13.18: Additional Problems

13 • Additional Problems 13 • Additional Problems

Visualizing Chemistry

Into how many peaks would you expect the ¹H NMR signals of the indicated protons to be split? (Green = Cl.) (a)

(b)

How many absorptions would you expect the following compound to have in its ¹H and ¹³C NMR spectra?

Sketch what you might expect the ¹H and ¹³C NMR spectra of the following compound to look like (green = Cl):

How many electronically nonequivalent kinds of protons and how many kinds of carbons are present in the following compound? Don’t forget that cyclohexane rings can ring-flip.

(b)

Chemical Shifts and NMR Spectroscopy

Problem 13-29

The following ¹H NMR absorptions were obtained on a spectrometer operating at 200 MHz and are given in hertz downfield from the TMS standard. Convert the absorptions to δ units. (a)

436 Hz

(b) 956 Hz (c) 1504 Hz

1H NMR absorptions were obtained on a spectrometer operating at 300 MHz. Convert the chemical shifts from δ units to hertz downfield from TMS. (a)

2.1 δ

(b) 3.45 δ (c) 6.30 δ (d) 7.70 δ

3) shows a single sharp absorption at 7.3 δ. (a)

How many parts per million downfield from TMS does chloroform absorb?

(b) How many hertz downfield from TMS would chloroform absorb if the measurement were carried out on a spectrometer operating at 360 MHz? (c) What would be the position of the chloroform absorption in δ units when measured on a 360 MHz spectrometer?

Why do you suppose accidental overlap of signals is much more common in ¹H NMR than in ¹³C NMR?

Is a nucleus that absorbs at 6.50 δ more shielded or less shielded than a nucleus that absorbs at 3.20 δ? Does the nucleus that absorbs at 6.50 δ require a stronger applied field or a weaker applied field to come into resonance than the one that absorbs at 3.20 δ?

¹H NMR Spectroscopy

(b)

(c)

(d)

(e)

The following compounds all show a single line in their ¹H NMR spectra. List them in order of expected increasing chemical shift:

CH₄, CH₂Cl₂, cyclohexane, CH₃COCH₃, H₂C $\text{=}$ CH₂, benzene

1H and ¹³C spectra? (a)

(b)

(c)

(d)

(e)

(f)

Problem 13-37

Propose structures for compounds with the following formulas that show only one peak in their ¹H NMR spectra: (a)

C₅H₁₂

(b)

C₅H₁₀ (c) C₄H₈O₂

(CH₃)₃CH

(b) CH₃CH₂CO₂CH₃ (c) trans-2-Butene

Predict the splitting pattern for each kind of hydrogen in isopropyl propanoate, CH₃CH₂CO₂CH(CH₃)₂.

(b)

(c)

(b)

(c)

Problem 13-42

The acid-catalyzed dehydration of 1-methylcyclohexanol yields a mixture of two alkenes. How could you use ¹H NMR to help you decide which was which?

1H NMR to distinguish between the following pairs of isomers? (a)

(b)

(c)

(d)

Problem 13-44

Propose structures for compounds that fit the following ¹H NMR data: (a)

• C₅H₁₀O
• 0.95 δ (6 H, doublet, J = 7 Hz)
• 2.10 δ (3 H, singlet)
• 2.43 δ (1 H, multiplet)

(b)

• C₃H₅Br
• 2.32 δ (3 H, singlet)
• 2.32 δ (3 H, singlet)
• 2.32 δ (3 H, singlet)

1H NMR spectra are shown. (a)

C₄H₉Br

(b)

C₄H₈Cl₂

¹³C NMR Spectroscopy

How many ¹³C NMR absorptions would you expect for cis-1,3-dimethylcyclohexane? For trans-1,3-dimethylcyclohexane? Explain.

13C NMR spectra of the following compounds? (a)

1,1-Dimethylcyclohexane

(b) CH₃CH₂OCH₃ (c) tert-Butylcyclohexane (d) 3-Methyl-1-pentyne (e) cis-1,2-Dimethylcyclohexane (f) Cyclohexanone

Suppose you ran a DEPT-135 spectrum for each substance in Problem 13.47. Which carbon atoms in each molecule would show positive peaks, and which would show negative peaks?

How could you use ¹H and ¹³C NMR to help distinguish the following isomeric compounds of the formula C₄H₈?

