15. Analytical Techniques:IR, NMR, Mass Spect

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 δ6.10, δ4.33, and δ7.52. Match each chemical shift with the compound. Explain how chemical shift correlates with pK_a.

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?

<IMAGE>

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

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