Exp 5.docx

TITLE: The Robinson Annulation Reaction. OBJECTIVE: 

To prepare an α,β-unsaturated cyclic ketone via sodium catalyzed Michael addition reaction and a base-catalyzed Aldol condensation reaction which is known as the Robinson Annulation reaction.



To calculate the percentage yield, obtain the melting point and NMR and FTIR spectrum for the product yield.

INTRODUCTION: The robinson annulation is the Michael addition followed by a simple aldol condensation may be used to build one ring onto another. The ring which is produced will be an α,βunsaturated cyclic ketone. In this experiment, we wil react ethyl acetoacetate with trans-chalcone prepared in the previous experiment to yield the product 6-ethoxycarbonyl-3,5-diphenyl-2cyclohexenone. The product 6-ethoxycarbonyl-3,5-diphenyl-2-cyclohexenone will have a structure like this:

6-ethoxycarbonyl-3,5-diphenyl-2-cyclohexenone

EXPERIMENTAL METHOD: 1) 1.2 g of the trans-chalcone prepared in experiment 4 was placed inside a 50 ml round bottom flask along with 0.75 g of ethyl acetoacetate and 25 ml of 95 % ethanol. 2) The flask was then swirled until all the solids dissolved. 3) Boiling stones were added next. 4) Then, the NaOH pallet weighing between 0.090-0.120 g was added to the flask. 5) The mixture was heated using a reflux apparatus until a gentle boil was observed. 6) The process was continued for 1 hour. After 1 hour, the solution was left to cool to room temperature. 7) Then, 10 ml of deionised water was added to the flask and the bottom of the flask was scratched to induce crystallization. 8) The mixture was then placed in an ice bath in a 500 ml beaker and was left to cool for 30 minutes. 9) The bottom of the flask was scratched to induce crystallization. The solid was then filtered using a vacuum and 4 ml of ice cold water was used to transfer the solid. 10) The solid was rinsed with 3 ml of 95 % ethanol and the solid was left to dry. 11) The dried solid was placed in a 100 ml beaker and reagent grade acetone was added to the solid. The solution was stirred using a spatula. 12) Then the liquid was transferred into centrifuge tubes using a Pasteur pipette. The solution was centrifuged for 2-3 minutes. 13) Then, the clear acetone was transferred into a pre-weighed Erlenmeyer flask. 14) The acetone was evaporated using a rotary evaporator. The yield was determined and the proton NMR spectrum was obtained.

RESULTS:

Mass of trans-chalcone

1.200 g

Mass of ethylacetoacetate

0.7504 g

Melting point

194oC-197ºC

Mass of final product

1.4237 g

OBSERVATION: Trans-chalcone was yellow solid and after ethyl acetoacetate and ethanol were added, the yellow solution was formed. After 10 mL water added, the solution was changed into cloudy yellow. After filtering with vacuum filter, white-yellow powder was formed. Before centrifuge, solution was yellow in color. After centrifuge, the solution became clear yellow and produced white precipitate. After been through with rotary evaporator, light yellow precipitate was formed. After ethanol was added, the solution became light yellow. After filtering with vacuum filter, light yellow crystal was produced. CALCULATIONS: C15H12O

+

Chalcone

C6H10O3 Acetyl acetoacetate

No. mole of ethyl acetoacetate =

𝑚𝑎𝑠𝑠 𝑜𝑓 𝑒𝑡ℎ𝑦𝑙 𝑎𝑐𝑒𝑡𝑜𝑎𝑐𝑒𝑡𝑎𝑡𝑒 𝑅𝑀𝑀 𝑒𝑡ℎ𝑦𝑙 𝑎𝑐𝑒𝑡𝑜𝑎𝑐𝑒𝑡𝑎𝑡𝑒 0.7504 𝑔

= 130.07 𝑔/𝑚𝑜𝑙 = 5.77 x 10-3 mol

C21H20O3

+

H2O

6-ethoxycarbonyl-3,5-diphenyl-2-cyclohexenone

No. of mole of chalcone =

𝑚𝑎𝑠𝑠 𝑜𝑓 𝑐ℎ𝑎𝑙𝑐𝑜𝑛𝑒 𝑅𝑀𝑀 𝑐ℎ𝑎𝑙𝑐𝑜𝑛𝑒 1.20 𝑔

= 254.25 𝑔/𝑚𝑜𝑙 = 4.72 x 10-3 mol The limiting reactant = chalcone. 1mole of chalcone produce 1 mol 6-ethoxycarbonyl-3,5-diphenyl-2-cyclohexenone 1

∴ mole of chalcone produce = 1 x 4.72 x10-3 mol 6-ethoxycarbonyl-3,5-diphenyl-2cyclohexenone = 4.72 x10-3 mol 6-ethoxycarbonyl-3,5-diphenyl-2-cyclohexenone Theoretical yield = (no. of mol of 6-ethoxycarbonyl-3,5-diphenyl-2-cyclohexenone) x( RMM of 6ethoxycarbonyl-3,5-diphenyl-2-cyclohexenone) = 4.72 x 10-3 mol x 320.382 g/mol = 1.5122 g Percentage of yield 𝐴𝑐𝑡𝑢𝑎𝑙 𝑦𝑖𝑒𝑙𝑑 𝑝𝑟𝑜𝑑𝑢𝑐𝑡

= 𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑦𝑖𝑒𝑙𝑑 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 x100 1.4237 𝑔

= 1.5122 𝑔/𝑚𝑜𝑙 = 94.15 %

QUESTIONS: 1. Why was it possible to separate the product from sodium hydroxide using acetone? 

It was possible to separate the product from sodium hydroxide using acetone because the organic product (6-ethoxycarbonyl-3,5-diphenyl-2-cyclohexenone) dissolves in acetone, while NaOH is insoluble in acetone. Thus NaOH will precipitate (cloudy). Upon centrifuging, NaOH is separated from the product.

2. The white solid that remains in the centrifuge tube after acetone extraction fizzes when hydrochloric acid is added, suggesting that sodium carbonate is present. How did this substance form? Give a balanced equation for its formation. Also give an equation for the reaction of sodium carbonate with hydrochloric acid. 

The white solid was the carbonate because the reaction was performed in air. Air contains carbon dioxide which reacts with the NaOH to form Na2CO3. CO2

Na2CO3

+

2NaOH

+

2HCl

Na2CO3

2NaCl

+

+

H2O

CO2

+

H2O

3. Draw a mechanism for each of the three steps in the preparation of the 6-ethoxycarbonyl3,5-diphenyl-2-cyclohexenone. You may assume that sodium hydroxide functions as a base and ethanol serves as a proton source.

3: Intermolecular aldol addition CH3 CH3

O O

O O

O

O

H2C

H CH2

O

OH

O

4: Heating CH3

CH3

O

O O

OH

O

O O

H

O

HO

HO

H

4. Indicate how you could synthesis trans-chalcone. 

Chalcones can be prepared by an aldol condensation between benzaldehyde and acetophenone in the presence of sodium hydroxide as a catalyst. Preparation of chalcone:

This reaction can be carried out without any solvent as a solid-state reaction. A mixture of benzaldehyde derivatives and acetophenone derivatives was dissolved in rectified spirit in a round‐bottomed flask equipped with a magnetic stirrer. Then NaOH solution was added drop wise to the reaction mixture on vigorous stirring. The reaction temperature was maintained between 20‐25 ˚C using a cold water bath on the magnetic stirrer. After vigorous stirring for , the reaction mixture was left to stand for overnight. The precipitate obtained was filtered, washed, dried in air and recrystallized from ethanol.