Structure Determination: Nuclear Magnetic Resonance Spectroscopy

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

Assign as many resonances as you can to specific carbon atoms in the ¹³C

 NMR spectrum of ethyl benzoate.

Assume that you have a compound with the formula C₃H₆O .

(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 1715cm^-1,

what functional group does it have?

(d) If your compound shows a single ¹H NMR absorption peak at 2.1$\mathrm{\delta }$,

what is its structure?

The compound whose ¹H NMR spectrum is shown has the molecular

formula C₄H₇O₂CI and has an infrared absorption peak at 1740cm-1.

Propose a structure.

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

a.

2.18(3H, singlet)

4.16(2H, doublet j=7Hz)

5.71(1H, triplet j=7Hz)

b. 

1.30(9H, singlet)

7.30(5H, singlet)

c. 

2.11(3H, singlet)

3.52(2H, triplet j=6Hz)

5.71(2H, triplet j=6Hz)

d.

2.15(2H, quintent j=7Hz)

2.75(2H, triplet j=7Hz)

3.38(2H, triplet j=7Hz)

7.22(5H, singlet)

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

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

Question: The NMR spectra of compound A, , are shown. Propose a structure for A, and assign peaks in the spectra to your structure.

Question: Propose structures for the three compounds whose NMR spectra are shown. 

$a. C_5{H}_{10}O$

$b. C_7{H}_{7}Br$

$c. C_8{H}_{9}Br$

Question:  The mass spectrum and ${}^{13}C$ 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 ${}^{13}C$ spectral data given below. On reaction with $B{H}_3$ , followed by treatment with basic $H_2{O}_2$, A is converted into B, whose ${}^{13}C$ spectral data are also given below. Propose structures for A and B. 

 Compound A Broadband-decoupled ${}^{13}C$ NMR: 26.8, 28.7, 35.7, 106.9, 149.7 d DEPT-90: no peaks DEPT-135: no positive peaks; negative peaks at 26.8, 28.7, 35.7, 106.9 d 

 Compound B Broadband-decoupled ${}^{13}C$ NMR: 26.1, 26.9, 29.9, 40.5, 68.2 d DEPT-90: 40.5 d DEPT-135: positive peak at 40.5 d; negative peaks at 26.1, 26.9, 29.9, 68.2 d

Propose a structure for compound C, which has ${\mathit{M}}^{\mathbf{+}}$= 86 in its mass spectrum, an IR absorption at 3400$\mathit{c}{\mathit{m}}^{\mathbf{-}\mathbf{1}}$ , and the following ${}^{\mathbf{13}}\mathit{C}$ NMR spectral data:

Compound C Broadband-decoupled ${}^{\mathbf{13}}\mathit{C}$ NMR: 30.2, 31.9, 61.8, 114.7, 138.4 d DEPT-90: 138.4 d DEPT-135: positive peak at 138.4 d; negative peaks at 30.2, 31.9, 61.8, 114.7 d

Compound D is isomeric with compound C (Problem 13-61) and has the following ${}^{\mathbf{13}}\mathit{C}$ NMR spectral data. Propose a structure.

Compound D Broadband-decoupled ${}^{\mathbf{13}}\mathit{C}$ NMR: 9.7, 29.9, 74.4, 114.4, 141.4 d DEPT-90: 74.4, 141.4 d DEPT-135: positive peaks at 9.7, 74.4, 141.4 d; negative peaks at 29.9, 114.4 d

Propose a structure for compound E, ${\mathbf{C}}_{\mathbf{7}}{\mathbf{H}}_{\mathbf{12}}{\mathbf{O}}_{\mathbf{2}}$ , which has the following ${}^{\mathbf{13}}\mathbf{C}$   NMR spectral data: 

