Exercise 8 (Carbonyl Compounds and Carbohydrates)

8/12/2019 Exercise 8 (Carbonyl Compounds and Carbohydrates)

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EXERCISE 8CARBONYL COMPOUNDS AND CARBOHYDRATES

I. INTRODUCTION

A. Carbonyl CompoundsAn important group of oxygen containing compounds are the carbonyl

compounds those that contain the carbonyl group, C = O. They areclassified as aldehyde or ketones depending on what groups are bonded tothe C = O group.

Aldehyde Ketone

Since oxygen is more electronegative than carbon, there is greater pielctron density at the oxygen end of the C = O bond. The carbonyl group istherefore polar, with carbon bearing the partial positive charge:

This property of the carbonyl group gives rise to a set of reactionscharacteristic of aldehydes and ketones nucleophilic addition. The ease ofaddition of the nucleophile depends on the degree of the crowding at thecarbonyl group. Ketones are thus less susceptible to nucleophilic additionthan aldehydes.

The compound 2,4-dinitrophenylhydrazine (2,4-DNP) may add to thecarbonyl group to form solid oily derivatives called 2,4-dinitrophenylhydrazones:

O

R

H

O

R

R'

R, R = alkyl or aryl

O+ -

aldehyde 2,4-DNP 2,4-dinitrophenylhydrazoneor ketone

O

O2N

NH

NH2NO2

+

O2N

NH

N

NO2

+ H 2O


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This reaction is chiefly important for the characterization of aldehydes andketones.

Carbonyl compunds possessing the structure can also

undergo the haloform reaction. The bond between the carbonyl and methyl iscleaved to give a carboxylate ion and a haloform:

Aldehydes and ketones are often differentiated from each other by theTollens test. Aldehydes are oxidized by Tollens reagent to yield thecorresponding carboxylic acid and a silver mirror.

B. Carbohydrates

Carbohydrates are polyhydroxy aldehydes or ketones and the derivativesor compounds which yield such on hydrolysis. Many of them are representedby the general formula C x(H2O) y, which led early workers to regard them ashydrates of carbon, hence the term "carbohydrates.

Carbohydrates which can not be hydrolyzed to simpler units are calledmnonsaccharaides. These may be classified accoding to the type of carbonylpresent (ketose or aldose) or the number of carbon atoms in the molecule(triose, tetrose, hexose, etc.). Carbohydrates may exist as hemiacetals orhemiketals due to an intramolecular reaction involving the C = O group andone of the OH groups in the chain. This reaction is shown below for D-glucose. The atom marked with an asterisk is the masked carbonyl carbon(hemiacetal carbom).

CH3

O

R

CH3

O

R+ 3X 2 + 4OH -

O-

O

R+ CHX 3 + 3X - + H 2O

H

O

R

Ag(NH 3)2 O

-

O

R NH4+ + Ag


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-D-glucopyranose D-glucose -D-glucopyranose

The most common ketose is a hexose, D-fructose. The cyclization of(hemiketal formation) of D-fructose is shown below:

-D-fructofuranose D-fructose -D-fructofuranose

In the cyclic form the monosaccharides can not undergo the reactionstypical of the carbonyl group. However, in aqueous solution, the cyclic formsexist in equilibrium with the open chain form. The presence of even smallamount of the open chain form allows the reactions associated with thecarbonyl group to take place. Moreover, in the open chain form, theinteraction between the carbonyl group and the ajacent hydroxyl group makesthe carbonyl group more susceptible to oxidation and more reactive tonucleophilic reagents.

