 |
Reply
| | | From: ramay (Original Message) | Sent: 2/26/2005 9:30 AM |
hi everyone
i wanted to ask that there are two reactions regarding oxidation of ethyne by KMnO4 either in cold or hot..if in cold then we get formic acid HCOOH ,,if in hot then oxallic acid ...can some one tell in detail that why i s the difference ,...i think that formic acid has got great oxiation as two molecules are formed from one ethyne ,,,so the hot kmno4 should make it,,,but taht is contrary to the reality,,,can u explain why ,,,,
take care
anis ramay |
|
 First
Previous
2-11 of 11
Next
Last
|
|
Reply
| | From:  ·Steve· | Sent: 2/26/2005 9:25 PM |
Hi Anis, yes, organic oxidations are often confusing and the mechanisms may not actually be known with certainty. To add to the confusion, different textbooks may state the conditions slightly differently, for example, "cold dilute KMnO4 in acidic solution" vs. "cold dilute KMnO4 in acidic solution", and we can vary this with "hot" and "concentrated" also. KMnO4 is used under different conditions depending on what is being oxidized - alkenes (two general outcomes are possible), alcohols, or alkyl groups on benzene rings.
For alkenes, the two general outcomes are
1) glycol formation if cold, dilute, alkaline KMnO4 is used. A sketch of the cyclic intermediate in this reaction can be seen here.
2) Oxidative cleavage of the carbon-carbon sigma bond results when hot, concentrated alkaline KMnO4 is used, forming ketones, carboxylic acids, or CO2 if the double bond is on the end of the chain.
The reaction of ethylene looks like a special case. I expect that the first product is ethylene glycol, HO-CH2CH2-OH. KMnO4 can oxidize alcohols to give the same products as chromium reagents do (1° alcohols --> carboxylic acids, 2° alcohols --> ketones, and 3° do not react). In the case of ethylene glycol, perhaps oxidation of the 1° alcohol groups occurring independently, converting each to the carboxylic acid group, is more favorable than the oxidative cleavage reaction that generally occurs. This would result in the product oxalic acid.
Formation of formic acid may be particular to the oxidation of ethylene also. In general, when a terminal alkene is oxidatively cleaved by hot, alkaline KMnO4, the products are RCOO- + CO2 (the carboxylate ion RCOO- is then protonated by adding aqueous acid to give the carboxylic acid RCOOH). I'm not sure why formic acid results instead of 2CO2 in the oxidaton of ethylene. Is this stated in your textbook, and if so, does it give any other examples of these KMnO4 alkene oxidations? Also note that formic acid is less oxidized than oxalic acid, because the oxidation state of carbon in formic acid is +2, whereas in oxalic acid it is +3. Thus, the more oxidized product oxalic acid is forming at higher temperature.
For accurate mechanistic details of these reactions (if available!), you will probably need to find an advanced textbook specializing in such reactions, or search the chemical research literature directly.
Steve |
|
Reply
| | | From: ramay | Sent: 2/27/2005 8:20 PM |
hi steve
thanks fro the reply ...i think it did solved my query..
keep up
thanks
anis
<st1:country-region><st1:place><st1:country-region><st1:place>
<o:p> viola</o:p>
<o:p></o:p>CONTACT ME
cell 92-300-3099175
</st1:place></st1:country-region></st1:place></st1:country-region>
Don't just search. Find. MSN Search Check out the new MSN Search! |
|
Reply
| | | From: ramay | Sent: 3/4/2005 8:05 PM |
hi again
if we do the same thing with ethyne or acetylene [[not ethene]] then
C2H2 + H2O+ 3[O] ------cold kmno4 ------>2HCOOH FORMIC ACID
AND
C2H2,+ [O]---------------HOT KMNO4---> HOOC=COOH OXALIC ACID
can u explan steve thi s difference and reason it a little
thanks
anis
|
|
Reply
| | | From: ·Steve· | Sent: 3/4/2005 10:12 PM |
It's unclear to me why these alkyne oxidations would give the same products as the alkene oxidations you mentioned previously. In both cases, "cold" KMnO4 gives the less oxidized product formic acid, and when "hot" KMnO4 is used, the more oxidized product oxalic acid results. I would not be surprised if the mechanisms are uncertain. Also, do we know how "clean" these reactions are? That is, are product mixtures actually obtained, or is formic or oxalic acid obtained as the only product in high yield? Often, phrases like, "low yields" and "unclear mechanism" accompany discussions of reactions using KMnO4 as the oxidant.
I can only crudely speculate here, but if a cyclic manganate intermediate forms with an alkyne leading to a diol of the type HO-CH=CH-OH, then possibly it could tautomerize to HO-CH2CHO (hydroxy acetaldehyde) which could be oxidized to oxalic acid under the reaction conditions. Formic acid formation requires cleavage of carbon-carbon bond, perhaps through some kind of an oxygen-bridged manganate intermediate analogous to the ozonide intermediate in alkene ozonolyses.
Perhaps the answers to these questions can be found in a specialized textbook of oxidation reactions or elsewhere in the chemical literature. I am curious to know where you found these reactions. Are they from your textbook? They are not in typical organic textbooks at least. That is why I thought these reactions might be special cases, either particular to ethene, and now ethyne also, or elso they are done under special reaction conditions that favor the formation of these otherwise atypical oxidation products. For instance, reaction conditions used in industrial preparations can involve high temperatures and/or pressures not used in ordinary laboratory benchtop preps, giving correspondingly unexpected products.
Steve
|
|
Reply
| | | From: ramay | Sent: 3/5/2005 6:07 PM |
hi
i have scanned my textbooks's section refering to the on going disscusison i think its just simple
anis |
|
Reply
| | | From: ·Steve· | Sent: 3/5/2005 7:17 PM |
Thanks Anis, I wish your book had more information about these reactions. I'll try to find out more about them. BTW, is the textbook you are using a special one for your class, or is it in more common use and available from sellers like Amazon.com? I'm curious who the authors are and what the title of the book is. Thanks! Steve |
|
Reply
| | | From: ramay | Sent: 3/6/2005 3:37 PM |
well i am afriad u canot get it threre as it is our text book being circulated in the provice sindh,,,pakistan,,its sindh textbookboard that publishes ,,its cheap not more that 80cents 40 rupee,,, see ya anis
Express yourself instantly with MSN Messenger! MSN Messenger Download today it's FREE! |
|
Reply
| | | From: ·Steve· | Sent: 3/6/2005 6:59 PM |
80 cents... I wish our textbooks sold for that! Here in the U.S. a new college organic textbook sells for 100+ dollars, although used books can be obtained sometimes for as little as a few dollars at used book stores. I will search my older textbooks and try to find out something about these reactions. I do not have any special topics texts that focus on oxidations. It may be that these are special cases for ethene and ethyne, or are carried out under special reaction conditions that optimize formation of these products. The "standard" products of KMnO4 oxidations of alkenes under mild conditions is glycol formation, while oxidative cleavage of the double bond occurs under stronger conditions, giving ketones R2C=O, carboxylic acids RCOOH, or H2CO3 (which always decomposes to CO2 + H2O). Alkyne oxidations under stronger conditions are supposed to give similar cleavage products, either carboxylic acids or H2CO3, depending on the position of the triple bond. I haven't seen any mention of alkyne oxidations under mild conditions that give less oxidized products in any of my usual texts. Perhaps there is not enough interest in these reactions, to warrant mention of them, or it may be that these are established reactions from early experiments that have not been characterized in greater detail in more recent times. Steve |
|
|
First
Previous
2-11 of 11
Next
Last
|
|
Free Forum Hosting
Important Announcement
Return to Organic