Selective Oxidation of Benzyl Alcohol to Benzaldehyde

Adapted from literature¹ by Keti Assor, Irvin Levy, Erin Thames and Rowan Walker

Background

Traditionally oxidation of alcohols to aldehydes requires the use of hazardous, heavy-metal reagents, such as pyridinium chlorochromate (PCC). In fact, PCC has been widely considered an appropriate agent for the oxidation, appearing in many undergraduate textbooks as the only viable method to produce aldehydes from alcohols. Both PCC and the usual solvent for its reaction, dichloromethane, are hazardous, cancer suspect agents. In this particular reaction, an environmentally safer oxidizing agent is formed from sodium molybdate to make an efficient catalyst that is activated by aqueous hydrogen peroxide. In addition, hydrogen peroxide serves not only as an activating agent, but also as the solvent for this transformation.

Purpose

This experiment provides an excellent example of selective oxidation. In addition it demonstrates the synthesis and use of a catalyst in reaction. Other green lessons include replacement of a hazardous reactant with a safer alternative, the elimination of hazardous solvents, and reduction of waste.

Procedure

Preparation of tetrakis(benzyltriethylammonium) octamolybdate catalyst

Scheme I:

Method:

To prepare the catalyst, sodium molybdate dihydrate (0.30 g; 1.2 mmol), and 4 M HCl (0.5 mL; 2.0 mmol) were added to a vial. About 1 mL of water was added to complete dissolution. Into a second vial benzyl tri-ethyl ammonium chloride (BTEAC) (0.525 g; 2.30 mmol) and ca. 3 mL water were stirred until dissolved. The BTEAC solution was heated at 70 °C with stirring. To the stirred solution was added the molybdate solution dropwise. After addition was complete, the solution was stirred for an additional five minutes, then removed from heat and vacuum filtered. The solid was washed with ca. 5 mL while on the filter. The catalyst produced could be used wet or saved for later use.

Preparation of Benzaldehyde

Scheme II:

Method:

For preparation of benzyaldehyde, benzyl alcohol (5 mL; 50 mmol) was added to a round bottom flask containing octamolybdate catalyst (0.25 g dry weight; 0.2 mol%). Next, hydrogen peroxide (60 mmol; 12 mL 15% or 60 mL 3%) was added to the flask. The mixture was then refluxed for one hour, then the apparatus was modified for simple distillation with glass centrifuge tubes as receivers, yielding benzaldehyde and water in the distillate. After collecting about 8 - 10 ml of distillate, the tubes were centrifuged. Water was removed with a disposable pipet and the product was dried over sodium sulfate, weighed and IR spectrum was recorded.

Prelab

1. Look up information, hazards, and safety on all materials used in this procedure.

2. Look up hazard information for PCC (pyridinium chlorochromate) and dichloromethane.

3. The original published procedure calls for the use of 15% hydrogen peroxide. Look up hazard information for this reagent and compare to the 3% solution. Which do you recommend that we use?

4. What makes the benzaldehyde oxidation reaction selective?

5. What is the atom economy of each reaction scheme?

6. What is the approximate e-factor of each method?

References

¹ Guo, Ming-Lin; Li, Hui-Zhen Li. Selective oxidation of benzyl alcohol to benzaldehyde

with hydrogen peroxide over tetraalkylpyridinium octamolybdate catalysts. Green Chem.

2007, 9, 421-423.

² ACS. Selection from “Introduction to green chemistry”. 2002. Web access:

http://domin.dom.edu/faculty/jbfriesen/chem254lab/atom_economy.pdf

³ Anastas, Paul T.; Warner, John C. Green Chemistry: Theory and Practice;

Oxford University Press: Oxford, 1998.

⁴ Levy, Irvin J. The goal is zero; E-factor as a green chemistry metric.

http://www.cs.gordon.edu/~ijl/visualizingWaste/