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Lecture Outline 1H NMR Spectra of Aromatic Compounds. (Text Sections 16.9, 16.10) Aromatic Hs Δ

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Lecture Outline 1H NMR Spectra of Aromatic Compounds. (Text Sections 16.9, 16.10) Aromatic Hs Δ Lecture outline 1H NMR spectra of aromatic compounds. (text sections 16.9, 16.10) Aromatic Hs δ 6.5 - 8.5 — downfield of vinylic Hs. Why? Of all the possible orientations that an aromatic ring can adopt relative to the applied magnetic field, orientations near the one below generate the largest effect. H B0 – π-e s circulate ("aromatic ring current") to create a magnetic field that opposes B0 in center of ring, and thus adds to it on outside. Protons outside the ring therefore feel an increased magnetic field (same effect as deshielding) and appear downfield (rel to vinylic Hs). In contrast, protons inside or above an aromatic ring feel a decreased magnetic field (same effect as shielding) and appear far upfield, sometimes at negative δ values. [18]-annulene perimeter Hs ! 8.14 - 8.67 outer Hs ! 9 methyl Hs ! – 4.25 inner Hs ! –3 Coupling between benzene ring Hs is nothing new — adjacent Hs couple (J ≈ 6 - 10 Hz) if they're different. Long-range (meta, "4-bond") coupling is also observed (J is small). How many different signals, and what splitting patterns would you expect for symmetrically substituted ortho, meta, and para benzenes (i.e. with two identical substituents on the ring)? What about unsymmetrically substituted o-, m-, and p-benzenes? The circulation of π-electrons in response to a magnetic field is not unique to aromatic compounds, although aromatic rings do show the most dramatic effects. Electrons in simple π- bonds and even e–s in σ-bonds can circulate and generate small local magnetic fields that influence the chemical shifts of nearby protons. This effect is responsible for the characteristic δ values of alkene and alkyne Hs — H H ... as well as those of Hs on sp3 Cs. (This is the real source of what we call "shielding".) NMR data tables can provide guidelines as to typical chemical shift ranges for certain structural units. The correct way to use a data table is from structural piece to δ value. Starting with a δ value and hunting for possible structural units is worse than useless. The table below, for example, is from your textbook. Notice that the δ values listed are average, or typical, or approximate δs. Note that they're for methyl groups only. The assumption is that a methylene will be a bit farther downfield and a methine a bit farther than that. How much farther varies unpredictably, so there's another bit of uncertainty. The table is useful for giving us an idea of how much certain substituents can be expected to affect the chemical shift of a particular set of Hs. In cases where several groups gang up on one set of Hs, the effects are usually additive. Let's see how these data might help us interpret the NMR spectrum below. Free radical bromination of 2-methylindene with 2 equivalents of NBS, initiated with AIBN and heat was expected to produce the dibromo compound shown. Maybe it did, maybe it didn't. Hmmmm. Let's take a look at the 1H NMR and see... Br NBS (2 equiv) PhH AIBN ! Br Here's the 400-MHz 1H NMR spectrum. Br H H (and its enantiomer) Br Are the δ values all reasonable? Integration? Splitting? Why are there two doublets around δ 4.5? Shouldn't there be a 2-H singlet there? Let's look more closely... .
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