[student8607]

I'm told there are three resonance structures for C2H2Cl2. I need to give shape, identify polarity, calculate E, sketch orbitals, etc.. BUT FIRST

I have:

H    Cl
..     ..
C = C         but I need 2 more structures
..     ..
H    Cl


Would this work?
H     H
..     ..
C = C
..     ..
Cl    Cl


How about?
Cl    H
..     ..
C = C
..     ..
H    Cl


Or are these three the same?

[nj_bartel]

dichloroethene has 0 resonance structures.  Are you sure you aren't looking for isomers?

[macman104]

It must be isomers.  The second two you posted are correct, the first one has 3 chlorines, which I imagine is just a typo, otherwise, you're on the right track.

[student8607]

Corrected the typo.

The book just says "structures" so that is why I assumed resonance structures.

In any case,
How would I predict the shape of these weird things?
Like would I just count 3 bonds to the Carbons and call it trigonal planar? I'm thinking no

[macman104]

Pretty much!  You're going to use VSPER to predict your bond angles and structure.

[student8607]

OK. So I have my structures and they are all trigonal planars.
I'm good to go for class tomorrow until my teacher adds on parts a.b.c.d.e.... to his questions. Ugh.

[student8607]

"which is nonpolar and which two are polar" explain.

I know that polarity depends on difference in electronegativities
0<x<2 --> polar
x=x --> nonpolar

but how does this apply to these structures?

[macman104]

If you look at each bond separately, which ones are polar?  Which way is the pull of electrons (take a look at the following page: http://www.molecularsoft.com/help/screens/structures/dipole.gif, the arrow shows dipole vector)?

Now, if you look at those vectors, a non-polar molecule will have dipole vectors that cancel out, while polar molecules will have an overall molecular dipole.

You can post a picture (use paint if you want), showing how you would draw out those vectors if you need help figuring that part out.

[student8607]

H    Cl
..     ..
C = C         
..     ..
H    Cl


H     H
..     ..
C = C
..     ..
Cl    Cl


Cl    H
..     ..
C = C
..     ..
H    Cl


I'm not really understanding.
C-H = 0.4 polar
C-Cl= 0.5 polar
So like all of the bonds are polar?

[macman104]

Well, I guess that is technically true, but for organic molecules, you can generally consider the C-H nonpolar.  Then only your C-Cl bonds are polar.  With that info, can you answer the question with the previous info I gave?

[student8607]

So A & B are polar and C is nonpolar?

If this is right I am going to jump for joy that I actually understand it!

[macman104]

Correct, A and B are polar, and C is not.  Remember though that the bond angle is 120 for double bonds, so the structure is quite as you've drawn, but the idea is correct.

[student8607]

OK, last part to the question

Sketch the orbitals involved in the central carbon-carbon bond and explain why so much energy is necessary for bond rotation to occurr?

I calculated that 599kJ/mol of energy is neeed based on the fact that it had a wavelength of 200nm
E = h c / wavelength
--> J/mol
--> kJ/mol

[macman104]

You're going to have to look for an image of pi bonding orbitals in your book.

[student8607]

Wowzers these pictures are intense. Haha.
I'll manage though.

Thanks a lot for your help.