EXERCISE 8 CARBONYL COMPOUNDS AND CARBOHYDRATES I.
INTRODUCTION A. Carbonyl Compounds An important group of oxygen – containing compounds are the carbonyl compounds – those that contain the carbonyl group, C = O. They are classified as aldehyde or ketones depending on what groups are bonded to the C = O group. R
R O
O
H
R, R’ = alkyl or aryl
R'
Aldehyde
Ketone
Since oxygen is more electronegative than carbon, there is greater pi elctron density at the oxygen end of the C = O bond. The carbonyl group is therefore polar, with carbon bearing the partial positive charge:
δ+
δ-
O
This property of the carbonyl group gives rise to a set of reactions characteristic characteristi c of aldehydes and ketones – nucleophilic addition. The ease of addition of the nucleophile depends on the degree of the crowding at the carbonyl group. Ketones are thus less susceptible to nucleophilic addition than aldehydes. The compound 2,4-dinitrophenylhydrazine (2,4-DNP) may add to the carbonyl group to form solid oily derivatives called 2,4dinitrophenylhydrazones: O2N
O
aldehyde or ketone
+
O2N
H2N
+ H2 O
N NH
NO2
2,4-DNP
NH
NO2
2,4-dinitrophenylhydrazone 2,4-dinitrophen ylhydrazone
This reaction is chiefly important for the characterization of aldehydes and ketones. O
Carbonyl compunds possessing the structure
H3C
R
can also
undergo the haloform reaction. The bond between the carbonyl and methyl is cleaved to give a carboxylate ion and a haloform: O R
O CH3
+
3X2 +
4OH-
-
R
O
+ CHX3
+ 3X- +
H2O
Aldehydes and ketones are often differentiated from each other by the Tollen’s test. Aldehydes are oxidized by Tollen’s reagent to yield the corresponding carboxylic acid and a silver mirror. O
O Ag(NH3)2
R
H
R
-
O NH4
+
+
Ag
B. Carbohydrates Carbohydrates are polyhydroxy aldehydes or ketones and the derivatives or compounds which yield such on hydrolysis. Many of them are represented by the general formula C x(H2O)y, which led early workers to regard them as hydrates of carbon, hence the term "carbohydrates”. Carbohydrates which can not be hydrolyzed to simpler units are called mnonsaccharaides. These may be classified accoding to the type of carbonyl present (ketose or aldose) or the number of carbon atoms in the molecule (triose, tetrose, hexose, etc.). Carbohydrates may exist as hemiacetals or hemiketals due to an intramolecular reaction involving the C = O group and one of the –OH groups in the chain. This reaction is shown below for Dglucose. The atom marked with an asterisk is the masked carbonyl carbon (hemiacetal carbom).
H 6
OH
5 4
H
* 1
3
6
1
O OH
OH
HO
O
2
2
HO
3
H
OH H OH
4
OH H
OH
5 4
O
* 1
OH
HO
3
2
OH
OH
OH 5
CH2OH 6
D-glucose
β-D-glucopyranose
α-D-glucopyranose
The most common ketose is a hexose, D-fructose. (hemiketal formation) of D-fructose is shown below:
The cyclization of
OH
HO
O
6
OH O HO * 2
5 4
OH
3
1
OH
HO H H
H
HO
6
1
O HO
5
OH OH
4
OH
*2 OH
OH
3
OH
β-D-fructofuranose
D-fructose
α-D-fructofuranose
In the cyclic form the monosaccharides can not undergo the reactions typical of the carbonyl group. However, in aqueous solution, the cyclic forms exist in equilibrium with the open chain form. The presence of even small amount of the open – chain form allows the reactions associated with the carbonyl group to take place. Moreover, in the open – chain form, the interaction between the carbonyl group and the ajacent hydroxyl group makes the carbonyl group more susceptible to oxidation and more reactive to nucleophilic reagents. Carbohydrates that can be hydrolyzed to two monosaccharide units are called disaccharides. The monosaccharide units in disaccharides are joined by a glycosidic linkage (specifically, an acetal or ketal linkage), which is really an ether linkage. Lactose is the dominant carbohydrate in milk and is made up of β-D-galactose and α-D-glucose:
OH OH
O
O
HO OH
OH
O
O OH
HO
OH
OH O
OH
OH
OH
OH
HO
OH
OH OH
OH
Lactose
β-D-galactose
α-D-gl;ucose
Sucrose is common table sugar. Its is composed of α-D-glucose and β-Dfructose: OH O
OH
OH HO
HO
O OH O
+
OH HO
HO O HO
OH
OH O HO OH
OH
OH OH
OH
sucrose
α-D-glucose
β-D-fructose
The glycosidic linkage joins the masked carbonyl group (hemiacetal or hemiketal site) of one cyclic unit to the second. If the point of attachment of the 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. O
O O
OH O
O O
H
However, if the potential carbonyl group of the second unit is also tied up in the glycosidic linkage, no such equilibrium will exist. The disaccharide will nnot undergo the reactions that are due to the free carbonyl group.
