For the following molecules, give the chemical shift for each ...

Question

For the following molecules, give the chemical shift for each indicated hydrogen.

(d) Chemical structure of a pyridine ring with a methyl group attached to nitrogen, hydrogens labeled H1 and H2.

Accepted Answer

All right. Hi, everyone. So for this question, let's go ahead and predict the chemical shifts of the labeled hydrogens with our starting material being 125 tri methyl, 25 dihydrate one HP. Now recall that the chemical shift in a proton in a more spectrum and a carbon 13 in a more spectrum for that matter as well is essentially telling you the location of that signal in the spectrum itself. Now, the chemical shift is expressed in units of parts per million. So when we discuss the position of that signal in the spectrum, we have to consider what factors affect the location itself. Eight. So to do that, we have to understand what makes a proton relatively shielded or shielding effects and d shielding effects. Now recall that electrons are which shield protons from NMR effects because they're in the exterior. So when we talk about a proton being relatively shielded or that proton having a shielding effect, what we're seeing here is that the proton in question is not affected or not as affected by the NMR process itself, right. So when we talk about a proton being shielded compared to others, we describe that signal as appearing more upfield, which I did not spell, right, give me one second, but we describe it as being more upfield or appearing more to the right side of the NMR spectrum, which means that the value of the chemical shift in parts per million is going to be lower or smaller. Now, by contrast, right, when we discuss ad shielding effect on a given proton, what we're doing here is we're discussing or describing protons that are more affected by the NMR process, right by NMR effects. So as a result of that right, ad shielded signal or D shielded proton is going to appear more downfield in a given nr spectrum. In other words, to the left of the spectrum itself, which means that the value of the chemical shift is going to be higher. Now, the most common or one of the common ways that a proton can appear more de shielded. For example is, if we have a proton that's connecting to an electron atom like oxygen, why? Because by having that oxygen pull electron density away from that hydrogen, then the proton inside the nucleus is going to be more de shielded, right? Because electrons are moving away and they're not guarding the nucleus as much. So the presence of electron withdrawing or electron groups surrounding a given proton can de shield it, whereas electron donating groups can shield it. Now, if we look at H A first depicted in red here H A is bound directly to one of the sp two hybridized carbons in this all Keen here, which means that H A is in the vanilli position. Now py bonds like this. All keen here are very interesting because they create what's called an anisotropic an isotropic effect. So let's talk about what that means, right? And to illustrate the example or an example of the anisotropic effect, I'm going to go ahead and draw a benzene ring on the bottom of the screen here. Now the term an isotropic means a non uniform. So the anisotropic or magnetic and isotropic effect describes a non uniform magnetic field. Because what is describing here is the fact that the pie system by itself creates its own magnetic field that can interact with an exterior or applied magnetic field. So if we take benzene, for example, right, like I have on the screen, let's say that the applied magnetic field of the NMR spectrometer is pointing upwards. Now the magnetic field of the pi system is going to appear as these concentric circles right orbiting the benzene itself or the benzene ring. So as a result of this circling right, this this orbiting pattern of the pi system, then different parts of the pie system are going to experience different magnetic fields. Because if we look at the interior of the pie system, right, if I look at the interior, then these arrows that represent the magnetic field are pointing downwards which goes against the applied magnetic field. So when the magnetic field induced by pi electrons is actually going against the applied magnetic field in the exterior that serves to actually shield those protons, which means that they would appear more upfield. Now, by contrast, if I look at the exterior right outside of the benzene ring than the magnetic field in this region is actually reinforcing the applied magnetic field because notice how they're both pointing upwards right in the same direction. So that in the exterior creates a b shielding effect. So the vanilli H A here is on the exterior of the magnetic field induced by the pi system of the ain, which means that that proton, that hydrogen is going to be de shielded. So it's t shielded. And in general, vinyl protons like H A are going to appear at around or in between 4.5 and six parts per million due to the D shielding caused by the magnetic and isotropic effect. So if we go ahead and re examine hp now, right, HB is connecting to a carbon that is connected to a nitrogen. Now, the presence of nitrogen which is electron is going to create a desiel effect because nitrogen is going to pull electron density towards itself, right, which creates a de shielding effect on the carbon adjacent to it and therefore the protons associated to that carbon. So therefore, right protons that are attached to carbons that are attached to electron atoms tend to fall in between approximately 2 to 3 parts per million because they definitely appear more de shielded or more downfield compared to all cane hydrogens that tend to have very low chemical shifts of about 1 to 2 parts per million. And there you have it right. So H A is going to appear in between 4.5 to 6 parts per million where is HB is going to appear between 2 to 3 parts per million. And this all depends on examining the shielding or de shielding effects that change where the chemical shift actually is. So if we stuck around to the end, thank you so very much for watching. And I hope we found this helpful.

For the following molecules, give the chemical shift for each indicated hydrogen.
(d)

Key Concepts

Chemical Shift

NMR Spectroscopy

Factors Affecting Chemical Shift

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