Lilly1234567890
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What do pure carbonyls mean exactly? Are they just any compounds with a carbonyl group. What is so 'pure' about it?


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alow
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This explains it:

https://books.google.co.uk/books?id=...rbonyl&f=false
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Lilly1234567890
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Thanks !


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What do pure carbonyls mean exactly? Are they just any compounds with a carbonyl group. What is so 'pure' about it?


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Hi so I've just come across this thread because I was coincidentally raising the same question after stumbling upon ChemRevise!

I know this is quite old now, however I'd like to ask - would mind explaining to me what you understand from the definition of a 'pure carbonyl'? I read the extract posted on Google books but I don't seem to be getting a solid understanding of what it means.

I would appreciate anyone's help! Thank you.
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Pigster
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(Original post by alow)
This explains it:
I know "pure" is defined in this case, but could the Q be more loosely using the term and be referring to the tautomerisation to an enol, which would show hydrogen bonding?
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Thesceptic
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(Original post by I <3 WORK)
Hi so I've just come across this thread because I was coincidentally raising the same question after stumbling upon ChemRevise!

I know this is quite old now, however I'd like to ask - would mind explaining to me what you understand from the definition of a 'pure carbonyl'? I read the extract posted on Google books but I don't seem to be getting a solid understanding of what it means.

I would appreciate anyone's help! Thank you.
I assume it means that there are no other functional groups (impurities) within the molecule that would significantly change its nature. That it does not have, for example, an alcohol group somewhere which would make it a molecule of mixed properties as opposed to a 'pure' molecule of only the major properties of the carbonyl group.
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charco
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(Original post by Thesceptic)
I assume it means that there are no other functional groups (impurities) within the molecule that would significantly change its nature. That it does not have, for example, an alcohol group somewhere which would make it a molecule of mixed properties as opposed to a 'pure' molecule of only the major properties of the carbonyl group.
Pigster has nailed it in the post above ...

--CH-C=O <==> --C=C-OH
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alow
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(Original post by Pigster)
I know "pure" is defined in this case, but could the Q be more loosely using the term and be referring to the tautomerisation to an enol, which would show hydrogen bonding?
Ah yeah I guess so.
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shengoc
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Pure carbonyls in this case prob refers to ketones/aldehydes. even carb acids usually are not referred to as carbonyls but rather as its own family of carb acids.

Tautomerism refers to vinyl alcohol in eqm more towards the carbonyl side. although this is not a main concept tested at a level usually.
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(Original post by Pigster)
I know "pure" is defined in this case, but could the Q be more loosely using the term and be referring to the tautomerisation to an enol, which would show hydrogen bonding?
I'm studying A2 at the moment and so I'm not too sure what 'tautomerisation' actually refers to. :confused: Would you mind clarifying this a little further? To give some more background, on ChemRevise it says the following:

"The smaller carbonyls are soluble in water because they can form hydrogen bonds with water."

And then very much adjacent to this it states: "Pure carbonyls cannot hydrogen bond, but bond instead by permanent dipole bonding."

(https://chemrevise.files.wordpress.c...er-organic.pdf) - pg 8

What really confuses me by the above two comments, is if 'smaller carbonyls' are able to hydrogen bond in water then is that to say they are not 'pure'? If not, then that comes back to the same question - what is a pure carbonyl?
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Pigster
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In a nutshell, carbonyls can rearrange themselves, e.g. from ethanal to ethenol.

Carbonyls can hydrogen bond TO water, but not to each other.

There is a lone pair on the carbonyl oxygen which can attract the delta positive hydrogen on water forming (what you might call) a hydrogen bond.

Whereas, one carbonyl molecule can't do the same to another as there isn't the delta positive hydrogen, which is required for a hydrogen bond. Instead they have a permanent dipole on both molecules, which will attract each other.
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(Original post by Pigster)
In a nutshell, carbonyls can rearrange themselves, e.g. from ethanal to ethenol.

Carbonyls can hydrogen bond TO water, but not to each other.

There is a lone pair on the carbonyl oxygen which can attract the delta positive hydrogen on water forming (what you might call) a hydrogen bond.

