How is this achiral?

[skyisblue]

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The molecule trans-cyclobutane-1,3-dicarboxylic acid is achiral and thus optically inactive because it has a plane of symmetry going thru the two carboxylic groups.

I can see how the cis version of this molecule is achiral bc of the 2 plane of symmetries that go thru it, but for the trans version? the two carboxylic groups are on opposite sides of the cyclic molecular plane for the trans version. How can the carboxylic grps be mirror images of each other here?

 

[Lonely Sol]

Well, the R-chain on each -COOH groups are the identical to each other, therefore it doesnt matter if the -COOH groups are cis or trans, it is achiral.

Imagine, its basically having like to identical R groups on a carbon, you it will be achiral then. For example, 2,2-dimethylbutane.

(someone correct me if i am wrong!)

 

[BodybldgDoc]

Lonely Sol is correct.

A carbon must have 4 different substituents to be a chiral center. If you notice that the carbon with the substituents H and COOH on either side of the molecule is also attached to two R groups that are the same therefore resulting in 2 different and 2 of the same subtituents and will not end up being chiral as you need to have 4 different substituents.
As far as the cis and trans issue is concerned, It will not affect chirality.

On the other hand If the question asked if this was a meso compound,only the cis version of the compound could be a meso and not the trans.

I think you are confused with the rules affecting chirality and the rules for a meso compound. Look up the rules and definitions for both to gain a better understanding.

 

[skyisblue]

Lonely Sol is correct.

A carbon must have 4 different substituents to be a chiral center. If you notice that the carbon with the substituents H and COOH on either side of the molecule is also attached to two R groups that are the same therefore resulting in 2 different and 2 of the same subtituents and will not end up being chiral as you need to have 4 different substituents.
As far as the cis and trans issue is concerned, It will not affect chirality.

On the other hand If the question asked if this was a meso compound,only the cis version of the compound could be a meso and not the trans.

I think you are confused with the rules affecting chirality and the rules for a meso compound. Look up the rules and definitions for both to gain a better understanding.

Alright, I see how there aren't 4 different substituents. BUT, what's up with the dashed line in the solution? Isn't that a plane of symmetry? OR does the trans verion have a plane of symmetry?

 

[BodybldgDoc]

Alright, I see how there aren't 4 different substituents. BUT, what's up with the dashed line in the solution? Isn't that a plane of symmetry? OR does the trans verion have a plane of symmetry?

In the trans compound there is no plane of symmetry. If you notice on the left side of the molecule, the COOH substituent is axial while the H substituent is in the equatorial position. On the right side of the molecule, the COOH substituent is equatorial while the H substituent is axial.
Therefore you would have
H | COOH
COOH | H <-- no symmetry

 

[Lonely Sol]

Alright, I see how there aren't 4 different substituents. BUT, what's up with the dashed line in the solution? Isn't that a plane of symmetry? OR does the trans verion have a plane of symmetry?

The dashed line in the middle is just to show you that the 2 chains are completely identical to each other, therefore the molecule is achiral!

 

[dentstd]

um, no...

the lines thru the molecule indicate where the plane of symmetry cross. In this case, there's a plane running up and down. The substituents (COOH's and H's) are within the plane! Thus, the front and back portions of the drawing are symmetric.

 

[skyisblue]

um, no...

the lines thru the molecule indicate where the plane of symmetry cross. In this case, there's a plane running up and down. The substituents (COOH's and H's) are within the plane! Thus, the front and back portions of the drawing are symmetric.

If it's within the plane, still, how is it symmetric OR more like how is one half a mirror image of the other half??

I don't get it.

 

[BodybldgDoc]

Its not a mirror image. It cant be. Its a trans molecule

 

[Lonely Sol]

If it's within the plane, still, how is it symmetric OR more like how is one half a mirror image of the other half??

I don't get it.

It doesnt have to be mirror image to be achiral! Thats meso compund, which should have a mirror image and plane of symmetry, which also give you an optically inactive compound

 

[skyisblue]

It doesnt have to be mirror image to be achiral! Thats meso compund, which should have a mirror image and plane of symmetry, which also give you an optically inactive compound

Yes, plane of symmetry means that one half is mirror image of the other half. Okay, so we established that there is a plane of symmetry bisecting this molecule. So this begs the question.....why is there not a mirror image?

I give up. Something like this better not be on my DAT.

 

[dentstd]

If it's within the plane, still, how is it symmetric OR more like how is one half a mirror image of the other half??

