Why are alkenes more likely to undergo incomplete combustion than alkanes? Watch

EvilScientist
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#1
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#1
Alkenes require less moles of oxygen to burn, per mole of alkene, compared with an equivalent alkane.

C2H4 + 302===> 2CO2 + 2H2O

compared with

C2H6 + 3.5O2===> 2CO2 + 3H2O

So why on earth are alkenes MORE likely to undergo incomplete combustion than alkanes if alkenes require LESS oxygen to burn?
Saying that alkenes have a higher proportion of carbon compared to alkanes isn't a sufficient explanation, so does anyone have any ideas? Is it because of the double bond...? Maybe because less water is formed as a product for alkenes... I don't know! Give me your thoughts.

https://www.toktol.com/notes/context...ion-of-alkenes says alkenes are more likely to undergo incomplete combustion

http://www.docbrown.info/page04/OilProducts05.htm says this too... (near bottom of page)

IT DOESNT MAKE SENSE

Really appreciate if you can help clarify this...
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charco
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#2
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(Original post by EvilScientist)
Alkenes require less moles of oxygen to burn, per mole of alkene, compared with an equivalent alkane.

C2H4 + 302===> 2CO2 + 2H2O

compared with

C2H6 + 3.5O2===> 2CO2 + 3H2O

So why on earth are alkenes MORE likely to undergo incomplete combustion than alkanes if alkenes require LESS oxygen to burn?
Saying that alkenes have a higher proportion of carbon compared to alkanes isn't a sufficient explanation, so does anyone have any ideas? Is it because of the double bond...? Maybe because less water is formed as a product for alkenes... I don't know! Give me your thoughts.

https://www.toktol.com/notes/context...ion-of-alkenes says alkenes are more likely to undergo incomplete combustion

http://www.docbrown.info/page04/OilProducts05.htm says this too... (near bottom of page)

IT DOESNT MAKE SENSE

Really appreciate if you can help clarify this...
The C=C double bond is stronger AND there is a higher proportion of carbon to hydrogen. In combustion the hydrogen ALWAYS turns to water, but the carbon may form carbon monoxide or even particulates of carbon.

The same argument extends to alkynes, which often produce micro (and macro) particulates of carbon on combustion. Burning ethyne in air produces very dirty smoke.

PS I have no idea why this thread has been reported!
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EvilScientist
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#3
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(Original post by charco)
The C=C double bond is stronger AND there is a higher proportion of carbon to hydrogen. In combustion the hydrogen ALWAYS turns to water, but the carbon may form carbon monoxide or even particulates of carbon.

The same argument extends to alkynes, which often produce micro (and macro) particulates of carbon on combustion. Burning ethyne in air produces very dirty smoke.

PS I have no idea why this thread has been reported!

Oh okay, just to understand how having a C=C double bond affects combustion: so if the carbon double bond is stronger, its harder to break and so less likely to be fully oxidised and therefore more likely to undergo incomplete combustion? Is that train of thought correct?

Also,
I was thinking about this last night, and I was wondering if this was correct: Assuming that the alkene is left to burn on its own, without any other heat input from other sources but itself(not being continuously heat up by a bunsen burner for example):

because less water is formed, less heat energy is produced overall by the alkene(formation of bonds releases energy), and so the carbons connected by a double bond are overall less likely to undergo complete combustion (because its less likely to break the double bond both because it is strong, and because there is not enough activation energy for the carbon=carbon double bond to break because less heat energy is produced from less water being produced). Is this right as well?

Wait... But when the hydrogens are oxidised, and they leave the alkene molecule as H20, shouldn't that completely destabilise the molecule and make it much easier to oxidise the structure that was the alkene so that the carbon should easily react with oxygen now? If H20 is gone, then you should have some carbon cation intermediates right because the oxygen took the Hs away, then the resulting structure should be very easy to oxidise right?

Hahahaha thanks for answering and you sort of made me understand it more, but i still don't understand it fully, if you have some ideas/answers, please share?:angel:
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User947387
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#4
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#4
(Original post by EvilScientist)
Alkenes require less moles of oxygen to burn, per mole of alkene, compared with an equivalent alkane.

C2H4 + 302===> 2CO2 + 2H2O

compared with

C2H6 + 3.5O2===> 2CO2 + 3H2O

So why on earth are alkenes MORE likely to undergo incomplete combustion than alkanes if alkenes require LESS oxygen to burn?
Saying that alkenes have a higher proportion of carbon compared to alkanes isn't a sufficient explanation, so does anyone have any ideas? Is it because of the double bond...? Maybe because less water is formed as a product for alkenes... I don't know! Give me your thoughts.

https://www.toktol.com/notes/context...ion-of-alkenes says alkenes are more likely to undergo incomplete combustion

http://www.docbrown.info/page04/OilProducts05.htm says this too... (near bottom of page)

IT DOESNT MAKE SENSE

Really appreciate if you can help clarify this...
Yes, because of the double bond, they also have a higher proportion of carbon in them, requiring additional oxygen to bring about full combustion.
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charco
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(Original post by EvilScientist)
Oh okay, just to understand how having a C=C double bond affects combustion: so if the carbon double bond is stronger, its harder to break and so less likely to be fully oxidised and therefore more likely to undergo incomplete combustion? Is that train of thought correct?

