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A LevelChemistryCIEExam Questions37. Analytical Techniques (AL)37.3 Carbon-13 NMR Spectroscopy

Carbon-13 NMR Spectroscopy (CIE A Level Chemistry)

Exam Questions

45 mins6 questions
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1a1 mark

This question is about the spectroscopy of benzene and some of its methylated derivatives.

Benzene is analysed using carbon-13 NMR spectroscopy. 

Suggest why benzene has only one peak in its spectrum.

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1b1 mark

The displayed formula of methylbenzene is shown in Fig. 1.1.

methylbenzene

Fig. 1.1

State the number of peaks that would appear in the low resolution 1H NMR spectrum of methylbenzene.

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1c3 marks

There are three isomers of dimethylbenzene shown in Fig. 1.2:

  • 1,2-dimethylbenzene
  • 1,3-dimethylbenzene
  • 1,4-dimethylbenzene

dimethylbenzene-isomers

Fig. 1.2

Complete Table 1.1 to show the number of expected peaks in the low resolution 1H NMR and 13C NMR spectra of the three dimethylbenzene isomers.

Table 1.1 

Isomer Number of peaks in 1H NMR spectrum Number of peaks in 13C NMR spectrum
1,2-dimethylbenzene    
1,3-dimethylbenzene    
1,4-dimethylbenzene    

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1a3 marks

During the production of an NMR spectrum, tetramethylsilane (TMS) is mixed with the sample.

 
i)
Give the structural formula of the standard reference chemical used for 1H NMR spectroscopy. 
 
[1]
 
ii)
Explain why tetramethylsilane is used as the standard reference chemical.
 
[2]

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1b1 mark

State the number of peaks in the C-13 NMR spectrum of 1,3-dichlorobenzene.

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1c2 marks
i)
Predict the number of peaks in the 13C NMR spectrum of ethylbenzene, shown in Fig. 1.1.
 
VRwFPzIK_ethylbenzene-asterisk
 
Fig. 1.1
 
[1]
 
ii)
The data in Table 1.1 should be used in answering this question.
 
One of the carbon atoms in the structure of ethylbenzene shown in Fig. 1.1 is labelled with an asterisk (*). Suggest a C-13 chemical shift range for this carbon environment.
 
Table 1.1
 
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<, cie-ial-data-table-arene 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 
 
[1]

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2a1 mark

Compound A contains the elements carbon and hydrogen forming an aromatic ring along with the elements oxygen and nitrogen.

Part of the mass spectrum of A is shown in Fig. 2.2.

 
compound-a-ms
 
Fig. 2.2
 

Give the identity of the molecular ion that gives rise to the peak at m / e = 76 in Fig. 2.2.

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2b3 marks

Suggest the structures of the three possible dinitrobenzene isomers of A that contain a benzene ring.

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2c1 mark

The C-13 NMR spectrum of compound A has four peaks.

Identify the structure of A. Explain your reasoning by labelling the different carbon environments in all the structures drawn in part (ii).

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3a
Sme Calculator4 marks

An unknown alcohol was analysed and found to contain 64.9% carbon, 13.5% hydogen and the rest oxygen. 

The results of mass spectrometry found the mass of the alcohol to be 74.12 g mol-1.

Determine the molecular formula of the unknown alcohol. Show your working.

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3b6 marks

The unknown alcohol can exist as four possible isomers.

i)
Using your answer to part (a), sketch the four possible isomers of the unknown alcohol.
 
[4]
ii)
For each isomer, deduce the number of chemical peaks expected in the 13C spectrum.
 
[2]

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3c3 marks

The 13C NMR spectra of one of the isomers is shown in Fig. 3.1.

cnF58aZe_10-1

Fig. 3.1

Deduce which isomer produced the spectrum shown in Fig. 3.1. Explain your answer with reference to Table 3.1.

Table 3.1

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<, cie-ial-data-table-arene 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 

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1a3 marks

Three hydrocarbons, L, M and N, have the molecular formula C8H10. Information about the number of peaks seen in the carbon-13 (13C) NMR spectrum of the three isomers is shown in Table 1.1.

Table 1.1

  Number of peaks
L 3
M 5
N 4

 

Suggest structures for compounds L, M and N.

 L

 

 

 

 
 

 M

 

 

 

 
 

 N

 

 

 

 

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1b3 marks

Complete Table 1.1 to give details of the proton NMR spectra for isomers L, M and N.

Table 1.1

  Number of peaks Relative Peak area
L    
M    
N    

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1c2 marks

Explain which of the three isomers, L, M or N has the highest melting point.

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2a
Sme Calculator3 marks

A chemist analyses a naturally occurring compound, K.

The percentage composition by mass is carbon 70.58%; hydrogen 5.92% and oxygen 23.50%.

The mass spectrum of the compound is shown in Fig. 5.1.

mass-spectrum-of-4-hydroxyphyenylethanone

Fig. 5.1

Determine the molecular formula of the compound K. Show your working.

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2b2 marks

The results of qualitative tests performed on compound K are shown in Table 5.1.

Table 5.1

Test Acidity Na2CO3 (aq) 2,4-DNPH Tollens' reagent
Observation pH 5.0 No reaction Orange precipitate No reaction

 

Identify the functional groups present in compound K. Explain your answer.

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2c3 marks

The carbon-13 (13C) NMR of compound K is shown in Fig. 5.2.

carbon-nmr-of-4-hydroxyphyenylethanone

Fig. 5.2

Table 5.1 

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<, cie-ial-data-table-arene 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


Suggest the structure of compound K using your answers to (a) and (b) along with information from Fig. 5.2 and Table 5.1. Explain your answer.

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