Carbohydrates that can be hydrolyzed to two monosaccharide units arecalled disaccharides. The monosaccharide units in disaccharides are joinedby a glycosidic linkage (specifically, an acetal or ketal linkage), which is reallyan ether linkage. Lactose is the dominant carbohydrate in milk and is madeup of -D-galactose and -D-glucose:

O

OHOH

OH

O H

OH

* 123

5

4

6

H OH

CH2OH

OH H

H OH

H OH

OH

2

1

3

4

5

6

O

OHOHOH

OH

OH

* 123

4

5

6

O OHOH

OH

OH

OH1* 2

34

5

6

OH

O

OH H

H

H

OH

OH

OH

O

OHOH

OH

OH OH

* 2

1

34

5

6


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Lactose -D-galactose -D-gl;ucose

Sucrose is common table sugar. Its is composed of -D-glucose and -D-fructose:

sucrose -D-glucose -D-fructose

The glycosidic linkage joins the masked carbonyl group (hemiacetal orhemiketal site) of one cyclic unit to the second. If the point of attachment ofthe second unit is not at its carbonyl group, i.e. there is a hemiacetal carbon,the cyclic disaccharide can exist in equilibrium with the free carbonyl form.

However, if the potential carbonyl group of the second unit is also tied upin the glycosidic linkage, no such equilibrium will exist. The disaccharide willnnot undergo the reactions that are due to the free carbonyl group.

OOH

OH

OH

O

OHO

OH

OH

OH

OH

O

OHOHOH

OH

OHOOH

OH

OH

OH

OH

O

OHOH

OH

OH

O

OOH

OH

OH

OH

O

OHOHOH

OH

OH

O OHOH

OH

OH

OH+

OO

O

OHO

O

O

H


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Monosaccharides and disaccharides are sometimes referred to as simplesugars or simply sugars.

Polysaccharides are high molecular weight polymeric carbohydrates madeup of many cyclic monosaccharide unit units. Those found in nature serve

either a structural or a nutritional function. Cellulose is the chief structuralmaterial of plants. It is a linear polymer of -D-glucose:

cellulose -D-glucose

Starch is the storage form of glucose in plants. It is a polymer of glucosebut the type of glycosidic linkage differs from that of cellulose. Starch is madeup of two components: amylose which has a linear structure and amylopectin,having a highly branched structure. Amylose and amylopectin contain a-D-glucose units.

amylose -D-glucose

amylopectin -D-glucose

O

OHOH

OH

OH

OH

O

OHOHOH

OH

OH

O

OOH

OH

OH

O

OOH

OH

OH

O

O

OOH

OH

OH

hydrolysis

hydrolysis

OH

OOH

OH

OH

O

O

OH

OH

OH

O

O

OH

OH

O

OH

O

OOH

OH

OH

O

OOH

OH

OH

O

O

OH

OH

OH

O

O

OOH

OH

CH2

O

O

OOH

OH

OH

hydrolysis O

OHOHOH

OH

OH


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There are a number of reactions used to characterize carbohydrates. TheMolisch reaction is a general test for carbohydrates either in the free orcombined form. In the presence of concentrated sulfuric acid, glycosidiclinkages are hydrolyzed to give monosaccharides.

nH2O + (C 6H10O5)n nC 6H12O6

The rate of hydrolysis depends on the solubility of the carbohydrate inwater. The monosaccharides formed are then dehydrated to furfural (orhydroxymethylfurfural) and other colored decomposition products.

a hexose hydroxymethylfurfural

Carbohydrates which, readily react with mild oxidizing agent are calledreducing sugars. The reducing property can be observed by the reaction withBenedicts reagent which consists of copper (II) hydroxide in aqueous solutioncomplexed with sodium citrate. The Cu 2+ complex ion has a bright deep bluecolor. A positive test is indicated by the reduction of the Cu 2+ ion to Cu (I) ,(Cu 2O) which is red brick in color and insoluble.

Reducing sugars may also be detected and differentiated from each otherby their reaction with phenylhydrazine. The reaction is more expensive thanthe formation of phenylhydrazones from simple carbonyl compounds. Theproducts of the reaction are called osazones. These are yellow, crystalline

solids with well defined melting points and crystalliine structure. They areuseful in the identification of simple sugars..

CHO

(CHOH) 4

CH 2 OH

OO

HOH

O

H

Cu 2+ , OH - O

OH-

+ Cu 2O

brick red

aldehyde 2,4-DNP 2,4-dinitrophenylhydrazoneor ketone

O

O2N

NH

NH2NO 2

+

O2N

NH

N

NO2

+ H 2O


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II. OBJECTIVES

1. To be acquainted with the chemical properties of carbonyl compounds andcarbohydrates.

2. To observe the differences in the chemical reactivity of aldehydes and

ketones.3. To apply chemical tests that distinguish the different types of carbohydrates.

