Tollen's reagent

[shal3]

OK so the DAT Destroyer says the Tollen's reagent suggests the presence of an aldehyde or an alpha-hydroxy ketone. BUT Kaplan says it's used to recognize ketones. SO WHICH ONE IS IT?? Kaplan gave a question with two of the answer choices being ketone and aldehyde. usind the destroyer method dives you aldehyde but kaplan said the correct answer is a ketone?? so which one?

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[shoombol tallah]

definately not with a ketone. Kaplan is wrong. The reason tollens agent reacts with an aldehyde and not a ketone is because an aldehyde can be further oxidized to carboxylic acid and ketone can not. When you do a tollens test also called silver mirror test Ag+ gets reduced to Ag forming a silver appearance. While the aldehyde get oxidized to a carboxylic acid. Hope that makes sense so know that Tollens test reacts with aldehyde and alpha hydroxy ketone and no ketones.

 

[kush87]

definately not with a ketone. Kaplan is wrong. The reason tollens agent reacts with an aldehyde and not a ketone is because an aldehyde can be further oxidized to carboxylic acid and ketone can not. When you do a tollens test also called silver mirror test Ag+ gets reduced to Ag forming a silver appearance. While the aldehyde get oxidized to a carboxylic acid. Hope that makes sense so know that Tollens test reacts with aldehyde and alpha hydroxy ketone and no ketones.

shoombol is right

 

[Contach]

can an alcohol get oxidized to a ketone or an aldehyde by Ag+?
will this produce a positive Tollen's test? Why not?

 

[vhlam]

has anyone encountered an error in the destroyer?

 

[shoombol tallah]

can an alcohol get oxidized to a ketone or an aldehyde by Ag+?
will this produce a positive Tollen's test? Why not?

hey contach im not really too sure about your question. I know a tollens test is used to determine the presence of an aldehyde when its down to ketone and aldehyde. I suppose if you wanted to find out if there was a alcohol in the mixture aswell you can try the lucas test which tests for primary, secondary, and tertiary alcohols based on their rate of reaction.

 

[Contach]

hey contach im not really too sure about your question. I know a tollens test is used to determine the presence of an aldehyde when its down to ketone and aldehyde. I suppose if you wanted to find out if there was a alcohol in the mixture aswell you can try the lucas test which tests for primary, secondary, and tertiary alcohols based on their rate of reaction.

Hi shoombol,
Does that mean that you can only use the tollens test if you've narrowed down your choices to a ketone and aldehyde? Because if so, this makes sense in terms of the OP's question. Since a ketone is "un-oxidizable", but an aldehyde is, the tollens will proceed. But say you put another oxidizable species like an alcohol into the mixture, then that will also give a positive tollens. No?

Wikipedia says taht you must do 2,4-DNPH first to narrow down the substance to one that has a carbonyl group. Then you can do Tollens' to narrow it down from there.

Thanks.
Super detective

 

[shoombol tallah]

yeah that sounds right . Im pretty sure tollens test is only used to decide between and aldehyde and ketone because like you said if you use another reducing agent such as alcohol you might still get the same positive result.

 

[joonkimdds]

yeah that sounds right . Im pretty sure tollens test is only used to decide between and aldehyde and ketone because like you said if you use another reducing agent such as alcohol you might still get the same positive result.

alpha-hydroxy ketone is a ketone so why is it an exception???

 

[D3ntalDud3]

what about the 2,4 DNP test? Could you use that to distinguish a ketone from an aldehyde also?

 

[joonkimdds]

what about the 2,4 DNP test? Could you use that to distinguish a ketone from an aldehyde also?

I don't think so. I think that test is used 1st to make sure that unknown chemical is aldehyde or ketone so that u can get rid of all the others except those two, and then u can go ahead and do the other tests to distinguish further details.



alpha-hydroxy ketone is a ketone so why is it an exception for Tollen's test?

and what's the point of doing both Chromic test and Tollen's test if both of them bring the same result? (+ for aldehyde in both test)

 

[chessxwizard]

The reason that Tollen's oxidizes keto sugars too is because they tautomerize into the aldo form under trace base.