The Periodic Table

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orthocresol

02:44

@andselisk I don't get it...

@user8718165 I assume you meant cation, not anion. Hyperconjugation is not just about what orbitals are present: it also crucially depends on how those orbitals are lined up. Simply having a C–H bond next to an empty p-orbital is not enough. The C–H bond must be pointing along the same axis as the p-orbital.

The same can be said of resonance. For there to be resonance stabilisation, the C=C pi bond must line up together with the empty p orbital.

So, you see how these two aims are contradictory. We cannot have the empty p-orbital aligned with both the pi bond, as well as the C–H bond (technically the sigma orbital), at the same time. So the molecule chooses resonance.

Btw, if you're going to talk about pi bonds between two carbons, then you should forget that a p orbital ever existed on those two carbons. You can't talk about a pi bond and a p orbital at the same time: the p orbital is consumed, forever, when making that pi bond.

Omega Krypton

@orthocresol Ca with atomic number 20, K 19

so 2019 ->HNY->2020

and 2019 <-time<- 2020

@andselisk nice one

orthocresol

user8718165

03:48

@orthocresol Yeah sir... I'm sorry I said anion. Thank you very much for help.

user8718165

04:17

@orthocresol just one last question please... What would happen if the empty p orbital of the cation aligned with the sp2 sigma C–H bond of the 2nd carbon instead of the pi bonds and the empty p orbital? I mean why isn't hyperconjugation favored over resonance? Is that just a matter of priority? Thank you sir.

1,3-feeds

Q: 2019: a year in moderation

It's New Year's Day in Stack Exchange land... A distinguishing characteristic of these sites is how they are moderated: We designed the Stack Exchange network engine to be mostly self-regulating, in that we amortize the overall moderation cost of the system across thousands of teeny-tiny sli...

5 hours later…

andselisk

09:08

@OmegaKrypton You got it;)

@orthocresol Adding to Omega's explanation: t' is a time-reversing translation, the best symbol I could come up with for time travel; HNY is Happy New Year.

2 hours later…

Hot nitric acid (HNQ)

11:19

Q: NMR magnetically equivalent protons for a 1,4-disubstituted benzene ring

Below I have drawn a molecule with random substituents $\ce{X}$ and $\ce{Y}$: If the distances from $\ce{H_b}$ and $\ce{H_a}$ to $\ce{H_d}$ are considered, they are different - $\ce{H_b}$ is 5 bonds away and $\ce{H_a}$ is 3 bonds away, so $\ce{H_b}$ sees $\ce{H_d}$ differently compared to $\ce...

organic-chemistry nmr-spectroscopy

1 hour later…

andselisk

12:38

I wonder what these socks are made of:

user8718165

12:53

@andselisk hello Happy new year :-)

andselisk

@user8718165 Yaay, HNY:)

user8718165

@andselisk yeah. How are you? BTW! This chat is really awesome :-)

andselisk

@user8718165 Sober and happy that it finally snowed (one can tell I rarely drive a car) ). How are you?

user8718165

@andselisk Wow! That's great :) I love snow. I'm fine too.

orthocresol

@andselisk I was just left wondering why t' indicated reverse time. :)

user8718165

13:09

@orthocresol Hii Ortho sir, could you please have a look at my question? Please :)

orthocresol

@user8718165 Yes. I am typing a reply. And don't call me sir.

That would be higher in energy than the alternative. It turns out that stabilisation derived from resonance is often far more effective than stabilisation derived from hyperconjugation. The molecule doesn't know what resonance or hyperconjugation is,
but it always wants to get to the lowest-energy configuration: this means having the pi orbital parallel to the empty p orbital.

user8718165

@orthocresol Okay. I'm sorry. I'll just call you Ortho :-) And wishing you a very happy new year 😊

orthocresol

Happy New Year!

user8718165

@orthocresol thank you so much :)

@orthocresol Is that because the sigma bond is more stable than the pi bond?

orthocresol

Hmm...

user8718165

13:17

@orthocresol okay... I was wondering why is resonance better than hyp. conj.? Does my reason make sense? I'm sorry if that's wrong.

orthocresol

I wish this chat had a way to show when people are typing. :)

It's a difficult question to answer and I think this will make more sense if you study MO theory in more detail.

Ultimately, though, it comes down to two factors: (1) orbital energies (2) overlap. The higher the energy of the filled orbital, the greater the stabilisation (generally). And the stronger the overlap, the greater the stabilisation.

user8718165

@orthocresol yeah. I studied just basic MO theory, is is a deeper concept?

@orthocresol yeah, like whatsapp... But there's none :(

orthocresol

I was thinking more of something along these lines: chemistry.stackexchange.com/q/52158/16683 That would mathematically illustrate how the two factors above affect the degree of stabilisation, but won't make much sense if you don't do QM.

Anyway, usually the orbitals involved in resonance win in both aspects. For example, the C=C pi orbital is higher in energy than the C–H sigma orbital.

user8718165

@orthocresol I'm so sorry. I know almost nothing about QM. Just learnt a bit about photons, HUP etc :(

@orthocresol yeah... fully got the 1st point... :-)

@orthocresol is there a similar argument for overlap? I'm not able to figure it out. Doesn't pi bond involve weaker overlap than the sigma?

orthocresol

13:36

@user8718165 Yes, but it's not that overlap that matters. It's the overlap between the filled orbital & the empty orbital.

user8718165

@orthocresol okay. Could you please explain it a little bit? I'll get it.

orthocresol

The pi bond is formed from two p orbitals. When you say the pi bond features weaker overlap, what you are really saying is that the overlap between those two original p orbitals is (relatively) weak. The overlap is thus not so much a property of the pi bond, but rather a property of the two interacting p-orbitals.

Likewise, the sigma bond does indeed feature stronger overlap, but that refers to the overlap between the constituent orbitals: a s orbital on H and an sp3 orbital on C.

user8718165

@orthocresol Got it. Thank you so much Ortho :-)

@orthocresol You explained it in so much detail. Nobody explained it this way. Awesome

@orthocresol Enjoy the holidays. See you later 🎉😊

orthocresol

You too.

3 hours later…

M.A.R.

16:39

M.A.R. is typing...

@orthocresol No ಠ_ಠ

Happy new year @Ortho @Yusuf @pH13 @And @Avnish @Skull @Geoff @Gau @Loong @Mith @Mart

And I've told Cowp already, so this is kinda awkward.

Loong

16:59

@M.A.R. happy new year

Martin - マーチン

Happy new year to all of you :D

3 hours later…

Hot nitric acid (HNQ)

20:31

Q: Water-stable organoaluminum compounds in which aluminum is part of the aromatic ring

The more common organoaluminum compounds, such as trialkylaluminum, decompose on contact with oxygen-containing molecules (water, ethanol etc). However, my guess is that organoaluminum compounds with an aluminum atom being part of the aromatic system might be more stable towards oxygen compounds,...

organic-chemistry

2 hours later…

Hot nitric acid (HNQ)

22:14

Q: Compounds with aluminum and nitrogen atoms directly bonded to each other as part of an aromatic ring?

Just hours ago, I asked a question about whether aromatic organoaluminum compounds with aluminum and carbon bonded together as part of an aromatic ring would be stable in water. The answer seemed to be that "it wouldn't even work at all since aluminum and carbon have relatively poor pi-overlap, a...

inorganic-chemistry

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