CSUS, CH06B Carbohydrates Instructor: J.T., P: 1 Carbohydrates

General Information a. The name carbohydrate literally means water compounds of carbon. b. The general formula for simple sugars is Cn(H2O)n.

This class of compounds are better described as Polyhydroxy aldehydes and ketones. Methods of Classification: Several methods are used to classify carbohydrates.

A) One is based on whether the carbohydrate can be broken down into smaller units.

1) Monosaccharides - cannot be broken down into smaller units by hydrolysis. Sometimes called simple sugars.

2) Disaccharides - can be broken down (hydrolyzed) into two monosaccharide units.

3) Oligosaccharide - can be broken into three to six monosaccharide units. 4) Polysaccharides - composed of 7 or more monosaccharide units.

B) Another method is based on the number of carbons found in a simple sugar.

1) If it has three carbons it is called a triose. 2) If it has four carbons it is called a tetrose. 3) If it has five carbons it is called a pentose. 4) If it has six carbons it is called a hexose.

C) Another method uses the kind of carbonyl group.

1) Aldose - a monosaccharide with an aldehyde group.

HCO

C

C

CH2OH

OH

OH

H

H

erythrose 2) Ketose - a monosaccharide with a ketone group.

2

CCC

CH2OH

CCH2OH

OH

HH

HOOHOH

fructose Note: Usually combine the carbonyl classification and the number classification together.

COH

C OHH

C HHO

C OHH

C OHH

CH2OH

CH2OH

C O

C HHO

C OHH

C OHH

CH2OH

COH

C OHH

CH2OHglyceraldehyde

aldoglucose fructose

aldo ketotriose hexose hexose Fischer Projection:

Horizontal lines represent bonds coming out of the plane of the paper. Memory Aid? The horizontal bonds were coming out to hug you! Vertical lines represent bonds going behind the plane of the paper.

3 Cyclic structures can also be represented using Fischer diagrams

Haworth Projection:

The ring structure representations are termed "Haworth Projections. In contrast to the ring structures of the aromatic carbohydrates or the heterocyclic rings, the pyranose ring is not plane, so that is should be written as Haworths projection formulas:

C

H

CH2OH

O

OH

HO

H OH

H

OHH

CH

O

CH2OH

HOHO

OH

HO 1

2

5

3

4

1 6

4

3

2

6

5

4I. Molisch's Test is a chemical test for the presence of carbohydrates

A. Molisch's reagent (-naphthol dissolved in ethanol)/ Sulfuric acid. B. Reactions:

C5H10O5 (pentose) + (conc.) H2SO4 C5H4O2 + 3 H2O C5H4O2 (furfural) + 2 C10H8-OH (-naphthol) colored product

II. Seliwanoff's test is used to distinguish between aldehyde and ketone hexoses A. Seiwanoff reagent: Resorcinol in diluted HCl ) Results: Ketohexoses- cherry red solution (~ 1 min)

Ketopentoses- blue-green solution.

Aldose- no color change.

Disaccharides- no color change.

Note: sucrose will hydrolyze (by heating) to give Glucose (an aldohexose) and Fructose (a ketohexose)

CHO

OHH

HHO

OHH

OHH

CH2OH

O

OH

OHCH2OH

OH

H2COH OHOH2C CHO

-3H2O

D-Glucose

D-Fructofuranose 5-hydroxy-methyl furfural

H3O+

5This product (5-hydroxymethylfurfural) then reacts with resorcinol to form a deep red solution.

OHOH2C CHO

5-hydroxymethyl furfural

+HO OH

Resorcinol(colorless)

H+

R

OH

HO

OHO

R= 5-hydoxy furfuryl (red)

III. Benedict's test for reducing sugars:

A. Benedicts Reagent: sodium carbonate, sodium citrate dihydrate, and copper sulfate pentahydrate in water.

B. Reaction:

R-CHO + 2Cu2+ +5OH- R-COO- + Cu2O (s, red) + 3H2O CHO

OHHHHO

OHH

OHH

CH2OH

+ 2 Cu2+ + 4 OH-

COOH

OHH

HHO

OHH

OHH

CH2OH

D-Glucose

D-Gluconic acid

+ Cu2Oredppt

+ 2H2O

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IV. Barfoed's test: a general test to distinguish between monosaccharides and disaccharides.

