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December 03, 2016, 03:39:45 pm

### AuthorTopic: Combustion of cyclohexane and cyclohexene  (Read 20079 times) Tweet Share

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#### akira88

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##### Combustion of cyclohexane and cyclohexene
« on: May 12, 2010, 04:09:05 pm »
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Why is it that cyclohexane produces a larger flame that lasts for longer when lit with a flame than cyclohexene which has a smaller flame and doesn't last as long?
I hope that question makes sense!
Any help would be appreciated, thanks
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#### Martoman

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##### Re: Combustion of cyclohexane and cyclohexene
« Reply #1 on: May 12, 2010, 06:09:02 pm »
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A guess? look at the equations.

$C_6H_{12} +9O_2 \rightarrow 6CO_2 +6H_2O$ (cyclohexane)

$2C_6H_{10} + 17O_2 \rightarrow 12CO_2 + 10H_2O$ (cyclohexene)

In the same volume of air say, you need twice as many cyclohexenes as you do cyclohexanes to combust. So cyclohexanes will more readily combust.
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#### kenhung123

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##### Re: Combustion of cyclohexane and cyclohexene
« Reply #2 on: May 12, 2010, 06:11:50 pm »
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Can you number functional groups on cyclo molecules or do you just ignore them?

#### akira88

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##### Re: Combustion of cyclohexane and cyclohexene
« Reply #3 on: May 12, 2010, 06:18:00 pm »
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Cyclohexene has a double bond, and so would have a higher burn rate since more energy is released from the double bond vs. the single bonds of cyclohexane.
That was the type of answer I was looking for, but how would it explain the cyclohexane producing a larger flame than the cyclohexene?
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#### akira88

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##### Re: Combustion of cyclohexane and cyclohexene
« Reply #4 on: May 12, 2010, 06:18:34 pm »
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Can you number functional groups on cyclo molecules or do you just ignore them?
Ignore them, you have no need to number them
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#### Richiie

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##### Re: Combustion of cyclohexane and cyclohexene
« Reply #5 on: May 12, 2010, 06:24:34 pm »
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All i can find in my book is that cyclohexane is very flammable.
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##### Re: Combustion of cyclohexane and cyclohexene
« Reply #6 on: May 12, 2010, 06:28:50 pm »
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A guess? look at the equations.

$C_6H_{12} +9O_2 \rightarrow 6CO_2 +6H_2O$ (cyclohexane)

$2C_6H_{10} + 17O_2 \rightarrow 12CO_2 + 10H_2O$ (cyclohexene)

In the same volume of air say, you need twice as many cyclohexenes as you do cyclohexanes to combust. So cyclohexanes will more readily combust.

And perhaps cyclohexene combustion doesn't last as long because of both the extra oxygen consumed and extra carbon dioxide produced, smothering the flame. I'm not sure though, this is just conjecture.
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#### Martoman

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##### Re: Combustion of cyclohexane and cyclohexene
« Reply #7 on: May 12, 2010, 06:40:16 pm »
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Yeah its guesswork. but it has some grounds in logic.
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#### kenhung123

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##### Re: Combustion of cyclohexane and cyclohexene
« Reply #8 on: May 12, 2010, 07:46:42 pm »
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Can you number functional groups on cyclo molecules or do you just ignore them?
Ignore them, you have no need to number them
But what if the functional group is not next to each other e.g. 2 Cl groups
the Cl's can be positioned anywhere in cyclohexene and still be called dichlorohexane?

#### Mao

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##### Re: Combustion of cyclohexane and cyclohexene
« Reply #9 on: May 12, 2010, 09:23:04 pm »
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A guess? look at the equations.

$C_6H_{12} +9O_2 \rightarrow 6CO_2 +6H_2O$ (cyclohexane)

$2C_6H_{10} + 17O_2 \rightarrow 12CO_2 + 10H_2O$ (cyclohexene)

In the same volume of air say, you need twice as many cyclohexenes as you do cyclohexanes to combust. So cyclohexanes will more readily combust.

What do you mean? the mole ratio is 1:9 and 1:8.5, they're pretty similar.
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#### akira88

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##### Re: Combustion of cyclohexane and cyclohexene
« Reply #10 on: May 12, 2010, 09:28:00 pm »
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Can you number functional groups on cyclo molecules or do you just ignore them?
Ignore them, you have no need to number them
But what if the functional group is not next to each other e.g. 2 Cl groups
the Cl's can be positioned anywhere in cyclohexene and still be called dichlorohexane?
Hmm I dunno, I highly doubt VCAA would give something like that in an exam though. But if you're wondering, I'm guessing you'll just number it with the lowest numbers. Eg. If they are next to each other, 1,2-dichlorohexane instead of 4,5-dichlorohexame for example
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#### Martoman

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##### Re: Combustion of cyclohexane and cyclohexene
« Reply #11 on: May 12, 2010, 09:30:59 pm »
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A guess? look at the equations.

