Green Oxidation of Alcohols
Using Sodium Tungstate and Hydrogen Peroxide

[adapted from Hulce, M. and Marks, D.W. J. Chem. Educ. 2001, 78, 66-7]

Introduction

Alcohol oxidations are one of the most important functional group transformations in organic chemistry. Primary alcohols can be oxidized to aldehydes or carboxylic acids and secondary alcohols to ketones. Historically, such oxidations were carried out with oxochromium(VI) reagents like CrO₃ and pyridinium chlorochromate. More recently, derivatives of the hypochlorite ion (ClO^-) have been used. Both types of oxidizing agents have a serious limitation in that they are harmful to the environment. Chromium(VI) reagents are known carcinogens (featured in Erin Brockovich) and hypochlorite reacts with a variety of aromatic compounds to yield carcinogenic PCP's. Green chemistry is a rapidly expanding area of chemistry that espouses the following 12 principles:

1. Prevention. It is better to prevent waste than to treat or clean up waste after it has been created.
2. Atom Economy. Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product.
3. Less Hazardous Chemical Syntheses. Wherever practicable, synthetic methods should be designed to use and generate substances that possess little or no toxicity to human health and the environment.
4. Designing Safer Chemicals. Chemical products should be designed to effect their desired function while minimizing their toxicity.
5. Safer Solvents and Auxiliaries. The use of auxiliary substances (e.g., solvents, separation agents, etc.) should be made unnecessary wherever possible and innocuous when used.
6. Design for Energy Efficiency. Energy requirements of chemical processes should be recognized for their environmental and economic impacts and should be minimized. If possible, synthetic methods should be conducted at ambient temperature and pressure.
7. Use of Renewable Feedstocks. A raw material or feedstock should be renewable rather than depleting whenever technically and economically practicable.
8. Reduce Derivatives. Unnecessary derivatization (use of blocking groups, protection/ deprotection, temporary modification of physical/chemical processes) should be minimized or avoided if possible, because such steps require additional reagents and can generate waste.
9. Catalysis. Catalytic reagents (as selective as possible) are superior to stoichiometric reagents.
10. Design for Degradation. Chemical products should be designed so that at the end of their function they break down into innocuous degradation products and do not persist in the environment.
11. Real-time analysis for Pollution Prevention. Analytical methodologies need to be further developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances.
12. Inherently Safer Chemistry for Accident Prevention. Substances and the form of a substance used in a chemical process should be chosen to minimize the potential for chemical accidents, including releases, explosions, and fires.

In this experiment you will oxidize either a primary or secondary alcohol using a sodium tungstate/hydrogen peroxide/phase transfer agent catalytic system. These reaction conditions fulfill the Principles of Green Chemistry because water is the reaction solvent and it is a catalytic system. The phase transfer agent is the [CH₃(C₈H₁₇)₃N]HSO₄ which is a mixture of Aliquat 336 and sodium bisulfate. Read about the the role of phase transfer agents (or catalysts) in organic reactions. The generic reaction and oxidation mechanism is shown below:

Possible alcohols:

Pre-Lab

Type up your normal introduction. You will be assigned an alcohol. Look up the physical properties of the alcohol and the corresponding ketone or aldehyde. Calculate the quantities (either by volume or weight) that you will need of the alcohol and other reagents before coming to class. Those numbers go in your table of reagents.

Experimental

NOTE: The Aliquat phase transfer catalyst is very viscous: break off the tip of a Pasteur pipet to make it easier to dispense. The 30% hydrogen peroxide is dispensed via a 50 mL burette. It is important to maintain the water bath at 80-90^oC.

1. Oxidizing the Alcohol

Fill a small dish with water and heat it to 80-90^oC on your hotplate-stirrer. While the water bath is warming, add a spin vane to a 5mL conical flask. Add 140 micromol of Na₂WO₄*2H₂O, 140 micromol of NaHSO₄*H₂O, 140 micromol of Aliquat 336, and 16 mmol of 30% H₂O₂. Stir this mixture rapidly at room temperature for 5 min, then add 14 mmol of your alcohol to it. Add a water-cooled reflux condenser and stir the mixture in the hot water bath for 1 hour, maintaining the bath temperature at 80-90^oC.

2. Isolating/Purifying the Aldehyde/Ketone Product

After 1 h, cool the flask in an ice bath for a few minutes. Add 3 mL of water and 7 mL of diethyl ether to the flask (you will need to transfer the contents to a separatory funnel when you run out of room). Mix the layers well and allow them to separate. Carefully remove the aqueous layer (be sure to pick the correct layer!). Wash the organic layer with 3 mL saturated sodium thiosulfate (Na₂S₂O₃ is a reducing agent which destroys the leftover hydrogen peroxide), let the layers separate and remove the aqueous layer.

Dry your sample with anhydrous magnesium sulfate, then filter it through a pad of neutral alumina. Evaporate the ether using a gentle stream of air in the hood. Weigh the tube to determine the crude yield of ketone or aldehyde.

Obtain a ¹H NMR and IR on your product.

Write up a normal discussion for the experiment. In your discussion, include a section where you discuss what aspects of this experiment which were not Green.