Qualitative Tests of Carbohydrates

1237 words 5 pages
Final Lab Report

Submitted by

Brittany Fitzgerald

Prepared for

Nancy Cook
Chemistry 3501

November 26, 2012

As a result of the Benedict’s test on various sugar solutions, it was found that galactose, mannose, arabinose, ribose, lactose, fructose, maltose, and cellobiose tested positively and therefore are considered reducing sugars. Glucose, starch, sucrose, and methyl-D-glucopyranoside on the other hand tested negatively and therefore are considered non-reducing sugars.
Table 1. Results of Benedict’s test on various sugar solutions Sugar Solution | Color | Clarity | Glucose | Bright blue with orange tint on top | Clear | Galactose | Brown
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Fructose produced a drastic color change after only 2 minutes, where as sucrose on the other hand produced a slight change in color after 3 minutes. Due to the results found in this portion of the experiment, it is thought that fructose is a ketose and that sucrose is neither a ketose nor an aldose, but it is a mixture of the two because it is formed from both fructose (ketose) and glucose (aldose).
Glucose and fructose are hexose sugars that differ only in the arrangement of the groups present on carbon 1 & 2. When either of these sugars are warmed in a weak alkaline solution, it is converted to an intermediate enediol form which contains a hydroxyl group at each end of the double bond.
As a result of the Seliwanoff’s test on various sugar solutions, it was found that all the different sugar solutions developed a change in color; however it was the solutions containing fructose that developed the biggest overall change. It is expected that these solutions would produce a positive test as fructose is a ketose and it develops a positive Seliwanoff’s test on its own. The fructose and Ba(OH)2 solution as well as the fructose and distilled water solution developed a peachy color almost immediately. Whereas the glucose and Ba(OH)2 solution and the glucose and distilled water solution developed a very light beige color near the end of the 5 minute heating