Oxidation of Alcohols
Use of an Environmentally Friendly
(Green) Oxidant
Oxidation of Alcohols
Oxidation of alcohols to carbonyl compounds is one of
the most important reactions of alcohols.
O H
C H
[O]
O
C
In the oxidation of an alcohol, two hydrogens are
removed by the oxidizing agent – one from the carbinol
carbon (blue H) and one from the hydroxyl oxygen (red
H).
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Oxidation of Secondary Alcohols
Secondary alcohols are oxidized cleanly to ketones by a
variety of oxidizing agents.
OH
R CH
O
[O]
R C
R'
R'
Tertiary alcohols are not oxidized under mild conditions
because there is no hydrogen on the carbinol carbon.
OH
no
[O]
R C R
X reaction
R'
Oxidation of Primary Alcohols
Primary alcohols are oxidized to aldehydes,
but many reagents can oxidize the aldehyde
further to a carboxylic acid.
OH
R CH2
[O]
O
R C
H
O
[O]
?
R
C
OH
A key to preventing oxidation beyond the
aldehyde is the use of anhydrous oxidation
conditions.
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Oxidation of Alcohols
Laboratory scale oxidations of alcohols
have frequently been carried out using
chromium salts as the oxidizing agents.
However, chromium (VI) salts are toxic and
potentially carcinogenic and require careful
handling and appropriate disposal. The true
story and the movie ‘Erin Brockovich’
involved inappropriate disposal of
chromium salts.
Oxidation of Alcohols
In today’s lab, we will use an
environmentally friendly procedure in
which household bleach (6% NaOCl) is the
oxidizing agent and the reduced by-product
is sodium chloride, a chemical easily
disposed of in small amounts!
The reaction is conducted in acetic acid, so
the active oxidizing agent is hypochlorous
acid, HOCl.
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Hypochlorite Oxidation of Alcohols Mechanism
The initial steps of the mechanism involve nucleophilic
substitution at chlorine by the alcohol oxygen.
a)
HOCl + CH3CO2H
Cl OH2 + CH3CO2
CH O Cl
H
+ H2O
CH OH + Cl OH2
b)
CH O Cl
CH O Cl + CH3CO2
H
c)
+ CH3CO2H
Hypochlorite Oxidation of Alcohols Mechanism
The final and key step of the mechanism
involves the elimination of the elements of
HCl from the chlorine-substituted alcohol.
Chlorine is transformed from oxidation state of
+1 to -1 (a 2-electron change) in this reaction.
C
H2O
H
O Cl
C
O
+ H3O + Cl
4
Oxidation of a Unknown Alcohol
In today’s experiment, the objective is to
oxidize one of four (4) secondary alcohols to
the corresponding ketone using bleach as the
oxidizing reagent. Your TA will assign each
pair of students a specific starting material
bottle.
OH
OH
OH
OH
cyclopentanol
cyclohexanol
3-heptanol
2-heptanol
Oxidation of a Unknown Alcohol
Today, you will purify the oxidation
product by distillation to obtain a boiling
point range for your product.
You will also need to obtain an IR
spectrum of both your distilled product
and of the alcohol which was your
starting material for this oxidation.
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Identity of Alcohol and Ketone!
You should be able to determine the identity
of your starting alcohol and product ketone.
You will use the observed boiling point of
your product, the infrared spectrum of your
product, and the infrared spectrum of your
starting material to determine their structures.
Reference spectra of the starting alcohols and
of the product ketones will be posted for
comparison with the IR spectra that you obtain
on your products.
Oxidation of a 2o Alcohol – Exptl.
You will use an Erlenmeyer flask with a
stirbar as your reaction flask. You will
insert a thermocouple into the reaction
mixture to monitor the temperature. Do not
allow temperature to stay above 45 oC.
Make sure the temperature does go above
30 oC.
An excess of oxidant is necessary. You will
use iodide-starch indicator paper to
determine if you have excess oxidant at the
end of the reaction.
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Oxidation of a 2o Alcohol – Exptl.
To test the solution for
presence of excess
bleach, remove the
thermocouple from the
flask and touch the end
of the thermocouple to a
piece of starch iodide
paper and observe the
color. Replace the
thermocouple into the
flask.
Place used test strips
on watch glass until
time to clean up.
Oxidation of a 2o Alcohol – Exptl.
Question - What color does starch-iodide paper
turn if excess oxidant is present?
(Write down an answer)
Check your answer with your neighbor
The starch-iodide paper turns blue-black when it
is treated with an oxidant.
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Oxidation of a 2o Alcohol
CLEANUP PROCEDURES
The aqueous phases from the extractions
may be washed down the sink.
Place the contaminated test strips and other
paper products in the container for solid
waste.
Dispose of the dichloromethane and the
product in the organic waste jar.
SAFETY
Glacial acetic acid is a dehydrating agent,
an irritant, and causes burns. It is much
stronger than the dilute solutions found in
vinegar. Dispense it in a hood and avoid
contact with skin, eyes, and clothing.
Sodium hypochlorite solution can emit
chlorine gas, which is a respiratory and eye
irritant. Dispense it in a hood and keep your
graduated cylinder and reaction flask in your
mini-hood.
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SAFETY
Set up your reaction DIRECTLY in front of
your mini-hood intake above your stirrer.
Have an ice bath ready in case the reaction
temperature exceeds 45 oC.
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