$C_6H_{12} +9O_2 \rightarrow 6CO_2 +6H_2O$ (cyclohexane)

$2C_6H_{10} + 17O_2 \rightarrow 12CO_2 + 10H_2O$ (cyclohexene)

In the same volume of air say, you need twice as many cyclohexenes as you do cyclohexanes to combust. So cyclohexanes will more readily combust.

Similar yeah, but it facilitates a direct 1:2 ratio because you can't have a fraction of a molecule.

What do you mean? the mole ratio is 1:9 and 1:8.5, they're pretty similar.
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#### akira88

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##### Re: Combustion of cyclohexane and cyclohexene
« Reply #12 on: May 12, 2010, 09:32:02 pm »
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A guess? look at the equations.

$C_6H_{12} +9O_2 \rightarrow 6CO_2 +6H_2O$ (cyclohexane)

$2C_6H_{10} + 17O_2 \rightarrow 12CO_2 + 10H_2O$ (cyclohexene)

In the same volume of air say, you need twice as many cyclohexenes as you do cyclohexanes to combust. So cyclohexanes will more readily combust.

What do you mean? the mole ratio is 1:9 and 1:8.5, they're pretty similar.
Do you think the difference in the length of combustion and the flame size are all due to the double carbon-carbon bonding? And why is this the case? Thanks a lot if you can answer this!
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#### Mao

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##### Re: Combustion of cyclohexane and cyclohexene
« Reply #13 on: May 12, 2010, 09:50:41 pm »
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A guess? look at the equations.

$C_6H_{12} +9O_2 \rightarrow 6CO_2 +6H_2O$ (cyclohexane)

$2C_6H_{10} + 17O_2 \rightarrow 12CO_2 + 10H_2O$ (cyclohexene)

In the same volume of air say, you need twice as many cyclohexenes as you do cyclohexanes to combust. So cyclohexanes will more readily combust.

What do you mean? the mole ratio is 1:9 and 1:8.5, they're pretty similar.

Similar yeah, but it facilitates a direct 1:2 ratio because you can't have a fraction of a molecule.

Ah, but it doesn't, because the equation describes a mole ratio. It's true to say that 1 molecule of cyclohexene cannot react with 8.5 molecules of dioxygen, because it's impossible to have half a molecule, but 1 mole of cyclohexene reacts with 8.5 moles of dioxygen, which is perfectly reasonable. So the increase in volume is less than 10%, which doesn't matter that much.

The double bond is more reactive and a single bond, but I can't give a good answer as to why the flame is smaller (intermediates of the reaction) and why the time is shorter (reaction kinetics). I'll probably pin it down to the stability of the two molecules, and conclude reactivity of the double bond is greater, thus faster reaction. As for the size of the flame, that's probably with how much cyclohexane you used and whether it was spread out or not.
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#### akira88

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##### Re: Combustion of cyclohexane and cyclohexene
« Reply #14 on: May 12, 2010, 10:35:57 pm »
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A guess? look at the equations.

$C_6H_{12} +9O_2 \rightarrow 6CO_2 +6H_2O$ (cyclohexane)

$2C_6H_{10} + 17O_2 \rightarrow 12CO_2 + 10H_2O$ (cyclohexene)

In the same volume of air say, you need twice as many cyclohexenes as you do cyclohexanes to combust. So cyclohexanes will more readily combust.

What do you mean? the mole ratio is 1:9 and 1:8.5, they're pretty similar.

Similar yeah, but it facilitates a direct 1:2 ratio because you can't have a fraction of a molecule.

Ah, but it doesn't, because the equation describes a mole ratio. It's true to say that 1 molecule of cyclohexene cannot react with 8.5 molecules of dioxygen, because it's impossible to have half a molecule, but 1 mole of cyclohexene reacts with 8.5 moles of dioxygen, which is perfectly reasonable. So the increase in volume is less than 10%, which doesn't matter that much.

The double bond is more reactive and a single bond, but I can't give a good answer as to why the flame is smaller (intermediates of the reaction) and why the time is shorter (reaction kinetics). I'll probably pin it down to the stability of the two molecules, and conclude reactivity of the double bond is greater, thus faster reaction. As for the size of the flame, that's probably with how much cyclohexane you used and whether it was spread out or not.
Thanks a lot! I just thought the size of the flame was a characteristic due to it happening more than once when I did the prac.
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