III. PROCEDURE

The following representative substances will be used in this exercise:

aldehydes acetadehyde, benzaldehydeketones acetone, cyclohexanonemonosaccharides glucose, fructose

polysaccharides starch, cellulose (cotton fiber)Note: It is advisable to start with part B first.

A. Solubility Behavior*Perform this test on benzaldehyde, acetone, glucose, starch

1. Place 2 mL water in a test tube.2. Add 15 drops of liquid or 0.2 grams liquid sample.3. Examine carefully for homogeneity.

B. Hydrolysis of Di- and Polysaccharides*Perform this using sucrose, starch, and cellulose

1. Place 0.5 g of sample in a test tube.2. Add 5 mL water and 1 mL dilute HCl.3. Heat the mixture in boiling water for 30 min. and cool.4. Neutralize the hydrolysates with dilute NaOH using phenolphthalein as

indicator.

NOTE: Keep the hydrolysates for the Benedicts test.

C. Chemical Reactivity of Carbonyl Compounds

1. Reaction with Tollens reagent*Use acetaldehyde and acetone as samples.

a. Place 3 mL of Tollens reagent in a test tube.b. Add 4 drops of the sample.


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c. Shake to mix, then heat gently in a water bath.

2. Iodoform Test*Perform this test on acetone and cyclohexanone.

a. Transfer about 1 mL of sample in a test tubeb. Add 2 mL distilled water followed by 1 mL I 2 /KI solution.c. Add 10% NaOH dropwise while shaking until the iodine color

disappears and the solution is faintly yellow.d. Examine the tube and not the odor.e. If no change is observed, shake the tube then heat gently in a water

bath for 1 2 minutes.

3. Reaction with 2,4 DNP*Perform this test on acetone and acetaldehyde

a. Place 2 mL of 95% ethanol in a test tube.b. Add 3 drops of the sample.c. Add 1 mL of 2,4 DNP reagent.d. Shake and allow to stand for 5 minutes, then examine.

D. Color Reactions of Carbohydrates

1. Molisch Test*Perform this test on cyclohexanone, glucose, sucrose and starch

a. Place 10 drops of 1% aqueous sample in a test tube.b. Add 2 mL water, followed by 2 drops of the Molisch reagent. (10%

naphthol in ethanol)c. Shake and then add 2 mL concentrated H 2SO 4 slowly along the sides

of the test tube. (Take care not to agitate the contents of the tube)d. Note the color at the interface of the layers.

2. Bendicts Test*Perform this test on glucose, fructose, sucrose, lactose, and hydrolysatesfrom Part B.

a. add 10 drops of sample to a 0.5 mL Benedicts reagent in a test tube.b. Heat the mixture in a boiling water bath for 10 minutes.c. Note the final appearance of the mixture.

3. Osazone Formation*Perform this test on glucose, fructose, sucrose and lactose. Perform this atthe start of the period

a. Take 0.2 g of each sample and place in separate test tubes.


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b. Add 4 mL phenylhydrazine-HCl/NaCH 3COOH in each test tubesimultaneously.

c. Place the tubes in a boiling water bath.d. Shake the tubes occasionally.e. Note the time of immersion and time of precipitation of each osazone.

IV. QUESTIONS

1. Explain the solubility behavior of the samples used based on their chemicalstructures.

2. What is the structural requirement for the haloform reaction of the carbonylcompounds

3. How would you rate the oxidizability of aldehydes and ketones: easilyoxidizable, oxidizable, not oxidizable? Justify your answer using specific

chemical tests.4. Account for the differences in the reaction of starch, sucrose, and their

hydrolysates with Benedicts reagent.

5. Whatv is the structural requirement for the reducing property ofcarbohydrates.

6. Give a simple chemical test to differentiate between the following. Showequations for the reactions.

a. butanone and butanal (butyraldehyde)b. 2-propanol and acetonec. glucose and butanald. sucrose and lactosee. glucose and 1-pentanol

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Exercise 8 (Carbonyl Compounds and Carbohydrates)