[Sapana]

Hey I need help with one question. Why is lactose a reducing sugar?
Please help.

[Bicster]

B/c in a solution it forms a ketone

[Hotflashn]

Originally Posted by Bicster

B/c in a solution it forms a ketone

Lactose forms a ketone? I am not challenging, I am questioning. I thought lactose was a diasaccarides of galactose and glucose, both aldehydes. Aldehydes oxidize, ketones don't. I thought some ketoses could form an aldehyde through tautorism (spelling?) going to an endiol, like fructose going to glucose or mannose. I thought ketoses were not typically oxidized (unless they turned into a "quickie aldehyde" endiol.)

I thought lactose was a reducing sugar because it has a hemiacetal on its (glucose) reducing end and because it can reduce the oxidizing agent (ex: Benedict's, Tollens, or Fehling's reagent or NAD+ biologically) In my o chem class lab we reduced Cu2+ to Cu+ from Fehlings reagent. The glucose portion of the lactose opened the chain to give an aldehyde that then resulted in a carboxylate: coo- Aldehydes yield carboxilic acid when oxidized.

I could be wrong, o chem is still new to me, and my learning curve is high, please let me know because if I am wrong it is because I don't understand and I NEED to understand before my final!

[a_damsel]

it contains a free hemiacetal. thus, it is able to form additional glycosidic linkages. the definition of a reducing sugar is a carbohydrate that gives a positive test with tollens and benedict's solution. both aldoses and ketoses show positive results by these test.

ketoses cannot be oxidized. the two tests mentioned above causes ketose to undergo a structural change allowing it to resemble an aldose. it is this aldose that then goes on to react with the chemicals in benedict's and tollens.