Скачать или смотреть Chapter 11: Proton Nuclear Magnetic Resonance | Introduction | Organic Chemistry by Clayden-Warren

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In this video, we will give introduction to Proton Nuclear Magnetic Resonance (NMR). This is the series of videos on the book of Organic Chemistry by Clayden - Greeves - Warren. This organic chemistry video tutorial focuses on Chapter 11 (Proton Nuclear Magnetic Resonance (NMR)).

The differences between carbon and proton NMR
We used 13C NMR in Chapter 3 as part of a three-pronged attack on the problem of determining molecular structure. Important though these three prongs are, we were forced to confess at the end of Chapter 3 that we had delayed the most important technique of all—proton (1 H) NMR—until a later chapter because it is more complicated than 13C NMR. This is that delayed chapter and we must now tackle those complications. We hope you will see 1 H NMR for the beautiful and powerful technique that it surely is. The difficulties are worth mastering for this is the chemist’s primary weapon in the battle to solve structures. Proton NMR differs from 13C NMR in a number of ways. • 1 H is the major isotope of hydrogen (99.985% natural abundance), while 13C is only a minor isotope (1.1%) • 1 H NMR is quantitative: the area under the peak tells us the number of hydrogen nuclei, while 13C NMR may give strong or weak peaks from the same number of 13C nuclei • Protons interact magnetically (‘couple’) to reveal the connectivity of the structure, while 13C is too rare for coupling between 13C nuclei to be seen • 1 H NMR shifts give a more reliable indication of the local chemistry than that given by 13C spectra We shall examine each of these points in detail and build up a full understanding of proton NMR spectra. The other spectra remain important, of course. Proton NMR spectra are recorded in the same way as 13C NMR spectra: radio waves are used to study the energy level differences of nuclei, but this time they are 1 H and not 13C nuclei. Hydrogen nuclei have a nuclear spin


Protons on saturated carbon atoms
Chemical shifts are related to the electronegativity of substituents
We shall start with protons on saturated carbon atoms. If you study Table 11.1 you will see that the protons in a methyl group are shifted more and more as the atom attached to them gets more electronegative.

Proton chemical shifts tell us about chemistry
The truth is that shifts and electronegativity are not perfectly correlated. The key property is indeed electron withdrawal but it is the electron-withdrawing power of the whole substituent in comparison with the carbon and hydrogen atoms in the CH skeleton that matters. Methyl groups joined to the same element, say, nitrogen, may have very different shifts if the substituent is an amino group (CH3–NH2 has δH for the CH3 group = 2.41 p.p.m.) or a nitro group (CH3–NO2 has δH 4.33 p.p.m.). A nitro group is much more electron-withdrawing than an amino group.

Methyl groups give us information about the structure of molecules
It sounds rather unlikely that the humble methyl group could tell us much that is important about molecular structure—but just you wait. We shall look at four simple compounds and their NMR spectra— just the methyl groups, that is. The first two are the acid chlorides on the right.

Chemical shifts of CH2 groups
Shifts of the same order of magnitude occur for protons on CH2 groups and the proton on CH groups, but with the added complication that CH2 groups have two other substituents and CH groups three. A CH2 (methylene) group resonates at 1.3 p.p.m., about 0.4 p.p.m. further downfield than a comparable CH3 group (0.9 p.p.m.), and a CH (methine) group resonates at 1.7 p.p.m., another 0.4 p.p.m. downfield. Replacing each hydrogen atom in the CH3 group by a carbon atom causes a small downfield shift as carbon is slightly more electronegative (C 2.5 p.p.m.; H 2.2 p.p.m.) than hydrogen and therefore shields less effectively.

Chemical shifts of CH groups
A CH group in the middle of a carbon skeleton resonates at about 1.7 p.p.m.—another 0.4 p.p.m. downfield from a CH2 group. It can have up to three substituents and these will cause further downfield shifts of about the same amount as we have already seen for CH3 and CH2 groups. Here are three examples from nature: nicotine, the compound in tobacco that causes the craving (though not the death, which is doled out instead by the carbon monoxide and tars in the smoke), has one hydrogen atom trapped between a simple tertiary amine and an aromatic ring at 3.24 p.p.m. Lactic acid has a CH proton at 4.3 p.p.m..

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