Violets from Lemons:
Synthesis of Beta-Ionone from Citral
Perfumes from violet flowers have been highly valued throughout history. Napoleon ordered
violets to be planted at Josephine’s grave. When Napoleon was exiled to St. Helena, he took some of
the violets from the grave and wore them in a locket until his death. Beta-ionone is an unsaturated
ketone that is one of chemicals responsible for the smell of violets and roses. In this lab, Beta-ionone
will be synthesized from citral, a citrus scent that is the main component of lemon-grass oil. The first
step is an aldol condensation with citral (a mixture of E and Z isomers) and acetone. The resulting
pseudoionone product is cyclized to Beta-Ionone using sulfuric acid and acetic acid.
O
H
O
O
NaOEt, EtOH
E isomer of citral
E isomer of pseudoionone
O
O
H2SO4 /CH3COOH
E isomer of pseudoionone
(after free rotation of sigma bonds)
Beta-ionone
(violets)
Day 1 – Synthesis of pseudoionones
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Place a 125 mL Erlenmeyer flask on a weigh boat on the scale. Add 3 g of citral and remove the
flask from the scale. Add 15 mL of acetone and a magnetic stir bar.
Cool the mixture in an ice/salt bath placed on a magnetic stirrer. The ice/salt bath should be
around – 8 degrees Celsius before proceeding. When the temperature has been reached, turn
on the magnetic stirrer.
While you are waiting for the mixture to cool, add two pipetfulls (about 3 mL) of a 2.25 M
sodium ethoxide solution in ethanol to a 50 mL beaker.
When the ice/salt bath is cold enough, add all of the sodium ethoxide solution dropwise over a
period of about 10 minutes. If you need to, you can tape the pipet to the Erlenmeyer.
After the addition of the base, keep stirring the reaction mixture in the ice bath for an additional
15 minutes.
6. When the reaction is complete, neutralize the reaction mixture by the addition of enough 2 M
HCl to turn the reaction permanently yellow (~ 4.2 mL of HCl will be necessary). Swirl the
beaker to accurately judge the persistence of the yellow color.
7. Cover the flask with parafilm or aluminum foil/tape and put it aside for tomorrow. Clean up.
Day 2 – Isolation of pseudoionones
1. Transfer the contents of the flask to a separatory funnel. Wash the Erlenmeyer flask with 10 mL
of MTBE and transfer the wash to the separatory funnel.
2. Add 6 mL of water and extract the product into the organic solvent. After the layers separate,
collect the lower aqueous layer in a beaker and leave the organic layer in the sep funnel.
3. Make a 10% solution of NaCl in distilled water by adding 1 g of NaCl to 10 mL of distilled water.
After the NaCl dissolves, add the 10 mL of salt solution to the organic layer in the sep funnel and
extract.
4. Collect the lower aqueous layer in the aforementioned aqueous beaker. Collect the organic
layer in a separate beaker.
5. Dry the organic layer by adding enough anhydrous magnesium sulfate until the powder flows
freely in the beaker (doesn’t stick anymore to the bottom). Set up a gravity filtration using a 50
mL beaker and a short stem funnel with filter paper.
6. Pour the organic layer through the funnel with filter paper. Wash the mag sulfate leftovers in
the beaker with 1 pipetfull of MTBE and filter (to try to collect all of the product).
7. After the filtration is complete, boil off the MTBE with the hot plate set on low. You should
obtain about 3 mL of pseudoionones as a dark brown liquid. White smoke is a good indication
that all of the MTBE is gone. Pour the pseudoionones into the small container provided and put
the cap on tightly.
8. Clean up: The aqueous layer from the extraction can go down the drain. The filter paper with
the mag sulfate should be washed out in the sink. The rinsed filter paper can go in the trash.
Clean all of the glassware except for the sep funnel. Any organic residue can be cleaned with
acetone.
Day 3 – Synthesis of Beta-ionone
1.
Carefully add 2.6 mL of acetic acid to a 10 mL graduated cylinder. Using the same graduated
cylinder, SLOWLY add 3.4 mL of sulfuric acid to reach the 6 mL mark.
2. Carefully add the two acids and a magnetic stir bar to a clean and dry 125 mL Erlenmeyer flask.
Cool the Erlenmeyer in an ice bath on top of a magnetic stirrer. Turn on the magnetic stirrer.
Clean the acidic 10 mL beaker now. Be careful with the strong acid residue!
3. Using the 1.0 mL marking on a pipet, add 2 mL of pseudoionones dropwise over a period of 15
minutes. If you need to, you can tape the pipet to the flask. As the pseudoionones are added,
the solution turns red-brown and becomes extremely viscous.
4. After the addition is complete, remove the flask from the ice bath and keep stirring at room
temperature for another 15 minutes. Do not throw out the ice bath yet.
5. When the reaction has about 1 minute remaining, add about 20 mL of ice water to a small
beaker. Add about 10 mL of MTBE to the small beaker with the ice water.
6. When the reaction is complete, add the ice water/MTBE mixture to the Erlenmeyer flask. Swirl
the contents of the flask until the top organic layer turns brown (Beta-ionone is a dark brown
liquid). Cover the flask with parafilm or aluminum foil/tape for tomorrow.
7. Any extra pseduoionones go in the class waste beaker. Clean all of the organic glassware with
acetone.
Day 4 – Isolation of Beta-ionone
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Transfer the contents of the flask to a separatory funnel. Add ~ 5 mL of MTBE to the flask to try
to dissolve any Beta-ionone left behind. Add the MTBE to the sep funnel.
Extract the product into the organic layer and separate the aqueous and organic layers in
separate beakers. Extract the aqueous layer again with ~ 5 mL of MTBE. Combine the aqueous
and organic layers.
Add 1 g of sodium bicarbonate to 20 mL of distilled water to create a 5% solution. Add 2 g of
NaCl to 20 mL of distilled water to create a 10 % solution. Combine the two solutions to obtain
40 mL of an aqueous extracting solution.
Extract the organic layer with ~ 20 mL of the aqueous extracting solution. Be extremely careful
to vent after EVERY “rock” of the sep funnel. The buildup of pressure from the CO2 produced is
quite strong. After several mixes, let the layers separate (takes a minute). Collect the lower
aqueous layer. Extract the organic layer again with the remaining ~ 20 mL of the aqueous
extracting solution. Follow the same precautions as before.
Dry the organic layer by adding enough anhydrous magnesium sulfate until the powder flows
freely in the beaker (doesn’t stick anymore to the bottom). Set up a gravity filtration using a 50
mL beaker and a short stem funnel with filter paper.
Pour the organic layer through the funnel with filter paper. Wash the mag sulfate leftovers in
the beaker with 1 pipetfull of MTBE and filter (to try to collect all of the product).
After the filtration is complete, boil off the MTBE with the hot plate set on low. You should
obtain about 1 mL of beta-ionone as a dark brown liquid. White smoke is a good indication that
all of the MTBE is gone. The beta-ionone can be poured into microcentrifuge tubes.
Clean up: The aqueous layer from the extraction can go down the drain. The filter paper with
the mag sulfate should be washed out in the sink. The rinsed filter paper can go in the trash.
Clean all of the glassware except for the sep funnel. Any organic residue can be cleaned with
acetone.