EXERCISE 7 ORGANIC DERIVATIVES OF WATER I.
INTRODUCTION In addition to carbon and hydrogen, the element oxygen is found in many important organic compounds. Among the classes of oxygen – containing are alcohols, phenols, and ethers, also known as organic derivatives of water because they may be seen to arise from the replacement of one or both of the hydrogen atoms of water by an organic group. In alcohols, one hydrogen is replaced by an aliphatic group, in phenols it is replaced by an aromatic group and in ether, both hydrogen atoms are replaced.
H
O
H
H
water
O
O
H
R
alcohol R = alkyl
R
Ar
phenol Ar = aryl
O R'
ether R,R’ = alkyl or aryl
The hydroxyl group (-OH) is the functional group of alcohols and phenols. The O – H bond is polar with the greater electron density around the oxygen atom. Under the proper conditions, the O – H bond can be made to split heterolytically to release a proton: R −O − H
+
R − O-
base
+
H – base
At the same time, the oxygen atom acts as the Lewis base because of its unshared pair of electrons. Hence, the oxygen atom is a site for attack by Lewis acids or electrophiles:
R–O–H
+
E+
R O
+
H
E
The nature of the substituent bonded to the hydroxyl group greatly affets the reactivity of alcohols and phenols. Thus, alcohols are classified as primary (10), secondary (20) or tertiary (30), based on the type of carbon bearing the hydroxyl group. In phenols, the hydroxyl group is directly bonded to an aromatic system. The aromatic ring makes the hydroxyl group more acidic than it is in alcohols. Ethers are relatively inert molecules. The ether linkage C – O – C, is quite stable and can be cleaved only under drastic conditions. The unshared pairs of electrons on the oxygen atom, however, make the ethers susceptible to electrophilic attack.
OH
Alcohols of the type
R
undergo a unique reaction with halogens
CH3
in the presence of a base. This is called haloform reaction. The mixture of halogen and base produces a hypohalite which is an oxidizing agent: 2OH− +
OX−
X2 →
X−
+
+
H2O
hypohalite O
The hypohalite first oxidizes the alcohol to a methyl carbonyl form R
CH3
Which subsequenctly undergoes the haloform reaction:
OH
O
X2/OH R
CH3
R
O
X2/OH CH3
R
O
+
-
CHX3
The compound classes considered in this exercise can be further differentiated via their reactions with oxidizing and reducing agents. Below is a summary of the functional group transformation for the oxidizable compoundsd: O
[O] 0
1 alcohol:
R
O
[O]
OH R
R
H
aldehyde OH
2 alcohol: R
carboxylic acid
O
[O]
0
OH
CH3
R
R'
ketone [O] Phenol:
30 alcohol or ether
OH
mixture of products
[O] no reaction
Potassium permanganate is a powerful oxidizing agent with a characteristic purple color due to the permanganate ion. When it reacts with oxidixazable compound, the purple color is discharged and a brown to black precipitate of MnO2 is formed together with the hydroxide ions. In this exercise, a small amount of acid is added to the reagent to neutralize any hydroxide ions produced from the reaction with oxidizable impurities. Tollen’s reagent is a mild oxidizing agent which, consists of ammoniacal silver nitrate prepared by dissolving silver nitrate in excess ammonium hydroxide. The oxidizing agent here is the silver ion which, exists as a stable complex Ag(NH3)+. Upon reaction, the silver ion is reduced to elemental silver (Ag) which, adheres to the clean glass walls of the container to form a mirro. Easily oxidizable compounds give positive test with Tollen’s reagent. Whether an alcohol is primary, secondary, or tertiary is shown by the Lucas test which, is based on the difference in reactivity of the alcohol toward hydrogen halides. Alcohols of no more than six carbon atoms are soluble in the Lucas reagent, a mixture of concentrated hydrochloric acid and zinc chloride. The alkyl chlorides formed are insoluble. The appearance of a cloudiness thus indicates the formation of a chloride from the alcohol. The reactions for 30 and 20 alcohols are: H
20 alcohol:
R
H OH
+
HCl
ZnCl2
R'
R'
R''
30 alcohol:
R'
II.
