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Video Solution: In each of the following groups of compounds, identify the o...

Problem

In each of the following groups of compounds, identify the one with the largest heat of combustion and the one with the smallest. In which cases can a comparison of heats of combustion be used to assess relative stability? 

(b)  cis-1,2-Dimethylcyclopentane, methylcyclohexane, 1,1,2,2-tetramethylcyclopropane 

Video Transcript

So, now given that we know it's cyclopentane is going to have the highest in combustion, what about these? Well, we can figure this out using a combination of a few things, right? we can talk about our IHD, we can also talk about the number of carbons that we have and we can figure out our heat of combustion, okay? So, using our IHD and the number of carbons we're going to figure out our heat of combustion. So, here we've got an IHD of one, right? here we got an IHD of as well and same thing here, since we don't have any pi bonds but we do have one ring in each one. So, each ring contributes one IHD, right? So, how many carbons do we have in each? in this we've got a cyclopentane with 2 methyls, so that means that we've got seven carbons. So, our formula for this one is going to be C7H we're going to have H14, right? This one over here is going to be also with seven carbons, cyclohexane plus a methyl we're going to have C7H14 and this one is it's got a propyl, right? It was got a cyclopropane and I've got 1, 2, 3, 4, so that's going to give us seven carbons, right? So, also C7H14, so now with that in mind we can actually figure this out because we've got the same number of carbons and the same number of hydrogens. So, all we think about is ring strain at this point, so guys actually it turns out for the one that's going to have the highest heat of combustion it's going to be this one right here, cyclopropane, because cyclopropane has, is under a lot of ring strain, right? It's actually forming something called banana bonds where you have your bond stretching out, right? It's a little bit uncomfortable for the molecule, actually extremely uncomfortable but that's okay, this next one down here, cyclopentane is not as stable as cyclohexane because we know cyclohexane like to off soon that proper chair shape, right? So, this is actually going to be number three with our lowest heat of combustion and this is going to be the one with the the middle heat of combustion, right? So guys it turns out that the more unstable your molecule is the higher the heat of combustion you're going to get, right? So, just remember that as your general rule of thumb, alright? So guys let's move on and heat of combustion per bond, right? Not overall.

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