Hi stef44-ga:
There are three isomers of C2H2Cl2:
1) 1,1-dichloroethylene, or (Cl2)C=CH2. There is a net dipole towards
one direction of the molecule; namely, that pointing towards the
chlorines. As "drawn" above, the negative dipole is to the left, while
the positive dipole is to the right. Thus, this molecule is polar.
2) cis-1,2-dichloroethylene. Since the chlorines are pointed in the
same direction, on the same side of the double bonds, there is a net
dipole towards the direction of the chlorines. This molecule is polar.
3) trans-1,2-dichloroethylene. The chlorines are on opposite sites of
the molecule. Since the chlorines are pointed in opposite directions,
the dipole moments cancel out and there is no net dipole on the
molecule. This molecule is non-polar.
Two of three possible isomers of C2H2Cl2 are polar.
You can make drawings of these molecules to convince yourself that 1)
and 2) are polar, while 3) is not. Very good drawings of 2) and 3) are
available in the document below, on page 3.
If you need further help, please request a clarification of answer and
I'll see what I can do!
Note: In my answer, I use ethylene. Ethylene is the same as the IUPAC ethene.
Hope this helps,
- supermacman-ga
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- My knowledge of chemistry, backed up by
http://webpub.allegheny.edu/employee/a/adeckert/courses/110/exam5key.doc
- polar dichloroethylene |
Clarification of Answer by
supermacman-ga
on
25 Apr 2004 21:56 PDT
With regards to whether all cis- are polar and all trans- are non-polar:
Not necessarily. It all depends on the situation.
1,2-dichloro-1-fluoroethylene (ClF=CHCl), for example, is polar,
because there are more halogens on the left side than the right side.
Both this cis- form (chlorines both on top or both on bottom) and the
trans-form are polar.
Basically, you need to look at the molecule as a whole. If it seems
symmetrical, then it might be non-polar. If there is a net direction
in polarity, then the whole molecule might be polar. Drawing pictures
is the most important thing in solving problems such as these.
Hope this helps,
- supermacman-ga
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Clarification of Answer by
supermacman-ga
on
25 Apr 2004 22:06 PDT
Benzene never forms cis-trans isomers because it has a delocalized
electron configuration - the double-bonds are not really double-bonds.
They are spread over the entire molecule - this is called resonance.
As such, they act differently from standard double-bonds, and form a
completely different class of chemicals, called aromatics.
Here are good pages with more information about benzene:
http://en.wikipedia.org/wiki/Benzene
http://www.chemhelper.com/benzenetutorial.html
As you can see from the Wikipedia article, benzene is circular, so if
you really wanted, you could say it's all cis-. However, since benzene
doesn't really have double-bonds (they're just drawn with them), we
don't call it cis-. Besides, benzene never forms trans- isomers, so
the cis- is understood and implied.
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