budullewraagh
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Synthesis of Sugars from Aldehydes+Alkaline Earth Hydroxides?
i was reading the synthetic use of metals in organic chemistry, and in the Mg/Ca/Ba section, i came across a short section on the production of sugars
from the hydroxides of Mg, Ca and Ba. it's very vague and gives no indication whatsoever of the mechanisms involved before it goes on to talk
about acrolein bromide+Ba(OH)2-->acrolein.
what i do see is that trioxylethylene+Ca(OH)2-->slightly yellowish syrup called "methose"
does anybody know how the reaction happens?
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chemoleo
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I think it's similar to the reaction of i.e. acetaldehyde and formaldehyde to form the sugar pentaerythritol. The mechanism of that is well
known. Even formaldehyde and acetone works as well, to form a dimer of pentaertythritol.
Those reactions are also done in the presence of either Ca(OH)2 or Ba(OH)2.
Essentially, C beta of acetaldehyde is reactive, and has each H reacting with CHO to from the CH2OH, which is then reduced in due course.
Look up the pentaerythritol synthesis, that will help you.
PS What is ' trioxylethylene'?
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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budullewraagh
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i meant trioxymethylene. it was early in the morning. sorry
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chemoleo
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If you call it paraformaldehyde people are more likely to understand... I had to google it to get what you mean.
Anyway, so yes, this supports it. You might get more interesting sugars with paraformaldehyde and other aldehydes, though, even something like
glyceraldehyde, acrolein, etc.
From personal experience, such preps are unfortunatley rather cumbersome, as the products often don't crystallise.
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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budullewraagh
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could i make sugars with ketones instead of aldehydes?
| Quote: | Originally posted by chemoleo
Even formaldehyde and acetone works as well, to form a dimer of pentaertythritol. |
This answers your own question. Chemoleo
[Edited on 24-9-2005 by chemoleo]
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trilobite
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I remember a lecturer telling that acidic condensation of formaldehyde gave among many other things glucose and it's enantiomers in an experiment
performed in the 19th century, but I haven't tried to find more information on the subject. The inspiration for the experiment surely originated
from the fact that formaldehyde and glucose have the same empirical formula, CH2O. I presume it is not too selective a synthesis. 
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budullewraagh
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actually chemoleo i meant with ketones only and a lack of aldehydes. is that possible? could i use acetone only and see a reaction?
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Mumbles
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Yes, you will see a reaction, but I am not sure if it goes the same mechanism as the other reactions mentioned here. Acetone and barium hydroxide
will make diacetone alcohol.
http://www.orgsyn.org/orgsyn/prep.asp?prep=CV1P0199
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trilobite
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Oh, now I read the thread through properly. How do you define a sugar? To my knowledge it is essential that a sugar has, in it's open chain form,
a carbonyl group and several hydroxyl groups so, that it can cyclisize to form a ring through an oxygen atom (hemiacetal formation). Traditionally it
also has an empirical formula of a carbohydrate (CH2O)x, but there are exceptions like deoxyribose, so the definition has been expanded since old
times. Pentaerythritol at least is not a sugar nor a carbohydrate.
The reaction that is being discussed here is called Aldol condensation. The name originates from aldol, CH3-CH(OH)-CH2-CO-H, the
self-condensation product of acetaldehyde (originally simply called aldehyde, from alcohol dehydrogenatum). However, in my opinion it is not very
useful in synthesizing sugars since the problem is to be able to put several hydroxyl groups in a row to adjacent carbons in a chain. It is always
the alpha carbon (the carbon next to a carbonyl group) of a ketone/aldehyde that is reacting with a carbonyl group of another molecule, be it
formaldehyde, acetaldehyde, acetone, benzaldehyde or whatever -- you won't get a hydroxyl group in that position unless the aldehyde/ketone you
start with already contains a hydroxyl group in that position. I don't know how well it would work in practice, but theoretically if you used
alpha-hydroxyacetaldehyde HO-CH2-CO-H, the self condensation would result in HO-CH2-CH(OH)-CH(OH)-CO-H and (HO-CH2-CH(OH))2-C(OH)-CO-H among higher
condensation products, both of which are able to form a five-membered ring: (the empty space between lines makes it hard to draw clear vertical bonds)
| Code: |
CH2-CH(OH)-CH(OH)-C-OH
'--------O--------'
HO-CH2-CH-OH
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CH2-CH(OH)-C(OH)-C-OH
'--------O-------'
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Another question is why isn't there a carbonyl group in pentaerythritol if it is formed through Aldol condensation? First acetaldehyde reacts
three times with formaldehyde, always forming a bond between the alpha carbon of the acetaldehyde molecule and the only carbon of a formaldehyde
molecule. This reaction gives (HO-CH2)3-C-CO-H, whose only carbonyl group, the one coming from acetaldehyde, is then reduced to a hydroxyl by the
excess formaldehyde present in the reaction mixture, giving pentaerythritol (HO-CH2)3-C-CH2-OH or C-(CH2-OH)4.
With all respect, what I can't understand is people asking questions about organic chemistry without studying even the elementary reactions
themselves, it's not like the information isn't available on the net, even if one hasn't got access to an academic library (which is
the only acceptable excuse of not going there to read the books, in my opinion). Please do not hold yourself down. I must add that the Aldol
condensation is an irreplaceable cornerstone of organic synthesis, one of the oldest known ways to form a C-C bond, with applications, variations and
advances being developed even today. Anyone who wants to understand organic synthesis has to understand this reaction as well.
[Edited on 27-9-2005 by trilobite]
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unionised
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Trioxymethylene is another name for s- trioxane and isn't the same as paraformaldehyde (which is a linear polymer).
Once, long ago, before the details of photosynthesis were worked out, it was thought that formaldehyde was an intermediate in the production of
carbohydrates. (A quick google for "formaldehyde hypothesis" might help here.)
There are probably a number of definitions of sugars and carbohydrates. For the latter, you can always go back to the origin of the name. If the stuff
is a hydrate of carbon ie has the formula
C(x) H(2y) O(y) or Cx (H2O)y it's a carbohydrate.
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