Bond lengths

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Emma Randolph 1J
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Joined: Fri Sep 28, 2018 12:29 am

Bond lengths

Postby Emma Randolph 1J » Mon Oct 29, 2018 12:38 pm

I'm confused as to how to find/ if we have to know how to find the bond length of experimentally observed structures. For example, in lecture Professor Lavelle went over how Benzene has double and single bonds, with the double bonds being 1.34A and the single bonds being 1.54A, but the experimentally observed length is 1.39A. Where did the 1.39 come from? That's what I'm confused on

Kristen Kim 2K
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Re: Bond lengths

Postby Kristen Kim 2K » Mon Oct 29, 2018 1:30 pm

I think the experimentally observed lengths will just be given, but they are basically going to be in between the single bond and double bond lengths because the actual length should be an average of the lengths from the resonance structures.

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Re: Bond lengths

Postby EllerySchlingmann1E » Mon Oct 29, 2018 2:13 pm

Yes, the experimentally observed bond lengths for benzene are known/given quantities. In benzene, the fact that the bond lengths are the same matters because it makes the structure symmetrical and therefore lower energy, but you don't have to just know the bond lengths of resonant structures, they would be given if relevant.

JiangJC Dis2K
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Re: Bond lengths

Postby JiangJC Dis2K » Mon Oct 29, 2018 3:38 pm

I believe that bond lengths can only be found experimentally, so it's unlikely that we will be needing them in this class. I felt that Lavelle was using the experimental data of bond lengths to depict to us the general pattern that in terms of length: triple bonds<double bonds<single bonds. Also to demonstrate the idea that the real lewis structure is an average its multiple resonance structures. Hope that helps :)

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Re: Bond lengths

Postby deepto_mizan1H » Mon Oct 29, 2018 3:44 pm

The experimentally observed bond lengths were shown in lecture to probably display how our model cannot fully grasp the true nature of the molecule's arrangement, as we have to find an average to find the true length and arrangement. A double bond is shorter than a single bond, but it does not mean our molecule is actually asymmetric or jaggedly shaped. Dr Lavelle mentioned the "double bond character" which depicts how the bond shares characteristics of both in order to accommodate electrons but keep it at an efficient and symmetric energy level. 1.39 and all the in-the-middle values we found are from averages and experimentation, and to just help us to understand what these models can (and cannot) show us.

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Re: Bond lengths

Postby EllaBerry » Mon Oct 29, 2018 7:36 pm

We will not have to find these bond lengths. In lecture, he used the lengths to show that the more bonds, the shorter the length of the bonds. Therefore a single bond will have a shorter length than a double bond. Also, he used it to show how in resonance structures the true length of the bonds are an average of the various lengths of the different numbers of bonds. There isn't actually a greater electron concentration where a double bond is drawn, instead the electrons are dispersed evenly. Using the average length of the bond was meant to demonstrate this.

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Re: Bond lengths

Postby juliasloan_4g » Mon Oct 29, 2018 10:29 pm

you will not need to calculate bond lengths. Professor Lavelle just included this in order to show that Lewis structures have a limitation, and this is why we need to show resonance

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