15. Analytical Techniques:IR, NMR, Mass Spect

Identify the indicated set of protons as unrelated, homotopic, enantiotopic, or diastereotopic.

How can 1,2-, 1,3-, and 1,4-dinitrobenzene be distinguished by

a. ¹H NMR spectroscopy?

Replace Hₐ, H₆, and H꜀ in methyl benzene with a deuterium. What is the relationship between the three products you obtain? Based on this, how many signals would you expect for these hydrogens in the ¹H NMR spectrum?

Replace Hₐ and H₆ in acetone with a deuterium. What is the relationship between the two products you obtain? Based on this, would you expect the two hydrogens to give one or two signals in the ¹H NMR spectrum?

For the following compounds, which pairs of hydrogens (H_a and H_b) are enantiotopic hydrogens?

Identify the indicated set of protons as unrelated, homotopic, enantiotopic, or diastereotopic.

If rotation is restricted, as in the case of the molecule shown, the hydrogens labeled a and b are nonequivalent. Why?

Phenyl Grignard reagent adds to 2-methylpropanal to give the secondary alcohol shown. The proton NMR of 2-methylpropanal shows the two methyl groups as equivalent (one doublet at δ1.1), yet the product alcohol, a racemic mixture, shows two different 3H doublets, one at δ0.75 and one around δ1.0.

(a) Draw a Newman projection of the product along the C1–C2 axis.

(b) Explain why the two methyl groups have different NMR chemical shifts. What is the term applied to protons such as these?

If the imaginary replacement of either of two protons forms enantiomers, then those protons are said to be enantiotopic. The NMR is not a chiral probe, and it cannot distinguish between enantiotopic protons. They are seen to be “equivalent by NMR.”

(a) Use the imaginary replacement technique to show that the two allylic protons (those on C₃) of allyl bromide are enantiotopic.

For the following compounds, which pairs of hydrogens (H_a and H_b) are enantiotopic hydrogens?

1.

Identify the indicated set of protons as unrelated, homotopic, enantiotopic, or diastereotopic.