Compound E Broadband-decoupled ${}^{\mathbf{13}}\mathbf{C}$   NMR: 19.1, 28.0, 70.5, 129.0, 129.8, 165.8$\mathbf{\delta }$ DEPT-90: 28.0, 129.8 $\mathbf{\delta }$  DEPT-135: positive peaks at 19.1, 28.0, 129.8$\mathbf{\delta }$ ; negative peaks at 70.5, 129.0 $\mathbf{\delta }$

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

a. Compound F Broadband-decoupled ${}^{\mathbf{13}}\mathbf{C}$ NMR: 27.6, 29.3, 32.2, 132.4 d DEPT-90: 132.4 d DEPT-135: positive peak at 132.4 d; negative peaks at 27.6, 29.3, 32.2 d

b. Compound G Broadband-decoupled ${}^{\mathbf{13}}\mathbf{C}$ NMR: 25.1, 27.7, 39.9, 56.0 d DEPT-90: 56.0 d DEPT-135: positive peak at 56.0 d; negative peaks at 25.1, 27.7, 39.9 d

3-Methyl-2-butanol has five signals in its ${}^{\mathbf{13}}\mathbf{C}$NMR spectrum at 17.90, 18.15, 20.00, 35.05, and 72.75 d. Why are the two methyl groups attached to C3 non- equivalent? Making a molecular model should be helpful.

A ${}^{\mathbf{13}}\mathbf{C}$  NMR spectrum of commercially available 2,4-pentanediol, shows five peaks at 23.3, 23.9, 46.5, 64.8, and 68.1 d. Explain.

Carboxylic acids $\mathbf{\left(}{\mathbf{RCO}}_{\mathbf{2}}\mathbf{H}\mathbf{\right)}$ 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: ${\mathbf{M}}^{\mathbf{+}}$ = 88

IR: 1735 ${\mathbf{cm}}^{\mathbf{-}}$

${}^{\mathbf{1}}\mathbf{H}$NMR: 1.11$\mathit{\delta }$ (3 H, triplet, J 5 7 Hz); 2.32$\mathit{\delta }$ (2 H, quartet, J 5 7 Hz); 3.65 $\mathit{\delta }$(3 H, singlet) ${}^{\mathbf{13}}\mathbf{C}$  NMR: 9.3, 27.6, 51.4, 174.6 $\mathit{\delta }$

Nitriles (RC≡N) react with Grignard reagents (R'MgBr). The reaction product from 2-methylpropanenitrile with methyl magnesium bromide has the following spectroscopic properties. Propose a structure.

MS: ${\mathbf{M}}^{\mathbf{+}}$ =86

 IR: 1715 ${\mathbf{cm}}^{\mathbf{-}}$

${}^{\mathbf{1}}\mathbf{H}$ NMR: 1.05 $\mathbf{\delta }$ (6 H, doublet, J 5 7 Hz); 2.12$\mathbf{\delta }$ (3 H, singlet); 2.67 $\mathit{\delta }$(1 H, septet, J 5 7 Hz) ${}^{\mathbf{13}}\mathbf{C}$  NMR: 18.2, 27.2, 41.6, 211.2 $\mathit{\delta }$

The proton NMR spectrum is shown for a compound with the formula ${\mathbf{C}}_{\mathbf{5}}{\mathbf{H}}_{\mathbf{9}}{\mathbf{NO}}_{\mathbf{4}}$. The infrared spectrum displays strong bands at 1750 ${\mathbf{cm}}^{\mathbf{-}}$ and 1562 ${\mathrm{cm}}^{-}$ and a medium-intensity band at 1320 ${\mathrm{cm}}^{-}$. The normal carbon-13 and the DEPT experimental results are tabulated. Draw the structure of this compound.

The proton NMR spectrum of a compound with the formula C₅H₁₀O isshown. The normal carbon-13 and the DEPT experimental results aretabulated. The infrared spectrum shows a broad peak at about3340 cm^-1 and a medium-sized peak at about 1651 cm^-1. Draw the structure of thiscompound.

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.