tollens reagent

does tolllens reagent oxidize alcohols and giv a positive test for thm too
wat bout fehlings and benedicts?

9 Answers

11
Pritish Chakraborty ·

Tollen's reagent acts on alpha hydroxy ketones, mind this little fact. It's because they tautomerise into ene-diols which eventually tautomerise to give an aldehyde group instead of keto group(such as fructose). Fehling's reagent also acts on alpha-hydroxy ketones.

See, in Fehling's reagent, due to presence of excess OH- ions, aromatic aldehydes, hindered ketones, those having no alpha hydrogen etc undergo Canizzarro reaction instead(highly basic medium). Since Tollen's reagent doesn't have these ions in excess, these compounds can still give Tollen's test.

Tollen's and Fehling's reagents are not used for alcohols at all. They are used to test presence of carbonyl/aldehyde group.

(Benedict's reagent is basically the same as Fehling's reagent, it uses citrate ions instead of tartarate ions.)

Anonymous ·

The tautomer of an alpha-hydroxy ketone, an ene-diol, coordinates with silver(I) in a similar manner that the pi bond of an alkyne coordinates to silver(I). It will activate the alkene/alkyne for insertion/elimination.

Ag (I) may coordinate with CO2 from atmosphere or solution and insert carboxylate as it Ag (I) reductively eliminates to Ag(0). In this case, the alkyne is left with a carboxylate (acid). The ene-diol should break apart, forming a carboxylate and an aldehyde 'leaving group' if you will from the metal. Another insertion of CO2 gives another carboxylate.

An alternate mechanism of direct oxidation of the activated carbons without CO2 would also be correct.

Drawing mechanism may be tricky, you can find publications to back up this logic. Just know it rather than guess mechanisms.

1
none ·

.... actually instead of CO2 from atmosphere, probably just coordination of Ag(I) to ene-vicinal diol to form metallocyclopropane, then attack of water at the activated carbon to oxidize one position, silver is reduced to Ag(0), and eliminates from the oxidized carbon. Deprotonation of the gem diol gives a carboxylate, concerted with the silver organometallic complex leaving.

1
none ·

... and to answer why not oxidize a 'normal' alcohol but oxidize an alpha-hydroxy ketone... Ag(I) I imagine is a better pi-acceptor. It is larger and coordinates to pi-bonds. Hence oxidation of an aldehyde, but not an alcohol. It will also coordinate and insert into alkenes (ene-diol) and also alkynes depending on conditions.

Alcohol isn't a good ligand for Ag(I), I guess aquo complex could be formed in sol'n. You can check the pka of alcohol vs ene-diol tautomer to see if an alkoxide would be formed. RO- would be a stronger donor, maybe could coordinate better than ROH.

Sorry for all the posts :/ will stop ruminating.

1
utd4ever ·

tollens benedicts and fehlings test is given only by aldehydes
sorry by mistake i typed it as alcohols lol

21
govind ·

It is given by Aldehydes..alcohols give..lucas test and Victor Mayer test..

1
Arka Halder ·

agree with govind.
lucas test or victor mayer test determines whether an alcohol is primary,secondary or tertiary.
aldehydes give silver mirror with tollen's and red ppt with fehlings.

3
Manmay kumar Mohanty ·

When adding the aldehyde or ketone to Tollens' reagent, the test tube is put in a warm water bath. If the reactant under test is an aldehyde, Tollens' test results in a silver mirror. If the reactant is a ketone, it will not react because a ketone cannot be oxidized easily. A ketone has no available hydrogen atom on the carbonyl carbon that can be oxidized - unlike an aldehyde, which has this hydrogen atom.

Tollens' reagent is also a test for alkynes with a triple bond in the 1-position. A yellow precipitate of the metal acetylide is formed in this case.

Tollens' reagent also gives a positive test with formic acid (methanoic acid), as does Fehling's reagent.

source wikipedia

Note: Fehling's solution can only be used to test for aliphatic aldehydes, whereas Tollens' reagent can be used to test for both aliphatic and aromatic aldehydes.

1
Arka Halder ·

1° alcohols are generally oxidised to alcohols and 2° alcohols to ketones by PCC(Pyridinium chlorochromate).
as manmay said:
Fehling's solution can only be used to test for aliphatic aldehydes, whereas Tollens' reagent can be used to test for both aliphatic and aromatic aldehydes.
fehlings reagent is a weaker oxidising agent than tollens reagent due to which the above happens. Ketones can be distinguished from aldehydes by using such weak oxidising agents which dont oxidise them but oxidise aldehydes.

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