Green Chemistry: As American as Apple Pie. Synthesis of Banana Oil Using Green Chemistry. AAS in Chemical Technology. Background

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1 Synthesis of Banana il Using Green hemistry Pamela Brown Simon Dexter New York ity ollege of Technology Brooklyn, NY Green hemistry: As American as Apple Pie As educators we have the opportunity to influence future generations of chemists The laboratory presented today was developed with an undergraduate, Simon Dexter, as an honors project in General hemistry II. AAS in hemical Technology Background General hemistry I and II (8 credits) rganic hemicstry I and II (1 credits) Analytical hemistry I and II ( 9 credits) Introduction to Physical hemistry (3 credits) Science Research Skills (3 credits) Physics I and II ( 8 credits) Pre-alculus ( 4 credits) English I and II ( 6 credits) Psychology (3 credits) Liberal Arts Electives ( 6 credits) Synthesis of esters, especially 3- methylbutylacetate, banana oil, is a classic experiment performed in organic chemistry labs across the country. Traditionally, the Fisher esterification is performed: H 2 H H 2 H H = H H 2 H 2 H 2 isopentanol acid isopentyl acetate water 3-methylbutanol acetic acid= 3-methylbutylacetatewater Eq. 1

2 Background The reaction uses sulfuric acid as catalyst and requires about one hour of reflux to reach equilibrium where: Keq = 4.2 At equilibrium the yield is only about 67%. An ether, such as ethyl ether is added to extract the banana oil. The banana oil is recovered by fractional distillation. Microwave Induced Heating Microwave radiation rapidly heats polar substances. Polar substances try to align themselves with the electric field. Since the frequency is 245 MHz the molecules don t have time to line up one way before they have to line up the other way. The resulting friction causes the solution to heat up. 3-Methylbutylacetate Synthesis with Microwave Heating Stoichiometric amounts of acetic anhydride (7. ml=.74 moles) and 3-methylbutanol (8. ml =.74 moles) are mixed in an HP5 pressure vessel and heated in a EM MARS Microwave oven with temperature control at ambient pressure for 8 minutes at 11, after a 2 minute ramp to the reaction temperature. 3-methylbutylacetate acetic anhydride= 3-methylbutylacetate acetic acid H H 2 H H 2 H == H H 2 H Determination of Yield The products are titrated using 2. M NaH. The sample separates into 2 phases, an organic phase containing essentially pure 3-methylbutylacetate (average yield = 9%), and an aqueous phase containing sodium acetate. Titration results can be used to calculate the yield. Acetic Anhydride water! 2 Acetic acid Acetic Acid NaH! Sodium acetate water Moles 3-methylbutylacetate produced = 2X initial moles of acetic anhydride - moles of acetic acid in product = 2X initial moles of acetic anhydride - moles of NaH to titrate product

3 Why is this an example of green chemistry? Eliminates the need for using sulfuric acid as a catalyst, and ether for the extraction Eliminates the need for fractional distillation Reduces energy consumption Additional Applications Last summer an undergraduate, Alyse Rich, supported by an NIH Bridges grant and a high school student, Vincent ng, supported by an AS SEED grant studied the rates of reaction using microwave heating and compared them to conventional heating. Reactions Studied 3-methylbutanol acetic anhydride _ 3-methylbutylacetate acetic acid H H 2 H H 2 H == H H 2 H ethanol acetic anhydride _ ethylacetate acetic acid H H == 3 H 2 H 2 H Determination of Rate Equation A! B First rder Reaction:-d[A]/dt = k[a] Plot of ln[a] vs t is a straight line, slope = -k Second rder Reaction d[a]/dt = k[a] 2 Plot of 1/[A] vs t is a straight line, slope = k

4 Procedure Two equimolar samples of acetic anhydride and alcohol were prepared (~15 ml total volume) ne was heated for a given time and temperature (T=8, 9, 11 with 3-methylbutanol and T=7, 75, 82 with ethanol) in a EM Microwave reactor with temperature control. The second was heated in a constant temperature oil bath. The % of unreacted acid = RH and the % yield of ester was determined by titration. Plots of % RH vs time, ln (% RH) vs time and % RH -1 vs time were prepared to determine the reaction order, rate constant and activation energy Plot of 1/ % RH vs Time at 8 for microwave heating slope = k Figure 1- Banana oil synthesis- Microwave Heating min ramp to 8 = time ml 2M NaH time (min) % Yield time (min) 1/(% reactant) /% reactant % Yield /%reactant vs time(min)-8 microwave time (min) % Yield vs Time - Microwave Heating - 8 y =.16x.99 R 2 = Time (minutes) Arrhenius Equation: k = A exp (-E/RT) Plot of ln k vs 1/T for microwave heating- slope = -E/R Figure 5 - ln k vs 1/T(K) - banana oil synthesis - microwave heating k Temperature() 1/T(K) ln(k) Experimental Activation Energies Reaction E a, Activation Energy (kj/mole) A, Arrhenius onstant ln k Determination of Activation Energy and Arrhenius onstant- Ln k vs 1/T(K) /T(K) y = x R 2 =.9659 Ethylacetate synthesis conventional heating x 1 24 Ethylacetate synthesis microwave heating x 1 13 Banana oil synthesis microwave heating and conventional heating x 1 7

5 Discussion With ethanol, the rate of reaction was 2-3 times greater with microwave heating than conventional heating. With 3-methylbutanol the rates were equal. This suggests that the alignment of molecules due to the oscillating electric field produced by the microwaves may have a catalytic effect on smaller alcohols. onclusion Introducing Green hemistry Labs into the curriculum reduces waste generated by the college and creates a culture of respect for the environment Encouraging students to develop these labs is an opportunity to develop short-term student projects with tangible results