properties of carbohydrates

Molisch’s Test with acid

• most general test

• Converts the carbohydrate to an aldehyde via dehydration reaction

• This test is named after Czech-Austrian botanist Hans Molisch, who is credited with its discovery

positive reaction of Molisch test

purple ring

positive reaction to all carbohydrates except

trioses and tetroses

Moore’s test alkali or base (NaOH)

• for presence of reducing sugar

moore’s test positive reaciton

• yellow to dark brown

• smells like caramel

Fehling’s test-reducing property

complex compound of Cu2+ or cupric ion.

fehling’s test positive reaction

• red precipitate

• no reaction in aromatic aldehydes

tollen’s test-reducing property

consists of silver ammonia complex in ammonia solution

Tollen’s test positive reaction

• gray black precipitate

• bright

• shiny silver mirror

Nylander’s Test- reducing property

for glucose in the urine, making use of a solution that contains bismuth subnitrate.

Nylander’s test positive reaction

brown or black precipitate

benedict’s test-reducing property

detects the presence of reducing sugars. This is a test for blood sugar and urine glucose

benedict’s test positive reaction

• red precipitate

(reason why it is detected by the benedict’s reagent)

Nonreducing sugars such as sucrose do not react with Fehling’s, Tollen’s, Nylander’s, and Benedict’s reagents. Can only produce positive result with the reagent when it is heated

Polymers

are broken down by hydrolysis, which is essentially the reverse of condensation.

Polysaccharides

such as starch and glycogen are broken down into thousands of glucose molecules during hydrolysis.

Bial’s Orcinol test for Pentoses

• Pentose - blue-green solution

• Hexose - muddy brown solution

Osazone test for Reducing Sugar

Formation of yellow osazone crystals

Mucic Acid test for Galactose

• Galactose - produces mucic acid

• Lactose - produces mucic acid

Seliwanoff’s test for Keto Sugars

• Ketoses - cherry red solution

• Aldoses - requires longer time to achieve cherry red

Iodine Test for Starch

• Starch - blue-black color

• Glycogen - reddish brown

• Glucose, Lactose, & Fructose - yellow

This is a hydrolysis reaction

because water (hydro) is used to break (lyse) a bond. When a bond is broken energy is released.

Fehling's test is considered

positive when the solution turns from blue to red. To test the presence of starch chemically, an iodine solution is used. If it turns from red to black or blue, the test is positive.