geometric isomerism

[mundane]

I have uploaded an image. Can someone help me explain why does one show G.I while the other doesn't? 

[studiot]

1 hour ago, mundane said:

I have uploaded an image. Can someone help me explain why does one show G.I while the other doesn't?

 

Does this help?

 

[mundane]

sort of. can you explain? 

[studiot]

I thought a picture was worth 1000 words ?

 

 

The left hand one  corresponds to your left hand one.

The (top) end has one hydrogen and one alkyl group as shown.
Swopping these over generates a different isomer  since the R group is now closer to the nearest oxygen.

In the right hand picture swopping two identical R groups does not alter the relative disposition of constituent atoms  - they are the same molecule from front and back.

Edited March 4, 2020 by studiot

[mundane]

19 minutes ago, studiot said:

I thought a picture was worth 1000 words ?

 

 

The left hand one  corresponds to your left hand one.

The (top) end has one hydrogen and one alkyl group as shown.
Swopping these over generates a different isomer  since the R group is now closer to the nearest oxygen.

In the right hand picture swopping two identical R groups does not alter the relative disposition of constituent atoms  - they are the same molecule from front and back.

in that case, please look at figure 1 and tell me why can't it show GI while figure 2 can. 

[studiot]

It's basically the same reason why there are no isomers of chloromethane.

Your G1 and  G2 are not the same structure.
In G1 the diether ring is symmetrical, in G2 it is not.

This means that which ever way round you attach the H and R groups, to G1, you can rotate the molecule to achieve the other configuration.
In G2  there is no rotation that will achieve this.

[mundane]

13 minutes ago, studiot said:

It's basically the same reason why there are no isomers of chloromethane.

Your G1 and  G2 are not the same structure.
In G1 the diether ring is symmetrical, in G2 it is not.

This means that which ever way round you attach the H and R groups, to G1, you can rotate the molecule to achieve the other configuration.
In G2  there is no rotation that will achieve this.

In G1 and G2, one R and one H are attached on the top, yet only G2 shows isomerism. 

The diether ring in G3 isn't symmetrical either, yet it doesn't show GI. 

[Sensei]

Do you have ball-and-stick model? Buy it from e.g. eBay or Amazon and make models of compounds that you draw on paper, and you should see what studiot is talking about.

 

 

[Strange]

On 3/4/2020 at 3:21 PM, mundane said:

In G1 and G2, one R and one H are attached on the top, yet only G2 shows isomerism. 

The diether ring in G3 isn't symmetrical either, yet it doesn't show GI. 

The key thing is, can the two forms be made the same by one or more rotations.

So in your G1, if you were to create an isomer by swapping the R and H at the top, then you could just rotate it by 180° to get back to G1.

In G3, if you were to swap the orientation of the diether ring you can still rotate the whole molecule by 180° and get back to where you stated.

But in G2 if you swap the R and H at the top, you can't rotate it to get back to the molecule you started with. You have to reflect it. In other words, you have a chiral pair (like a pair of gloves, rather than a pair of socks).

If this is not clear from looking at the diagrams, then you need to build a model as Sensei suggests.

(I bought a ball and stick kit when I was doing chemistry but never used it because I can see the symmetries immediately from the drawings. Which is bizarre because I have almost total aphantasia and can't tell left from right!)

[Ghideon]

On 3/4/2020 at 3:02 PM, studiot said:

there are no isomers of chloromethane

 

On 3/4/2020 at 7:09 PM, Sensei said:

ball-and-stick model?

@mundane you could use the ideas above in digital tools* if you prefer. Here is an example of chloromethane (did not have time to create an animation of it in rotation)

. 

Here is an example view of two types of But-2-ene. You cant rotate one to get to the other. 

*) There are free tools available online

 

[Strange]

18 minutes ago, Ghideon said:

Here is an example view of two types of But-2-ene. You cant rotate one to get to the other. 

This also highlights the importance of the double bond. That forces the relative alignment of the two halves of the molecule to stay the same. That is a key reason that isomerism exists (otherwise you could just rotate half the molecule and get to the other form).

[mundane]

3 hours ago, Strange said:

The key thing is, can the two forms be made the same by one or more rotations.

So in your G1, if you were to create an isomer by swapping the R and H at the top, then you could just rotate it by 180° to get back to G1.

In G3, if you were to swap the orientation of the diether ring you can still rotate the whole molecule by 180° and get back to where you stated.

But in G2 if you swap the R and H at the top, you can't rotate it to get back to the molecule you started with. You have to reflect it. In other words, you have a chiral pair (like a pair of gloves, rather than a pair of socks).

If this is not clear from looking at the diagrams, then you need to build a model as Sensei suggests.

(I bought a ball and stick kit when I was doing chemistry but never used it because I can see the symmetries immediately from the drawings. Which is bizarre because I have almost total aphantasia and can't tell left from right!)

oh! now I get it! thanks alot! 

1 hour ago, Ghideon said:

 

@mundane you could use the ideas above in digital tools* if you prefer. Here is an example of chloromethane (did not have time to create an animation of it in rotation)

. 

Here is an example view of two types of But-2-ene. You cant rotate one to get to the other. 

*) There are free tools available online

 

thanks for the explanation, bud! 

[studiot]

Chemtube is a good resource

https://www.chemtube3d.com/category/organic-reactions/stereochemistry/