July 03, 2020, 12:58:20 PM
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Topic: Sigma conjugation in ethanal  (Read 302 times)

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Offline Nswtu

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Sigma conjugation in ethanal
« on: June 19, 2020, 06:08:06 PM »
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 :)

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