Tollen's Test: Organic and Bio Confusion

[justadream]

Is the "Tollen's Test" (using AgNO3 to test for an easily oxidizable carbonyl group) different from the Tollen's test used to test if a sugar is a "reducing sugar"?

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[NextStepTutor_1]

Is the "Tollen's Test" (using AgNO3 to test for an easily oxidizable carbonyl group) different from the Tollen's test used to test if a sugar is a "reducing sugar"?

No, the Tollens' test(s) that you mentioned are not different. They are one and the same.

You'll find that when you can use the Tollens' Reagent [Ag(NH3)2] to test whether a compound can be oxidized, it makes no difference whether that compound is a carbohydrate or something else. Tollens' Reagent is produced by AgNO3.
For example, if you wish to test whether glucose or a generic aldehyde are capable of oxidation (i.e. are reducing agents), you can use the Tollens' Test which uses Ag(NH3)2 as the reagent. If the compound is capable of oxidation, the reaction will proceed producing an oxidized product and monatomic silver that will accumulate and produce the mirror that is indicative of a positive Tollens' test. If the reactant is not capable of being oxidized (such as a ketone) the reaction will not proceed, no mirror will be created, and the test will be concluded as negative.

Hope this helps some!



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[justadream]

So even though the Tollen's reagent is often used to test for aldehyde presence (oxidizing an aldehyde), it can also be used to test for whether a sugar (e.g., a sugar with a hemiacetal group) is oxidizable? In the latter case, what is the hemiacetal oxidized into?

 

[NextStepTutor_1]

So even though the Tollen's reagent is often used to test for aldehyde presence (oxidizing an aldehyde), it can also be used to test for whether a sugar (e.g., a sugar with a hemiacetal group) is oxidizable? In the latter case, what is the hemiacetal oxidized into?

Yes, that's right. You will mostly likely see it when testing for a reducing sugar (an aldose).
Recognize that a hemiacetal or acetal sugar will be a cyclical structure (it's linear, less stable structure presents an aldehyde).

A hemiacetal can be oxidized into an ester (the -OH group in the hemiacetal is oxidized to a carbonyl group). Keep in mind, however, that a hemiacetal is not oxidized into an acetal (the central carbon retains the same number of bonds to oxygen)

A hemiacetal placed in a acidic conditions with an alcohol can produce an acetal. The -OH group of the acetal is protonated by the acidic conditions making it a strong leaving group, the central carbon (now positively charged after the leaving group leaves) is bounded to the oxygen in the alcohol that acts as a nucleophile, the alcohol binds to central carbon and the alcohol's H is deprotonated from the oxygen leaving an acetal.