[Unknown008]

So far, I've tried to find why cis-polyethyne does not conduct electricity whilst trans-polyethyne does. I've got no satisfactory answer until now, unfortunately.

For example, I found this on wikipedia:
Conjugated system - Wikipedia, the free encyclopedia

But it doesn't tell me why cis doesn't conduct while trans does...

I've seen this answer on a forum I got through a Google search.

the trans isomer will have (relatively) straight chains and can align alongside one another allowing some degree of crosslinking and overlapping of the delocalised electrons.

The cis-isomer contains 'kinks' (yah Ray Davies rocks) preventing good alignment.

While this may be true, I have found nothing to confirm this... any idea?

Thanks!

[DrBob1]

Unknown008
The forum you cite sounds correct to me. You sound like you think this is too simple an explanation. Don't underestimate the power of a double bond!

You can read this answer because the photons coming from the screen to your eye isomerize one double bond in a retinal molecule from a trans to a cis configuration.

A triglyceride made with oleiic acid (cis-octadecenoic acid) melts at 5 oC; if it is made with elaidic acid (the trans isomer) the melting point rises to 42 oC.

Look at the differences between hevea (natural rubber) and gutta percha or balata rubber. Cis/trans isomerization!

These aren't electrical effects, but they show how symmetry and molecular packing affect many physical properties.

If I think of another really killer example I'll send it too.

[KISS]

About half-way doen the page there is a cis trans pic side by side.

Cis?trans isomerism - Wikipedia, the free encyclopedia


One seems to have a "dotted", for lack of a better word, line bond to Cl. If this is a notation for a weak bond, then I think you have an answer.

What does the notation mean?

[DrBob1]

KISS

In this case you have a different kind of cis/trans isomerizaton ---- not in an alkene but in a cycloalkane. The bold dagger lines indicate a CH3- group that is oriented above the plane of the ring and the dotted lines show a CH3- group that is below the plane of the ring.

We still have the same concept: cis groups are on the same side of the double bond or ring and trans groups are across the double bond or ring.

It may be interesting to note that the cis and trans isomers are geometric isomers of each other and that the trans isomers form a pair of enantiomers. They are non-superimposable mirror images. So here we have a total of three distinct compounds!

[KISS]

I'm just saying if the bond is weak it can break and provide a free electron.

What does the dotted line actually mean?

[DrBob1]

KISS
The dotted line shows the stereochemistry of the Cl groups. Thedotted line is oriented below the cyclohexane ring. Take a look at the structure shown, the Trans Cl atoms are up and down while the cis atoms are both up. This is a little hard to see because one of them is AXIAL while the other is in a Equatorial conformation and sticks out to the side. But the H atom goes straight down so the Cl atom must be up.
There is little difference in bond strength here. Certainly no attempt is made to imply it. Should the C-Cl bond break we would expect to get a carbocation and a chloride ion, not radicals.
R-Cl -----> R+ + Cl-

[Unknown008]

Thank you both! :)

Well, I know that the cis and trans conformations contribute to distinct physical properties, it's just about electrical conductivity that I'm not sure as yet.

I understand that cis molecules have higher boiling/melting point. The molecule can make stronger bonds on one side only, to usually form dimer like molecules and hence they will clutch together less strongly. While in the trans conformation, the molecule will tend to form chains and make some giant chain and hence, have a lower boiling/melting point.

And KISS, you'll see that there are two types of C-Cl bond shown. They are the means by which chemists show the 3 dimensional bonds in a molecule. A 'dotted' bons represents a bond that protrudes into paper, and a 'bold' bond represents one bond that protrudes out of paper while the normal straight bond represents a bond along the plane of paper.

One thing you caught my attention though.

It may be interesting to note that the cis and trans isomers are geometric isomers of each other and that the trans isomers form a pair of enantiomers. They are non-superimposable mirror images. So here we have a total of three distinct compounds!

I was taught that to have optical isomers, there needs to be a carbon molecule with four different groups attached to it. But if we are talking about alkenes, there are only three groups attached to the central carbon atom :confused:


Back to the topic.

From my logic, since there is a delocalisation of electrons, both the cis and trans conformations should conduct electricity well... I'll try to draw a picture to illustrate my thoughts.

[Unknown008]

Okay...