I don't get it.

it's within the plane, so it's symmetric. Note that the only thing coming OUT of hte plane is the triangular wedge. The only thing going into the plane, the other triangular wedge.

it's like an airplane being cut in left/right halves.

 

[dentstd]

Yes, plane of symmetry means that one half is mirror image of the other half. Okay, so we established that there is a plane of symmetry bisecting this molecule. So this begs the question.....why is there not a mirror image?

I give up. Something like this better not be on my DAT.

planes of symmetry and mirror images are different concepts. TWO DIFFERENT MOLECULES can be mirror images of each other. Planes of symmetry exists WITHIN ONE molecule.

 

[Lonely Sol]

Yes, plane of symmetry means that one half is mirror image of the other half. Okay, so we established that there is a plane of symmetry bisecting this molecule. So this begs the question.....why is there not a mirror image?

I give up. Something like this better not be on my DAT.

There is nothing that states that they have to be mirror image to be chiral or achiral! I am not sure what you mean but what dentsd might make sense to you about airplane cutting!

 

[BodybldgDoc]

planes of symmetry and mirror images are different concepts. TWO DIFFERENT MOLECULES can be mirror images of each other. Planes of symmetry exists WITHIN ONE molecule.

How is there a plane of symmetry when the molecule cant be superimposed on itself? Please do explain.

 

[dentstd]

How is there a plane of symmetry when the molecule cant be superimposed on itself? Please do explain.

Lots of people seem to mix the two concepts (symmetry and mirror images) together. It's a common mistake that causes lots of confusion.

Mirror images: we take ONE object, and reflect it across a mirror. As a result, we get TWO objects (the original, and the mirror image). If those two objects are non-superimposable, then it's chiral.

Plane of symmetry: we take ONE object. we try to find a plane of symmetry cutting thru that object. (we never deal with TWO objects)

...we DON'T first find a plane of symmetry then try to superimpose that. Nor do try to superimpose molecules after finding a plane of symmetry. The two concepts are DIFFERENT concepts.

 

[BodybldgDoc]

Lots of people seem to mix the two concepts (symmetry and mirror images) together. It's a common mistake that causes lots of confusion.

Mirror images: we take ONE object, and reflect it across a mirror. As a result, we get TWO objects (the original, and the mirror image). If those two objects are non-superimposable, then it's chiral.

Plane of symmetry: we take ONE object. we try to find a plane of symmetry cutting thru that object. (we never deal with TWO objects)

...we DON'T first find a plane of symmetry then try to superimpose that. Nor do try to superimpose molecules after finding a plane of symmetry. The two concepts are DIFFERENT concepts.

I think what skyisblue is asking is that if you cut the molecule down the middle perpendicularly to the dashed line. Will the left half of the molecule be a mirror image of the right half? and that would be no. would you agree?

 

[skyisblue]

I think what skyisblue is asking is that if you cut the molecule down the middle perpendicularly to the dashed line. Will the left half of the molecule be a mirror image of the right half? and that would be no. would you agree?

Yeah, that's what i've been asking....

 

[dctrgreen]

four different substituents to the same Carbon atom=chiral

anything less is achiral. bottom line. no matter how you cut it, twist it, or stand it on its head.

someone correct me if i am wrong.

 

[dentstd]

I think what skyisblue is asking is that if you cut the molecule down the middle perpendicularly to the dashed line. Will the left half of the molecule be a mirror image of the right half? and that would be no. would you agree?

if that's her question, then the answer is a def no. the substituents are reversed after the reflection

 

[dentstd]

four different substituents to the same Carbon atom=chiral

anything less is achiral. bottom line. no matter how you cut it, twist it, or stand it on its head.

someone correct me if i am wrong.

only if there's one stereocenter

 

[Lonely Sol]

Wow, it took 20 posts to explain an achiral molecule, I need to work on my explaining skill!

 

[RozhonDDS]

Sky, as you can see in the picture that there are NO wedges, thus sterochemistry is not present, thus there are not 4 differen't things on the carbon and there is a plane of symmetry which further concludes that the imiage is Achiral or for short (there aren't 4 differen't molecules/compounds coming off any carbon)

 

[BodybldgDoc]

if that's her question, then the answer is a def no. the substituents are reversed after the reflection

ALLELUYAH. After 20 something posts there is finally a consensus on the answer to SkyisBlue's TRUE question.