Why N-Butane & Isobutane Have Different Autoignition & Flash Points

[jadelamlam]

The autoignition temp of n-butane(straight chain) is 288 degree celsius while that of
isobutane(branched chain) is 460 degree celsius.
The flash point of n-butane is -60 degree Celsius while that of isobutane is -83 degree celsius.

I was confused by these..Why does n-butane have a higher flash point but a lower autoignition temperature?
(same case in n-octane and isooctane)

 

[pa5tabear]

Good question!

I'm curious to know how the atomic chemistry affects the physical properties.

 

[aroc91]

The flash point seems to be a function of vapor pressure. The intermolecular interactions of isobutane are weaker than those of n-butane because it's a compact molecule and doesn't stack as neatly with itself. This makes it more volatile and gives it a higher vapor pressure.

Here's the n-butane vapor pressure curve-

and the isobutane curve-

The autoignition temperature is a function of the activation energy of combustion of the molecule, so basically, how stable the molecule is. The stability of a molecule isn't related to the strength of its intermolecular interactions, so I don't think you make any correlation between a molecule's flash point and autoignition temp. Sort of apples and oranges.

 

[JohnRC]

Straight chain hydrocarbons (aliphatic, saturated) all have much lower autoignition temperatures than their branched chain isomers, but quite similar flash points.

The reason lies in the detailed mechanism of the combustion reactions. A warning before I get very far -- a state of the art model of the combustion of ethane (C2H6) needs to consider about 200 reaction steps; combustion reaction mechanisms are very complicated!

Combustion reactions generally proceed via free radical mechanisms. For autoignition, a reaction needs to escalate when one or a small number of reactive free radicals from the immediate environment are able to build up in a few steps to a self-sustaining reaction.

This can only happen at a low temperature if there is a radical explosion, or a branched chain reaction, where a prominent series of steps in the overall reaction system leads to an increase in the number of reactive free radicals that are driving the reaction.

Such a series of steps usually looks something like this:

•OH + R--H --> R• + H2O

R• + O=O --> R--O--O•

R--O--O• + R--H --> R--O--O--H + R•

R--O--O--H --> R--O• + •OH

In these equations, RH is the hydrocarbon that is undergoing combustion, and • is a symbol that denotes a very reactive free radical with an odd unpaired electron. You can see that in the previous three steps the number of free radicals was conserved, but that in the last step, 2 new free radicals were generated.

For hydrocarbons with straight chain molecular structures, these types of reaction steps are a major part of the mechanism; for those with branched structures there are more attractive alternatives that suppress the last step shown here -- for example, a rearrangement reaction to an alcohol plus a ketone.

It is a little ironical -- branched chain molecular structures proceed via simple radical chain mechanisms, while straight chain molecular structures react via branched chain combustion mechanisms, which can get quite explosive and unstable.

An autoignition can thus occur at a quite low temperature for a hydrocarbon with a straight chain molecular structure, but a much higher temperature will be needed to produce a self-sustaining reaction in its isomer with a branched chain molecular structure.

With a flash point test, the tiny spark or flame of the test produces enough free radicals and/oror thermal feedback to initiate a self-sustaining reaction for either type of molecule