15. Analytical Techniques:IR, NMR, Mass Spect

Which of the labeled hydrogens will be most de-shielded? 

Which compound possesses a hydrogen with the highest chemical shift for its 1H NMR signal? 

Which of the following chemical shift values could plausibly correspond to the protons in divinyl ether (CH₂=CH–O–CH=CH₂)

Predict the approximate chemical shifts of the protons in the following compounds. (c) CH3-O-CH2CH2CH2Cl (d) CH3CH2-C=C-H

The NMR spectrum of toluene (methylbenzene) was shown in Figure 13-11. (a) How many different kinds of protons are there in toluene? (b) Explain why the aromatic region around d7.2 is broad, with more than one sharp absorption.

Predict the chemical shifts of the protons in the following compounds. (a)

(b)

In a 300-MHz spectrometer, the protons in iodomethane absorb at a position 650 Hz downfield from TMS. (a) What is the chemical shift of these protons? (b) What is the chemical shift of the iodomethane protons in a 60-MHz spectrometer? (c) How many hertz downfield from TMS would they absorb at 60 MHz?

Without referring to Table 14.1, label the proton or set of protons in each compound that gives the signal at the lowest frequency a, at the next lowest b, and so on. h.

Without referring to Table 14.1, label the proton or set of protons in each compound that gives the signal at the lowest frequency a, at the next lowest b, and so on. e.

Label each set of chemically equivalent protons, using a for the set that will be at the lowest frequency in the 1H NMR spectrum, b for the next lowest, and so on. Indicate the multiplicity of each signal. a.

Explain the relative chemical shifts of the benzene ring protons in Figure 14.18.

Without referring to Table 14.1, label the proton or set of protons in each compound that gives the signal at the lowest frequency a, at the next lowest b, and so on. c.

Explain why the chemical shift of the OH proton of a carboxylic acid is at a higher frequency than the chemical shift of an OH proton of an alcohol.

Which underlined proton (or sets of protons) has the greater chemical shift (that is, the higher frequency signal)? a.

b.

Which underlined proton (or sets of protons) has the greater chemical shift (that is, the higher frequency signal)? c.

d.

The methyl hydrogens in propane appear at a chemical shift of 0.9 ppm, whereas the methyl hydrogens of propene appear around 2.5 ppm. Explain.

Alkene hydrogens usually appear at similar chemical shifts between 5 and 6 ppm. The alkene hydrogens in the structure shown are very different. Rationalize the difference in chemical shifts between Hₐ and H₆ in the structure shown.

(•) For the following molecules, give the chemical shift for each indicated hydrogen. (d)

(•) For the following molecules, give the chemical shift for each indicated hydrogen. (e)

(•) For the following molecules, give the chemical shift for each indicated hydrogen. (b)

Compound A with molecular formula C6H10 has two peaks in its 1H NMR spectrum, both of which are singlets (with ratio 9 : 1). Compound A reacts with an acidic aqueous solution containing mercuric sulfate to form compound B, which gives a positive iodoform test (Problem 58) and has an 1H NMR spectrum that shows two singlets (with ratio 3 : 1). Identify A and B.

The 1H NMR chemical shifts of nitromethane, dinitromethane, and trinitromethane are at d 6.10, d 4.33, and d 7.52. Match each chemical shift with the compound. Explain how chemical shift correlates with pKa.

Which set of underlined hydrogens has its 1H NMR signal at a higher frequency? a. CH3CH2CH3 or CH3OCH2CH3 b. CH3CH=CH2 or CH3OCH=CH2

How would you expect the IR and ¹H NMR spectra for propanamide and N,N-diethylpropanamide to differ?

The chemical shifts of the C-2 hydrogen in the spectra of pyrrole, pyridine, and pyrrolidine are 2.82 ppm, 6.42 ppm, and 8.50 ppm. Match each heterocycle with its chemical shift.

[18]-Annulene shows two signals in its 1H NMR spectrum: one at 9.25 ppm and the other to the right of the TMS signal at -2.88 ppm. What hydrogens are responsible for each of the signals? (Hint: Look at the direction of the induced magnetic field outside and inside the benzene ring in Figure 14.6.)

An unknown compound (C₇H₆O) gives the IR spectrum shown here. At what chemical shifts would you expect to see signals in the ¹H NMR spectrum?

Predict the theoretical number of different NMR signals produced by each compound, and give approximate chemical shifts. Point out any diastereotopic relationships. (c) Ph—CHBr—CH2Br (d) vinyl chloride

b. Which pairs are diastereotopic hydrogens?

Predict the theoretical number of different NMR signals produced by each compound, and give approximate chemical shifts. Point out any diastereotopic relationships. (a) 2-bromobutane (b) cyclopentanol