How could you use ¹H NMR, ¹³C NMR, and IR spectroscopy to help you distinguish between the following structures?

Assign as many resonances as you can to specific carbon atoms in the ¹³C NMR spectrum of ethyl benzoate.

General Problems

3H₆O. (a)

How many double bonds and/or rings does your compound contain?

(b) Propose as many structures as you can that fit the molecular formula. (c) If your compound shows an infrared absorption peak at 1715 cm^–1, what functional group does it have? (d) If your compound shows a single ¹H NMR absorption peak at 2.1 δ, what is its structure?

The compound whose ¹H NMR spectrum is shown has the molecular formula C₃H₆Br₂. Propose a structure.

The compound whose ¹H NMR spectrum is shown has the molecular formula C₄H₇O₂Cl and has an infrared absorption peak at 1740 cm^–1. Propose a structure.

1H NMR data: (a)

• C₄H₆Cl₂
• 2.18 δ (3 H, singlet)
• 4.16 δ (2 H, doublet, J = 7 Hz)
• 5.71 δ (1 H, triplet, J = 7 Hz)

(b)

• C₁₀H₁₄
• 1.30 δ (9 H, singlet)
• 7.30 δ (5 H, singlet)

(c)

• C₄H₇BrO
• 2.11 δ (3 H, singlet)
• 3.52 δ (2 H, triplet, J = 6 Hz)
• 4.40 δ (2 H, triplet, J = 6 Hz)

(d)

• C₉H₁₁Br
• 2.15 δ (2 H, quintet, J = 7 Hz)
• 2.75 δ (2 H, triplet, J = 7 Hz)
• 3.38 δ (2 H, triplet, J = 7 Hz)
• 7.22 δ (5 H, singlet)

Long-range coupling between protons more than two carbon atoms apart is sometimes observed when π bonds intervene. An example is found in 1-methoxy-1-buten-3-yne. Not only does the acetylenic proton, H_a, couple with the vinylic proton H_b, it also couples with the vinylic proton H_c, four carbon atoms away. The data are:

Construct tree diagrams that account for the observed splitting patterns of H_a, H_b, and H_c.

The ¹H and ¹³C NMR spectra of compound A, C₈H₉Br, are shown. Propose a structure for A, and assign peaks in the spectra to your structure.

1H NMR spectra are shown. (a)

C₅H₁₀O

(b)

C₇H₇Br

(c)

C₈H₉Br

The mass spectrum and ¹³C NMR spectrum of a hydrocarbon are shown. Propose a structure for this hydrocarbon, and explain the spectral data.

Compound A, a hydrocarbon with M⁺ = 96 in its mass spectrum, has the following ¹³C spectral data. On reaction with BH₃, followed by treatment with basic H₂O₂, A is converted into B, whose ¹³C spectral data are also given. Propose structures for A and B.

Compound A

• Broadband-decoupled ¹³C NMR: 26.8, 28.7, 35.7, 106.9, 149.7 δ
• DEPT-90: no peaks
• DEPT-135: no positive peaks; negative peaks at 26.8, 28.7, 35.7, 106.9 δ

Compound B

• Broadband-decoupled ¹³C NMR: 26.1, 26.9, 29.9, 40.5, 68.2 δ
• DEPT-90: 40.5 δ
• DEPT-135: positive peak at 40.5 δ; negative peaks at 26.1, 26.9, 29.9, 68.2 δ

Propose a structure for compound C, which has M⁺ = 86 in its mass spectrum, an IR absorption at 3400 cm^–1, and the following ¹³C NMR spectral data:

Compound C

• Broadband-decoupled ¹³C NMR: 30.2, 31.9, 61.8, 114.7, 138.4 δ
• DEPT-90: 138.4 δ
• DEPT-135: positive peak at 138.4 δ; negative peaks at 30.2, 31.9, 61.8, 114.7 δ

Compound D is isomeric with compound C (Problem 13.61) and has the following ¹³C NMR spectral data. Propose a structure.

