[UCSD_student]

I know that alkyl groups are considered to have a positive inductive effect compared to a hydrogen, this is because the carbons on the alkyl groups pull electron density from their hydrogens and share this electron density with other carbons in the chain, and this can be seen with the decreasing acidity of alkanes as they get longer (the positive inductive effect of the alkyl groups destabilizing the conjugate acid of the longer chain alkanes). However, I've also learned that in NMR, more substituted carbons are deshielded and thus the chemical shift for those carbons is shifted to the left, or downfield (an RCH₃ would show up at around 10 ppm, and a R₃CH would show up around 15 for example), and I've been told it's because carbons are more electronegative than hydrogens, resulting in more "sharing" of electrons and ultimately pulling electron density. This seems to me to be two conflicting properties of the alkyl groups, and I can't think of an explanation to resolve this.

[Archer]

I know that alkyl groups are considered to have a positive inductive effect compared to a hydrogen, this is because the carbons on the alkyl groups pull electron density from their hydrogens and share this electron density with other carbons in the chain, and this can be seen with the decreasing acidity of alkanes as they get longer (the positive inductive effect of the alkyl groups destabilizing the conjugate acid of the longer chain alkanes). However, I've also learned that in NMR, more substituted carbons are deshielded and thus the chemical shift for those carbons is shifted to the left, or downfield (an RCH₃ would show up at around 10 ppm, and a R₃CH would show up around 15 for example), and I've been told it's because carbons are more electronegative than hydrogens, resulting in more "sharing" of electrons and ultimately pulling electron density. This seems to me to be two conflicting properties of the alkyl groups, and I can't think of an explanation to resolve this.

You are right, this phenomenon in ¹³C NMR is to do with the relative electron withdrawing properties of a carbon relative to a proton (see attahced, this figure relates to the same phenomenon being observed in ¹H NMR also, but the theory is the same for the carbon they are bonded to)

In your case with the acids there is more electron density in a CH₃ relative to an H you are observing what happens when a chemical change takes place i.e. ionisation (or lack thereof), in the same way as aromatic substitution, alkyl groups are generally known to be electron donating but only when stabilising a carbocation. And these too are relative electron donating properties, alkyl groups are very weak donators and that is why in Friedel Crafts you see O,M & P products (albeit in small quantities).

I hope this helps.



Edit: this is wrong, see later posts.

[orgopete]

I've been told it's because carbons are more electronegative than hydrogens, resulting in more "sharing" of electrons and ultimately pulling electron density.

I don't know that I can answer your question, but I don't agree with this part of the explanation. Although I am not seeking to be flagged for my contrary opinion, but Pauling's electronegativity theory is simply wrong and organic chemistry reveals this more than other disciplines. A premise that carbon is more electron donating contradicts carbon being more electron withdrawing. Electronegativity arguments are for the tests, electron withdrawing is for the lab. Carbon has electrons and can inductively donate them compared to a proton. This will match acidity, reactivity, electrophilic aromatic substitution patterns, etc. This will include the acidity (or basicity) of the anion of methane versus isobutane. The electron donation is revealed in the C-H bond length of isobutane being slightly longer. I think you will find the electron donation properties of carbon to be generally constant. (Please report any exceptions you know of or think might be true.)

That leaves the NMR explanation. I don't know enough about this phenomena to explain the shifts. From my practical experience, the shifts found were always better than those predicted. That may suggest the theory leading to the predictions are in need of improvement. HBr is a stronger acid than HF, but carbons and hydrogens attached to CF are shifted more. This suggests the theory predicting HF should be a stronger acid (more ionic) than HBr also results in the NMR shifts.

[Archer]

That leaves the NMR explanation. I don't know enough about this phenomena to explain the shifts. From my practical experience, the shifts found were always better than those predicted. That may suggest the theory leading to the predictions are in need of improvement. HBr is a stronger acid than HF, but carbons and hydrogens attached to CF are shifted more. This suggests the theory predicting HF should be a stronger acid (more ionic) than HBr also results in the NMR shifts.

I apologise for my previous explanation, it is fair to say that I was wrong. What better place to be wrong but on an open forum.

Ignore my first post  and feel free to throw rotten eggs at me as I pass you by in the street.