Also,
I was thinking about this last night, and I was wondering if this was correct: Assuming that the alkene is left to burn on its own, without any other heat input from other sources but itself(not being continuously heat up by a bunsen burner for example):

because less water is formed, less heat energy is produced overall by the alkene(formation of bonds releases energy), and so the carbons connected by a double bond are overall less likely to undergo complete combustion (because its less likely to break the double bond both because it is strong, and because there is not enough activation energy for the carbon=carbon double bond to break because less heat energy is produced from less water being produced). Is this right as well?

Wait... But when the hydrogens are oxidised, and they leave the alkene molecule as H20, shouldn't that completely destabilise the molecule and make it much easier to oxidise the structure that was the alkene so that the carbon should easily react with oxygen now? If H20 is gone, then you should have some carbon cation intermediates right because the oxygen took the Hs away, then the resulting structure should be very easy to oxidise right?

Hahahaha thanks for answering and you sort of made me understand it more, but i still don't understand it fully, if you have some ideas/answers, please share?:angel:
You seem to be getting there although you are confusing several ideas. First of all, activation energy only affects the rate of the reaction not the products. However, you are correct in noting that the energy released by producing water would have more difficulty breaking C=C double bonds.

Secondly, combustion does not involve ionic intermediates, it is a free radical process, which is why, once initiated, it is a fast reaction.
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EvilScientist
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#6
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(Original post by EvilScientist)
Oh okay, just to understand how having a C=C double bond affects combustion: so if the carbon double bond is stronger, its harder to break and so less likely to be fully oxidised and therefore more likely to undergo incomplete combustion? Is that train of thought correct?

Also,
I was thinking about this last night, and I was wondering if this was correct: Assuming that the alkene is left to burn on its own, without any other heat input from other sources but itself(not being continuously heat up by a bunsen burner for example):

because less water is formed, less heat energy is produced overall by the alkene(formation of bonds releases energy), and so the carbons connected by a double bond are overall less likely to undergo complete combustion (because its less likely to break the double bond both because it is strong, and because there is not enough activation energy for the carbon=carbon double bond to break because less heat energy is produced from less water being produced). Is this right as well?

Wait... But when the hydrogens are oxidised, and they leave the alkene molecule as H20, shouldn't that completely destabilise the molecule and make it much easier to oxidise the structure that was the alkene so that the carbon should easily react with oxygen now? If H20 is gone, then you should have some carbon cation intermediates right because the oxygen took the Hs away, then the resulting structure should be very easy to oxidise right?

Hahahaha thanks for answering and you sort of made me understand it more, but i still don't understand it fully, if you have some ideas/answers, please share?:angel:
Oh wait, I think you probably end up with negative carbon ions (carbanions) instead of carbocations..... Maybe that explains it... Ohhh I think i got it!! So I think the Hydrogens that are taken away are H+, so leave their electron with carbon, and so you get carbanions, and since these carbons have more electrons, they can get a full outer shell, and so won't need to bond with oxygen because of the full outer shell, and thats why you get soot, because you get a stable carbon with a full outer shell.... is this somewhat right?
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EvilScientist
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(Original post by charco)
You seem to be getting there although you are confusing several ideas. First of all, activation energy only affects the rate of the reaction not the products. However, you are correct in noting that the energy released by producing water would have more difficulty breaking C=C double bonds.

Secondly, combustion does not involve ionic intermediates, it is a free radical process, which is why, once initiated, it is a fast reaction.
I thought activation energy was the minimum energy needed for a certain reaction to start, I was just thinking maybe the activation energy to produce CO2 is higher than the activation energy to produce CO/C? So CO and C are more likely to be produced?
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EvilScientist
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(Original post by EvilScientist)
Oh wait, I think you probably end up with negative carbon ions (carbanions) instead of carbocations..... Maybe that explains it... Ohhh I think i got it!! So I think the Hydrogens that are taken away are H+, so leave their electron with carbon, and so you get carbanions, and since these carbons have more electrons, they can get a full outer shell, and so won't need to bond with oxygen because of the full outer shell, and thats why you get soot, because you get a stable carbon with a full outer shell.... is this somewhat right?
Oh wait I realised that this argument doesn't mean alkenes are more likely to undergo incomplete combustion, because you could easily apply this idea to alkanes... damnit.. I think I got the gist of it but thanks charco for helping me understand this anyway!
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