Preparation of Benzaldehyde from Benzyl Alcohol





by Magpie

Sept 7, 2013





Introduction

This procedure is for the preparation of ~ 19mL of benzaldehyde by the oxidation of benzyl alcohol using sodium persulfate.



Normally it is difficult to stop the oxidation of an alcohol at the aldehyde using common oxidants. The oxidation continues to the carboxylic acid. For some unknown reason this preparation is an exception, and the aldehyde can be expected to be produced in a ~65% yield.







CAUTION

The reaction is highly exothermic and can result in a runaway if not carefully controlled. Adequate personal protective equipment, including safety goggles, is advised.



Be aware that benzyl alcohol and benzaldehyde are flammable.



Procedure



A. Preparation of Reagents

Pour 263mL of water into a beaker. Add 69g of sodium persulfate and mix until the salt is dissolved. Set aside.



Measure out 30mL of benzyl alcohol. Set aside.



(Note 1)



B. Equipment Set-Up

Set up a 500 mL round bottomed flask (RBF) with a Claisen adapter. Support the RBF using a ringstand with wire mesh pad. Provide as much space as possible under the RBF for heating with a Bunsen burner. Place a magnetic stir bar in the RBF. Attach a reflux column to the short neck of the Claisen adapter. Using a 2nd ringstand attach a small pressure-equalizing funnel to the long neck of the Claisen adapter. All fittings are to be ground-glass, coated with a thin film of silicone grease, and clamped.



Have a magnetic stirrer and supports available for later placement under the RBF. Have a Bunsen burner available for heating the RBF. Provide cooling to the reflux column.



C. Reaction with Reflux

Temporarily remove the reflux column. Using a funnel placed in the Claisen neck, add the sodium persulfate solution prepared above to the RBF. Reconnect the reflux column.



Pour the benzyl alcohol prepared above into the pressure equalizing funnel (valve closed!). Install the plug in the funnel fill port.



Turn on the cooling water to the reflux column. Light the Bunsen burner and set at low heat.



Add ten drops of benzyl alcohol to the RBF via the pressure equalizing funnel. Steadily but gently heat the RBF until the benzyl alcohol is actively reacting. Extinguish the Bunsen burner. Place the magnetic stirrer under the RBF and provide vigorous stirring of the RBF contents. When the reaction subsides add another 10 drops of benzyl alcohol. Continue to add the benzyl alcohol in this manner, 10 drops at a time. This will keep the reaction under good control yet still keep the mix hot enough to support the reaction. The RBF will be so hot that you cannot touch the bottom: an estimated 80°C.



D. Steam Distillation

When the contents of the RBF have cooled somewhat set the RBF up for indirect steam distillation as follows: Remove the reflux column, pressure-equalizing addition funnel, and Claisen adapter. Install a distillation adapter, condenser, vacuum adapter, and a 250mL RBF receiver. Install a thermometer, or a plug, in the top of the distillation adapter. Place a 270 watt, 500mL Glas-Col mantle (or equivalent) under the pot.



Set the mantle power at about 75% and distill over the benzaldehyde as a milky suspension in water as fast as the condenser can handle it. An insulated blanket placed over the pot may be helpful here. You should soon become aware of a very pleasant new smell in the lab – benzaldehyde! Continue the distillation until the distillate runs clear.



E. Separation and Drying

Place the distillate in a separatory funnel. Let the mix set for awhile then separate the benzaldehyde (sp gr = 1.046) from the upper water layer, catching it in an Erlenmeyer flask. Add several spatulas of anhydrous MgSO4 and swirl to dry the benzaldehyde. Cork and let stand, swirling occasionally. Let it stand overnight. If a water layer is found on the bottom of the flask the next day remove it using a Pasteur pipette.



F. Fractional Distillation under Vacuum

(Note 2)



Place the crude benzaldehyde in a 50mL RBF and set up for fractional distillation under vacuum. Attach a Claisen adaptor to the pot and install an ebulliator (bubbler) in the short neck. Pack the column with a stainless steel scrub pad or broken glass and attach it to the long neck of the Claisen. Use a 25mL RBF as receiver. Install a thermometer in the open port of the distillation adapter. All ground glass connections must be well sealed with silicone grease and clamped. Place an 80 watt 100mL Glas-Col mantle (or equivalent) under the pot. Insulate the pot and Claisen adaptor.



Turn on the cooling water to the condenser, establish vacuum, adjust the bubbler, and set the mantle heat at 70%.



At 83mmHg absolute pressure (26.75”Hg vacuum) the benzaldehyde will come over at ~93°C at ~ 20 drops/minute. Stop the distillation when the temperature of the vapor at the still head starts to drop. There will be ~5mL remaining in the pot. There should be about 19mL of benzaldehyde in the

receiver.



Attachment: fractional distillation of benzaldehyde.doc (110kB)

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Qualitative Testing Results

Some qualitative tests were run on the product:



1. Permanganate test: Positive: turned purple to brown in 1 minute. Addition of 1-2 drops of 10% NaOH hastened this reaction.



2. Fehling’s test: Negative: no red or brown Cu2O seen. With fructose an immediate, strong, positive brown Cu2O formed.



3. Tollen’s test: Positive silver mirror with 2 drops of benzaldehyde.



4. Addition rxn with NaHSO3: Positive: gross quantity of white ppt formed.





Notes

1. Being aware of the susceptibility of aldehydes to air oxidation, CO2 was used as a cover gas whenever possible, including its use as feed to the bubbler. However, the advantage of this measure has not been determined.

2. It is preferable to combine several batches of crude benzaldehyde before doing the vacuum fractionation.



References

1. Credit is given to Fleaker and NERV of the ScienceMadness forum for their original suggestion of using sodium persulfate for the preparation of benzaldehyde.













[Edited on 7-9-2013 by Magpie]







The single most important condition for a successful synthesis is good mixing - Nicodem