PROCEDURE: • Prepare 3 test tubes and label as follows: “glucose”, “galactose”, and “lactose” • Add 1 mL of each of the test sugars respectively. • Carefully place 1 mL of concentrated Nitric Acid into each tube and mix thoroughly for a minute. • Using a masking tape, label 3 glass slides with the name of the sugar samples. • Hold a slide with a wooden test tube holder along one end. Transfer some drops of the mixture onto the slide and heat over an alcohol lamp. Do not scorch. Continue doing this until one mL or half of the sample has been transferred. • Cool the slide to room temperature. • Examine the crystals formed under the microscope.
A. What is the principle involved in the mucic acid test? Oxidation of most monosaccharides by nitric acid yields soluble dicarboxylic acids. However, oxidation of galactose yields insoluble mucic acid. Lactose will also yield a mucic acid due to hydrolysis of glycosidic linkage between its glucose and galactose subunits. Being insoluble, galactosaccharide acid crystals separate out.
b. What is the purpose of the test?
This test is used in the identification of GALACTOSE.
C. What components make up the mucic acid test?
H2O and HNO3
D. Provide the type equation used in the test. • HC=O-(CHOH)4-COOH (aq) + 2 HNO3 (aq) --> HOC=O-(CHOH)4-COOH (s) + H2O (l) + 2 NO2 (g)
When galactose is oxidized by nitric acid, insoluble mucic acid is formed. A positive result is the formation of a precipitate. Lactose is a dimer of glucose and galactose. The nitric acid will hydrolyze the dimer and so lactose will also give a positive result. Galactose is converted to Saccharic acid on heating with HNO3(a strong oxidizing agent). Mucic acid (galactaric acid) which is formed from galactose due to the oxidation of both aldehyde & primary alcoholic group at C1&C6.
E. Show the sample equation involved in the reaction between mucic acid reagent and your sample/s which gave a positive result.
F(a). Provide a possible explanation for the positive results obtained with your samples. F(b). Provide a possible explanation for the negative results obtained with your samples.
G. Explain why only galactose forms the white sandy crystals.
H. Lactose has also been known to form crystals in the mucic acid test. Rationalize why is this possible.
The Nitric Acid first catalyzes hydrolysis of lactose to glucose and galactose then the Nitric acid oxidizes both of these sugars to be tetrahydroxy acids. Mucic acid crystallizes out, which is a positive test.