Monosaccharides and disaccharides are sometimes referred to as “simple sugars” or simply “sugars”. Polysaccharides are high molecular weight polymeric carbohydrates made up of many cyclic monosaccharide unit units. Those found in nature serve either a structural or a nutritional function. Cellulose is the chief structural material of plants. It is a linear polymer of β-D-glucose: OH
OH OH O O
OH
O
O O
H
OH
OH
O
OH
O
O OH
hydrolysis
OH
OH
OH HO
OH
OH
cellulose
β-D-glucose
Starch is the storage form of glucose in plants. It is a polymer of glucose but the type of glycosidic linkage differs from that of cellulose. Starch is made up of two components: amylose which has a linear structure and amylopectin, having a highly – branched structure. Amylose and amylopectin contain a-Dglucose units. OH
OH
O O
O
OH
OH
O
OH
OH
OH O OH
O
O
OH
OH
OH
α-D-glucose OH O
OH
O
OH
OH
OH
OH
O
O OH
OH
amylopectin
O
hydrolysis
OH HO
O
O
O O
O CH2
OH
OH
OH
OH
OH
O O
OH HO
amylose OH
O
hydrolysis
OH OH
OH O OH
α-D-glucose
There are a number of reactions used to characterize carbohydrates. The Molisch reaction is a general test for carbohydrates either in the free or combined form. In the presence of concentrated sulfuric acid, glycosidic linkages are hydrolyzed to give monosaccharides.
nH2O
+
(C6H10O5)n
→
nC6H12O6
The rate of hydrolysis depends on the solubility of the carbohydrate in water. The monosaccharides formed are then dehydrated to furfural (or hydroxymethylfurfural) and other colored decomposition products.
O
CHO O
(CHOH)
HO
4
H
CH 2 OH
a hexose
hydroxymethylfurfural
Carbohydrates which, readily react with mild oxidizing agent are called reducing sugars. The reducing property can be observed by the reaction with Benedict’s reagent which consists of copper (II) hydroxide in aqueous solution complexed with sodium citrate. The Cu 2+ complex ion has a bright deep blue color. A positive test is indicated by the reduction of the Cu 2+ ion to Cu (I) , (Cu2O) which is red brick in color and insoluble. O
Cu2+, OH-
H
O
+
Cu2O
OH-
brick red
Reducing sugars may also be detected and differentiated from each other by their reaction with phenylhydrazine. The reaction is more expensive than the formation of phenylhydrazones from simple carbonyl compounds. The products of the reaction are called osazones. These are yellow, crystalline solids with well – defined melting points and crystalliine structure. They are useful in the identification of simple sugars.. O 2N
O
aldehyde or ketone
+
O2N
H2N
+ H2O
N NH
2,4-DNP
NO 2
NH
NO2
2,4-dinitrophenylhydrazone
II.
OBJECTIVES 1. To be acquainted with the chemical properties of carbonyl compounds and carbohydrates. 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 ketones monosaccharides polysaccharides
acetadehyde, benzaldehyde acetone, cyclohexanone glucose, fructose 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. 2. 3. 4.
Place 0.5 g of sample in a test tube. Add 5 mL water and 1 mL dilute HCl. Heat the mixture in boiling water for 30 min. and cool. Neutralize the hydrolysates with dilute NaOH using phenolphthalein as indicator.
NOTE: Keep the hydrolysates for the Benedict’s test. C. Chemical Reactivity of Carbonyl Compounds 1. Reaction with Tollen’s reagent *Use acetaldehyde and acetone as samples. a. Place 3 mL of Tollen’s reagent in a test tube. b. Add 4 drops of the sample.
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 tube b. 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. b. c. d.
Place 2 mL of 95% ethanol in a test tube. Add 3 drops of the sample. Add 1 mL of 2,4 – DNP reagent. 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 2SO4 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. Bendict’s Test *Perform this test on glucose, fructose, sucrose, lactose, and hydrolysates from Part B. a. add 10 drops of sample to a 0.5 mL Benedict’s 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 at the start of the period a. Take 0.2 g of each sample and place in separate test tubes.
b. Add 4 mL phenylhydrazine-HCl/NaCH 3COOH in each test tube simultaneously. 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 chemical structures. 2. What is the structural requirement for the haloform reaction of the carbonyl compounds 3. How would you rate the oxidizability of aldehydes and ketones: easily oxidizable, 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 Benedict’s reagent. 5. Whatv is the carbohydrates.
structural
requirement
for
the
reducing
property
6. Give a simple chemical test to differentiate between the following. equations for the reactions. a. b. c. d. e.
butanone and butanal (butyraldehyde) 2-propanol and acetone glucose and butanal sucrose and lactose glucose and 1-pentanol
of
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