BackThe NMR spectrum of toluene (methylbenzene) was shown in Figure 13-11.
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(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.
Which compound possesses a hydrogen with the highest chemical shift for its 1H NMR signal?
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.
[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.)
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.
Which underlined proton (or sets of protons) has the greater chemical shift (that is, the higher frequency signal)?
c.
d.
Explain the relative chemical shifts of the benzene ring protons in Figure 14.18.
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Which of the labeled hydrogens will be most de-shielded?
Predict the chemical shifts of the protons in the following compounds.
(a)
(b)
Which pairs are diastereotopic hydrogens?
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.
Which of the labeled protons absorbs energy most upfield in the 1H NMR?
For the following molecules, give the chemical shift for each indicated hydrogen.
(b)
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.
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.