The anomylous desheilding observed in 1°, 2° and 3° carbons bearing alkyl groups appears to be an ainisotropic effect which is obvisously either not observed in C-H bonds or is considerably weaker than in of C-C bonds. I say anomylous desheilding because the anisotropic effect increases the magnetic field experienced by the carbon or proton rather than pulling electrons away from it.

The effect is similar to that of alkenes, but weaker and the opposite of alkynes.

Please see attahced to aid the explanation.

[Archer]

I would like to thank UCSD_student and orgopete for making me stop and think about what I have been taught and taken as correct.

This afternoon I have poured over five text books on NMR to try and crack this so I hope that I am right but would welcome any challenges to this theory.

That's what I like about this forum and why I post here, you get challenged on your incorrect axiomatic fundamental theories causing you to think back to the basics.

The fact is that carbon does desheild the carbon it is bonded to. You don't really need any more detail than that do interpret NMR. When an academic then tells you it is because of electron withdrawing of the alkyl group you and it gets past your "what?" filter and gets entrenched in your brain as correct so you never question it again.

I know I have posted this misinformation previously on this forum, now I will have to locate it and highlight my error. May be a bit too late for the OP of that thread who will probably go on to propagate this common misconception.

[TheUnassuming]

Fantastic find and explanation Archer! 
I've never taken a graduate level spectroscopy course so I missed out on many of the more interesting aspects, and this one was definitely glossed over in my undergrad spectroscopy course.  I've propagated the wrong explanation before as well, so thank you for the lesson!

[orgopete]

... it is fair to say that I was wrong.

Been there, done that, but wait...

Now, as good as that explanation was, I can imagine it was offered for why certain atoms may be shifted upfield or downfield. However, if it were a bona fide truth, then why aren't predictions always correct?

Sometimes wrong ideas are right for wrong reasons and sometimes seemingly right ideas are actually wrong. I'm not going to hold my breath waiting for corrections. Five books? This doesn't sound like something everyone agreed on. I don't think any apologies are needed for that.

[UCSD_student]

Regardless of whether or not these explanations are actually correct, what I was looking for was a reasonable explanation which makes sense to me and which I can agree with. Thanks Archer for providing one to me. So if I'm understanding this correctly, alkyl groups do have a positive inductive effect on the carbon to which they are bonded to, but due to anisotropically induced magnetic fields from the electrons of the sigma C-C bond the hydrogens (and carbons) which lie perpendicular to the direction of the field are thus deshielded as is the case with alkenes (and opposite to alkynes in which the hydrogens lie parallel to the direction of the magnetic field), and this effect is stronger than the inductive effect, resulting in deshielding, correct?

[Archer]

Five books? This doesn't sound like something everyone agreed on. I don't think any apologies are needed for that.

I am sure you have read your fair share of NMR text books, if you can get past the first page you are doing well! I remember as an undergraduate thinking "Vector theory? I just want to assign these chemical shifts to my compounds how does vector theory get me there?"

Anyway, it was not a case of disagreement, most of them just quoted the chemical shift or the factor by which these influence theoretical chemical shifts. I need to pour over these books again tomorrow.

Only one pertained towards it being an anisotropic effect. If I recall this was an old text. The thing is that this is such a minor part of shielding that it seems glossed over. Although I shall spend more time going over the texts again.

As far as I can tell, from what I have read (and hopefully understood) the theory fits well with branched alkanes, however where it struggles a bit (I need to read some more) is when the anisotropic effect of methyl substituted polyaliphatic ring systems (i.e. steroids) are considered. The B_local (δ+) is a through space effect and when protons or ¹³C nuclei are in this space through a rigid ring system the comparative shielding does not follow the predictions.

As I said this is the best I can come up with in one afternoon ( while working at the same time) I will look into this in more detail to confirm or refute this theory.

[Archer]

Regardless of whether or not these explanations are actually correct, what I was looking for was a reasonable explanation which makes sense to me and which I can agree with. Thanks Archer for providing one to me. So if I'm understanding this correctly, alkyl groups do have a positive inductive effect on the carbon to which they are bonded to, but due to anisotropically induced magnetic fields from the electrons of the sigma C-C bond the hydrogens (and carbons) which lie perpendicular to the direction of the field are thus deshielded as is the case with alkenes (and opposite to alkynes in which the hydrogens lie parallel to the direction of the magnetic field), and this effect is stronger than the inductive effect, resulting in deshielding, correct?

