[charzhino]

Simple question, what compound is the most electron withdrawing to the carbon on the methyl attached. Does the benzene ring or pyridine ring make this particular sp3 carbon more electron defficient and why? Thanks

[Honclbrif]

What's your reasoning so far?

[charzhino]

What's your reasoning so far?

I suspect the benzene system is more electron withdrawing because the pyridine system is electron deficient as the nitrogen lone pair is not involved in the aromatic system. Therefore there are more carbon p-orbtial electrons in the benzene system making it more delocalized and able to withdraw electron desnsity from substiuents easily. But im not sure and could be the opposite way around

[Honclbrif]

"the nitrogen lone pair is not involved in the aromatic system. Therefore there are more carbon p-orbtial electrons in the benzene system..."

The e- pair in the nitrogen sp2 orbital is not involved in the system, but the e- pair in the p orbital is. If it were not, the ring would not be aromatic.

[charzhino]

ok, so does that mean they both have the same degree of delocalisation? And does delocalisation have any influence for electron pulling power?

[Honclbrif]

Delocalization does have an effect on "electron pulling power". The strongest effects tend to occur when you can move electron density from one site to another through resonance. Can that happen in this case?

[charzhino]

Delocalization does have an effect on "electron pulling power". The strongest effects tend to occur when you can move electron density from one site to another through resonance. Can that happen in this case?

No it cannot happen in the pyridine system. So therefore it is a weaker electron withdrawing system than benzene hence the methyl will be more upfield (desheilded) in NMR?

[Honclbrif]

The 1H shifts for the pyridine and toluene methyls are close enough as to make no difference (you can look them up on SDBS). However, just because electrons can't be moved around by resonance, they can still move through the sigma framework. What phenomenon causes unequal distribution of electrons within bonds?

Modification: "does that mean they both have the same degree of delocalisation", they do not have the same degree of delocalization. The delocalization energy of pyridine is 117 kJ·mol−1 vs. 150 kJ·mol−1 in benzene (via Wikipedia, because I'm feeling kind of lazy). However, since you established that electron density is not being moved to or from the methyl via resonance, this shouldn't make a huge difference. Speculation: it may explain the NMR results if pyridine has a weaker ring current.

[BluePill]

Think about magnetic anisotropy. These big words will help you a lot.