Organic Chemistry / Alkane Combustion
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Solution: In each of the following groups of compounds, identify the o...

Question

In each of the following groups of compounds, identify the one with the largest heat of combustion and the one with the smallest. (Try to do this problem without consulting Table 2.3.)

(e) Ethylcyclopentane, ethylcyclohexane, ethylcycloheptane 

Video Transcript

Hey everyone. So, for this question we're comparing our three compounds. Notice that it's a five membered ring, a six membered ring and seven membered ring, so let's go ahead and draw that out first, we have a 5 membered ring, a six membered ring and a seven membered ring, with all of them having a CH2, CH3 group, some are coming off. Well, I want you guys to realize is that when comparing these three structures you should notice that is one on the left is going to have seven carbons, the next one is going to have eight carbons and the last one will have nine carbons. So, why is that important? we're comparing seven to eight to nine carbons, so notice that we're increasing the number of carbons throughout all these structures. Now, do you remember the trend that states that when we increase the number of carbons, what we're going to do to the heat of combustion? ironically, we're also going to increase it, okay? Because what that means is when we have the greater heat of combustion we're going to have the lower stability, alright? So, note that the one that has the nine carbons in it will have the great heat of combustion, our next one will have both in between, right? And then all the way to the left will have the lower heat of combustion, okay? So I hope that makes sense, we talked about our trend that stated that as we increase the number carbons we're going to increase the heat of combustion and that's the main rule we use to distinguish between these three compounds, okay? So I hope that made sense.