Preparation and identification of Esters


  1. Title

Preparation and identification of Esters 

  1. Aim of the experiment

* To synthesize/prepare esters, methylbutanoate and ethylbutanoate.

* To write the chemical equations for the formation of methylbutanoate and

ethylbutanoate using structural formulas.

  1. Materials/Apparatus

Equipment/ Glasswares                       Chemical / Reagents

2 test tubes                                           Methanol

Water soluble marker                          Ethanol

Test tube rack                                      Concentrated sulphuric acid

Eye dropper                                         Butanoic acid

2 x 250 ml beakers

Hot plate

10 ml graduated cylinder

Thermometer

Lab apron & Safety goggles

Water bath

  1. Safety Precautions
  • Concentrated sulphuric acid is a strong oxidizing agent and highly corrosive. It is dangerous and can burn skin, eyes, and clothing very badly. Avoid contact with skin or clothes. Wear gloves and proper eye protection when using this substance. Use only in a fume hood. Use exactly as directed. If it is spilled, wash immediately before the acid has a chance to cause a burn, and inform the instructor.
  • Butanoic acid (all organic liquid acids) is toxic and corrosive to skin, eyes, and clothing. While working on this experiment, wear safety goggles, full face shield, gloves, and lab apron. Wash spills and splashes off your skin and clothing immediately, using plenty of water.
  • All burners and other flames in the laboratory must be extinguished before you start this experiment.
  • Methanol is highly flammable. It is poisonous if swallowed. Its vapour is harmful to the eyes, lungs and skin and other organs.
  • Ethanol is flammable, poisonous and its toxic.
  • Detect odours of these esters with caution because vapors produced may be harmful. Breathing the vapours of some of these esters can cause sore throat, dizziness, headache, and drowsiness. Hold the test tube 30 cm away and 15 cm below your nose. Waft a small amount of vapor or odour from the ester toward your nose, sniffing cautiously, once or twice. Do not breathe deeply while sniffing.
  • Before leaving the laboratory, wash your hands thoroughly with soap and water.
  1. Safety audit on:
  • Ethanol
  • Methanol
  • Butanoic acid
  • Sulphuric acid
  1. Procedure/Method
  2. Put on your lab apron and safety goggles.
  3. Label the two test tubes 1 and 2 with your water soluble marker and place them in the test tube rack.
  4. Into test tube 1, pour 5cm3 of methanol and in test tube 2, 5cm3 of ethanol and then add 6cm3 of butanoic acid as indicated in Table 1 below. Smell the mixture in each test tube as per safety precaution, above (waft!).

Table 1 –Reagents for preparation of esters

Test tubeConstituents
15 cm3 methanol +  6 cm3 butanoic acid
25 cm3 ethanol + 6 cm3 butanoic acid
  1. Add 4 drops of concentrated sulphuric acid to each test tube.
  2. Pour about 150 ml of tap water in a 250 ml beaker. Place the test tubes in the water and heat the water on a hot plate to a temperature of about 60˚C – 75˚ Leave the test tubes in the hot water bath for 15 minutes.
  3. Cool the test tubes by immersing them in cold water in another beaker.
  4. Add 5 ml of distilled water to each of the test tubes.

Carefully note the odour of the contents of each of the test tubes in your copy of Table 2 in your notebook. Hold the test tube about 30 cm away from your nose and gently waft the vapour towards your nose without inhaling deeply. Each of the odours should be somewhat familiar to you. Alternatively, the contents of the test tube may be poured into a beaker half full of water and the odour above it detected carefully.

  1. Dispose of all materials following the reagent disposal instructions.
  2. Before leaving the laboratory, wash your hands thoroughly with soap and water.
  1. Observations and recording

               Table 2 – Odours of Esters

Test tubeAroma(smell)Constituents
1 Methanol + butanoic acid
2 Ethanol + butanoic acid
  1. Analysis and interpretation
  • Identify the esters formed in relation to the odours smelt.
  • Draw the structural formulae of the formation of the two esters.
  1. Conclusion

Make a list of the odours you were able to detect and the ester responsible for

that odour.

  1. Factors influencing results

Mention all limitations and challenges in this experiment.

  1. Conceptual questions (Questions related to the experiment)

Questions

  1. Write down the general equation for the reaction of an alcohol and a carboxylic acid                                                 (3)
  2. Name the ester formed in each of the test tubes (Table 2, above).         (4)
  3. What is the function of sulphuric acid in these reactions?                                     (2)
  4. How would you dilute a concentrated sulphuric acid solution? (2)
  5. Write 5 safety precautions that you took during this experiment             (5)
  6. Give four (4) uses of esters (4)

Calculating the Equilibrium Constants


Calculating Kc from Known Equilibrium Amounts

  • Write the equilibrium expression for the reaction.
  • Determine the molar concentrations or partial pressures of each species involved.
  • Subsititute into the equilibrium expression and solve for Kc.