Trans-polyethyne:


Cis Polyethyne:


The black dots are the electrons that are in the pi bond and I'm making as if they are already delocalised and the red lines are the path that they take.

[I didn't put the hydrogen atoms to make it look simpler]

[DrBob1]

Unknown008
You are completely correct about the optical isomers. My statement only pertains to the dichlorocyclohexanes. Cis/trans isomerization in alkenes has no optical activity consequences. (That's not totally true - Unknown, take a look at optically active ALLENES, a type of moleculle-as-a-whole assymetry. I think you will enjoy that if you are not already familiar with them)

Now back to the conductive polymers that were the original subject of your question. To be a good conductor you want your molecules to line up in great parallel lines. The conjugated double bonds can then move the electrons along the chain. Think of these molecules as nanowires. The trans alkenes are much more likely to be linear rather than cyclic and thus much more efficient at moving electrons along a wire form point A to point B. Every cis double bond sends the chain off at an angle of 120 o and impedes the conductivity. In manufacture, I would think that the polymers would be [I]oriented[I] by stretching to maximize their linear nature.

The conducting polymers you are contemplaiting are like one dimensional graphite molecules. Just as graphite conducts because it comes in large flat sheets, these polymers work best as long, straight linear molecules. Cis double bonds are kinks in the chain and screw it up.

I just came up for air and saw the structures you posted. Make them ten times longer and you will more easily see the effect of a random cis double bond.

[Unknown008]

Thanks! I did find allenes interesting :)

=\

In both polymers, the bond angles are ...

I have not convinced myself yet... :(

Okay, another picture. I put the same number of carbon atoms, and the angles are more easily identified. I don't see how the trans is more linear than the cis... :(



EDIT:
Okay, I got this from a friend, thinking that it's more like this:

[DrBob1]

I can't figure how to paste my ChemDraw figures to AskMe. Rats!

In your figure the grey line is deceptive. Notice how, in the cisoid figure it runs parallel to the double bonds but in the trans figure above, it intersects the double bonds. Make the polymer chain longer - four trans alkenes, a cis alkene, and four more trans alkenes. You will see you get a kink in the chain.

I wish I could show it.

[Unknown008]

Well, what I did was draw the picture, then uploaded it on an image hosting site. I copied/pasted the link here.

What you can try too, is to press "Go Advanced" in the reply box. Then, look for 'Manage Attachments' below the reply box. A window will pop up and ask you to browse the image. Upload, then post.

If you want to insert the image somewhere in the middle of the post, click on the 'paperclip' button in the 'Go Advance' page and click on the attachment you already uploaded. A link will appear in your post. You can move it around as you wish. :)

I'll be waiting for the image then.

[DrBob1]

I will try your suggestion, but I am so computer illiterate that it may not be enough! But thanks.

But I had a further thought: are you wondering about the conductivity of ALL CIS polyethyne? I don't think this is stable enough to be used for actual materials. I think the reason for this is that cis double bonds are slightly more sterically hindered (eclipsing H atoms) than the trans isomer. In a macromolecule like polyethyne the accumulation of strain disrupts the whole thing.

The structures I wanted to post had a chain of trans double bonds with a random cis thrown in. That shows a big kink. I never really thought about all cis!

Lastly, I hope anybody reading this fascinating discourse realizes we are talking about polyethyne (polyacetylene) not the familiar polyethylene -- which, of course has no double bonds at all!

[Unknown008]

Why not all cis polyethyne? Anything on the matter will be welcome :)

[DrBob1]

I am no expert in this field, not even well acquainted with it. But that never stopped me from having an opinion!

I believe that the all cis polymer can be synthesized, but it is not thermally stable and spontaneously changes to the trans form. I have no idea what temperature, etc is needed to cause this. Or how long a life (or half life) the cis polyethyne enjoys.

[Unknown008]

Seems like my friend removed the image from her photobucket... anyway...

I had a discussion with my teacher today, and he told me that both polymers conduct. It's just that the trans configuration conduct better. So, that means that the cis does conduct, but to a much lower extent than trans.

[DrBob1]

This confirms what I have seen on the internet. Furthermore, the polymers must be doped with Iodine to give them their high conductivity.

If you draw their structures the trans form makes a nice direct zig-zag across the page while the cis isomer takes a very tortured, loopy path. I really think this is important.

Hummm? I wonder what sodium or potassium would do as a dopant?