Compound D

• Broadband-decoupled ¹³C NMR: 9.7, 29.9, 74.4, 114.4, 141.4 δ
• DEPT-90: 74.4, 141.4 δ
• DEPT-135: positive peaks at 9.7, 74.4, 141.4 δ; negative peaks at 29.9, 114.4 δ

Propose a structure for compound E, C₇H₁₂O₂, which has the following ¹³C NMR spectral data:

Compound E

• Broadband-decoupled ¹³C NMR: 19.1, 28.0, 70.5, 129.0, 129.8, 165.8 δ
• DEPT-90: 28.0, 129.8 δ
• DEPT-135: positive peaks at 19.1, 28.0, 129.8 δ; negative peaks at 70.5, 129.0 δ

Compound F, a hydrocarbon with M⁺ = 96 in its mass spectrum, undergoes reaction with HBr to yield compound G. Propose structures for F and G, whose ¹³C NMR spectral data are given below.

Compound F

• Broadband-decoupled ¹³C NMR: 27.6, 29.3, 32.2, 132.4 δ
• DEPT-90: 132.4 δ
• DEPT-135: positive peak at 132.4 δ; negative peaks at 27.6, 29.3, 32.2 δ

Compound G

• Broadband-decoupled ¹³C NMR: 25.1, 27.7, 39.9, 56.0 δ
• DEPT-90: 56.0 δ
• DEPT-135: positive peak at 56.0 δ; negative peaks at 25.1, 27.7, 39.9 δ

3-Methyl-2-butanol has five signals in its ¹³C NMR spectrum at 17.90, 18.15, 20.00, 35.05, and 72.75 δ. Why are the two methyl groups attached to C3 nonequivalent? Making a molecular model should be helpful.

A ¹³C NMR spectrum of commercially available 2,4-pentanediol, shows five peaks at 23.3, 23.9, 46.5, 64.8, and 68.1 δ. Explain.

Carboxylic acids (RCO₂H) react with alcohols (R′OH) in the presence of an acid catalyst. The reaction product of propanoic acid with methanol has the following spectroscopic properties. Propose a structure.

MS: M⁺ = 88

IR: 1735 cm^–1

¹H NMR: 1.11 δ (3 H, triplet, J = 7 Hz); 2.32 δ (2 H, quartet, J = 7 Hz);

3.65 δ (3 H, singlet)

¹³C NMR: 9.3, 27.6, 51.4, 174.6 δ

Nitriles (RC$\text{≡}N)\; react\; with\; Grignard\; reagents\; (R\prime MgBr).\; The\; reaction\; product\; from\; 2-methylpropanenitrile\; with\; methylmagnesium\; bromide\; has\; the\; following\; spectroscopic\; properties.\; Propose\; a\; structure.$

MS: M⁺ = 86

IR: 1715 cm^–1

¹H NMR: 1.05 δ (6 H, doublet, J = 7 Hz); 2.12 δ (3 H, singlet);

2.67 δ (1 H, septet, J = 7 Hz)

¹³C NMR: 18.2, 27.2, 41.6, 211.2 δ

The proton NMR spectrum is shown for a compound with the formula C₅H₉NO₄. The infrared spectrum displays strong bands at 1750 and 1562 cm^–1 and a medium-intensity band at 1320 cm^–1. The normal carbon-13 and the DEPT experimental results are tabulated. Draw the structure of this compound.

Normal CarbonDEPT-135DEPT-90     14 ppm Positive No peak  16 Positive No peak  63 Negative No peak  83 Positive Positive 165 No peak No peak

The proton NMR spectrum of a compound with the formula C₅H₁₀O is shown. The normal carbon-13 and the DEPT experimental results are tabulated. The infrared spectrum shows a broad peak at about 3340 cm^–1 and a medium-sized peak at about 1651 cm^–1. Draw the structure of this compound.

Normal CarbonDEPT-135DEPT-90     22.2 ppm Positive No peak  40.9 Negative No peak  60.2 Negative No peak 112.5 Negative No peak 142.3 No peak No peak

The proton NMR spectrum of a compound with the formula C₇H₁₂O₂ is shown. The infrared spectrum displays a strong band at 1738 cm^–1 and a weak band at 1689 cm^–1. The normal carbon-13 and the DEPT experimental results are tabulated. Draw the structure of this compound.

Normal CarbonDEPT-135DEPT-90     18 ppm Positive No peak  21 Positive No peak  26 Positive No peak  61 Negative No peak 119 Positive Positive 139 No peak No peak 171 No peak No peak