Experiment
A Greener Oxidation of Alcohols
9
Background
In the past, alcohol oxidations were commonly accomplished using chromium-based reagents; your
lecture textbook may still refer to these reaction conditions. However, chromium reagents and their
byproducts are toxic and harmful to the environment (a topic famously explored in the movie Erin
Brokovich) and are steadily being replaced with more environmentally friendly reagents, such as
sodium hypochlorite (bleach, NaOCl). The byproducts of sodium hypochlorite oxidation are salt and
water.
In this experiment, we will prepare cyclohexanone by oxidation of cyclohexanol using
hypochlorite. We will use common household bleach that is a 5.25% (0.75 M) aqueous solution of
sodium hypochlorite. The overall (unbalanced) reaction is shown below.
!
To ensure that the reaction goes to completion, it is necessary to use an excess of bleach. Starchiodide paper may be used to test for the presence of NaOCl; if the paper turns blue-black within a few
seconds the solution contains hypochlorite. The exact reaction mechanism is not known but a plausible
mechanism will be given in lab. It is known that the mechanism does not involve free radicals and that
yields are better in acidic rather than basic conditions. This is why we use acetic acid, which reacts with
the sodium hypochlorite to give hypochlorous acid, which then reacts with the alcohol.
Experiment 9 – A Greener Oxidation of Alcohols
Procedure
1. In a 250 mL Erlenmeyer flask, place 1.5 mL of cyclohexanol and
about 3.5 mL of glacial acetic acid.
Danger
!
!
Glacial acetic acid is corrosive
and its vapors are highly
irritating. Wear gloves and do
not breathe the vapors.
2. Cool the resulting solution in an ice-bath. Add 30 mL of
household bleach drop-wise over 5 minutes with swirling.
During the addition, ensure that the temperature of the solution
does not exceed 50°C.
Danger
!
3. After addition, remove the solution from the ice bath. Stir the
solution at room temperature for 1 hour. Test the solution every
10 minutes with starch-iodide paper. If the paper does not turn
blue-black, the test is negative and the oxidant has been used up.
Add more sodium hypochlorite (bleach) about 1 mL at a time
until you get a positive test.
4. Add small portions (half the size a pea) of solid sodium
thiosulfate (NaS2O3) with stirring to the reaction mixture. This
reduces excess hypochlorite. Continue adding NaS2O3 until the
starch-iodide test comes out negative.
5. Slowly add 3M sodium hydroxide to neutralize the remaining
acetic acid in solution (you can, of course, calculate the volume
of base needed to neutralize the acid). Add sodium hydroxide
until the solution is basic (red litmus paper turns blue).
!
Sodium hypochlorite is
corrosive and hazardous to the
environment. Wear gloves and
do not breathe the vapors.
Experiment 9 – A Greener Oxidation of Alcohols
8. Add about 1 g of sodium chloride to the solution to increase the
ionic strength. This makes the polar cyclohexanone less soluble
in the water, and more likely to dissolve in the organic layer.
9. Pour the solution into a separatory funnel, then rinse the sides of
the Erlenmeyer with 5 mL dichloromethane and add the rinse to
the separatory funnel. Shake, then drain the dichloromethane
layer and set it aside. Extract the aqueous layer with another 5
mL of dichloromethane. Combine the two dichloromethane
extractions in an Erlenmeyer flask. Remove residual water from
the dichloromethane by adding small portions of anhydrous
sodium sulfate until no more clumping occurs.
10. Use gravity filtration with fluted filter paper to separate the
drying agent from the dichloromethane/cyclohexanone mixture
into a pre-weighed 100 mL beaker (use a beaker, not an
Erlenmeyer flask). WARM (do not heat) the beaker on a hot plate
set to lowest setting. The last 1 mL or so that does not evaporate
is the cyclohexanone; there should be no more dichloromethane
smell at the end of the heating. Reweigh the beaker to obtain the
mass of cyclohexanone.
Experiment 9 – A Greener Oxidation of Alcohols
Clean Up and Waste Disposal
1. Dispose of all aqueous waste down the drain.
2. Place your product in the vial provided by your instructor.
3. Wash and dry all glassware and equipment.
4. Return all glassware and equipment to their proper locations.
Experiment 9: A Green Oxidation of Alcohols
Name
Section
Partner(s)
Volume of cyclohexanol
_______________ mL
Mass of cyclohexanol
_______________ g
Mass of product
_______________ g
Yield
_______________ %
1. Write a plausible mechanism for the hypochlorous acid oxidation of cyclohexanol under acidic
conditions.
Be sure to staple the duplicate pages from your laboratory notebook at the back of this laboratory data sheet before
handing in to your instructor.