When the aldehyde is oxidized, the silver I ions are reduced to silver metal. Aldehydes have a proton attached to the carbonyl carbon which can be abstracted, allowing them to be easily oxidized to form carboxylic acids.
The lack of this hydrogen, makes ketones generally inert to these oxidation conditions. Nevertheless, ketones can be oxidized but only under extreme conditions. Oxidation of hexanal to form hexanoic acid using Tollens Reagent. There are a wide variety of reagents which can cause the oxidation of aldehydes to carboxylic acids. The most common reagent for this conversion is CrO 3 in aqueous acid also called Jones Reagent.
This reaction generally gives good yields at room temperature. Oxidation of hexanal to form hexanoic acid using Jones Reagent. The oxidation of aldehydes occur through the reversible nucleophilic addition of water to the carbonyl to form a gem-diol functional group. This addition reaction is discussed in greater detail in Section Because ketones do not have hydrogen atom attached to their carbonyl, they are resistant to oxidation.
You add a drop of sodium hydroxide solution to give a precipitate of silver I oxide, and then add just enough dilute ammonia solution to redissolve the precipitate. To carry out the test, you add a few drops of the aldehyde or ketone to the freshly prepared reagent, and warm gently in a hot water bath for a few minutes.
Figure 1: Tollens' test for aldehyde: left side positive silver mirror , right side negative. Image used with permission from Wikipedia.
Aldehydes reduce the diamminesilver I ion to metallic silver. Because the solution is alkaline, the aldehyde itself is oxidized to a salt of the corresponding carboxylic acid. The electron-half-equation for the reduction of of the diamminesilver I ions to silver is:. Combining that with the half-equation for the oxidation of an aldehyde under alkaline conditions:. Fehling's solution and Benedict's solution are variants of essentially the same thing.
Both contain complexed copper II ions in an alkaline solution. Both solutions are used in the same way. A few drops of the aldehyde or ketone are added to the reagent, and the mixture is warmed gently in a hot water bath for a few minutes. Figure 2 : Fehling's test. Left side negative, right side positive. And you can see this is the one with the OH down relative to the plain of the ring, and if you're thinking about which anomer this is you can compare this OH to this CH two OH group, which is up relative to a flat plain of the ring.
And since they're on opposite sides, this would be the trans, they're trans to each other. This is the alpha anomers, so this is the alpha, alpha glucose form in the cyclic hemiacetal form. So the other possibility, of course, would be to add that OH up relative to the plain of the ring.
So since we have a plainer aldehyde here, we could have added the OH up, and then they'd be on the same side as the CH two OH, so they're sis to each other. So this would be the Beta anomer, so that would be Beta glucose, the cyclic form of it. And the cyclic hemiacetal is actually favored, so we talked about that in the video on hemiacetal formation.
So glucose spends most of it's time in the cyclic forms, in the Beta and the Alpha form. But it's in equilibrium with the open chain form, with the open chain form containing our aldehyde functional group. This is what we need to react with Tollen's reagent. So when we oxidize the aldehydes, we're going to form a carboxylate anion, and when we oxidize the aldehyde we're going to reduce the silver.
So the silver ion's go from Ag Plus to Ag and forming our silver mirror. And the reason that glucose is used is because this is a highly water soluble, so it just makes this reaction a lot easier. And because of all these OH groups on the glucose molecule.
So you can form your silver mirror this way. You can make some really cool silver mirrors using glucose. And let me show you pictures of a couple of these things. So over here on the left my students actually made me an ornament. So you can do this reaction using a glass ornament. And then you can put your ornament on a chemist's tree. So let me go ahead and write on here Chemist Tree if it's the holiday season.
Here is actually my wife holding the ornament that my students made for us. And then you can actually make flat mirrors out of this too. Ben Norris Ben Norris 41k 8 8 gold badges silver badges bronze badges. Apparently they form after some more steps benzoe-, isobutyric- and acedic acid. However this might happen I did not read much further as it pained my eyes and brain. I hope this unfogs the mystery of the paper a little. BTW: there was a previous version in russian.
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