A level 화학 C-NMR , H-NMR 문제

1a

2 marks

This question is about the NMR analysis of various organic compounds.

Name and draw the structure of the chemical that is commonly used as a standard in NMR spectroscopy.

1b

3 marks

Fig. 1.1 shows the structures of compounds A, B, and C.

Compound A is pentane, with the chemical formula C₅H₁₂. Compound B is 2-methylbutane and compound C is 2,2-dimethylpropane, which are both isomers of pentane.

State the number of hydrogen peaks that would be expected in low resolution ¹H-NMR spectrum of each isomer.

1c

3 marks

More structural details can be deduced using high resolution ¹H NMR.

Explain why the methyl groups in 2-methylbutane, compound B, give a doublet splitting pattern while the methyl groups in 2,2-dimethylpropane, compound C, give a singlet splitting pattern.

1d

3 marks

Carbon-13 NMR is also commonly used to distinguish chemicals.

Predict the number of peaks in the carbon-13 NMR spectra of compounds A, B, and C.

2a

3 marks

Compound X is a carbohydrate.

Compound X contains 62.1% C, 10.3% H, and 27.6% O by mass.

(i) Show that the empirical formula of compound X is C₃H₆O. [2]

(ii) The empirical formula of compound X is C₃H₆O and the M_r is 58.0. Deduce the molecular formula of compound X. You must explain your reasoning.

Molecular formula: ..............................

................................................................................ [1]

2b

2 marks

There are several possible isomers of compound X.

Draw the structures of two isomers of compound X that contain a carbonyl group.

Isomer 1:

Isomer 2:

2c

3 marks

A sample of a different isomer of compound X is cyclopropanol, which was analysed by NMR spectroscopy.

(i) Predict the number of peaks in the carbon-13 NMR spectrum of cyclopropanol. [1]

(ii) Cyclopropanol was dissolved in CDCl₃ and the proton NMR spectrum of this solution was recorded as shown in Fig. 2.1.

Suggest one change that can be made to the solvent used for the proton NMR and how this will affect the spectrum produced. [2]

1a

2 marks

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.

(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]

1b

3 marks

The structure of methyl cinnamate is shown in 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

Environment of proton Example chemical shift range, δ / ppm

• Alkane –C_H3, –C_H2–, >C_H–: 0.9 – 1.7
• Alkyl next to C=O C_H3–C=O, –C_H2–C=O, >C_H–C=O: 2.2 – 3.0
• Alkyl next to aromatic ring C_H3–Ar, –C_H2–Ar, >C_H–Ar: 2.3 – 3.0
• Alkyl next to electronegative atom C_H3–O, –C_H2–O, –C_H2–Cl: 3.2 – 4.0
• Attached to alkene =C_HR: 4.5 – 6.0
• Attached to aromatic ring H–Ar: 6.0 – 9.0
• Aldehyde H_COR: 9.3 – 10.5
• Alcohol RO_H: 0.5 – 6.0
• Phenol Ar–O_H: 4.5 – 7.0
• Carboxylic acid RCOO_H: 9.0 – 13.0
• Alkyl amine R–N_H–: 1.0 – 5.0
• Aryl amine Ar–N_H2: 3.0 – 6.0
• Amide RCON_HR: 5.0 – 12.0

1c

2 marks

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.

1d

5 marks

The proton NMR spectrum of another isomer of C₆H₁₂O₂ is shown in 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

Chemical shift, δ/ppm: 3.8, 3.5, 2.6, 2.2, 1.2

Integration value: 0.6, 0.6, 0.6, 0.9, 0.9

Splitting pattern: triplet, quartet, triplet, singlet, triplet

(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]

1e

1 mark

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

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

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

2a

2 marks

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.

Complete Table 2.1 to suggest which compound is responsible for each spectrum? Explain your answer.

2b

2 marks

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).

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

2c

3 marks

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

Describe and explain the splitting patterns of the spectrum.

2d

2 marks

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

3a

1 mark

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

3b

3 marks

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

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

Table 3.1

• δ / ppm environment of proton number of ¹H atoms responsible for the peak splitting pattern
• 1.2 terminal methyl groups next to CH₂: 6 triplet
• 2.3, 3.0, 7.1 - 7.4 attached to the aromatic ring: 3 overlapping peaks
• 9.0

Table 3.2

• Alkane –C_H3, –C_H2–, >C_H–: 0.9 – 1.7
• Alkyl next to C=O C_H3–C=O, –C_H2–C=O, >C_H–C=O: 2.2 – 3.0
• Alkyl next to aromatic ring C_H3–Ar, –C_H2–Ar, >C_H–Ar: 2.3 – 3.0
• Alkyl next to electronegative atom C_H3–O, –C_H2–O, –C_H2–Cl: 3.2 – 4.0
• Attached to alkene =C_HR: 4.5 – 6.0
• Attached to aromatic ring H–Ar: 6.0 – 9.0
• Aldehyde H_COR: 9.3 – 10.5
• Alcohol RO_H: 0.5 – 6.0
• Phenol Ar–O_H: 4.5 – 7.0
• Carboxylic acid RCOO_H: 9.0 – 13.0
• Alkyl amine R–N_H–: 1.0 – 5.0
• Aryl amine Ar–N_H2: 3.0 – 6.0
• Amide RCON_HR: 5.0 – 12.0

3c

1 mark

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

1a

3 marks

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.

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

1b

4 marks

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

• Test: Addition of H₂O - Observation: Forms separate layers
• Test: Na₂CO₃(aq) - Observation: No visible change
• Test: 2,4-DNPH - Observation: No visible change
• Test: Tollens' reagent - Observation: No visible change

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

1c

3 marks

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

Table 1.2

• Hybridisation of the carbon atom
• Environment of carbon atom Example Chemical shift range δ / ppm
• sp₃ alkyl C_H3–, C_H2–, –C_H<, >C<: 0="" 50="" li="">
• 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="" li="">
• 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.

1d

3 marks

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

2a

1 mark

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.

2b

3 marks

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]

2c

8 marks

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.

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