One-pot alkoxylation of phenols with urea and 1,2-glycols

Article abstract of DOI:10.1002/jccs.201000027

A one-pot epoxide-free alkoxylation process has been developed for phenolic compounds. The process involves heating phenols and urea in 1,2-glycols at 170-190 °C using Na₂CO₃/ZnO as co-catalysts under atmospheric conditions. During the course of this new alkoxylation reaction, a five-membered ring cyclic carbonate intermediate, ethylene carbonate (EC) or propylene carbonate (PPC), was produced in-transit as the key intermediate and was subsequently consumed by phenols to form alkoxylated ether alcohols as final products in excellent yields. For instance, phenol, bisphenol A (BPA), hydroquinone and resorcinol were converted into their respective mono-alkoxylated ether alcohols on each of their phenolic groups in 80-95% isolated yields. In propoxylation of phenols, this approach shows great product selectivity favoring production of high secondary alcohols over primary alcohols in isomeric ratios of nearing 95/5. Since ammonia (NH₃) and carbon dioxide (CO₂) evolving from the reaction can be re-combined in theory into urea for re-use, the overall net-alkoxylation by this approach can be regarded as a simple condensation reaction of phenols with 1,2-glycols giving off water as its by-product. This one-pot process is simple, safe and environmentally friendlier than the conventional alkoxylated processes based on ethylene oxide (EO) or propylene oxide (PO). Moreover, this process is particularly well-suited for making short chain-length alkoxyether alcohols of phenols.

Full text of DOI:10.1002/jccs.201000027

Journal of the Chinese Chemical Society, 2010, 57, 167-173
167
One-Pot Alkoxylation of Phenols with Urea and 1,2-Glycols
Hsing-Yo Lin (
Department of Chemical Engineering, National Chung-Hsing University, Taichung, Taiwan, R.O.C.
) and Shenghong A. Dai* (
)
A one-pot epoxide-free alkoxylation process has been developed for phenolic compounds. The pro-
cess involves heating phenols and urea in 1,2-glycols at 170-190 °C using Na2CO3/ZnO as co-catalysts
under atmospheric conditions. During the course of this new alkoxylation reaction, a five-membered ring
cyclic carbonate intermediate, ethylene carbonate (EC) or propylene carbonate (PPC), was produced
in-transit as the key intermediate and was subsequently consumed by phenols to form alkoxylated ether al-
cohols as final products in excellent yields. For instance, phenol, bisphenol A (BPA), hydroquinone and
resorcinol were converted into their respective mono-alkoxylated ether alcohols on each of their phenolic
groups in 80-95% isolated yields. In propoxylation of phenols, this approach shows great product selec-
tivity favoring production of high secondary alcohols over primary alcohols in isomeric ratios of nearing
95/5. Since ammonia (NH3) and carbon dioxide (CO2) evolving from the reaction can be re-combined in
theory into urea for re-use, the overall net-alkoxylation by this approach can be regarded as a simple con-
densation reaction of phenols with 1,2-glycols giving off water as its by-product. This one-pot process is
simple, safe and environmentally friendlier than the conventional alkoxylated processes based on ethyl-
ene oxide (EO) or propylene oxide (PO). Moreover, this process is particularly well-suited for making
short chain-length alkoxyether alcohols of phenols.
Keywords: Alkoxylation; Ethoxylation; Propoxylation; Urea; Green process.
1. INTRODUCTION
detergents, surfactants, and waterborne polymeric interme-
diates.5 Typically, the alkoxylation reaction is achieved by
introducing EO or PO into an enclosed pressure reactor
containing an initiator at about 120-150 °C in the presence
of a base catalyst such as NaOH or KOH. Depending upon
the amount of epoxides added to the reactor, different
chain-lengths of the alkoxylated products can be obtained.
However, there is always a possibility of an un-controlled
chain-reaction due to rapid exothermic reactions releasing
the strained ring energy of the epoxides. Thus, low boiling,
volatile EO or PO in pressurized conditions does pose a po-
tential dangerous situation.
Development of an environmentally friendly process
for practical industrial chemical transformations is an im-
portant consideration nowadays. Society demands it; gov-
ernments encourage it, and sustainability of the chemical
industry hinges upon it. The basic approach in designing
green chemical processes is to avoid using hazardous re-
agents and halogen-derived chemicals in the overall syn-
thetic scheme. In addition, recycling of by-products from
the reaction to eliminate wastes and pollutants should also
be emphasized.1,2 Herein, we report an efficient one-pot
alkoxylation of phenols with urea and 1,2-glycols that
meets these general green chemistry principles.
Since the use of EO and PO has inherent danger of
rapid exothermic reaction, finding alternative raw materi-
als for alkoxylation under atmospheric condition is highly
desirable. Ethylene carbonate (EC) and propylene carbon-
ate (PPC) are the potential good candidates for filling this
gap. First of all, EC and PPC are high boiling liquids with
little ring-strain.6 Alkoxylation with EC/PPC has been in-
vestigated previously.7 In fact, short-alkoxylated products
of phenols have been obtained by using EC or PPC. For in-
The most commonly used alkoxylating agents such as
ethylene oxide (EO) and propylene oxide (PO) are versatile
industrial chemicals. For instance, long-chained polyal-
koxyether polyols, major derivatives of alkoxylated prod-
ucts of EO and PO, are widely used in the syntheses of poly-
urethanes and polyesters.3,4 Making of hydrophilic poly-
alkoxyethers especially from polymerization of EO are
well known for their utilities in the syntheses of cleaners,
* Corresponding author. E-mail: shdai@dragon.nchu.edu.tw

