Isomer Question trans 2 butene

[ssjsike]

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I am reviewing ek 1001 o chem #129. The question states: How many carbon-carbon bonds in the structure shown below have conformers.

(Image is of trans 2 butene)

The answer stated is 2, and the reasoning is thus: The double bond cannot rotate, so there are no conformers for the double bond; however, the two single bonds can rotate.

If the double bonds can't rotate, how can conformational isomers exist? Please help!

 

[geeyouknit]

I am reviewing ek 1001 o chem #129. The question states: How many carbon-carbon bonds in the structure shown below have conformers.

(Image is of trans 2 butene)

The answer stated is 2, and the reasoning is thus: The double bond cannot rotate, so there are no conformers for the double bond; however, the two single bonds can rotate.

If the double bonds can't rotate, how can conformational isomers exist? Please help!

This doesn't seem possible. I guess the single bonds could rotate, but it's all hydrogen, so I don't know why that's an isomer.
I know there have been cases where the EK 1000 questions have made mistakes with answers. This seems like one. Check on their website to see if you can find the list of errors.

 

[tn4596]

They are right.
Double bond cannot rotate due to the pi bonding (2 pi orbital overlapping each other top and bottom). single bond only have 2 sp3 orbitals overlapping like a sticks connecting the atoms therefore they can just rotate around it.

Cis or trans isomers can be created from reactions. Reaction such as elimination reaction usually leave multiples possible products.

 

[Majik]

The double bond is fixed, but the two single bonds can still rotate freely. Maybe it might help to see this with a picture:

The double bond is fixed because the they are basically "tied down" by side-side overlap of 2 p-orbitals - one from each carbon. The bond to bond connectivity for each single bond though is an sp2 hybridized carbon with an sp3 hybridized carbon (head to head overlap). This is why these two bonds are able to rotate.

 

[geeyouknit]

They are right.
Double bond cannot rotate due to the pi bonding (2 pi orbital overlapping each other top and bottom). single bond only have 2 sp3 orbitals overlapping like a sticks connecting the atoms therefore they can just rotate around it.

Cis or trans isomers can be created from reactions. Reaction such as elimination reaction usually leave multiples possible products.

Cis and trans have nothing to do with conformational isomers. Conformational isomers are just isomers that can randomly and quickly switch between each other, like if it rotated a single bond and it became the same thing, or if the chair conformation flipped and it became the same thing.

I understand the single bonds on the end can rotate freely, but all the things on the carbons are hydrogens. No matter how you rotate it, you're just rotating 3 hydrogens around. Why would these be isomers?

 

[MD Odyssey]

Cis and trans have nothing to do with conformational isomers. Conformational isomers are just isomers that can randomly and quickly switch between each other, like if it rotated a single bond and it became the same thing, or if the chair conformation flipped and it became the same thing.

I understand the single bonds on the end can rotate freely, but all the things on the carbons are hydrogens. No matter how you rotate it, you're just rotating 3 hydrogens around. Why would these be isomers?

There are different conformations because the hydrogens can be gauche to each other or anti. Build a model and you'll see.

 

[geeyouknit]

There are different conformations because the hydrogens can be gauche to each other or anti. Build a model and you'll see.

Ah, ok, that makes sense. Thanks.

 

[ssjsike]

The double bond is fixed, but the two single bonds can still rotate freely. Maybe it might help to see this with a picture:

The double bond is fixed because the they are basically "tied down" by side-side overlap of 2 p-orbitals - one from each carbon. The bond to bond connectivity for each single bond though is an sp2 hybridized carbon with an sp3 hybridized carbon (head to head overlap). This is why these two bonds are able to rotate.

Thanks for the image Majik. It helped a lot!