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Topic: Sigma conjugation in ethanal  (Read 396 times)

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Naruto

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Sigma conjugation in ethanal
« on: December 10, 2021, 05:08:46 AM »
I'm new to this forum so sorry if this post is in the wrong place!

I will be starting my degree this year and have been doing some reading, but an explanation of the σ conjugation in ethanal has confused me, and I was hoping somebody would be able to clarify some things for me?

My first question is about σ conjugation in general. I've seen MO diagrams demonstrating how the interaction between an occupied σ MO and unoccupied p orbital can result in an overall lowering of energy. I know this allows us to explain why alkyl groups stabilise cations as more conjugation is possible, but what is the best way to think about this? Should we think of each individual σ MO as interacting separately with the p orbital, or consider all the MOs together? My initial thought was the latter, having seen how surface plots of the resulting MOs show delocalisation over all the σ bonds. Would this then form many MOs, as the number of MOs interacting is equal to the number of MOs formed? If so, how would these MOs relate to one another in terms of energy? Sorry if I'm unclear, I don't really know how to best word this.

My second question relates to ethanal. The book I am reading states we can think of a pi MO, contributing to the bonding in the methyl group, as arising from the interaction between the methyl carbon 2p and the 1s AOs of the two out of plane hydrogens, and that the interaction between this MO and the π system results in σ conjugation. I would have thought the methyl carbon was sp3 hybridised, so how is the 2p able to do this? Wouldn't three MOs be formed in total from this interaction? My guess was that two of these MOs would be filled and it is the one lower in energy that interacts with the π system, but I could be completely off. And how does the rest of the bonding around this carbon work if one 2p is involved in bonding to two hydrogens?

Sorry for all the questions! It's probably clear I'm very new to chemistry, so any help would be appreciated :)

Offline Babcock_Hall

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Re: Sigma conjugation in ethanal
« Reply #1 on: December 10, 2021, 11:24:28 AM »
I am not at all well versed in molecular orbital theory, but there were a couple of things that puzzled me in your question.  You mention both MOs and AOs.  MOs are combinations of AOs; in some sense the AOs have been replaced by MOs [in the molecule].  I also did not understand what you meant by hybridization.  In the form of MO theory with which I am familiar, one does not hybridize the orbitals.  On the other hand, I don't see a problem with predicting the approximate geometry of a molecule using hybridized atomic orbitals.
« Last Edit: December 10, 2021, 01:31:03 PM by Babcock_Hall »

Offline Corribus

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Re: Sigma conjugation in ethanal
« Reply #2 on: December 10, 2021, 12:24:16 PM »
In the form of MO theory with which I am familiar, one does not hybridize the orbitals.
True. Hybridization is an extension of valence bond theory, which is distinct from molecular orbital theory. Both involve combining AOs, but hybridized orbitals are formed by combining AOs centered on a single nucleus, and then the hybridized orbitals on adjacent nuclei interact to form bonds. In contrast, MOs are formed by mixing (non-hybridized) atomic orbitals on adjacent nuclei. Valence bond theory tends to treat each pair of adjacent nuclear interactions (bonds) as more or less independent of whatever else is going on around them - i.e., electrons are only shared between two nuclei. Molecular orbital theory tends to treat bonding electrons as spread/shared over the entire nuclear framework. As you can imagine, neither picture holds a monopoly on accuracy. For example, in bigger molecules, treating sigma bonding electrons as spread over vast molecular orbitals seems very inaccurate. There are also plenty of examples where the hybridization model falls apart.

Anyway, the OP seems to be mixing some different theories together, which makes the post a little hard to follow. It may be helpful, Naruto, if you gave a specific example of the type of MO diagrams you are referring to when you say "I've seen MO diagrams demonstrating how the interaction...". Also we don't usually speak in terms of sigma bond conjugation.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

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