Experiment 4: Properties of Carbohydrates

Carbohydrates

Originates from the fact that many, but not all members of this class have the general molecular formula Cn(H2O)n and thus were considered hydrates of carbon

Monosaccharides

Simplest carbohydrates which can either be polyhydroxy aldehydes or polyhydroxy ketones

Polyhydroxy aldehydes

Have the general structure A and are referred to as aldoses

Polyhydroxy ketones

Have the general structure B and referred to as ketoses

3-6 carbons

How many carbons does monosaccharide have?

3C

Trioses

4C

Tetroses

5C

Pentoses

6C

Hexoses

Ribose, Glucose, and Fructose

Examples of monosaccharides

Ribose

Glucose

Fructose

Cyclic hemiacetal

Most monosaccharides exist in a cyclic form the aldehydes or ketone group reacts with one of the -OH groups on the other end of the same molecule to form a _______

a-D-glucose and B-D-glucose

Different anomers of glucose

Inter-converted

The rings are constantly opening and closing again. In this way the a and B forms can be?

Disaccharides

Two monosaccharides may be combined with the loss of one molecule of water

Glycosidic bond

The bond that connects 2 monosaccharide units in a disaccharide

Sucrose and Lactose

Examples of disaccharides

Table Sugar

Other term for sucrose

Milk Sugar

Other term for Lactose

Polysaccharide

Consists of many monosaccharides linked together

Starch, Pectin, Glycogen, and Cellulose

Examples of polysaccharides

Hans Molisch

Discovered the Molisch’s Test

Molisch’s Test

Involves the addition of Molisch’s reagent to the analyte and the subsequent of a few drops of concentrated sulfuric acid (H2SO4)

Purple or purplish-red ring

Confirms the presence of carbohydrates in the analyte

Tetroses and trioses

Exceptions for positive reaction for Molisch’s test given by almost all carbohydrates

Reducing Sugar

Carbohydrate that is oxidized by a weak oxidizing agent in basic aqueous solutions

Non-reducing Sugar

Not-oxidized

Aldoses

True reducing sugar because they contain the aldehyde functional group

Ketoses

Do not contain aldehyde (they contain ketone)

They also show reducing properties

Keto-enol tautomerization

Ketoses in basic aqueous solutions undergo ______ converting them to mixture of aldoses.

Free anomeric carbon

The only disaccharides that are reducing sugars are those that contain_______

It has the capacity to open and expose the aldehyde group

Monosaccharides

All _____ are reducing

Polysaccharides

All_____ are non-reducing

Benedicts test

Based of the fact that Cupric Ion (Cu2+) will oxidize aliphatic aldehydes including a-hydroxyaldehydes such as aldoses

Can also be used to quantitatively estimate the amount of sugar in a solution

This is also used to detect glucose in the urine

Brick red precipitate / vary from red to orange to green

Positive test for Benedicts Test

Green Color

Combination of blue solution and some are orange precipitate from Benedict’s test

Blue Solution

Negative Test for Benedict’s Test

Green/Yellow ppt

Traces of reducing sugar in Benedict’s Test

Orange red ppt

Moderate traces of reducing sugar in Benedict’s Test

Picric Acid Test

Saturated picric acid solution is used in oxidizing agent

Yellow

Color of picric acid solution

10% Na2CO3

The environment is made alkaline by _______

Mahogany red colored solution

Positive test for Picric Acid Test

Picric acid to Picramic Acid

The mahogany red colored solution is due to what kind of reduction?

Moore’s Test

At basic environment, this carbonyl groups can undergo aldol condensation

caramel with a brown color and characteristic odor

Positive result for Reducing Sugar (Moore’s Test)

Barfoed’s Test

Like Benedict’s test, it depends on the reducing properties of sugars. However, because of the specific conditions employed for the test, it is possible to distinguish between monosaccharides and disaccharides

Test for Monosaccharides

Cupric Ion (Cu2+)

Oxidizing agent used in Barfoed’s test just like Benedict’s test

Brick red precipitate of Cu2O within 2-3 minutes

Positive test for Barfoed’s test in Monosaccharides

Disaccharides in Barfoed’s Test

Require a longer time, providing the precipitate only after about 10 minutes in Barfoed’s test

Seliwanoff’s Test

Used to distinguish aldoses from ketoses

Test for Ketoses

Seliwanoff’s Reagent

Dehydrates ketoses more rapidly to give furfural derivatives

Furfural Derivatives

Undergo condensation reaction with resorcinol

Cherry red complex solution (2 minutes)

Positive test for Seliwanoff’s test for ketoses

Bial’s Test

Used to distinguish pentoses from hexoses

Test for pentoses

This distinction is based on the color that develops in the presence of the reagent

Bluish green solution

Positive reaction of pentose for Bial’s test which reacts with orcinol and ferric ion

Muddy-brown, yellow or gray solution

Positive reaction of hexoses for Bial’s test which are dehydrated and reacts with orcinol and ferric ion

Polysaccharides

Have highly coiled structures, such as starch

Iodine test

A very specific test for polysaccharides

Iodine

Forms a coordinate complex between the helically coiled polysaccharide chain and iodine centrally located within the helix due to the adsorption

Brown

Original color of iodine

Different colors

What colors do polysaccharides produce?

Ribose

5 carbon ring glucose

Fructose

Contains ketone

Benedict’s Reagent

Fehling’s Reagent

Tollen’s Reagent

Nylander’s Reagent

Picric Acid with Sodium Carbonate

Weak oxidizing Agent

Darkish Violet

Starch

Darkish orange/Brown

Glycogen

Light Yellow

Inulin

Cloudy Yellow

Cellulose