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p.282 of Clayden:Organic Chemistry 2nd ed.

I was studying the 1H NMR chapter of Clayden: Organic Chemistry 2nd ed. until I came across this case and became curious about integral numbers of protons because it didn't seem right. Then I measured them and the ratios from left to right were 1:1:2.16:3.28. However it should have been 1:1:4:6 according to the structure of compound. Why is it?

I am also curious about the chemical shift of red and green protons. both should be around 5.7 according to additive guidelines for determining chemical shift provided in the book. Basically saying for CH proton we should start from 1.7 and add 2 for each of Halogens or ethers present. The red proton seems ok but the green one is too far from expected value. Can you provide an explanation?

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    $\begingroup$ Iirc 100 MHz doesn't give you a very good spectrum. But I'm out of the game for too long. $\endgroup$ Commented Jun 28, 2023 at 21:05
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    $\begingroup$ @Martin-マーチン it is not very good in terms of resolution. But a 100 is not very different from a 300 in terms of accuracy of integrals (or should not be, at least). $\endgroup$ Commented Jun 29, 2023 at 22:04

2 Answers 2

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The error in the integrals in the 100 MHz 1H NMR spectrum of the diethyl acetal of dichloroacetaldehyde is only one of the problems. The spectrum is simulated and it is incorrect. ChemDraw produces a similar spectrum and it is also incorrect (Fig. 1). The methylene protons at δ 3.70 are not chemically nor magnetically equivalent but rather they are non-equivalent and diastereotopic.

Fig. 1
Consider the 90 MHz spectrum of acetaldehyde diethyl acetal from the SDBS Spectral Data Base (Fig. 2). Notice that the pattern in the region of δ 3.5 is more complex than a simple quartet. Each methylene carbon bears two protons, Ha and Hb, that are coupled to each other as well as to the methyl group. This issue has been addressed previously on ChemSE.

Fig.2a

Fig.2b

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In general, you should not expect NMR integrals to be perfect. There are several reasons for this. Two main ones are:

  1. The integral in the spectrum depends on the proportion of the spins that are aligned with the magnetic field before the experiment—this is called the polarisation. In principle, you would like this quantity to be the same for all the spins in the molecule. In practice, it isn't, because different spins relax at different rates.

  2. Different spins are affected differently by the radiofrequency used. For a given pulse length and power, you will excite a different proportion of one spin than another.

Avoiding these and getting quantitative results requires setting up your NMR experiment carefully. This isn't done routinely because it takes time (in particular, you need to use a longer relaxation delay, i.e. a larger gap between experiments to allow spins to regain their polarisation). Furthermore, super-accurate integrals are not often needed for routine analysis.

For more information see section 4.1.2 of Claridge, High-Resolution NMR Techniques in Organic Chemistry (3rd ed.).

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