Download Experiment 2. Oxidation of an Alcohol (see Ege`s: pp 515-519)

Chem 216 S11 Notes - Dr. Masato Koreeda
Topic: _Experiment 2_ page 1 of 3.
Date: May 10, 2011
Experiment 2. Oxidation of an Alcohol (see Ege’s: pp 515-519)
R
O-H
[ox]
C
R'
H
R
1o -alcohol: R = alkyl or aryl; R' = H
R
C O
H
R
C O
R'
2o -alcohol: R/R' = alkyl or aryl
aldehyde
C O ketone
R'
• A number of chromium (VI)-based reagents have been developed for the oxidation of alcohols, e.g., Jones’
reagent [CrO3/H2SO4/acetone/water]. The oxidation of a primary alcohol by the use of the Jones’ reagent results
in the formation of mostly a carboxylic acid.
• Pyridinium chlorochromate (abbreviated as PCC; developed in 1975 by E. J. Corey) is one of the mildest and
yet highly versatile reagents used for the oxidation of alcohols. With this reagent, the oxidation of a primary
alcohol cleanly produces the corresponding aldehyde.
Mechanism of the PCC oxidation of an alcohol:
R
R'
more electrophilic than the
Cr in CrO3 because of Cl
O-H
C
O
O
Cr
Cl
O
H
H
O
R
C
R'
H
O
Cr O
O
Cl
N
H
-Cl
N
H
R
C O
R'
+
oxidation
step
O
R
R'
O Cr
O
C
B:
O
Cr O
R'
C
H
O
Cr O
O
N
H
Cl
still Cr+6
still Cr+6
B:
H
O
R
O
H
O
Cr+4
(chromium is reduced!)
extremely acidic!
often referred to as
"Chromate" ester
N
H
Cl
• Due to the suspected toxicity of Cr(VI)-based reagents, a number of alternate reagents have been developed.
Oxidation of secondary alcohols can be efficiently achieved using much environmentally safer reagents such as a
5.25% (0.75M) sodium hypochlorite (NaOCl) solution available in the grocery store as household bleach.
Mechanism of the oxidation with bleach:
Na-OCl +H3O
OH
Na
Cl
H
O
H
+ H-O-Cl
H2O
H3O
+
oxidation
step
O
+
H
H2O
H2O
Cl
O
Cl
O
+
ketone
H
H
Cl
O
H
H2O
O H + Cl
H
+
H
H
O H
Chem 216 S11 Notes - Dr. Masato Koreeda
Topic: _Experiment 2_ page 2 of 3.
Date: May 10, 2011
Notes for experimental procedures:
1. The TLC spot of neither any of the alcohol we use nor the ketone product could be visible under the
254 nm UV lamp. Use the vanillin stain (this stain contains vanillin and conc. sulfuric acid in ethanol).
After developing the spots, evaporate off the solvent from the plate and strongly heat the plate on a
heater.
2. The ketone product is to be collected by the steam distillation.
3. The distillate obtained is to be extracted with ether.
4. Make the 2,4-DNP [2,4-dinitrophenylhydrazone] derivative of the ketone product and recrystallize
the 2,4-DNP derivative from methanol (obtain the mp data as well as IR).
O
N
O
H
R
C O
+
R'
ketone
N N
H H
O
N
O
H+
O
N
O
R
R'
2,4-Dinitrophenylhydrazine
O
N
O
N N
H
+ H2O
2,4-Dinitrophenylhydrazone
2,4-DNP derivative: usually highly crystalline
and sharp melting solids
Mechanism of the 2,4-DNP formation:
See Ege’s book (pp 575-576). Please note the mechanism for the formation of 2,4-DNP is essentially
identical with that described for the formation of oximes. Simply replace the OH in the oxime
formation with NHAr in the 2,4-DNP formation [Ar = 2,4-dinitrophenyl].
This N reacts with the ketone C=O!
The internal NH lone pair electron is delocalized throughout the
aromatic pi system and two nitrogroups by extensive resonance.
O
N
O
H
N N
H H
O
N
O
O
N
O
H
N N
H H
O
N
O
O
N
O
H
N N
H H
O
N
O
not basic, not nucleohpilic
O
O N
H
N N
H H
O
O N
O
N
O
H
N N
H H
O
N
O
Chem 216 S11 Notes - Dr. Masato Koreeda
Date: May 10, 2011
Topic: _Experiment 2_ page 3 of 3.
Steam distillation setup [see: textbook, pp 141-150 (distillation) and 153-155 (steam
distillation)].
Modified setup for this experiment:
Extraction procedure [see: textbook, pp 128-137 (extraction) and 137-140 (drying)]
• The organic layer (i.e., ether in this case) used for extraction from an aqueous solution
contains about 2%! of water in it. Thus, this organic layer needs to be dried.
• The organic layer is next washed with a roughly equal volume of brine (saturated
aqueous NaCl).
Brine is highly ionic, thus expelling non-ionic organic
compounds/solvents out of the aqueous layer (i.e., brine).
“brine and water-immiscible organic solvents separate better → drier organic layer”
• The brine washed organic layer still contains a trace amount of water. In order to
further remove water in the organic layer, an inorganic drying agent, for example,
anhydrous MgSO4, Na2SO4, or CaCl2, is used by taking advantage of the following
chemical reactions with water.
MgSO4 + 7 H2O → MgSO4•7H2O
Na2SO4 + 10 H2O → Na2SO4•10H2O
CaCl2 + 2 H2O (or 6 H2O) → CaCl2•2H2O (or CaCl2•6H2O)
These are semi-transparent crystalline water-containing inorganic salts. Water
molecules are not simply adsorbed on the surface of these inorganics. These
water molecules are ligands and are part of their crystalline structures.
• Remove the drying agent by gravity filtration and evaporate the organic solvent from
the filtrate.