Carbohydrates - Carbohydrates Color Tests and Polarimeter...

Carbohydrates:
Color Tests and Polarimeter Experiments
Anna Krajec
10/20/11
Lab Section 01

Tables and Graphs
Determining DNS Unknown

Questions
1. Why should a reagent control be run with all color reactions?
A reagent control is always a good idea because it indicates a negative result each time and thus
can be a comparison for the other solutions. It made it distinctively clear which solutions had
reacted positively and those that had not.
2. What functional group or groups will reduce Benedict’s solution?
Aldehydes will reduce Benedict’s solution, provided they are a part of a hemiacetal linkage.
Ketones will also reduce it provided that they are a part of a hemiketal linkage, since in the
presence of a base, they will be converted into aldehydes.
3. What happens to the sugar when the copper in Benedict’s reagent is reduced?
The aldehyde is oxidized to a carboxylic acid when the cupric ions are reduced to cuprous oxide.
4. When sucrose is hydrolyzed what monosaccharides are produced? Would hydrolyzed
sucrose give a positive or negative Benedict’s test? Explain your answer.
Sucrose is hydrolyzed to yield glucose and fructose. Glucose is an aldohexose and fructose is a
ketohexose. When not hydrolyzed, sucrose would give a negative result because the alpha(1-2)
linkages between the glucose and fructose lock the rings shut. When hydrolyzed, however,
sucrose would give a positive result since, alone, glucose and fructose are both reducing sugars.