E vs Z and stability?

[BrazilianRider]

So I was wondering if what I had in my head is correct:

1) You determine whether or not a molecule is E or Z based on whether or not the two biggest substituents are on opposite sides or the same side, respectively.

2) If you are given a problem asking whether a certain E configuration or Z configuration is more stable, the most stable one would be the molecule that has the most sterically hindered groups trans to each other?

I'm pretty sure #1 is right, but #2 confuses me. I came across a Bootcamp problem where there was an alkene, and on C1 there was a methyl and a benzene attached, and then on C2 there was like a propyl group then a methyl. In the question, it asked which configuration would be more stable, E (with the benzene and propyl groups on opposite sides of the double bond) or Z (with the benzene and the propyl groups on the same side of the double bond). The answer ended up being that Z was more stable (even though normally E is more stable), because benzene is sp2 and less sterically hindered or something to that effect, so having the methyl and propyl on opposite sides was actually more favorable.

So this leads me to this question: If you have an alkene with a chlorine and a propyl on C1, then a butyl and a methyl on C2, would the Z conformation (Chlorine and butyl on same side) be more stable since the methyl is more sterically hindered than Chlorine? Or would the E conformation be more stable because Chlorine is still sp3 (the lone pairs count in hybridization.... I think?)?

Thanks in advance guys, and sorry if this is poorly worded!

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[BrazilianRider]

Awesome. I'll go the same route you went and ignore the answer, and hope it leads me to the same scores you got

 

[Triflate]

Ah this question... I was wondering when someone else might ask about this! So I, too, came across this puzzling question and decided to ask the man who runs "Mastering Organic Chem", Dr. James Ashenhurst... I sent him a screenshot of the question (which I've also attached to this post) so there'd be no confusion concerning either the question or the supplied answer...

He sent back the following email:



"Dear [gobinoob],

Your answer is correct in my book, not only for steric reasons, but also for reasons that the writers never even thought about. First of all it's wrong that just because the phenyl group is planar it has less steric bulk than methyl. To directly compare steric bulk by another measure, on a cyclohexane ring for example, the A value of a phenyl group is about 3.0 vs 1.7 for a methyl group, meaning that a phenyl group is considerably more disfavored in the axial position.

There are other problems as well. If you look at compound I and draw in the hidden hydrogens, there's a hydrogen pointing right at the t-butyl group, whereas this is not the case for the methyl. Secondly, if you make a model, while it is possible to get the phenyl group to adopt a conformation where it won't interact with the t-butyl as much, the phenyl will be at 90 degrees to the alkene double bond, which will bring it out of conjugation (making it less stable). Just based on conjugation stabilization alone, compound II should win. Anyways, the writers should have picked a halogen instead of a phenyl; then their point would've been valid."


I wasn't sure what to make of all this (I, for one, have no where close to the level of ochem knowledge needed to even talk about this, lol) but, in the end, I decided to just disregard the question. I like to think the writers of the DAT will tend to avoid controversial or "conflicting" topics like this, ie those that include multiple sources claiming opposite things. Hope this helped and good luck!


View attachment 181232

Dr. Ashenhurst is right. I was actually going to email Ari about this, but decided to just forget it and let it go. Something like this on the DAT won't show up. You can actually get into even more detail with molecular orbital theory, but at that point, it's wayyyyy over the scope of the DAT. You won't see this at all.

 

[Dotoday]

Ha, yeah no worries. However, your answer is right anyway. Looks like the book made a mistake here.

 

[Dotoday]

How did you feel about Bio questions on Bootcamp? Why are their explanations so short? I feel like they did not fully explain the whole answer.