The methyl hydrogens in propane appear at a chemical shift of ...

Question

The methyl hydrogens in propane appear at a chemical shift of 0.9 ppm, whereas the methyl hydrogens of propene appear around 2.5 ppm. Explain.

Accepted Answer

All right. Hi, everyone. So this question says that the methyl highlighted in the isoptin structure appears at 0.87 parts per million while that of two methyl, but two E appears at 1.56 parts per million. Excuse me, rationalize this observation. So before we go ahead and talk about what chemical shift actually tells us in a proton NMR spectrum. What I want to point out here is the primary difference between both protons that are listed here because the first one highlighted in red belongs to the methyl proton of isoptin, which is an A proton. However, the one highlighted in blue, which corresponds to that of two methyl buttu E appears at 1.56 parts per million and that one isn't a lilic proton because recall that the aic position refers to carbons that are adjacent to a double bond but not part of it. And the reason why I feel the need to bring this up first is because the chemical shift of any given proton on a proton and a mar spectrum is so heavily dependent on its chemical environment, right? And when we discuss chemical environment, we have to differentiate or rather distinguish what makes a proton relatively shielded and what makes that a different proton relatively d shielded. Now, to understand this concept of shielding versus de shielding, we have to recall that the electrons surrounding a proton right inside of a nucleus are would protect the proton in question from NMR effects. Now, when a proton is referred to as shielded, right, what it's saying is that the proton in question is not exposed or it's not exposed, at least to a great extent to the NMR effects. So as a result, right, protons that are relatively shielded, that are not subject to nr effects to such a great extent, they're described as appearing up fields in the NAR spectrum. In other words, to the right, right. And when we say that a proton appears to the right of the spectrum in its more upfield, then the value of the chemical shift in parts per million is going to be smaller. Now, by contrast, right, when we say that a proton is d shielded, then what we're seeing is that this proton, the one that's de shielded is more exposed to the Anmar effect. Now, there are multiple ways that this can happen. The most common one being, for example, protons that are on carbons that are next to electron atoms because the electron atom is going to pull electron density away from the nucleus and therefore expose the proton more. So when we describe the location of D shielded protons on an NMR spectrum, we describe them as downfield, we describe them as downfield and appear to the left of the spectrum. Therefore, having a higher value in parts per million of the chemical shift. So when comparing our values again for the chemical shifts here, the oop proton is going to be upfield, whereas the AIC proton is going to be downfield. So with this comparison in mind, right, we have to ask ourselves, why is the aic proton more down field, more d shielded compared to the O proton? Because if we examine the structure of two methyl, but two in right, the aly carbon is bound to another carbon. So there's no difference in electron negativity per se. But the fact that the aly carbon is right next to a pi bond is going to play a role in answering this question. So the reason here if you recall has to do with the fact that the pie system create what's known as magnetic a nice. So choppy. So let's get into what this actually means. Right. Now, the word an isotropic in an an isotropy refers to or rather means quote unquote, non uniform. So when we describe magnetic and isotropy, we're referring to a non uniform magnetic field. Now, when we talk about the magnetic and isotropic effect and isotropic, excuse me, it may be easier to imagine this with a benzene ring, for example, now what the magnetic and isotropic effect is essentially saying is that electrons in pi systems such as akines aromatic molecules etc will interact with the applied field of the NMR spectrometer, which will then create a non uniform magnetic field that neighboring protons might experience differently. So if we go back to our benzene ring right, that I drew on the screen here for the sake of the example, then the magnetic field that's caused by the pie system looks a little bit like concentric circles, right? And these concentric circles are going to affect how neighboring protons are going to experience the overall magnetic field. Why? Because if we examine the center of the P system, then what's happening is that these field lines as we approach, the center are going to oppose the magnetic field of the exterior, right or the applied magnetic field. So imagine here that the applied magnetic field is going upwards. So if we examine the center of the pie system, right, the magnetic field of the P system is actually opposing that of the applied magnetic field, which actually is going to result in shielding. But if we examine the exterior right, if we examine the outer region of the pi system, then this magnetic fields is actually aligning with the applied magnetic field. And when it aligns with the exterior or applied magnetic field, then this is equivalent to D shielding. So because my a lilic proton here is in the exterior of the Pie system because it's next to the proton or rather the pie bond, but not part of it that creates an effect equivalent to D shielding according to the magnetic and isotropic effect. So let's go ahead and recap our answer. So our answer here as to why we observe the observation that we do observe is because the presence of an SP two carbon atom creates magnetic an isotropy which D shields the nearby proton and shifts the NMR signal downfield. So there you have it, the presence of an sp two carbon atom creates magnetic and isotropy which DeShields the nearby proton and shifts the NMR signal downfield. And the reason why that happens is because the pie system of the A K create a non uniform magnetic field, which actually is going to align with the exterior of the pie system, namely the aic proton here and therefore make the signal appear more downfield. So with all that being said, thank you so very much for watching. And I hope we found this helpful.

The methyl hydrogens in propane appear at a chemical shift of 0.9 ppm, whereas the methyl hydrogens of propene appear around 2.5 ppm. Explain.

Key Concepts

Chemical Shift in NMR Spectroscopy

Shielding and Deshielding Effects

Effect of Pi Bonds on Chemical Shift

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