Whereas, one carbonyl molecule can't do the same to another as there isn't the delta positive hydrogen, which is required for a hydrogen bond. Instead they have a permanent dipole on both molecules, which will attract each other.
Okay thank you. Also, so when they refer to carbonyl molecules as having permanent dipole-dipole bonds, why are they specifying that 'pure carbonyls' have such bond. What's the difference between a pure carbonyl and any other normal carbonyl we usually refer to?
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Pigster
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Pure carbonyls simply refer to the theoretical molecule that has no conversion into the enol and hence is not able form to hydrogen bonds to other carbonyl molecules.
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(Original post by Pigster)
Pure carbonyls simply refer to the theoretical molecule that has no conversion into the enol and hence is not able form to hydrogen bonds to other carbonyl molecules.
I really do apologise now for my slowness in understanding.

Though I still don't seem to be grasping the meaning of a 'pure' carbonyl or any associated conversions to 'enols'. Perhaps, let us shift to the context of ChemRevise for now. (pg 8) https://chemrevise.files.wordpress.c...er-organic.pdf

I would like to know please:

What is the difference between referring to the carbonyl molecules as pure ["Pure carbonyls cannot hydrogen bond, but bond instead by permanent dipole bonding"] or just simply stating that 'carbonyl molecules cannot hydrogen bond, but bond instead by permanent dipole bonding'? Is the usage of pure carbonyls or carbonyls interchangeable in this context?

I hope I'm making sense here.
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Pigster
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In theory a test tube could contain ethanal. As in, every molecule in the test tube were ethanal molecules.

Such a test tube is impossible, though. If you poured ethanal into a test tube, a handful of the molecules would convert into ethenol. The liquid you thought was pure ethanal was in fact a mixture of mostly ethanal with a bit of ethenol mixed in.

You can't stop the conversion, it is just a fact of carbonyls that they will convert. Or least a tiny amount will. It is an equilibrium that lies well to the carbonyl side.
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emilysmith268
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(Original post by Pigster)
In theory a test tube could contain ethanal. As in, every molecule in the test tube were ethanal molecules.

Such a test tube is impossible, though. If you poured ethanal into a test tube, a handful of the molecules would convert into ethenol. The liquid you thought was pure ethanal was in fact a mixture of mostly ethanal with a bit of ethenol mixed in.

You can't stop the conversion, it is just a fact of carbonyls that they will convert. Or least a tiny amount will. It is an equilibrium that lies well to the carbonyl side.
I've just been lurking trying to understand this lol think I get it now but can I just check?
  • Carbonyls are in equilibrium with enols because of tautomerism
  • the equilibrium heavily favours the carbonyl side
  • carbonyls can form H bonds with water but not other carbonyls because no partial charge on a hydrogen
  • enols can form H bonds with water and other enols because OH group so lone pair on O and delta +ve on H
  • saying 'pure carbonyls' in this context is referring to the theoretical circumstances in which no enols are formed

is that right?
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Pigster
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(Original post by emilysmith268)
is that right?
T'is a brilliant summation.
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charco
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(Original post by Pigster)
In a nutshell, carbonyls can rearrange themselves, e.g. from ethanal to ethenol.
.
Does the type of nut matter?
Is it a catalyst?
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Pigster
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Inside the nutshell, the tautomerisation may or may not have happened and until you crack open the nut to check, there is actually a superposition of states and the molecule is both the carbonyl AND enol form at the same time.

Damn stupid cat if you ask me. Just get on and grow opposable thumbs and crack open the nut.
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(Original post by Pigster)
In theory a test tube could contain ethanal. As in, every molecule in the test tube were ethanal molecules.

Such a test tube is impossible, though. If you poured ethanal into a test tube, a handful of the molecules would convert into ethenol. The liquid you thought was pure ethanal was in fact a mixture of mostly ethanal with a bit of ethenol mixed in.

You can't stop the conversion, it is just a fact of carbonyls that they will convert. Or least a tiny amount will. It is an equilibrium that lies well to the carbonyl side.
Ah thank you so much, this makes a lot more sense! Now just one more question - so if carbonyls possess this feature, could you say that all carbonyls are impure?
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