[elite engineer]

I'm very confused with the concept of molecular stability at this point. So, I'm under the impression that high octane fuel is composed of molecules that can withstand a high compression ratio (i.e. 1:12) and not combust, until a spark is introduced. At first i thought I was right because looking at the octane ratings of xylene and toluene, their octane ratings were 116.5 and 114 respectively. I assumed this due to the alkylbenzene structure...with benzene being very stable. However I noticed that methane had a much higher octane rating (120). Also propanol (118) and propane (112) had similar ratings. It seems like this throws my theory out the window. Can some please explain?

~EE

[kamiyu]

I could be wrong, but as far as I know, the octane rating is not about the molecular stability.

It is about the tendency of giving sudden thrust when combustion, which gives the driver trouble during driving.

[Arkcon]

Like kamiyu:, I don't understand what you mean by molecular stability, and what structures you associate with it.  Furthermore, your examples are poor choices as automotive fuel.   So that's another thing to think about, if you're being practical.

[Babcock_Hall]

I could be way off base here, but I wonder whether or not you are conflating thermodynamic stability with reactivity.  I am not saying that the two have absolutely nothing whatsoever to do with each other, only that they they are sometimes closely related and sometimes not.

[elite engineer]

I could very well be thinking the wrong ideas here, which is why I would really appreciate some clarity here.

kamiyu: so higher octane rating correlates with higher thrust/ more energy? This may be the case since methane has 55.5 MJ/kg. But it has to is some way correlate with the molecules stability. Why does 87 octane prematurely detonate in a high compression cylinder, but 93 octane doesn't? My thought is the 93 octane has additives (like xylene, etc) that when under a high pressure still retain stability and don't combust, therefore allowing the piston to fully compress before the spark is introduced..(i.e. maximum pressure in the cylinder = more energy).

Arkcon: By molecular stability, I mean how benzene has a lower resonance energy than say cyclohexane, b/c of it's double structure. I would imagine it's lower resonance energy translates it is "less" reactive under the same conditions that cyclohexane would combust.

Babcock_Hall: I think is this scenario they are related. Once again, the additives in 93 octane, such as toluene, xylene, etc, have a lower resonance energy, and are therefore more stable. This stability allows them to remain somewhat unreactive in a high compression environment. Right?...(BTW, do you belong to the "Original Science Forums"..I'm a member there too..your name looks familiar)

[Babcock_Hall]

I could very well be thinking the wrong ideas here, which is why I would really appreciate some clarity here.

Babcock_Hall: I think is this scenario they are related. Once again, the additives in 93 octane, such as toluene, xylene, etc, have a lower resonance energy, and are therefore more stable. This stability allows them to remain somewhat unreactive in a high compression environment. Right?...(BTW, do you belong to the "Original Science Forums"..I'm a member there too..your name looks familiar)

Yes, I am BabcockHall over at Science Forums.  One of my reservations is that non-aromatic alkanes have different octane numbers; therefore, one cannot invoke resonance for them.  From what I have read methyl test-butylether also raises octane, but it is not stabilized by resonance.

[elite engineer]

Babcock_Hall: so by what mechanism do o non-aromatic alkanes (methyl butyether, propane, etc.) increase octane rating?

[Yggdrasil]

The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine.

http://auto.howstuffworks.com/fuel-efficiency/fuel-consumption/question90.htm

So, the OP is correct about the definition of octane rating.  I don't think kamiyu's explanation is correct.

Higher octane ratings correlate to higher activation energies: the amount of applied energy required to initiate combustion. Since higher octane fuels have higher activation energy requirements, it is less likely that a given compression will cause uncontrolled ignition, otherwise known as autoignition or detonation.

https://en.wikipedia.org/wiki/Octane_rating

So Babcock_hall is correct that you should not be thinking about the thermodynamic stability of the compound, but rather its kinetic stability (for more discussion see http://chem.libretexts.org/Core/Physical_Chemistry/Equilibria/Chemical_Equilibria/Principles_of_Chemical_Equilibria/Kinetically_vs_Thermodynamically_Stable).

Unfortunately, I also don't really understand how molecular structure relates to reactivity with oxygen enough to explain why certain compounds have higher octane numbers than others.  Because octane number relates to the activation energy, you have to think about the thermodynamic stability of the transition state for the reaction in relation to the thermodynamic stability of the reactants.

[kamiyu]

The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine.

http://auto.howstuffworks.com/fuel-efficiency/fuel-consumption/question90.htm

So, the OP is correct about the definition of octane rating.  I don't think kamiyu's explanation is correct.

Higher octane ratings correlate to higher activation energies: the amount of applied energy required to initiate combustion. Since higher octane fuels have higher activation energy requirements, it is less likely that a given compression will cause uncontrolled ignition, otherwise known as autoignition or detonation.

https://en.wikipedia.org/wiki/Octane_rating

So Babcock_hall is correct that you should not be thinking about the thermodynamic stability of the compound, but rather its kinetic stability (for more discussion see http://chem.libretexts.org/Core/Physical_Chemistry/Equilibria/Chemical_Equilibria/Principles_of_Chemical_Equilibria/Kinetically_vs_Thermodynamically_Stable).

Unfortunately, I also don't really understand how molecular structure relates to reactivity with oxygen enough to explain why certain compounds have higher octane numbers than others.  Because octane number relates to the activation energy, you have to think about the thermodynamic stability of the transition state for the reaction in relation to the thermodynamic stability of the reactants.

Hi there

I did not say OP's definition is incorrect. I just said the relationship between octane rating and molecular stability is not clear.

[Enthalpy]

Hi Elite Engineer, nice to see you here!

The octane number relates essentially with the autoignition temperature. There could be more secondary subtleties, but it suffices for reasoning.

Compression in the cylinder heats the air, and to avoid a premature ignition in a gasoline engine, the fuel must not autoignite at the temperature that results from the compression. So to increase the compression ratio and gain efficiency, the fuel's autoignition temperature hence octane number must be high.

Knocking relates with the tendency to detonate instead of burning gently. It has a different number to characterize a fuel, but as things happen, both numbers a strongly linked.

It's the opposite need in Diesel engines, where the fuel must guarantee autoignition at the temperature attained by air compression.

Alas, simple thoughts seem to stop here. I know no simple way to estimate, nor even guess, an autoignition temperature. For sure, it doesn't relate with a hydrocarbon's heat of formation. Some general trends:

• Longer alkyl molecules and tails ignite more easily; Diesel oil has longer and straighter molecules, while high octane gasoline is branched.
• Aromatics need a higher temperature.
• I've seen no clear relation with the weakest C-H nor C-C bond in a hydrocarbon. This must not be the whole picture.

[orgopete]

I don't KNOW anything, so these are just my opinions. This seems as though it is related to two different reactions, a reforming reaction and an oxidation reaction. The catalysts used in platinum reforming reactions should simply give the more thermodynamically favored products. Branching and ring forming reactions must be more stable configurations of longer and straight chained reactions as noted by Enthalpy. Presumably this same reaction should occur in the absence of a catalyst, just heat.

I further guess that if one tried to generate these intermediates in the presence of an oxidant, the reforming reaction will be shunted by oxidation.

If I had to speculate, I'd consider carbon as weakly electron deficient and hydrogen further electron depleting. If we consider alkenes, in which the carbon nucleus is even more electron deficient, more substitution give a more stable alkene. If I have assessed the driving forces correctly, then going from straight chained to branched should be favored because carbon can be a better electron donor than a hydrogen. I speculate these properties can be found in the acidity of alkanes, CH4>RCH3>R2CH2>R3CH, and C-H bond lengths.