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I'm in class right now and my teacher put this up on the board as a representation of 1,2-dichloroethane. He also projected a model. I'm thinking ... his board drawing is wrong. Shouldn't the chlorines also be on dashes too? There's no way the chlorines can be in the same plane as the carbon atoms; the model makes it too obvious.

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  • $\begingroup$ If you look at the model you posted, there's a plane that goes through both carbons and both chlorines. If you make that plane the sheet of paper, then you can connect these atoms with straight lines. Also, keep in mind that there can be rotation about the C-C bond, so there are many possible correct drawings. The only beef I have with the teachers drawing is that the C-C-Cl bond angles are 90 degrees, which is wrong. But, depending on the information that's being conveyed, that might not matter. $\endgroup$ – canadianer May 28 '14 at 16:57
  • $\begingroup$ I see, thank you! And yes the right angles were a bit jarring. $\endgroup$ – Dissenter May 28 '14 at 17:11
  • $\begingroup$ Your drawing of dichloroethane is another possibility of writing it and there are multiple more ways. Lewis-type structures project a 3D object into a two dimensional plane. It can therefore never be accurate enough and you will find multiple representations of the same molecules in different books. $\endgroup$ – Martin - マーチン May 29 '14 at 10:17
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The teacher's drawing is correct. Of course there are other possible conformers that could be drawn, but there is nothing wrong with the one that is pictured. Basic rule: a dashed wedge indicates a bond projecting behind the plane of the screen, a bold wedge indicates a bond projecting out in front of the plane of the screen, and a simple solid line indicates a bond existing in the plane of the screen. Also note that this drawing notation can be used for any molecule, not just carbon systems.

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  • $\begingroup$ Could you elaborate on drawing wedge/dash diagrams correctly or link me to a resource? Thank you! $\endgroup$ – Dissenter May 28 '14 at 16:02
  • $\begingroup$ Does this help? chemwiki.ucdavis.edu/Organic_Chemistry/Chirality/… $\endgroup$ – ron May 28 '14 at 17:38
  • $\begingroup$ Wait simple solid lines mean existing in the plane of the screen? What about the chlorines? These project into the screen (they are the red balls in the picture above), but these are also coplanar with the carbons - i.e. we can draw a single plane with both the carbons and chlorines on the plane. How about the simple solid lines just mean the ligands are coplanar (if not necessarily in the same plane as the screen)? $\endgroup$ – Dissenter May 28 '14 at 20:38
  • $\begingroup$ The red balls are in the model. The model is a 3D object. When we draw a 3D molecule in 2D, a simple solid line means in the plane of the page or screen. In the teachers drawing, the carbons are also in this plane. $\endgroup$ – ron May 28 '14 at 20:55

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