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Can anyone explain why my textbook only gives the following resonance structures?

Write the important resonance structures for each of the following:

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I also put two resonance case where the double bond electrons are moved on the individual carbon atoms.

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    $\begingroup$ @Freddy Thank you for taking care of this site. Please do not use markup in the title field, see here for details. In short: It looks ugly in the search engines and results may not be found. $\endgroup$ – Martin - マーチン Sep 5 '14 at 3:53
  • $\begingroup$ @Martin Sorry i didn't take that in consideration. $\endgroup$ – Freddy Sep 5 '14 at 4:07
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Your resonance structure is perfectly alright. But the resonance structure with the negative and positive charges won't be very important and here is why: For one thing, it contains seperated charges and usually those resonance structures are most important that contain as few charges as possible. This is simply a matter of electrostatics - seperating charges and holding them apart from one another costs energy. A second reason has to do with the "electronic origin" of this resonance charge: It describes a conjugation between a free electron pair of $\ce{Br}$ and the $\pi$ system of the double bond - more accurately put the filled free-electron-pair-orbital can interact with the $\pi^{*}$ orbital of the $\ce{C=C}$ bond to form two new orbitals: an in-phase combination that is lower in energy and will contain the two electrons from the free electron pair and an out-of-phase combination that is higher in energy and will remain unoccupied. This stabilizes the system. The stronger the overlap between the free-electron-pair-orbital and the $\pi^{*}$ orbital of the $\ce{C=C}$ bond and the closer they are in energy the better the stabilization of the system through this conjugation is and the more important is the second resonance structure. But in the case at hand the $\pi$-overlap between the bromine orbital and the $\ce{C=C}$ bond is very weak (the reason for this you can find here) and so the conjugation is very weak, too. Thus, this resonance structure is not very important.

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  • $\begingroup$ "I also put two resonance case where the double bond electrons are moved on the individual carbon atoms." What about those? Sorry if I was not clear, but that was my main question :O! $\endgroup$ – yolo123 Sep 5 '14 at 2:55
  • $\begingroup$ @yolo123 Do you mean each carbon gets one electron from the double bond or one carbon gets two (thus becoming negatively charged) or the other gets none (thus becoming positively charged)? $\endgroup$ – Philipp Sep 5 '14 at 3:07
  • $\begingroup$ Just to add a little factoid, separated charges on a MOLECULE can be favorable under certain conditions. Amino acids such as glycine are known to be zwitterions (neutral molecules that contain separated charges) in solution across a wide pH range (~3-11). $\endgroup$ – LordStryker Sep 5 '14 at 12:50

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