EXPERIMENT 11: OXIDATION OF ALCOHOLS by TKf5O5r7

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									           SODIUM HYPOCHLORITE OXIDATION: PREPARATION OF CAMPHOR

     Oxidation has a somewhat different meaning in organic
chemistry than you have been used to in inorganic, where
oxidation is defined as a process involving loss of electrons,
as in the conversion of Fe+2 to Fe+3. Oxidation of organic
compounds also normally involves electron transfer, but because
the valence of carbon usually remains at four, it is more useful
to define oxidation as either (a) loss of hydrogen atoms or (b)
addition of oxygen or other electronegative atoms. Thus, all of
the following reactions are classified as oxidations:
                                                  H
                                       CrO3
                              OH                      O



                                       KMnO4                  OH
                               CH 3                       C

                                                              O


                                                      OH
                                       H2O2

                                       OsO4
                                                          OH


     Oxidations of alcohols provides one of the most general
methods for the preparation of carbonyl compounds; oxidation of
primary alcohols affords aldehydes (or carboxylic acids, if
oxidation is continued), while oxidation of secondary alcohols
affords ketones. Tertiary alcohols cannot be oxidized without
breaking carbon-carbon bonds.

                                       H                           OH

                                   R          O               R
  R         OH                                                          O

Primary alcohol               Aldehyde                             Carboxylic Acid


      R'                       R'

 R                        R            O
           OH                                            The most
common oxidizing agents for the conversion of alcohols to
aldehydes and ketones are chromium trioxide (CrO3) and its
relatives, chromate and dichromate ions (CrO4-2 and Cr2O7-2), as
well as a series of modified forms of CrO3, such as Collins
reagent, in which CrO3 is complexed with pyridine (Collins
reagent is used in nonaqueous media, and is especially useful
for oxidizing primary alcohols to aldehydes without
overoxidation to the carboxylic acid). KMnO4 can also be used to
oxidize alcohols.

     An alternative oxidizing agent for the preparation of
ketones, sodium hypochlorite (NaOCl) in acetic acid, was
introduced by Stevens, Chapman, and Weller in 1980 (J. Org.
Chem. 45, 2030). This reagent offers several advantages: it is
cheap (sodium hypochlorite is the reagent in Clorox and
"swimming pool chlorine"); it oxidizes secondary alcohols
rapidly and in high yield; and it avoids the problem of
disposing of toxic wastes associated with chromium and manganese
reagents.

     The example chosen to illustrate the oxidation of alcohols
is the sodium hypochlorite oxidation of isoborneol to camphor:

            H3C                             H3C   CH 3
                      CH 3




                              NaOCl

                                  CH3CO2H
                             OH

            H3C                             H3C
                                                  O
Camphor is a bridged bicyclic ketone widely distributed in
nature, especially in trees of the Far East. It is used as a
plasticizer for the production of celluloid film, for smokeless
powders and explosives, as an insect repellent, and for
medicinal purposes (you will recognize the characteristics odor
of Vicks). Borneol, one of the stereoisomeric related alcohols,
is found in certain trees of Borneo, and isoborneol, used in
today's experiment, is made commercially from pinenes (the C10
hydrocarbons of turpentine).

    Safety and Waste Disposal

     1. Sodium hypochlorite (Clorox) is a strong oxidizing
agent and bleach; be careful to keep it off your skin and
clothing. Wash spills with water.
     2. Excess Clorox, as well as the filtrate from the
collection of camphor, can be poured down the drain if diluted
well with water.
     3. If you distill the methylene chloride at the end of the
experiment, put the distillate in the bottle labeled "RECOVERED
METHYLENE CHLORIDE"

                     EXPERIMENTAL PROCEDURE

     Dissolve 5 g of isoborneol in 15 mL of glacial acetic acid
in a 125-mL Erlenmeyer flask. Add 50 mL of Clorox by the mL
over 5 minutes, cooling the flask as necessary to keep the
internal temperature in the range 15-25ºC. Allow the mixture to
stand at room temperature for one hour with occasional swirling.
A positive KI-starch test should be obtained at this point.

     Add saturated NaHSO3 solution carefully until the yellow
color of the reaction mixture disappears and the KI-starch test
is negative. Pour the mixture over 100 mL of brine and ice,
collect the solid by filtration on a Buchner funnel, and wash it
with saturated NaHCO3 solution until foaming is no longer
evident. Press the solid as dry as possible on the funnel.

     Dissolve the solid 20 mL of methylene chloride, separate
from a water layer if necessary, and dry the solution over
anhydrous sodium sulfate. After filtering the drying agent, boil
off the methylene chloride on a hot plate or in a hot-water bath
in the hood. Alternatively, you may set up a simple
distillation, heating with a hot-water bath, to remove the
methylene chloride. Camphor is quite volatile, so be careful not
to overheat or some of your product will be lost. Weigh the dry
product and calculate the yield. After determining the percent
yield, prepare the 2.4 Dinitrophenylhdrazone derivative:
Dissolve about 0.2 g of your crude camphor in 5 mL of 95%
ethanol, add 10 mL of the 2,4-dinitrophenylhydrazine reagent
solution, heat to boiling on a hot plate for 3 minutes, and
allow the mixture to cool. Add water dropwise until the
turbidity just clears. Crystallization usually occurs within a
few minutes. Filter the yellow precipitate by suction, wash
with 2-3 mL of water, and allow drying to occur. If the time
permits, determine the melting point (reported mp 164 ºC).

    Questions:     (Answer these in your notebook write-up).

     1. Write a balanced equation for the oxidation for the
oxidation is isoborneol by NaOCl.

     2. What do you actually observe in a "positive Kl-starch
test"? What does it mean is the Kl-starch test mentioned at the
end of the first paragraph in the Experimental Procedure is
positive?

     3. What is the purpose of adding NaHSO3 at the end of the
oxidation? Write a balanced equation for the reaction it
undergoes.

    Notes.
    1. Commercial Clorox contains about 6.5% NaOCl.

     2. Preparation of 2,4-dinitrophenylhydrazine reagent:
Add 50 mL of concentrated sulfuric acid to 10 g of 2,4-
dinitrophenylhydrazine in a 500-mL Erlenmeyer flask and mix
well. Add 75 mL of water dropwise (CAREFUL! HEAT EVOLUTION),
with stirring or swirling, until solution is complete. To this
warm solution add 250 mL of 95% ethanol. This scale provides
enough reagents for 35 students. The solution should be
prepared fresh each day.

     3. For alternative oxidations of secondary alcohols with
Clorox, see
     (a)      Oxidation of cyclohexanol: N.M. Zuczek and P.S.
              Furth, in J. Chem Educ. 58, 824 (1981).
     (b)      Oxidation of cyclic and acyclic secondary
              alcohols: R.A. Perkins and F. Chau, in J. Chem.
              Edu. 59, 981 (1982).
     (c)      Oxidation of 9-fluorenol: C.S. Jones and K.
              Albizati, in J. Chem. Educ. 71, A271 (1994).

								
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