Example: Calculate the value of the equilibrium constant, Kc , for the system shown, if 0.1908 moles of CO2, 0.0908 moles of H2, 0.0092 moles of CO, and 0.0092 moles of H2O vapor were present in a 2.00 L reaction vessel were present at equilibrium.

CO2 + H2 -- > CO + H2O

Equilibrium Expression for Reaction

  • Since Kc is being determined, check to see if the given equilibrium amounts are expressed in moles per liter (molarity).  In this example they are not; conversion of each is requried.[CO2] = 0.1908 mol CO2/2.00 L = 0.0954 M
    [H2] = 0.0454 M
    [CO] = 0.0046 M
    [H2O] = 0.0046 M
  • Substitute each concentration into the equilibrium expression and calculate the value of the equilibrium constant.

Solving for K

Top

Calculating K from Initial Amounts and One Known Equilibrium Amount

  • Write the equilibrium expression for the reaction.
  • Determine the molar concentrations or partial pressures of each species involved.
  • Determine all equilibrium concentrations or partial pressures using an ICE chart.
  • Substitute into the equilibrium expression and solve for Kc

Example:   Initially, a mixture of 0.100 M NO, 0.050 M H2, 0.100 M H2 O was allowed to reach equilibrium (initially there was no N2 ).  At equilibrium the concentration of NO was found to be 0.062 M.  Determine the value of the equilibrium constant, Kc , for the reaction:

2NO + 2 H2 --    > N2 + 2 H2O

Equilibrium Expression

  • Check to see if the amounts are expressed in moles per liter (molarity) since Kc is being .  In this example they are.
  • Create an ICE chart that expresses the initial concentration, the change in concentration, and the equilibrium concentration for each species in the reaction.  From the chart you can determine the changes in the concentrations of each species and the equilibrium concentrations.  From the example, we start with the folowing information.
NO H2 N2 H2O
Initial Concentration (M) 0.100 0.0500 0 0.100
Change in Concentration (M)

– 2 x

– 2 x

+ x

+ 2 x

Equilibrium Concentration (M) 0.062

The change in concentration of the NO was (0.062 M – 0.100M) = – 0.038 M.  Thus -2 x = – 0.038 and x = 0.019. Note: the negative sign indicates a decreasing concentration, not a negative concentration.  The changes in the other species must agree with the stoichiometry dictated by the balance equation.  The hydrogen will also change by – 0.038 M, while the nitrogen will increase by + 0.019 M and the water will increase by + 0.038 M.  From these changes we can complete the chart to find the equilibrium concentrations for each species.

NO H2 N2 H2O
Initial Concentration (M) 0.100 0.0500 0 0.100
Change in Concentration (M) – 0.038 – 0.038 + 0.019 + 0.038
Equilibrium Concentration (M) 0.062 0.012 0.019 0.138
  • Substitute the equilibrium concentrations into the equilibrium expression and solve for Kc.

Solving for K

Top

Calculating Kc from Known Initial Amounts and the Known Change in Amount of One of the Species

  • Write the equilibrium expression for the reaction.
  • Determine the molar concentrations or partial pressures of each species involved.
  • Determine all equilibrium concentrations or partial pressures using an ICE chart.
  • Substitute into the equilibrium expression and solve for Kc.

Example:  A flask is charged with 3.00 atm of dinitrogen tetroxide gas and 2.00 atm of nitrogen dioxide gas at 25oC and allowed to reach equilibrium.  It was found that the pressure of the nitrogen dioxide decreased by 0.952 atm.  Estimate the value of Kp for this system:

N2O4 -->2 NO2

K<sub>p</sub >  expression

  • Check to see that the given amounts are measured in appropriate pressure units since Kp is to be .  In this example they are (atmospheres).
  • Create an ICE chart and calculate the changes in pressure and equilibrium pressures for each species.
N2O4 NO2
Initial Pressure (atm)(kpa) 3.00 2.00
Change in Pressure (atm)(kpa) + 0.476 – 0.952
Equilibrium Pressure (atm)(kpa) 3.476 1.048
  • Substitute the equilibrium pressures into the expression for Kp and solve for Kp.

Solving for K<sub>p</sub >

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