[Professor 0110]

Hello everyone,

I have a quick query about three different isomers. I understand the principles of molecular polarity - especially in terms of how it relates to boiling points - but I am unsure as to how to apply it in the case of three different isomers of dichloroethene. To my viewing, there appears to be discrepancies or perhaps something I am missing. Below are geometrical depictions of the three isomers as well as the boiling points. I have questions beneath, and if you could answer them, it would be so appreciated!

The first one is 1, 1 dichloroethene



It's boiling point is 32 degrees Celsius

The second isomer is trans 1, 2 dichloroethene



It's boiling point is 47.35 degrees Celsius.

The third isomer is cis 1, 2 dichloroethene



It's boiling point is 60.15 degrees Celsius


Now my main questions is why the boiling point of 1,1 dichloroethene is so much lower than cis 1, 2 dichloroethene when the arrangement seems exactly the same, except the chlorines are down the bottom in cis and the hydrogens are at the top. Don't both molecules have the same polarity with the highly electronegative chlorines at one end and the positive hydrogens at the other end?

Also, my second question is why trans 1, 2 dichloroethene has such a high boiling point compared with 1, 1 dichloroethene, because 1, 1 dichlorethene looks like it has the negative chlorines on one side and the positive hydrogens on the other whereas with trans 1, 2 dichloroethene, we don't seem to have an overall electronegativity on any one side.

Any help with this problem will be so much appreciated!

Thank you for your time,

Ben

[AWK]

1. cis - polar, trans - nonpolar

[enahs]

Look into the polarizability of the different molecules.

[Professor 0110]

Thanks for the replies, AWK and enahs.

If trans 1, 2 is dichloroethene is non-polar, then why does it have a higher boiling point than 1, 1 dichloroethene when 1, 1 dichloroethene looks almost exactly the same as cis 1,2 dichloroethene (in that there are chlorines on one side and hydrogens on the other)? I would have thought 1, 1 dichloroethene is also polar because of the electronegative elements on one side, and the least electronegative elements on the other side?

Thank you for your time,

Ben

[Vidya]

Hi
1,2 dichloroethane (trans ) is non polar and has less boiling point than cis isomer.
1,1 dichloroethane is more polar than trans but less polar than cis form.Here two chlorine atoms are not on the same side of the plane,hence the net dipole is going to be lesser than cis form .Now the question is comparison of the trans and 1,1 dichloroethane boiling points .I feel that 1,1 dichloroethane is sterically more unsatble because of two big atoms on the same C than trans form and this results in less fitted moecular structure.This can be the reason for less boiling point of 1,1 dichloroethane .

[Professor 0110]

Thank you so much for your reply, Uma!

I just have one more question. You referred to the fact that the 1,1 dichloroethene does not have the chlorine atoms on the same side of the "plane". Is the plane equivalent of the x-axis? If so, why does the fact that the chlorines are not on the same side of the "plane" affect the polarity? The cis 1,2 dichloroethene which is most polar has the chlorines gathered all on one side, and the hydrogens on the other. It's the same for 1, 1 dichloroethene, so I am still not sure why the net polarity isn't close to being the same for both...just in different directions.

Any further help will be much appreciated! But I'm definitely getting to the bottom of this, so that is good.

Thank you for your time.

Ben

[AWK]

Is the plane equivalent of the x-axis?

Plane has two dimensions

[fledarmus]

A little more discussion on polarizability -

The Cl atoms have lone pairs in p orbitals which can interact with the p orbitals of the C=C bond. Imagine a resonance structure with a lone pair from one Cl moving down to form a double bond with C (leaving Cl⁺). If both Cl are on the same carbon, the two electrons from the C=C pi-bond must move out onto the opposite carbon, leaving C^-. If the Cls are on opposite carbons, the C=C pi electrons can form a double bond to the other Cl instead, leaving Cl^-. Since Cl is more electronegative than C, it is happier to have the additional negative charge, so the charge is easier to delocalize on the 1,2-dichloroethenes than on 1,1-dichlorethene. It is easier to "induce" a charge on the 1,2 system - if a Cl atom approaches a negative charge (like the dipole of an adjacent molecule), the electrons find it easier to flow towards the other Cl atom, leaving the first Cl atom slightly positive and generating a small attractive force to the dipole of the adjacent molecule. The other Cl atom on that molecule is now slightly more negative and induces more charge flow on the next molecule, and so forth. This collection of induced attractive forces increases the attraction between the molecules and therefore the boiling point. (I wish I knew how to draw this on the computer, it's much easier to see with pictures! But draw the resonance forms and you might be able to figure out what I mean).

⁺Cl=C-C=Cl^-  vs ⁺Cl=C-C^-
                              /
                            Cl

That polarizabitily effect is enough to make both of the 1,2-dichloroethenes higher boiling than the 1,1-dichloroethene, and the difference in dipole moments between the two 1,2-dichlorethenes (trans having the dipoles opposed and canceling, and cis having them in similar directions and adding) accounts for the higher boiling point of the cis 1,2-dichloroethene.

[Vidya]

two Cl atoms on the same C and two Cl atoms on different C atoms .This can also make a big difference in the polarity of the bonds .

[Professor 0110]

Wow, thanks a million for the great response, fledarmus, and you too, Uma. I'm still trying to digest what it all means, but I think I will be able to piece it all together.

This is very very good. I'll digest all this information and if I am still unsure I'll post any further questions. I'll definitely have to draw some diagrams, fledarmus, but the information you posted is a goldmine!

Thanks again!

Ben

[Professor 0110]

Okay, I have worked my way through the information, and I've got another question. Any help much appreciated!

Okay, I understand the principles you laid out fledarmus, but the dichloroethene systems do not have chlorine double bonds with carbon. There's the carbon - carbon double bond, but chlorine just has a single bond because of the hydrogen atoms. So, I am wondering if the hydrogen atoms and the fact that its only single chlorine bonds has any affect upon the principle outlined

Thanks everyone,

Ben

[fledarmus]

No, the Cl atoms do not have double bonds with carbon. They do, however, have lone electron pairs in p orbitals which can interact with the electrons in the p-orbitals forming the carbon-carbon double bond. This is frequently shown by drawing the resonance structures I attempted to draw. The actual structure of the 1,2-dichloroethenes is somewhere between the structure as it is normally drawn (with the double bond between the two carbons) and the resonance structures, with double bonds between Cl and C and charges on the Cl. The bonds between Cl and C are slightly shorter than you would expect from a Cl-C single bond, and the bond between the carbons is slightly longer than you would expect from a normal C=C.

There is a picture here http://books.google.com/books?id=QV6cwXA9XkEC&pg=PA111&lpg=PA111&dq=chlorine+delocalization&source=bl&ots=qfAwVBGsli&sig=JyWE6vS8rGp35cMQXiOvIvHZHLQ&hl=en&ei=emgMTsL-DfK40AHNqtW6Dg&sa=X&oi=book_result&ct=result&resnum=5&ved=0CDUQ6AEwBA#v=onepage&q=chlorine%20delocalization&f=false which shows the delocalization of electrons from the C=C of vinyl chloride to the Cl atom. Vinyl chloride has a single Cl attached to the ethene.

If this doesn't make sense, send me a private message and I will be happy to email some pictures and descriptions to you that might make it easier to follow.

[Professor 0110]

Thank you, fledarmus. I have sent you a private message.