Lab: Phase Transfer Catalysis
These experiments introduce the concept of phase transfer catalysis, which is a way to change the
solubility properties of a reagent so that two reactants (say an organic compound and a salt)
which would normally not dissolve in the same solvent, can be brought together.
1.
Phase Transfer Oxidation
We will oxidize an alcohol (benzhydrol, or diphenyl methanol) in ethyl acetate solution with
bleach (which is a solution of NaOCl in water) to the ketone benzophenone (diphenyl ketone):
The oxidizing -OCl anion is insoluble in ethyl acetate or other nonpolar organic solvents with
most cations. However, the phase transfer catalyst tetrabutylammonium hydrogensulfate (Bu4N+
HSO4-) is able to partition the oxidant, hypochlorite anion, between the two phases. Partitioning
occurs because the cation Bu4N+ can form an ion pair with many different anions. Because the
ion pair is a salt, it will normally be water soluble. However, because of all the carbon atoms in
Bu4N+, ion pairs with it will be soluble in organic solvents, too. Thus, -OCl can be brought into
the organic phase.
In many cases oxidations of alcohols are carried out with chromium complexes. However, such
oxidants are carcinogenic and disposal of reduced chromium byproducts is difficult and
expensive. Phase transfer catalysis permits chemists to use a cheaper and safer oxidant.
Experimental Procedure:
You will carry out the following operations:
-
Perform a phase transfer oxidation of benzhydrol to benzophenone with bleach in the
mixed solvent system water/ethyl acetate, using the phase transfer catalyst
tetrabutylammonium hydrogensulfate.
-
Follow the course of the reaction with TLC, using UV visualization.
-
Isolate the organic product.
-
Recrystallize the benzophenone product from hexane.
-
Recalculate yield of your product.
Detailed Procedure
In a 100 mL Erlenmeyer flask, add 30mL of a bleach solution, 30 mL of ethyl acetate, 0.30 g of
tetrabutylammonium hydrogensulfate and 2.00 g of benzhydrol. Stopper the flask and shake it
with magnetic stirrer.
Take a TLC of the reaction every 5 to 10 minutes to follow the course of the oxidation. Take the
sample for the TLC from the top ethyl acetate layer, which is lighter than water. The TLC solvent
is 5:1 hexane-ethyl acetate. Visualize the reactant and product with the UV lamps. The TLC
plates have a fluorescent additive that glows green under UV light. If a compound absorbs UV
light (which usually means it has at least two double bonds in a ring or along a chain) it will
quench the fluorescence and appear as a dark spot. After about 30 minutes, almost all of the
starting material will have reacted. You may not see complete disappearance of the benzhydrol.
After no more than 40 minutes, work up the reaction. Wash the organic layer twice with saturated
NaCl solution (what organic chemists call brine) and twice with water. Dry with anhydrous
magnesium sulfate. Evaporate off the ethyl acetate with rotary evaporator. The benzophenone
will be a yellow syrup (mp 47-48 oC) that should crystallize once it cools to room temperature.
(Note that even if your material does not solidify, you may be able to obtain crystalline material
by following the recrystallization procedure). The weight of the benzophenone corresponds to the
crude yield, since there are probably traces of starting material or other impurities in the sample.
You must purify the benzophenone by recrystallization. We will recrystallize the benzophenone
product from hexane. Add approximately 10 mL of hexane to the product, and warm it in the hot
water bath until the benzophenone dissolves. Allow the solution to cool slowly to room
temperature, during which time crystals should start to form. If you want to maximize recovery,
once the sample has reached room temperature, you can cool it. Isolate the benzophenone by
vacuum filtration. You can use a little extra hexane to aid in the transfer and to wash the crystals
on the funnel. Once the crystals are dry weigh your product to get a yield of pure product.
1.
Phase Transfer Alkylation
We will obtain dialkyl peroxide (cumyl propyl peroxide) in the presence of phase transfer
catalyst and compare the rate of this process with the same reaction curried out without catalyst.
Organic peroxides are used in the industrial organic synthesis as auxiliaries, for example,
initiators of free radical reactions. The most often utilized method of preparing organic
dialkylperoxides lies in the alkylation reaction of alkyl hydroperoxides in the presence of alkaline
solutions.
The course of alkylation process of alkyl hydroperoxides with alkyl bromides in the presence of
NaOH aqueous solution and quaternary onium salt (Q+Cl-) can be demonstrated by means of the
following equations:
organic phase
O
Q ROO
+
ROOH
interphase
+
R'R’Cl
C
Cl
Na OH
Q
Cl
+
ROO
Na
+
O
C
R'R’OOR
OOR
H2O
aqueous phase
Na OH
(OH - )aq
+
(ROO- )
+
i
+
(ROOH) org
(Q+Cl - ) org
O
-
(Q ROO )
org
+
R'R’Cl
C
Cl
Na Cl
k1
k -1
k2
k -2
k3
org
(ROO- ) i
(1)
(Q+ROO- )
org
+
(Cl- ) aq
O
C
R'R’OOR
OOR org
+
+
(2)
-
(Q Cl )
where: org - organic phase, i - interphase, aq - aqueous phase
org
(3)
Experimental Procedure:
You will carry out the following operations:
-
Perform a phase transfer alkylation of cumene hydroperoxide with propyl bromide in the
presence of water NaOH solution and the phase transfer catalyst tetrabutylammonium
bromide.
-
Perform alkylation of cumene hydroperoxide with propyl bromide in the presence of
water NaOH solution without phase transfer catalyst.
-
Follow the course of the reaction with HPLC method.
-
Draw the course of both reactions (amount of product versus time).
-
Isolate the organic product.
-
Recalculate yield of your product.
Detailed Procedure
Into a thermostated three-necked 100 ml flask, equipped with a mechanical stirrer, a thermometer
and septum, a solution of 88% alkyl hydroperoxide 1.14 g (6.58 mmol) in 30 ml of hexane,
tetrabutylammonium bromide 0.2 g (0.66 mmol) and 0.52 g of 50% NaOH aqueous solution
(6,58 mmol) were introduced. Then a content of the flask was hited up to 50oC the solution of
propyl bromide 0.81 g (0.62 ml) (6.58 mmol) in 20 ml of hexane was added dropwise while
stirring. Afterwards, the reaction mixture was still stirred for about 3-4 h depending on reaction
rate. Samples were taken every 30 min from organic layer with syringe (1 ml to 5 ml analytical
flask and filled up with methanol) and analysed by HPLC. Finally, 20 ml of H2O was added,
layers were separated, the organic layer was washed with three 20 ml portions of 30% NaOH
solution. The organic phase was dried over magnesium sulphate and evaporated to dryness afford
cumyl propyl peroxide.
Analysis. HPLC will be performed using High Performance Liquid Chromatograph with
photodiode array detector and 2.0 x 250 mm cartridge column (LiChroCART, Purospher, RP C18
60A, 5 μm); solvent system included acetonitryle /water (90/10 v/v, flow rate 0.25 ml/min).