ester by qingyunliuliu


									                                  Preparation of Esters
     An ester is a chemical compound that is formed when an organic acid reacts with an alcohol. Esters fre-
quently have distinctive odors, and are found in the flavorings of many fruits and plants_ The reaction between an
organic acid and an alcohol is shown in Figure 1.

                     Organic Acid             Alcohol                 Ester

      Figure 1. The Reaction Between An Organic Acid and Alcohol to Form an Ester and Water
      In the diagram, Rand R' represent organic groups such as hydrocarbons. The -OH group from the acid
combines with the -H from the alcohol producing water molecules. The R' -0- group from the alcohol then attaches
to the carbon on the acid forming the ester. The reaetion is catalyzed by adding some concentrated sulfuric acid,
H2S04, Concentrated sulfuric acid is a strong dehydrating agent, and helps the reaction by removing the water
molecules as they are formed.
      If acetic acid and methanol are reacted, the reaction shown in Figure 2 occurs. The product is called methyl
acetate. The systematic name for acetic acid is ethanoic acid, and the product is also known as methyl ethanoate.

                Figure 2. The Reaction between Acetic Acid (Ethanoic Acid) and Methanol
       In this experiment we will prepare small quantities of several esters. The esters will be identified by their
distinctive odors. Then we will prepare a larger quantity of the ester ethyl acetate (ethyl ethanoate) and purify it by

 Part 1:
       Various combinations of acids and alcohols will be used which may contain the following:
       Organic acids:
                           Formic acid (methanoic acid)
                           Acetic acid (ethanoic acid)
                           Propionic acid (propanoic acid)
                        Butyric acid (butanoic acid)
                        Salicylic acid
                        Anthranilic acid

    Organic alcohol: Methyl alcohol (methanol)
                        Ethyl alcohol (ethanol)
                        Propyl alcohol (n-Propanol)
                        Isopropyl alcohol (isopropanol)
                        Butyl alcohol (butanol)
                        Isopentyl alcohol or isoamyl alcohol (isopentanol)
                        Octyl alcohol (octanol)
    Sulfuric acid, concentrated (18 M)
    Baking soda, NaHC03, to neutralize acid spills
Part 2:

     Acetic acid, concentrated, 17.4 M (glacial)
     Ethanol (ethyl alcohol, denatured alcohol)
     Sulfuric acid, concentrated (18 M)
     Sodium carbonate, N~C03·lOH20, solid Baking
     soda, NaHC03, to neutralize acid spills

Part 1:

                                                          Beaker, 400-mL
     Test tubes, 13- x 100-mm
                                                          Hot plate or Bunsen burner, ring and wire gauze
     Beaker, 250-mL
Part 2:
                                                          Boiling stone
     Erlenmeyer flask, 125-mL
                                                          Distilling flask
     Condenser with cork fittings
                                                          Hot plate, or Bunsen burner, ring and wire gauze
     Beaker, 400-mL for water bath                        Test tube, 18 x I50-mm and cork stopper Capillary
     Beaker, lOO-mL to collect distillate                 dropper
     Separatory funnel or test tube,
       15- x 125-mm, and stopper
       and return to the reaction vessel. Slowly run cold water through the condenser, in at the bottom and out at
       the top. Heat the flask in a hot water bath. Raise the temperature of the hot water until the mixture in the
       Erlenmeyer flask is gently boiling, and continue heating for about 15 minutes. Cool the mixture.

  2. Distillation of ethyl acetate.
       Pour the mixture (including the boiling stone) into a
       distilling flask and connect the condenser to the side                        SET UP A
       arm of the flask. Insert a thermometer in an aluminum                         DISTILLATION PROCESS
       foil-covered cork in the top of the flask with the 3.     2.
       thermometer bulb even with the side arm of the
       condenser. Heat the bottom of the distilling flask in a
       hot water bath until no more distillate is coming over.
       Record the temperature at which the distillation begins
       and the temperatures during and at the end of the
       distillation. Look up the boiling point of ethy 1 acetate
       and compare to the distillation temperature.
   3. Separation of the ethyl acetate from alcohol.
       During the distillation some of the unreacted alcohol
       will distill along with the ethyl acetate. Ethanol is very
       soluble in a saturated solution of sodium carbonate,
       while the ethyl acetate is only slightly soluble.              Do not distill to dryness.
                                                                      Use extreme caution when distilling mixtures
                                                                      containing flammable liquids.
                                                                      Do not distill liquids with boiling points below SO°C.
                                                                      Use only cold water in the condenser.
       Prepare a saturated solution of sodium carbonate in            If available, use an electric heating mantle instead of
       distilled water by combining 1.5 g N~C03·lOH20 with            a laboratory burner.

       5 mL distilled water in a 15 x 2s-mm test tube.
       Stopper with a cork, shake well and then allow any
       undissolved solid to settle. Pour the clear solution into a separatory funnel, or if none is available, into a
       second 18- x Is0-mm test tube. Add the distillate, stopper and shake for a minute. If using a separatory
       funnel, turn it upside down and open the stopcock occasionally to vent the system. If using a test tube,
       remove the stopper with caution-some pressure may have built up. Separate the two layers. A capillary
       dropper may be helpful in the separation if you are using a test tube. Measure the mass of the ethyl acetate
       produced. Pour a little of the ethyl acetate into 200 mL of water and cautiously note its odor.


     The solutions used to prepare the esters can be safely washed down the sink with a large amount of water
according to Flinn Suggested Disposal Method #26b. The ethyl acetate can be saved and used as a solvent, or can
be evaporated in the fume hood according to Flinn Suggested Disposal Method #18a. See the appendix.

     In your laboratory report include all of your observations, and answer the following questions:
    1. The density of ethanol is 0.79 g/mL. The density of acetic acid is 1.05 g/mL. Assuming that each
       substance was a pure substance, calculate the moles of each reactant used in part 2. Determine the limiting
       reactant, and calculate the theoretical yield of ethyl acetate. Use the actual yield to determine
           the percent yield of product.
     2. Why was sulfuric acid added to the mixture of acid and alcohol?

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