Phenol Synthesis
(Hock Method)
INTRODUCTION
1.Use of Phenol
Phenol is an important starting material for numerous intermediate and finished products. The
world production capacity for synthetic phenol is about 7x106 tonnes per year. The use of
phenol in the world is distributed as follows:
-
phenol resins
37%
-
bisphenol A
32%
-
ε-caprolactam
15%
-
alkylphenols
-
others: aniline, chlorophenols, antioxidants
2%
12%.
2. Manufacturing processes for phenol:
-
Sulphonation of benzene and heating the benzene sulphonate in molten alkali
hydroxide
-
Chlorination of benzene and hydrolysis of the chlorobenzene
-
Toluene oxidation to benzoic acid and oxidizing decarboxylation
-
Dehydrogenation of cyclohexane-cyclohexanone mixtures
-
Alkylation of benzene with propene, oxidation of obtained cumene and acid cleavage
of hydroperoxide (Hock process).
Besides synthesis, others sources (include tar and coke-oven water from coal cooking and
wastewater from cracking plant) are insignificant.
More than 90% of the world’s phenol production technology currently based on the cumene
hydroperoxide route. The Hock process is based on cumene oxidation to cumene
hydroperoxide and its cleavage in acidic media to phenol and acetone. Both reactions are
highly exothermic.
3. Liquid-phase oxidation of cumene
The key stage of Hock method is liquid-phase cumene oxidation with oxygen. Cumene is
oxidized according to the widely known free-radical chain mechanism as presented in the
scheme 1 (reaction 1-7).
Scheme 1
Oxidation of alkylaromatic hydrocarbons
Initiation
.
2In
(1) In2
.
(2) In + RH ki
Propagation
(3) R . + O2
(4) RO2. + RH
InH + R
.
RO 2
kp
Termination
.
(5) RO 2. + RO 2 kt
.
.
(6) RO 2 + R
.
.
(7) R + R
(8) ROOH
(9) 2 ROOH
.
ROOH + R
.
Non-radical
products
. .
RO + OH
.
.
RO + H2O + ROO
.
(10) RO + RH
(11) RO
.
ROH + R
O
R1
.
.
+ CH3
where R – PhC(CH3)2-, R1 – Ph
The oxidation process is often initiated by addition of free-radical initiators such as azocompounds or peroxides. The decomposition reaction of azo-compouds is described by the
following equation:
R'
N N R'
.
2R' + N2
The hydroperoxide obtained in the oxidation process may be partially decomposed into
alkoxyl or alkylperoxyl free radicals (reactions 8 and 9). The formed radicals may also initiate
subsequent chains of reactions.
The alkoxy radicals undergo hydrogen abstraction (reaction 10) or β-scission reaction
(reaction 11). The formed alcohol and ketone are the main by-products of the oxidation
process.
4. Acid-catalyzed cleavage of cumene hydroperoxide
The acid-catalyzed cleavage of cumene hydroperoxide follows an ionic mechanism. Cleavage
is catalyzed by strong mineral acid. Sulfuric acid is used almost exclusively as the catalyst in
industry.
The reaction is strongly exothermic. The heat of reaction is 256kJ/mol.
EXPERIMENTAL
In the frame of the laboratory classes the Hock synthesis of phenol will be studied. The freeradical oxidation of cumene and the acid cleavage of cumene hydroperoxide will be carried
out.
Cumene Oxidation in a Semi-Periodic Reaction
Put 20 ml of cumene into a semi-periodic reactor supplied with a bubbler, thermometer,
reflux condenser and heating jacket. Heat it up to 100oC. Weigh initiator (1,1’azobis(cyclohexanecarbonitrile)) (ACHN), put it into reactor and then pass the oxygen
through the liquid. During the process take samples every 30 min. and determine the contents
of hydroperoxide (HP) iodometrically.
Report:
1. Complete the following data:
Temperature
……………
ACHN concentration
……………
Oxygen flow
……………
2. Complete the following table:
1.
0.5
2.
1
3.
1.5
4.
2
5.
2.5
6.
