why reflux? (alcohol to ketone)

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TheGreaterGood
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#1
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#1
I understand that if we want to go from an alcohol to a carboxylic acid we need to reflux so that we don't just end up with an aldehyde.

However, in going from an alcohol to a ketone I know we need to heat the alcohol with acidified potassium dichromate but why do we need to reflux it?

possiby it is something to do with having a secondary alcohol maybe?
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EierVonSatan
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#2
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It's always nice not to lose your solvent through boiling/evaporation while heating :mmm:
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Connor3
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#3
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Erm take my answer with a pinch of salt, its been a while since I've done this.

But don't you reflux because alcohols in general are volatile and would escape into the atmosphere, lowering your yield. Refluxing condenses the vapour and returns it to the system
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TheGreaterGood
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#4
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(Original post by EierVonSatan)
It's always nice not to lose your solvent through boiling/evaporation while heating :mmm:
:rofl:

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TheGreaterGood
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#5
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(Original post by Connor3)
Erm take my answer with a pinch of salt, its been a while since I've done this.

But don't you reflux because alcohols in general are volatile and would escape into the atmosphere, lowering your yield. Refluxing condenses the vapour and returns it to the system
yeah looking at this answer and the answer above shows your right [s]thumbsup[/]s

thanks
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Hunarench95
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#6
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Otherwise when you heat it, it would evaporate and leave your reaction vessel.


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gingerbreadman85
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#7
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(Original post by TheGreaterGood)
I understand that if we want to go from an alcohol to a carboxylic acid we need to reflux so that we don't just end up with an aldehyde.

However, in going from an alcohol to a ketone I know we need to heat the alcohol with acidified potassium dichromate but why do we need to reflux it?

possiby it is something to do with having a secondary alcohol maybe?
Invariably in organic chemistry, if you are heating something, unless it is very gently indeed, you always do so under reflux. It allows you to maintain the temperature at the boiling point of the solvent, ensuring maximum rate of reaction without loss of solvent. Solvents are often chosen deliberately to be very volatile, enabling easy purification, but this results in solvent loss during heating being a significant problem without reflux. Reflux is so good at preventing solvent loss that continual heating for 24-48hrs is possible without adding extra solvent.

Oxidation of an alcohol to an aldehyde is a special case, as the aldehyde is more volatile than any of the reagents it is heated but under distillation conditions, maintaining a temperature below that of the boiling point of the solvent. This results in the aldehyde being given off as soon as it is formed, preventing it from reacting any further to form a carboxylic acid. In this case, reflux would be counterproductive as you WANT to allow volatiles to leave the reaction mixture, which is what reflux prevents.

You don't actually NEED reflux to get you to the carboxylic acid. Plenty of times groups of mine doing the experiment have not had time to reflux and have had to leave the mixture overnight until the next lesson, and in the morning found the colour change you'd expect for an oxidation. Aldehydes are also more reactive than alcohols, thus why they can be tested for with Tollen's reagent which does not react with alcohols. The reflux is simply there to ensure the reaction occurs in a reasonable time frame (20-30min, rather than 24hrs!). Obviously though there are reactions that need increased temperatures to take place at all (meeting the requirements of Ea and/or dG=dH-TdS).

Oxidation to a ketone on the other hand does not have this problem, as there is no possibility for a further reaction to take place. You could theoretically still do this under distillation, as the ketone is more volatile than the alcohol, however the rate of reaction is faster under reflux and it's considerably easier to carry out as a procedure. Thus reflux is the ideal.
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TheGreaterGood
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#8
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(Original post by gingerbreadman85)
Invariably in organic chemistry, if you are heating something, unless it is very gently indeed, you always do so under reflux. It allows you to maintain the temperature at the boiling point of the solvent, ensuring maximum rate of reaction without loss of solvent. Solvents are often chosen deliberately to be very volatile, enabling easy purification, but this results in solvent loss during heating to be a significant problem without reflux. Reflux is so good at preventing solvent loss that continual heating for 24-48hrs is possible without adding extra solvent.

Oxidation of an alcohol to an aldehyde is a special case, as the aldehyde is more volatile than any of the reagents it is heated but under distillation conditions, maintaining a temperature below that of the boiling point of the solvent. This results in the aldehyde being given off as soon as it is formed, preventing it from reacting any further to form a carboxylic acid. In this case, reflux would be counterproductive as you WANT to allow volatiles to leave the reaction mixture, which is what reflux prevents.

You don't actually NEED reflux to get you to the carboxylic acid. Plenty of times groups of mine doing the experiment have not had time to reflux and have had to leave the mixture overnight until the next lesson, and in the morning found the colour change you'd expect for an oxidation. Aldehydes are also more reactive than alcohols, thus why they can be tested for with Tollen's reagent which does not react with alcohols. The reflux is simply there to ensure the reaction occurs in a reasonable time frame (20-30min, rather than 24hrs!). Obviously though there are reactions that need increased temperatures to take place at all (meeting the requirements of Ea and/or dG=dH-TdS).

Oxidation to a ketone on the other hand does not have this problem, as there is no possibility for a further reaction to take place. You could theoretically still do this under distillation, as the ketone is more volatile than the alcohol, however the rate of reaction is faster under reflux and it's considerably easier to carry out as a procedure. Thus reflux is the ideal.
what a detailed answer!

Thanks
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