86 ORGANIC SYNTHESIS I. RATIONALE Organic chemicals are synthesized industrially for many reasons: some are perfumes or artificial flavors, some are dyes, some have other useful properties. In this exercise we want to look at two types of organic synthesis reactions. In one we will form esters. In the other we will form an amide polymer. II. PREREQUISITES 1. Make sure that you know what can happen to give you NO CREDIT for this exercise. 2. As required In GENERAL INFORMATION - LAB, be ready to turn in a COPY of your: A. Equipment list B. Flow Chart III. OBJECTIVES When you are finished with this exercise you should be able to: 1. Tell how to prepare an ester from an alcohol and a carboxylic acid. 2. Tell how to prepare an amide from an amine and an acyl chloride. 3. State a use for esters. 4. Name the type of polymer which forms from a diacid and a diamine. IV. LAB EXERCISE 1. Introduction An ester is the product of the endothermic reaction between an alcohol and a carboxylic acid: O O R-OH + HO-C-R' ---> R-O-C-R' + H2O 11/03/2004 87 Esters often have characteristic odors or tastes and are used for flavorings or perfumes. Some, such as novocaine or aspirin, are used medicinally as anesthetics or analgesics. An amide is the product of the endothermic reaction between a carboxylic acid and an amine: O O HO-C-R' + R-NH2 ---> R-NH-C-R' + H2O An amide can also be made in an exothermic reaction between an amine and an acyl halide: O O Cl-C-R' + R-NH2 ---> R-NH-C-R' + HCl The acyl halide is a relative of a carboxylic acid which must be made in an endothermic reaction. So, when we consider the energy which must be added to make the acyl halide, the synthesis of an amide is still an endothermic process even though one step is not. Esters can be made in a similar process, from an alcohol and an acyl halide. The acyl halide is called a high energy compound because it was produced in an endothermic reaction and can then be used to form an ester or an amide without the addition of heat. Biochemical systems use a similar technique to avoid high-temperature reactions in the body: The large amount of energy released in the body by redox reactions is released chemically to form the compound ATP. ATP is used in a rearrangement reaction to produce acyl phosphates, which are similar to acyl halides - they are high energy compounds. They acyl phosphates then react with alcohols to form esters or with amines to form amides. This happens in the body without the need for high temperatures normally associated with the nonbiological synthesis of amides and esters. Polymers are large molecules made up of repeating subunits. Proteins and polysaccharides are examples of polymers found in living things. Polymers form from the reaction between difunctional molecules - molecules which have two functional groups. The alcohols, amines and carboxylic acids in the reactions above are monofunctional compounds - they have only one reactive functional group, and when it reacts there is nothing further left to react. So no larger molecule can form. However, if a difunctional amine reacts with a difunctional carboxylic acid to form an amide, there is still one amine and one carboxylic acid left to react with other molecules: O O O O HO-C-R'-C-OH + NH2-R-NH2 ---> HO-C-R'-C-NH-R-NH2 + H2O This molecule could continue to react with more similar molecules to produce a very large polymer. 88 TABLE OF ESTERS Ester: ethyl butyrate Acid: 1 ml butyric acid Alcohol: 1 ml ethyl alcohol methyl salicylate 0.3 gm salicylic acid 1 ml methyl alcohol ethyl acetate 1 ml acetic acid 1 ml ethyl alcohol In this exercise we will synthesize esters with an endothermic reaction and will synthesize an amide polymer from a difunctional amine and a difunctional acyl halide. 2. Synthesis of some esters Clean and dry (by heating) three small test tubes. Label each to correspond to one of the esters from the TABLE OF ESTERS above. Heat about 200 ml of tap water to boiling in a 250 ml beaker. Turn off the flame. Place the amount of each acid and of each alcohol indicated in the TABLE OF ESTERS into the corresponding dried test tube. CAUTIOUSLY add 4 drops of concentrated sulfuric acid to each test tube. Mix until all solids are dissolved. BE CAREFUL WITH FLAMES - some of these compounds are highly flammable. Don't have flames and your chemicals in the same vicinity. Anyone with a fire will get NO CREDIT for the exercise. Then place each test tube in the hot water bath for 10 minutes. After 10 minutes in hot water, put 4 drops of the contents of one tube into a 150 ml beaker containing enough tap water to cover the bottom and record what the ester smells like. Repeat with a different beaker for each ester. What odors do you smell? Waste Disposal Pour the contents of each test tube and the contents of the beakers into the "Organic" container. 89 3. Synthesis of an amide polymer All of the reagents that you will use here are irritating or somewhat toxic. Avoid breathing a lot of the fumes. Keep as much as possible off your hands and wash your hands when you are finished. Avoid handling the polymer with your hands until after it has been washed and dried. Dissolve 5 drops of sebacyl chloride in 15 ml of heptane in a 50 ml beaker. Dissolve 0.25 gm (or 8 drops if it is a liquid) of 1,6-hexanediamine in 5 ml of 3% NaOH in a 50 ml beaker and label it. Fill a 250 ml beaker with purified water. Get a pair of tweezers and a wooden stick ready. Very slowly pour the hexanediamine solution down the inclined side of the beaker of sebacyl chloride solution so that it forms a layer on top without mixing. A film of polymer forms where the two layers meet. Reach down to this film with your tweezers, grab hold of the center, and slowly lift it out of the beaker - a long fiber will form and will continue to form as long as there is reactant left. Wind this fiber up on the wooden stick as you pull it out. When no more fiber forms (or when you get tired winding), place what you have in the 250 ml beaker of water to wash it. Stir gently, then decant the water into another "waste" beaker. Rinse again with 50 ml of 50% acetone. Pour off the acetone into your "waste" beaker and dry your polymer with a paper towel. The polymer which you have just formed is nylon. Describe the appearance and physical properties of your dried nylon. Waste Disposal: Place your nylon (unless you want to keep it), the solutions used to make it, and any wash solutions in your "waste" beaker into the"Organic" container.