3
Time T
VNa2SO3
CHP
CHP
[h]
[cm3]
[mol/dm3]
[%]
3. Draw a plot of cumene concentration vs. time of reaction
4. Calculate the rate of cumene hydroperoxide (rHP) formation by means of graphic
differentiation of kinetic’s curve and draw a plot rHP vs. cumene hydroperoxide
concentration
CHP [%]
rHP [%/h]
rHP [mol/dm3 s]
Acid Cleavage of Cumene Hydroperoxide to Phenol
The reaction will be carried out in a round bottom flask supplied with a magnetic stirrer,
thermometer, dropping funnel and destillation link with Liebig condenser and 100 ml
receiver flask.
Put 110 ml of acetone, 1 ml of H2O and 0.6 ml of concentrated H2SO4 into flask and
immersed the flask in a heating bath. Heat it up to temperature which is closed to boiling
point of the mixture. The temperatures in heating bath and inside reactor should be close to
each other. Prepare a solution containing 55 ml of solution of cumene hydroperoxide in
cumene (technical grade) and 183 ml of acetone. Slowly drop the prepared solution of
hydroperoxide to reactor containing catalyst. The decomposition reaction is exothermic. The
heat of reaction is used for:
-
evaporation of acetone which is collected in receiver flask
-
heating up the dropped solution of hydroperoxide from the room temperature to the
temperature of boiling mixture.
Determine the hydroperoxide concentration in the technical raw material and in the obtained
reaction product of its acid decomposition. Measure the amount of evaporated acetone and
determine the amount of reaction heat used for the evaporation. Determine amount of heat of
reaction used for heating up the dropped mixture of cumene hydroperoxide, cumene and
acetone.
Warning!: the hydroperoxide can decompose with explosion in the presence of acid and
some transition metals.
Report:
1. Complete the following data:
-
concentration of the technical cumene hydroperoxide [%]
-
concentration of the cumene hydroperoxide in the decomposition reaction product [%]
-
amount of decomposed hydroperoxide [mol]
-
room temperature
-
reaction temperature
2. Complete the following table:
Heat of reaction used for:
heat up the dropped hydroperoxide
heat up the dropped acetone
heat up the dropped cumene
evaporation of acetone
Summary ΣQ [kJ]
Q[kJ/mol of decomposed hydroperoxide]
amount [kg]
Q [kJ]
Iodometric Analysis of Hydroperoxides
Put 0.2 ml of sample into a 250 ml Erlenmeyer flask containing 20 ml of glacial acetic acid
and sweep the flask with carbon dioxide. Next add about 2 g of sodium iodide and place it in
a dark for 30 minutes. Then put 20 ml of water into the flask and titrate with 0.1 N solution of
sodium thiosulfate.
ROOH + 2I¯+ 2H+→ ROH + I2 +H2O
I2 + 2S2O32-→ 2I¯ + S4O62¯
CHP=VNa2S2O3/4 [mol/dm3]
Cumene
M=120.19 g/mol;
d=0.8618 g/cm3
cp =1.6 kJ/kgK
kp/kt1/2 =1.34*10-2 [dm3/(mol/s)]1/2 at 100oC
Cumene hydroperoxide
M=152.19 g/mol;
d=1 g/cm3 (70%)
cp =1.6 kJ/kgK
Acetone
d= 0.79g/cm3
cp =2,2kJ/kgK
Heat of vaporization 29kJ/mol
ACHN
M=244.25 g/mol
kd=9.414x10-5 [s-1]; half-life time τ1/2=123 [min.] at 100oC
Heat of decomposition reaction of cumene hydroperoxide
256kJ/mol
LITERATURE:
1. E.Grzywa, J.Molenda, Technologia podstawowych syntez organicznych, tom II, PWN,
2000.
2. Ullmann’s Encyclopedia of Industrial Chemistry, VCH Verlagsgesellschaft mbH,
Weinheim, 1991, vol.A19, pp299-311.
3. R.J. Schmidt, Applied Catal., A: Gen., 280, 89-103 (2005).