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

Question

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

(e) Chemical structure showing ester with labeled hydrogens: H2 on CH2 group and H1 on CH3 group.

Accepted Answer

All right. Hi everyone. So for this question, let's go ahead and provide a good estimate of the chemical shifts for each labeled hydrogen in the compound below. Now, the component question is methyl berate. So recall really quickly that the chemical shift in any given NMR spectrum is a measure of the location, right, the chemical shift, which is expressed in units of parts per million is essentially telling you the location of that particular signal in the spectrum itself. Now when we talk about what factors influence the location of a given signal, that conversation revolves around whether or not a given proton or a given signal is shielded or d shielded in comparison to other signals. So recall that electrons are what surround the proton inside of a nucleus and therefore shield it from the NMR effects. So when we talk about a particular signal or particular proton being shielded, then what that means is that the proton in question is being protected by the surrounding electrons right to the electron or proton, excuse me, is less affected from the NMR process itself. Now, as a result of the proton being less affected by NMR, then the particular signal that it would produce is described as appearing up field in the spectrum itself. And upfield refers 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 relatively small, right, it's going to be lower. Now, by contrast, we can also describe any given signal as being d shielded compared to others. And what D shielding refers to is there a proton is more affected by the NMR process because something is pulling electrons away from the nucleus, which exposes it even more. Now, a very common way that this happens if is for example, if you have a hydrogen or a proton connecting to an electron atom such as oxygen, because that oxygen will pull electron density towards itself leaving the proton more exposed, right, more D shielded. So if a given proton is de shielded, the net corresponding signal is going to be more downfield in the spectrum or on the left side, which also means that the value of the chemical shift is going to be higher or larger. So with that in mind, let's go ahead and analyze both of our protons here starting off with H A highlighted in red. Now, H A is the first carbon here that's directly adjacent to the carbon. Now, the proximity to the car bal is going to create a de shielding effect because the car bono itself is highly polar, right which means that it's going to pull electron density towards itself. However, hydrogens that are directly connecting to the car bottle are going to be more de shielded because they are in closer proximity to this highly polar carbo, right H A is a little bit different because while it is in close proximity to the car bono, it is not directly attached to it, right. So the D shielding is going to be to a lesser extent because it is one carbon away from the carbon though its still going to be in close proximity. Now, in comparison to a standard Alcaine, right, a standard Alcaine hydrogen recall that Alcaine hydrogens are the most shielded, which means that they tend to appear in between 1 to 2 parts per million in the NMR spectrum. So most up feel but hah A cannot be found within the range of 1 to 2 parts per million because its proximity to the car bono is going to affect, it's going to affect the signal, right, because it's going to appear more the shield. So because of the fact that it should be more de shielded to the Car Bono H A generally appears in between 2 to 3 parts per million because while it is de shielded, it's not going to be as de shielded as for example, a carbon that's connecting to an electron atom directly because that example applies more. So to HB right HB is connecting to a carbon that is bound to add oxygen directly. So recall it when we have a hydrogen that's connecting to a carbon that is bound to an electron atom, that signal tends to fall between the range of 2 to 4 parts per million. But we have to be a little bit more specific when considering the rest of the structure, right, because in addition to there being an electron oxygen right next to the carbon that bears HB we also have a car bal which again is highly polar, highly electron withdrawing. So therefore, there is slightly more of a de shielding effect, which means that of this range, right, of 2 to 4 parts per million, the signal for HP should appear more in the upper range right in the upper boundary. So for HB we can say that it's going to appear in between 3 to 4 parts per million, right, just to showcase that it should appear in the higher bound of the general R and there you have it right. So H A should appear in between 2 to 3 parts of a million, whereas HP should appear approximately between 3 to 4 parts per million. So with that being said, thank you so very much for watching and I hope you found this helpful.

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

Key Concepts

Chemical Shift

Shielding and Deshielding

Functional Groups and Their Influence

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