That's my understanding of it, although strictly speaking it's not really deshielding the nuclei as such it is more of an increase in the magnetic field which the nuclei are subjected to. Similar to spin-spin coupling and why a split peak is divided equally upfield and downfield, this too is not a shielding / deshielding but either an increase or decrease in the field B_Local of a fraction of the nuclei in the sample exposed to the change in the actual applied field B₀.

If you look at some correlation tables there is a slight issue with quaternary carbons which have a range of predicted values which is slightly upfield from tertiary carbons. These are just ranges but I would have expected it to be downfield. I need to look over some examples to see why this might be.

[Archer]

The more I read into this the more complicated it becomes.

propane, isobutane and neopentane have increasing δ values for C2 respectively as predicted. However C1 (CH₃ of neopentane has a more downfield shift than the quaternary carbon.

I need to sleep on this one and try again in the morning.

[orgopete]

The more I read into this the more complicated it becomes.

That's what I thought and that's why I didn't think you should be too apologetic. I wasn't even willing to give it a try. I also agree that many books try to help draw a correlation. That may be telling us something.

I only jumped in because of electronegativity theory. It would be helpful if I had my book finished, then you could see why I think electronegativity theory is so poor and how I provide an explanation consistent with the laws of physics. Pauling never recanted even though the rationale was to explain bonds stronger than predicted. He said bonds must always be stronger. The discovery of bonds weaker than predicted never caused him to recant. Even he must have thought his theory was too good not to be true.

[Archer]

I only jumped in because of electronegativity theory. It would be helpful if I had my book finished, then you could see why I think electronegativity theory is so poor and how I provide an explanation consistent with the laws of physics. Pauling never recanted even though the rationale was to explain bonds stronger than predicted. He said bonds must always be stronger. The discovery of bonds weaker than predicted never caused him to recant. Even he must have thought his theory was too good not to be true.

How much more of your book do you have left before its complete? Sounds like I could use a copy 

[Archer]

The more I read into this the more complicated it becomes.

propane, isobutane and neopentane have increasing δ values for C2 respectively as predicted. However C1 (CH₃ of neopentane has a more downfield shift than the quaternary carbon.

I need to sleep on this one and try again in the morning.

This anomalous chemical shift difference in neopentane can be explained by the α, β, γ and occasionally δ effect of neighbouring substituents. All texts give you formulae and data to use these to predict carbon chemical shifts. No surprises the calculated data fits the observed.

Most texts give you the numbers and formulae for branched alkanes and then swiftly move on to inductive effects of halogens and mesomeric effects of enol ethers but none I have seen address how carbons exert these α, β & γ effects. Not that I can see anyway.

This is so frustrating! If it's not inductive and anisotropic influence is unlikely to have a significant effect on long distance atoms due to the inverse square law how is this possible?

I am no physicist but is it possible that the large increase in the localised magnetic field on the quaternary carbon that this in turn induces an even greater magnetic field adjacent to it?

So we have B₀ from the magnet an increased B_Local at the quaternary carbon due to the anisotropic effect of the four C-C sigma bonds. This B_Local is the sum of B₀ and the fields generated by the four methyl groups. Could this induce a greater field on the methyl groups sigma bonds, making their B_Local slightly stronger than that of the induced field  at the quaternary carbon?

I'll draw another picture and see if one of our physicist friends can help out.

I am not happy with the answer "it just does" which is what seems to be the general consensus in the texts that I have read.

Maybe this makes zero sense, I am very tired if so then just ignore me until I stop filling in the gaps with nonsense.

[Irlanur]

This is so frustrating! If it's not inductive and anisotropic influence is unlikely to have a significant effect on long distance atoms due to the inverse square law how is this possible?

I am no physicist but is it possible that the large increase in the localised magnetic field on the quaternary carbon that this in turn induces an even greater magnetic field adjacent to it?

How I think about such problems: Explanations in (Organic) Chemistry are mostly rules. Rules means, that we don't REALLY understand it. But we can use these rules, and they can be extremely good and, most of all, very fast and effective. BUT, if we want a REAL and deep understanding, we can't ignore Quantum Mechanics and Computational Chemistry.