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As far as I know, the reason isooctane is better than octane in gasoline engines is because the octane is more prone to ignition by pressure. This makes it ignite when it is not supposed to, which damages the engine.

Isooctane, however, is somehow less prone to this. Why is that? Why is isooctane "more resistant to the potential of ignition incited by pressure"? How are the higher number of ligands involved in this?

andselisk
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A. Kvåle
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    Who says they are, by what quantitative measure? – Karl Dec 12 '20 at 20:30
  • @Karl That's what I've learned. Is this wrong? – A. Kvåle Dec 12 '20 at 20:36
  • @Karl Also, more heavily branched isomers have lower boiling points than their less branched isomer counterparts. I think that may be related. – A. Kvåle Dec 12 '20 at 20:44
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    Yes, branched isomers have a lower Bp., and that is of course one factor. The rest is debateable. Who taught you that, what was their reasoning? Do you know how the Diesel engine works? – Karl Dec 12 '20 at 20:45
  • @Karl Not quite sure who taught me that, I think it is a mash of stuff from different places. So, if the branched isomers are not more combustible than those lesser branched isomers, does this mean they are less combustible? Or is the amount of branches not the most defining factor behind combustion? I believe the diesel engine works by compressing the diesel hydrocarbons, which makes the combust? – A. Kvåle Dec 12 '20 at 20:56
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    Something tells me this is an extrapolation of the octane rating of a fuel, where 2,2,4-trimethylpentane (isooctane) is seen as a good fuel, whereas n-heptane is not. Whether this extrapolation is correct, and whether it has an explanation, I do not know. – Nicolau Saker Neto Dec 12 '20 at 22:14
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    Yes, diesel oil (very much linear alkanes) autoignites on compression. You don't want that in a regular gasoline engine, that's why gasoline has a high content of branched hydrocarbons. – Karl Dec 12 '20 at 22:14
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    @NicolauSakerNeto "Good" for a gasonline engine means no autoignition even at high compression. For a Diesel, you want autoignition without explosion. There is the cetane number for that. – Karl Dec 12 '20 at 22:21
  • @Karl Okay, so having branched hydrocarbons is good for making it "pressure-resistant", in the sense that it wont ignite before it is supposed to (before the fuse ignites it), and as such, won't cause knocking in the engine? How come linear alkanes are more prone to ignition due to compression than non-linear alkanes? – A. Kvåle Dec 12 '20 at 23:11
  • That's a tough one. You should ask a new question. To this one, there is no linear scale for "combustibility". There is the flash point, ignition temperature, vapour pressure, heat of combustion, .... – Karl Dec 13 '20 at 08:38
  • @NilayGhosh Yeah, I checked out that question and it did lead me to some good information about radical-based reactions, but the research I did also made me believe more in this notion that more heavily branched hydrocarbons are more combustible. This is because I thought that the propagation step would be more abundant in radicals due to the higher amount of ligands to cut off, made more available due to the higher surface area. – A. Kvåle Dec 13 '20 at 10:53
  • https://chemistry.stackexchange.com/questions/123093/deciding-the-order-of-heat-of-combustion-of-isomeric-alkanes/130811#130811 – user55119 Dec 13 '20 at 17:17
  • the real problem here is not that this is a bad question but that it is (essentially) a duplicate of https://chemistry.stackexchange.com/q/99333/81 – matt_black Feb 01 '21 at 11:36
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    "higher number of ligands" seems to be the wrong terms, ligand implies there is a metal complex. Do you mean "more branched" or something similar? – andselisk Feb 01 '21 at 12:14
  • @matt_black - you can use the "close" button at the bottom of the question to flag it as a duplicate of the question you linked. – Andrew Feb 01 '21 at 13:48

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