Protons directly attached to O, N, or S (or any other heteroatom, but these are the most important) also have signals in the NMR spectrum. We have avoided them so far because the positions of these signals are less reliable and because they are affected by exchange.

In Chapter 2 you met the antioxidant BHT. Its proton NMR is very simple, consisting of just four lines with integrals 2, 1, 3, and 18. The chemical shifts of the tert-butyl group (brown), the methyl group on the benzene ring (orange), and the two identical aromatic protons (green) should cause you no surprise. What is left, the ¹H signal at 5.0 ppm (pink), must be the OH. Earlier on in this chapter we saw the spectrum of acetic acid, CH₃CO₂H, which showed an OH resonance at 11.2 ppm. Simple alcohols such as tert-butanol have OH signals in CDCl₃ (the usual NMR solvent) at around 2 ppm. Why such big differences?

This is a matter of acidity. The more acidic a proton is—that is, the more easily it can escape as H+ (this is the defi nition of acidity from Chapter 8)—the more the OH bond is polarized towards oxygen. The more the RO–H bond is polarized, the closer we are to free H+, which would have no shielding electrons at all, and so the further the proton goes downfi eld. The OH chemical shifts and the acidity of the OH group are—to a rough extent at least—related. Thiols (RSH) behave in a similar way to alcohols but are not so deshielded, as you would expect from the smaller electronegativity of sulfur (phenols are all about 5.0 ppm, PhSH is at 3.41 ppm). Alkane thiols appear at about 2 ppm and arylthiols at about 4 ppm. Amines and amides show a big variation, as you would expect for the variety of functional groups involved, and are summarized below. Amides are slightly acidic, as you saw in Chapter 8, and amide protons resonate at quite low fi elds. Pyrroles are special—the aromaticity of the ring makes the NH proton unusually acidic—and they appear at about 10 ppm.

مواضيع ذات صلة

Some compounds can exist as a pair of mirror-image forms

Coupling in the proton NMR spectrum Nearby hydrogen nuclei interact and give multiple peaks

Exchange of acidic protons is revealed in proton NMR spectra

Non-aldehyde protons in the aldehyde region: pyridines

The aldehyde region: unsaturated carbon bonded to oxygen

Structural information from the alkene region

Electron-rich and electron-deficient alkenes

How electron donation and withdrawal change chemical shifts

Uneven electron distribution in aromatic rings

The benzene ring current causes large shifts for aromatic protons

Summary chart of proton NMR shifts

Chemical shifts of CH groups