMR spectrum of ethanedioic acid.

Lidocaine is used as a local anaesthetic. The structure of lidocaine is shown in Fig. 3.1.

Fig. 3.1

A sample of lidocaine was analysed by carbon-13 NMR spectroscopy.

Deduce the number of peaks in the carbon-13 NMR spectrum of lidocaine.

Lidocaine was dissolved in CDCl₃ and the proton NMR spectrum of this solution was recorded as shown in Fig. 3.2.

Fig. 3.2

Use the Data Booklet to complete Table 3.1 for the chemical shifts at δ 2.3 ppm, 3.0 ppm and 9.0 ppm.

Table 3.1

Table 3.2

Explain why the absorption at δ 1.2 ppm is a triplet.

Compound P is a naturally occurring chemical found in strawberries, apples and Parmesan cheese.

The percentage by mass is carbon 58.82%, hydrogen 9.80% and oxygen 31.38%.

The mass spectrum of compound P is recorded in Fig. 1.1.

Fig. 1.1

Calculate the molecular formula of compound P. Show your working.

Table 1.1 shows the results of qualitative tests performed on compound P.

Table 1.1

Deduce the functional groups present in compound P. Explain your answers with reference to Table 1.1.

The carbon-13 (¹³C) NMR spectrum of compound P is shown in Fig. 1.2.

Fig. 1.2

Deduce the functional group present in compound P, using your answer to (b) and information from Fig. 1.2 and the Data Booklet. Explain your answer.

The high-resolution proton (¹H) NMR spectrum of compound P is shown in Fig. 1.3.

Fig. 1.3

Deduce the skeletal formula of compound P, using your answers to (a), (b) and (c) and information from Fig. 1.3 and the Data Booklet. Explain your answer.

A chemist prepares and analyses some esters.

The chemist prepares an ester by reacting propan-2-ol with ethanoic anhydride.

Using structural formulae, construct an equation for the reaction of propan-2-ol with ethanoic anhydride.

A sample contains a mixture of two esters contaminated with an alkane and an alcohol.

The chemist attempts to separate the four organic compounds in the mixture using gas chromatography. The stationary phase in the gas chromatograph column is a liquid alkane.

i) Explain how a liquid stationary phase separates organic compounds in a mixture.

[1]

ii) Deduce the relative retention times of these four compounds using the alkane stationary phase. Explain your answer.

[2]

An ester is isolated from a perfume and analysed.

The percentage by mass of the ester is: carbon 66.63%, hydrogen 11.18% and oxygen 22.19%.

The mass spectrum and proton (¹H) NMR spectrum of the ester are shown in Fig. 2.1 and Fig. 2.2 respectively.

Fig. 2.1

Fig. 2.2

Table 2.1

i) Calculate the molecular formula of the ester. Show your working.

[3]

ii) Use Fig. 2.1, Fig. 2.2 and the Data Booklet to deduce the skeletal formula of the ester. Explain your reasoning.

[5]

Compound X contains carbon, hydrogen and oxygen only. It has a relative molecular mass (M_r) of 88.

The infra-red spectrum of X shows a strong absorption at 1745 cm⁻¹ but no absorption between 3200 cm⁻¹ and 3650 cm⁻¹.

The proton (¹H) NMR spectrum of X is shown in Fig 8.1

Fig 8.1

State the name of the compound used as a reference standard in proton (¹H) NMR spectroscopy and state two reasons why it is chosen for this purpose.

i) Using the IR data, identify the functional group present in X. Explain your answer.

[2]

ii) Use the proton (¹H) NMR spectrum to deduce the structural formula of X.

Explain how you reached your conclusion by referring to the chemical shifts and splitting patterns of the peaks.

[4]

A different compound, Y, contains a halogen atom. The mass spectrum of Y shows two molecular ion peaks, M⁺ and [M+2]⁺, of approximately equal height (1:1 ratio).

i) State the identity of the halogen present in Y.

[1]

ii) Explain why the M⁺ and [M+2]⁺ peaks appear in this ratio.

[2]