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

Methyl cinnamate, C₁₀H₁₀O₂, is a white crystalline solid used in the perfume industry.

The proton NMR spectrum of methyl cinnamate in the solvent CDCl₃ is shown in Fig. 1.1.

Fig. 1.1

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

[1]

ii) Explain why TMS is added to give the small peak at chemical shift δ = 0. 

[1]

The structure of methyl cinnamate is shown in Fig. 1.2. 

Fig. 1.2

The data in Table 1.1 should be used in answering this question.

 Identify the proton environment that gives rise to the peak at a chemical shift of 3.8 ppm in Fig. 1.1. Explain your answer.

Table 1.1

Proton NMR spectroscopy can be used to distinguish between isomers of C₆H₁₂O₂.

Draw the two esters with formula C₆H₁₂O₂ that each have only two peaks, both singlets, in their ¹H NMR spectra. The relative peak areas are 3:1 for both esters.

The proton NMR spectrum of another isomer of C₆H₁₂O₂ is shown in Fig. 1.3.

Fig. 1.3

 The integration values for the peaks in the proton NMR spectrum of this isomer are given in Table 1.2.

 Table 1.2

i) Deduce the simplest ratio of the relative numbers of protons in each environment in the isomer. 

[1]

 ii) The data in Table 1.1 should be used in answering this question.

 Describe and explain the splitting patterns of the peaks at δ = 3.5 and δ = 1.2.

 splitting pattern at δ = 3.5 ...................................................................................................

 reason for splitting pattern at δ = 3.5 ..................................................................................

 splitting pattern at δ = 1.2 ...................................................................................................

 reason for splitting pattern at δ = 1.2 .................................................................................. 

[4]

Four isomers of C₆H₁₂O₂, A, B, C and D, are shown in Fig. 6.4. 

Fig. 1.4

The C-13 NMR spectrum of one of the four isomers of C₆H₁₂O₂ is shown in Fig. 1.5.

Fig. 1.5

The data in Table 1.3 should be used in answering this question.

Identify which of the four isomers, A, B, C or D of C₆H₁₂O₂ produced the C-13 NMR spectrum shown in Fig 1.5.

Table 1.3

Hybridisation of the carbon atom	Environment of carbon atom	Example	Chemical shift range δ/ppm
sp3	alkyl	CH3–, CH2–, –CH<, >C<	0 – 50
sp3	next to alkene / arene	–C–C=C, –C–Ar	25 – 50
sp3	next to carbonyl / carboxyl	C–COR, C–O2R	30 – 65
sp3	next to halogen	C–X	30 – 60
sp3	next to oxygen	C–O	50 – 70
sp2	alkene or arene	>C=C<,	110 – 160
sp2	carboxyl	R−COOH, R−COOR	160 – 185
sp2	carbonyl	R−CHO, R−CO−R	190 – 220
sp	nitrile	R−C≡N	100 – 125

Ethane-1,2-diol, C₂H₆O₂, can be distinguished from ethanedioic acid, C₂H₂O₄, by a number of analytic techniques including MS, IR and NMR

 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 to suggest which compound is responsible for each spectrum? 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)

The data in Table 2.3 should be used in answering this question.

 Complete Table 2.2 to suggest which compound is responsible for each spectrum? Explain your answer.

 Table 2.2 

Table 2.2

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

Describe and explain the splitting patterns of the spectrum.

 Fig. 2.5

Suggest the number of proton NMR peaks and splitting pattern for 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.

Predict 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

Using Table 3.2 to complete Table 3.1 for the chemical shifts δ 1.2 ppm, 3.5 ppm and 5.5 ppm.

Table 3.1

Table 3.2

Explain the splitting pattern for the absorption at δ 1.2 ppm.

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

Determine 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

 Analyse the potential functional groups in compound P. Explain your answers.

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

Fig. 1.2

Table 1.2 

Hybridisation of the carbon atom	Environment of carbon atom	Example	Chemical shift range δ / ppm
sp3	alkyl	CH3–, CH2–, –CH<, >C<	0 – 50
sp3	next to alkene / arene	–C–C=C, –C–Ar	25 – 50
sp3	next to carbonyl / carboxyl	C–COR, C–O2R	30 – 65
sp3	next to halogen	C–X	30 – 60
sp3	next to oxygen	C–O	50 – 70
sp2	alkene or arene	>C=C<,	110 – 160
sp2	carboxyl	R−COOH, R−COOR	160 – 185
sp2	carbonyl	R−CHO, R−CO−R	190 – 220
sp	nitrile	R−C≡N	100 – 125

Identify the functional group(s) present in compound P using your answer in (b) and information from Fig. 1.2 and Table 1.2. Explain your answer.

The high-resolution proton NMR spectrum of compound P was recorded as shown in Fig. 1.3.

Fig. 1.3

Table 1.3

 Suggest the structure of compound P using your answers to (a), (b) and (c) and information from Fig. 1.3 and Table 1.3. 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, write an equation for the reaction of propan-2-ol and 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) How does a liquid stationary phase separate the organic compounds in a mixture? 

[1]

 ii) Predict the separation of these four compounds using the alkane stationary phase, including relative retention times. Explain your answer. 

[2]

Gas chromatography is often used in conjunction with other techniques such as mass spectrometry and NMR spectroscopy.

An ester is isolated from a perfume by gas chromatography and then analysed. 

The percentage by mass is carbon 66.63%, hydrogen 11.18% and oxygen 22.19%.

The mass spectrum and high-resolution proton NMR spectrum of the ester are recorded in Fig. 2.1 and Fig. 2.2 respectively.

Fig. 2.1

Fig. 2.2

Table 2.1

Use all of the information to draw the structure of the ester. Explain your answer.