Dipole Moment and Cis Vs Trans Alkenes

[manohman]

In the science workbook there's a question that asks "Which of the following molecules have a dipole moment" and the answer is that CCl3H has a dipole moment, which it does.

But there is also a cis alkene. wouldnt a cis alkene have a dipole moment as well?

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[DesertSeaDiver]

I think of a molecule's dipole moment as the sum of all the individual bond dipoles. Since a bond dipole is just a difference in electronegativity, and because C and H both basically have the same electronegativity, then a pure alkene structure doesn't have a significant dipole moment. In comparison to the trans alkene, the cis alkene does have a greater overall dipole but it's essentially still zero.

 

[baxt1412]

C-H has no dipole, it's equivalent to C-C or O-O, etc.


I can see how you'd get there though.

 

[manohman]

I think of a molecule's dipole moment as the sum of all the individual bond dipoles. Since a bond dipole is just a difference in electronegativity, and because C and H both basically have the same electronegativity, then a pure alkene structure doesn't have a significant dipole moment. In comparison to the trans alkene, the cis alkene does have a greater overall dipole but it's essentially still zero.

Okay thanks that makes sense.

What confuses me/threw me off was that we're normally taught that a cis alkene does have a dipole moment, not necessarily a large one, but at least some non zero dipole moment to demonstrate proof of concept in comparing cis vs trans.

But in choosing the best answer definitely CHCl3 because that is much more polar. Awesome!

 

[texan2414]

Good example of what the AAMC means by picking the best answer.