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Ashish Ahuja
Ashish Ahuja
03:42
@LalitTolani yeah kind of.. I know one of the fehling's solutions is CuSO4 and the other is some tartarate salt. On mixing them together we get a Cu-tartarate complex, and on reaction with some aldehyde the aldehyde gets oxidized to an acid, and the Cu(II) gets reduced to Cu(I) to form a reddish-brown ppt of Cu2O
Benedicts is the same thing, Cu2O ppt. Only difference is that instead of 2 solutions there is only one solution of Cu-citrate complex, which is more stable than the Cu-tartarate complex
Tollens is essentially diammine silver(I) ion, upon reaction with aldehyde its gets oxidized to give acid and Ag+ gets reduced to give a silver mirror of Ag
Fehlings, Benedicts don't work on aldehydes without alpha hydrogens (eg benzaldehyde, privaldehyde)
Btw even Barfoed's test is identical to Fehlings, Benedicts, only difference is the complexing ligand is acetate, which makes it even more stable and less reactive. If you see there's a nice pattern among all these 3 tests which makes them easy to remember
^^^ this is all I know
Ashish Ahuja
Ashish Ahuja
04:06
@Wolgwang yes, but looks like according to the latest iupac rules benzene is never taken as the substituent in such cases chemistry.stackexchange.com/questions/19229/…
but read the comments here:
@pikachu The old IUPAC rule (from 1979) actually was: “Choice between these methods is made according to the more appropriate of the following principles: (…) (b) treatment of a smaller of structure as a substituent into a larger. (…) In accordance with the principle (b) (…), a hydrocarbon containing a small cyclic nucleus attached to a long chain is generally named as a derivative of the acyclic hydrocarbon; (…)” — user7951 Mar 20, 2015 at 17:23
so I would stick with the 6 carbon rule
 
8 hours later…
Lalit Tolani
Lalit Tolani
11:59
@AshishAhuja Ah ok , this much is sufficient I think because I have seen some people learning how ag or cu attacks/ reacts with aldehydes
 
2 hours later…
hansika
hansika
13:44
@AshishAhuja hearing this for the first time, where is it?
Ashish Ahuja
Ashish Ahuja
14:07
@hansika ncert lab manual, test for carbohydrates and proteins. You should do it, it has come in mains before.
essentially the acetate complex is less reactive, and disaccharides are less prone to oxidation than monosaccharides, hence the cu(II) acetate complex reacts much slower with disaccharides than with monosaccharides, hence it can be used to distinguish the two
and the ppt formed is the same as with fehlings, benedicts, cu2o red-brown ppt
Lalit Tolani
Lalit Tolani
14:54
Those alpha hydroxy ketones which cannot form aldehydes on twice taotomerization will give tollen's test?
For eg consider this one 3 hydroxy butan 2 one
chemistry.stackexchange.com/questions/97592/… I got this but it doesn't show any mechanism just mentions that alpha hydroxy ketones give tollens test but how? here we can't form aldehyde @AshishAhuja @hansika
Ashish Ahuja
Ashish Ahuja
15:34
@AniruddhaDeb is here, I remember reading some answer of his on tollens :D
Aniruddha Deb
Aniruddha Deb
haha just hopped in after a long time :P got caught
Ashish Ahuja
Ashish Ahuja
@LalitTolani they usually do
Lalit Tolani
Lalit Tolani
@AshishAhuja do you know why or you just accepted it that they do ;-)
Ashish Ahuja
Ashish Ahuja
eg chemistry.stackexchange.com/questions/94620/… and you'll find many more questions on SE on this
although there is no clear answer for "all alpha-hydroxy ketones which cannot form aldehydes"
Lalit Tolani
Lalit Tolani
I have seen many and they contradict
@AshishAhuja yes
Ashish Ahuja
Ashish Ahuja
15:37
yup. Thing is if many compounds haven't actually been tested, there's no way to tell. And from what I remember not many have been
also its about reaction rates. If some compound reacts slowly, do you consider that a +ve test or not
Lalit Tolani
Lalit Tolani
1 min ago, by Lalit Tolani
@AshishAhuja do you know why or you just accepted it that they do ;-)
?
Ashish Ahuja
Ashish Ahuja
so its all a bit vague AFAIK
@LalitTolani the answer to the question I linked to gives a mechanism
I would expect all alpha-hydroxy ketones to undergo something similar. Although impossible to predict how fast
@AniruddhaDeb how's college btw
Lalit Tolani
Lalit Tolani
@AshishAhuja Hmm thanks , they should undergo something similar

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