R'' OH
R
Cl
R
+
HCl
ZnCl2
Cl
R
+ H2O
R'
OBJECTIVES 1. To be acquainted with the chemical properties of the organic derivatives of water. 2. To observe the differences in chemical reactivity of primary, secondary andb tertiary alcohols, phenols and ethers towards selected chemical reagents.
III.
PROCEDURE The following representative compounds will be used in this exercise: OH
OH H3C
OH
H3C
1 – butanol (n-butyl alcohol)
CH3
2 – butanol (sec-butyl alcohol)
H3C
CH3 CH3
2-methyl-2-propanol (tert-butyl alcohol) OH
CH3 H3C
O
CH3 CH3
diisopropyl ether
phenol
CAUTION Phenol crystals slowly liquefy in hot humid air. The yellow color of the sample is due to phenol oxidation products. The sample is suitable for the chemical tests, except where anhydrous samples are required. HANDLE PHENOL WITH CARE AS IT CAN CAUSE PAINFUL BURNS. IF SPILLED ON THE SKIN, WASH FREELY WITH 95% ETHANOL, THEN WITH SOAP AND WATER. Diisopropyl ether is highly volatile and flammable. Its vapor has a high tendency to form explosive hydroxides. Fire is therefore a hazard to be guarded against. STAY AWAY FROM OPEN FLAME WHEN WORKING WITH ETHER AND MINIMIZE EXPOSURE TO AIR.
A. Solubility behavior 1. 2. 3. 4. 5. 6. 7.
Place 2 mL water in a test tube. Add 15 drops of the sample. Cover the tube with cork stopper Shake to mix. Examine the mixture Repeat steps 1 – 5 using 1 mL of 10% NaOH as solvent. For the compounds insoluble in base, repeat steps 1 – 5 using 1 ml concentrated H2SO4 as solvent.
B. Chemical Reactivity 1. Reaction with potassium permanganate a. Place 3 mL dilute, slightly acidic permanganate in a clean test tube. b. Add 5 drops of the sample. c. Warm the tube gently in a water bath. d. Examine the mixture after 5 minutes 2. Reaction with Tollen’s reagent (ammoniacal silver nitrate) a. Place 3 mL Tollen’s reagent in a test tube. b. Add 4 drops of sample. c. Shake then heat gently in a water bath. d. Examine the mixture NOTE: This test should be done using a test tube with smooth walls and is scrupulously clean. Otherwise, the mirror will not deposit on the tube. 3. Lucas Test: reaction with HCl – ZnCl2 mixture a. Place 2 mL of the HCl – ZnCl2 reagent in a test tube. b. Add 5 drops of the sample c. Stopper the tube and shake d. Allow to stand e. Note the time for the appearance of an insoluble layer or emulsion. 4. Reaction with ferric chloride (FeCl3) a. Place 2 drops of sample in 2 mL water b. Add several drops of 2.5% aqueous FeCl3 c. Examine the mixture d. Compare the results with a blank using water. 5. Iodoform test: reaction with I2, NaOH a. Place about 1 mL of the sample in a test tube. b. Add 2 mL water. c. Add about 1 mL I2/KI solution. d. Add 10% NaOH solution dropwise with shaking until the iodine color disappears and the solution is faintly yellow. e. Examine the contents of the tube and note the odor. f. If the test seems negative, shake the tube. g. Heat gently in a water bath for 1 – 2 minutes h. Examine the mixture.
IV.
QUESTIONS 1. Explain the solubility behavior of the representative compounds in water as a function of: a. branching in the molecule b. relative proportions of hydrophilic bonds to hydrophobic bonds 2. Explain the acidity differences observed for the compounds used in this exercise in terms of the stability of the corresponding conjugate bases. 3. Based on the results of the oxidation test, classify the compounds tested according to the following categories: easily oxidizable, oxidizable, resistant to oxidation. 4. Discuss the reactivity differences of alcohols towards the Lucas reagent. 5. Suggest simple chemical test that will differentiate between the following pairs of compounds. Write the equations for the reactions involved. a. b. c. d. e.
phenol and isopentyl alcohol tert-butyl alcohol and isobutyl alcohol neopentyl alcohol and ether sec-butyl alcohol and neopentyl alcohol propene and butanol
