Tollens test

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akimhaneul

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Why is it that alpha hydroxy ketones will come out as positive for tollens test?

Also why is it that ketones in general will come out as negative for tollens test?


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Think about what the Tollens test does and how it works. Can you first explain how it works?
 
Think about what the Tollens test does and how it works. Can you first explain how it works?

It reduces silver to form a precipitate and oxidize the molecule of interest? This means that the molecule of interest must be able to reduce the silver.


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It reduces silver to form a precipitate and oxidize the molecule of interest? This means that the molecule of interest must be able to reduce the silver.

Yes - in other words, whatever Tollens' reagent is reacting with must be able to be oxidized by Tollens' reagent. Now, what do you know about how aldehydes and ketones can be oxidized? How is an aldehyde oxidized (in the most basic mechanistic terms - what strong bond must be broken)? How is a ketone oxidized?
 
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Yes - in other words, whatever Tollens' reagent is reacting with must be able to be oxidized by Tollens' reagent. Now, what do you know about how aldehydes and ketones can be oxidized? How is an aldehyde oxidized (in the most basic mechanistic terms - what strong bond must be broken)? How is a ketone oxidized?


Aren't aldehydes oxidized by removing the hydrogen attached to the carbon of the carbonyl group? Ketones do not have this so you can't oxidize it. Unless it was alpha hydroxy ketone, which can become aldehydes and then be oxidized.

hopefully I'm correct about this haha
 
Aren't aldehydes oxidized by removing the hydrogen attached to the carbon of the carbonyl group? Ketones do not have this so you can't oxidize it. Unless it was alpha hydroxy ketone, which can become aldehydes and then be oxidized.

Yes - aldehydes have alpha hydrogens which can be more easily removed to oxidize them. To oxidize ketones, you need to break a strong C-C bond, which requires very strong reagents without prior functionalization. For a sense of how strong those bonds are, there are only a few ways nature can even make/break C-C bonds - aldol being one of them.
 
So to recap, aldehydes can be oxidized up with the right choice of reagent, but ketones cannot (at least in the realm of introductory organic chemistry).
 
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