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


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.)

(b) 2-Methylpropane, pentane, 2-methylbutane 

Video Transcript

Hey everyone. Well, in this question we're going to be comparing our three different structures we have 2-methyl propane, so we can draw that out, we look something like this there's our methyl, we have a regular pentane, which is five carbons and we have a 2-methyl butene, so it's going to be a four carbon chain with the methyl group coming off at position two, so it's our job to figure out which one has a greater heat of combustion, and also have the lowest heat of combustion, okay? So, HOC, the greater heat of combustion, we can say that that's maybe the one that has the most carbons because increasing number of carbons will also increase our heat of combustion, so we're gonna say those carbons. Now, the lower heat of combustion there's another rule. So, for heat of combustion, the lower heat of combustion, that means the greater stability that's going to be for our branched, okay? So, branched compared to one that unbranched, if it's unbranched that means is going to have the greater heat of combustion, okay? And those are we're comparing isomers. So, first let's count all of our carbons for 2-methyl propane, how many did you come up with? there's four carbons, what about pentane? that one easy there's just five, right? What about 2-methylbutane? Well, here we also have five. So, using our first rule we can already state that our first one will have the lowest heat of combustion because it has the least amount of carbons of the three, but now we need to look at the other two, they're isomers of each other, so let's take a look at our second rule, which one is branched the 2-methylbutane branch, so what does branching do? that's going to lower our heat of combustion, so which one you think that I have the highest heat of combustion between the two? the one that's unbranched, which is our regular pentane, so this one has the greatest feat of combustion, the one on the right is somewhere in the middle and our 2-methyl propane is going to have the least, okay? So I hope this made sense, let me know if you have any questions.