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Topic: Stereochemistry of E2 and alkene reactions?  (Read 858 times)

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Offline sharbeldam

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Stereochemistry of E2 and alkene reactions?
« on: March 01, 2021, 05:23:06 AM »
There was this rule of alkene reactions that says:
Syn reaction + cis alkene gives Meso Product (if same groups on both carbons of the alkenes)
Cis + anti or trans + syn gives two enantiomers
anti reaction + trans alkene gives meso (if same groups on both carbon of alkene).

This topic is confusing to many students, first of all, what if the two groups on the carbons weren't the same, for instance in this reaction in the picture, it's a trans alkene with anti reaction, I would like a rule that would spare me from using models, according to my rule stated above, it should give meso (but groups aren't the same on both sides) so it should give R/S? or S/R? Hence two enantiomers? like is my logic correct, since it's an anti reaction, both carbons on the double bond need DIFFERENT configurations, and if it's a syn reaction, they should have SAME configuration?
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Offline Vidya

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Re: Stereochemistry of E2 and alkene reactions?
« Reply #1 on: March 02, 2021, 12:13:48 AM »
If you want to do it using rules only then remember meso means plane of symmetry and in that case your alkene has to be symmetrical alkene .Only symmetrical alkenes can give meso because they can have a plane of symmetry.
This compound shown by you is not a symmetrical alkene so you will not get meso.
In both types of electrophilic addition(syn or anti) it will give enantiomers mixture -means racemic
My advise is try to understand which will help you a lot and you need not to memorize  so many rules.
In the title you have written E2 but actually question is based on electrophilic addition.   

Offline sharbeldam

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Re: Stereochemistry of E2 and alkene reactions?
« Reply #2 on: March 02, 2021, 03:13:32 AM »
Thank you, yes i know it's not meso but how would the Br2 get in? I know anti, but do the two carbons need to be different configurations? because its anti.
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Offline AWK

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Re: Stereochemistry of E2 and alkene reactions?
« Reply #3 on: March 02, 2021, 04:10:01 AM »
Think for a minute and compare (E-2-butene with (E) -2-but-2-oic acid.
Br has the highest preference for both carbon atoms, so the configuration of the chiral atoms will be identical to that of the meso compound.
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Offline sharbeldam

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Re: Stereochemistry of E2 and alkene reactions?
« Reply #4 on: March 02, 2021, 04:26:01 AM »
I understand, please continue with me, I really want to make a video, explaining all that.

the product to look " anti " has many options, like these two, but since it's anti, I'd assume it's the product on the right *BECAUSE* the configurations are not the same, right? so an anti reaction would never give R,R product or S,S product, or is that logic false?


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Offline AWK

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Offline sharbeldam

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Re: Stereochemistry of E2 and alkene reactions?
« Reply #6 on: March 02, 2021, 05:29:03 AM »
I did actually watch that before asking my question, he does answer at min 11, a question similar to mine, but I would like an answer to my question, he doesnt go too deep into explanations , he just go by his rule.

Can an anti reaction produce R,R or S,S? when he drew the products at min 11:00, there are different ways of drawing them in anti position like i did in my previous post, do they have to be different configurations?
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Offline sjb

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Re: Stereochemistry of E2 and alkene reactions?
« Reply #7 on: March 02, 2021, 08:54:15 AM »
Syn and anti just relate to the geometry of addition, they have no fixed correlation to the configuration of the newly formed asymmetric centres.

Offline sharbeldam

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Re: Stereochemistry of E2 and alkene reactions?
« Reply #8 on: March 02, 2021, 09:08:58 AM »
Uh that complicates it, then how would you know the configurations ?
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Offline sjb

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Re: Stereochemistry of E2 and alkene reactions?
« Reply #9 on: March 02, 2021, 09:41:54 AM »
Syn addition just means both groups are added to the same side of the double bond (like for instance the OsO4 addition to alkenes; and anti means added to opposite sides (like Br2 addition to alkenes).

Sorry, I perhaps phrased my comment wrong - it has no correlation to the naming for the configuration.

Offline sharbeldam

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Re: Stereochemistry of E2 and alkene reactions?
« Reply #10 on: March 02, 2021, 09:45:17 AM »
Thank you sjb but perhaps my question isn’t clear, my problem is to know how to draw it, and to draw it right I need to know the configurations R/s)
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Offline sjb

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Re: Stereochemistry of E2 and alkene reactions?
« Reply #11 on: March 02, 2021, 12:01:05 PM »
You can still draw it by imagining (or using models) that for instance in the dihydroxylation case the two OH both attach to the top (or to the bottom) of the alkene, with the double bond in the plane of your paper, then mentally pushing the other two radicals on each end of the double bond down. A bit like how you might visualise the stereochemistry of a Fisher projection.

Offline sharbeldam

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Re: Stereochemistry of E2 and alkene reactions?
« Reply #12 on: March 02, 2021, 04:23:28 PM »
I understand, please continue with me, I really want to make a video, explaining all that.

the product to look " anti " has many options, like these two, but since it's anti, I'd assume it's the product on the right *BECAUSE* the configurations are not the same, right? so an anti reaction would never give R,R product or S,S product, or is that logic false?

Can you please look at these 2 pictures here, imagining it is hard, because in both pictures they look anti, isn't there a simple way to know which one is right? using models or imagining is very hard for me unfortunately
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Offline AWK

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Re: Stereochemistry of E2 and alkene reactions?
« Reply #13 on: March 03, 2021, 03:39:39 AM »
Left: trans-syn
Right: trans-anti
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Offline sharbeldam

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Re: Stereochemistry of E2 and alkene reactions?
« Reply #14 on: March 03, 2021, 04:34:57 PM »
then maybe this works everytime?

alkene + anti -> R,S And S,R

alkene + syn -> R,R and S,S
« Last Edit: March 03, 2021, 05:36:38 PM by sharbeldam »
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