Stereochemistry

[Ashish]

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Guys, what would the stereochemistry of the product when E-but-2-ene is treated with H2/Pt? It will form butane but what will be the stereochemistry of butane since the double bonds can't rotate for the syn-addition of Hydrogen?

Please shed some light on this topic.

Thanks!

 

[DAT Destroyer]

Guys, what would the stereochemistry of the product when E-but-2-ene is treated with H2/Pt? It will form butane but that will be the stereochemistry of butane since the double bonds can't rotate for the sun-addition of Hydrogen?

Please shed some light on this topic.

Thanks!

When hydrogen is added, the hydrogen adds to the double bond from the same side to give butane. There is no stereochemistry to show, just a simple butane molecule is produced.

Here is one for you to practice:

add D2/Pt to cyclobutene, here the stereochemistry is important. The cis isomer is produced and the compound is meso.

Hope this helps

Dr. Romano

 

[Ashish]

Thank you Dr. Romano for such a quick response. One more question if that is okay? So, during the epoxide opening of a nucleophilic attack in acid catalyzed condition, will the product go inversion? If so how do we detect what R/S product is formed if the carbon is chiral? I frequently get the stereochemistry of epoxide opening wrong. What about base catalyzed condition, is the stereochemistry retained in the product? Thank you!

 

[gangazi]

it goes Sn2 attack ("backside attack") so if the attacked side was R then the product has S streochem in both acid and base catalyzed

"hope this helps"🤣

 

[Ashish]

it goes Sn2 attack ("backside attack") so if the attacked side was R then the product has S streochem in both acid and base catalyzed

"hope this helps"🤣

But how do you tell if the reactant is S or R when it in an epoxide ring?

 

[gangazi]

But how do you tell if the reactant is S or R when it in an epoxide ring?

Well if it has stereocenter then you could assign R/S just like we assign R/S for all other molecules. A wedge or dash should be present in order for us to be able to assign R/S conformation ... "If not, dont care" - chad

 

[gangazi]

I recommend reading organic chemistry as a second language.. It helped me a lot, especially streochem

 

[DAT Destroyer]

Thank you Dr. Romano for such a quick response. One more question if that is okay? So, during the epoxide opening of a nucleophilic attack in acid catalyzed condition, will the product go inversion? If so how do we detect what R/S product is formed if the carbon is chiral? I frequently get the stereochemistry of epoxide opening wrong. What about base catalyzed condition, is the stereochemistry retained in the product? Thank you!

Anytime you hit a carbon that is chiral you will get an inverted product. The best way to do a problem if this comes up is to evaluate the stereochemistry of the expoxide you started with.

After you do the reaction, check the stereochemistry by the normal priority rules. You need to see a worked out example. If you can get your hands on the David Klein text there are worked out problems on this. If you can get your hands on the 2nd edition pages 655, 656, 657,660,661 show this nicely. If you can't get the 2nd edition surely they will be in the first.

Bottom line... always show the inverted product if a chiral carbon is attacked.

Hope this helps

Dr. Romano