A. Barfoed's Reagent: Copper acetate monohydrate, glacial acetic acid in water.

CHO

OHH

HHO

OHH

OHH

CH2OH

CH2OH

C O

HHO

OHH

OHH

CH2OH

OH-+ 2 Cu+2 + 4 OH-

COOH

OHH

HOH

OHH

OHH

CH2OH

+ Cu2O + 2 H2O

redppt.

D-fructose(+ D-mannose)

D-glucose(+ D-mannose)

D-gluconic acid(+D-mannonic acid)

Glucose is oxidized to gluconic acid. The copper (II) is reduced to copper (I). The rate of appearance of the red precipitate can distinguish reducing monosaccharides

from reducing disaccharides.

Note: It is more sensitive than the Benedicts test; it can be used to differentiate between reducing monosaccharides and reducing disaccharides. The reduction of Cu2+ to copper (I) oxide occurs more rapidly for monosaccharides than for disaccharides.

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V. Bial's test distinguishes a pentose (5 carbons) from a hexose (6 carbons) monosaccharide.

A. Bial'sReagent: orcinol, Concentrated HCl, ferric chloride hexahydrate B. Reaction:

CHO

OHH

OHH

OHH

CH2OH

D-ribose(an aldopentose)

C

OHH

OHH

CH2OH

O

CH2OH

D-ribulose(a ketopentose)

O CHO

furfural

-3H2O

The furfural then reacts with orcinol and ferric chloride to form a blue-green solution.

CH3

OHHO

O CHO

CH3

HO

RH

CH3HO

OHOH

H+

CH3

CH3

OHOH

HO+ FeCl3(oxidation)

orcinol(colorless)

furfural(colorless) R = furfuryl (colorless) R = furfuryl (blue-green)

O

R

The rate of formation and the color of the solution can be used to distinguish pentoses from hexoses. Pentose sugars form blue-green solutions; hexose sugars form reddish brown complexes.

VI. The iodine test is used to differentiate between starch and glycogen. I2 + KI ' I3- + K+

The amylose in starch forms helices where iodine molecules assemble, forming a dark blue or black color.

Dehydration: C12H22O11 + 11 H2SO4 12 C + H2SO4 . H2O

8 This is a list of some common monosaccharides:

Trioses:

o Aldotriose: glyceraldehyde

o Ketotriose: dihydroxyacetone

Tetroses:

o Aldotetrose: erythrose and threose

o Ketotetrose: erythrulose

Pentoses:

o Aldopentoses: arabinose, lyxose, ribose and xylose

o Ketopentoses: ribulose and xylulose

Hexoses:

o Aldohexoses: allose, altrose, galactose, glucose, gulose, idose, mannose and talose

o Ketohexoses: fructose, psicose, sorbose and tagatose Disaccharides - composed of two monosaccharide units.

2. Maltose - malt sugar.

a. Used in cereals, candies and the brewing of beverages. b. Composed of two D-glucose sugars joined by an -1,4 linkage.

O OHH

H

HH

H

O

HOHOH H H

OH OHOH

OH

CH2OH CH2OH

H

It contains a Hemiacetal functional group and has a reducing suger.

3. Lactose - milk sugar.

a. Found in milk and milk products. b. Composed of one galactose and one glucose unit joined by a -1, 4 linkage.

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OH

H

H

OH H

OH

OH

CH2OH

HO

CH2OH

OH

HOHH

H

OH

H

O

4. Sucrose - table sugar.

a. Product of sugar cane and sugar beets. b. Composed of one glucose and one fructose unit.

O O

OH

H

OH

H

H

O

HH

H

OH H

HOHCH2OHOH

CH2OHCH2OH

Polysaccharides - composed of many monosaccharide units.

5. Cellulose

a. Major structural material of plant cells. b. Consists of many glucose units joined by -1, 4 linkages. c. Disaccharide starting material is called Cellobiose.

OH

H

H

OH H

OH

OH

CH2OH

HO

CH2OH

OH

HOHH

HOHH

O

6. Starch

a. Storage form of glucose found in rice wheat, potatoes, grains and cereals. b. Consists of many glucose units joined by -1, 4 linkages. c. Maltose is the disaccharide starting material.

7. Glycogen a. Animal starch. Storage form of glucose found in the liver and muscle of

animals. b. Contains many highly branched glucose units.