168 J. Chin. Chem. Soc., Vol. 57, No. 2, 2010
Lin and Dai
stance, bis-hydroxyethyl ether of bisphenol A (BHE-BPA,
3ba), a raw material used in polyesters, was prepared sim-
ply by heating BPA and EC at about 160 °C in the presence
of a base.7
the condenser which would account for portion of urea
loss. Therefore, an excess of urea in 30% molar excess was
typically used to compensate for this apparent loss in the
solution. Similarly, in all our studies using bisphenols, high
yields of bis-ethoxylated diols were isolated in greater than
80% isolated yield (Table 1).
Recent literature8 indicated that, EC and PPC can be
synthesized from urea and 1,2-glycols, but there has not
been any report of alkoxylation accomplished by using
urea-1,2-glycol mixtures directly. Recently, we reported
the first application of this technology in polycarbonate re-
cycling,9 and it appears to us that this one-pot process has
great potential for the general alkoxylation of phenols.
Herein, we report the chemistry involved in this convenient
one-pot high yield synthesis using urea-glycol mixtures as
the raw materials.
2.2. Mechanism: IR Monitoring, By-product Forma-
tions and Optimization
To monitor the reactions involved in this one-pot re-
action, the changes of absorption spectra of the reaction
mixture in FTIR was followed closely, particularly paying
attention to the carbonyl regions. The composition of the
product mixture was also examined by 1H NMR and HPLC
to ascertain the product structures and their relative iso-
meric ratios. As shown in Fig. 1a, the absorption of iso-
cyanic acid, H-N=C=O (2211 cm-1), due to partial decom-
position of urea could be detected at a very early stage
when the temperature of the mixed solution reached 100
°C. Addition product of isocyanic acid with ethylene gly-
col, carbamate 7, was then observed in the solution as indi-
cated by the appearance of a 1711 cm-1 carbamate absorp-
tion.
2. RESULTS AND DISCUSSION
2.1. General One-Pot Ethoxylation Condition
The one-pot urea-ethylene glycol process for synthe-
sis of ethoxylated ether alcohols was outlined in Scheme I
(eq. 1, R1 = H). Phenols, urea and ethylene glycol were
mixed and heated together in solution. In the case of our
model study with phenol, we found that mono-ethoxylation
product, 2-phenoxyethanol (3aa), was isolated in 89%
yields at 160 to 180 °C by using urea-ethylene glycol in the
presence of catalysts (Na2CO3 and ZnO). The reaction was
monitored by IR, checking the disappearance of carbonyl
absorptions at 1711, 1736, 1774 and 1802 cm-1. The reac-
tion time took approximately 4-6 hours to complete. Dur-
ing the course of the reaction, ammonia evolved from the
mixture rapidly, and some urea deposits could be found on
Ethylene glycol could also directly react with urea to
form carbamate 7 by substitution reaction which is shown
Table 1. Reaction combinations and yields for the alkoxylated
products
Phenolic
Yield
(%)b
Entry
1,2-Glycol Product Isomer ratioa
compound
1
2
3
4
5
6
7
8
9
1a
1a
1b
1b
1c
1c
1d
1d
1e
2a
2b
2a
2b
2a
2b
2a
2b
2a
3aa
3abc
3ba
–
– (94/6e)
–
89
92
88 (83f)
3bbd 92:1:0 (95/5e)
93
91 (85f)
93
Scheme I One-pot alkoxylation of phenols using 1,2-
glycols-urea catalyzed by Na2CO3 and ZnO
(eq. 1)
3ca
3cbd
–
91:2:0 (92/8e)
3da
–
99
3dbd 91:1:0 (91/9e)
3ea
92
86 (77f)
–
a Isomer ratios of 2°-2°, 2°-1° and 1°-1° isomeric propoxylated
ether alcohols (by HPLC analyses).
b Isolated yields of mono-alkoxylated product(s) for each phenol
group.
c A mixture of two isomers, 1-phenoxypropan-2-ol and 2-
phenoxypropan-1-ol.
d Mixtures of 2°-2°, 2°-1° and 1°-1° isomeric alcohols.
e Isomeric ratios of (2°/1°) of all mono-propoxylated alcohols (by
NMR analyses).
f Isolated yields by re-crystallization.

One-Pot Alkoxylation of Phenols with Urea and 1,2-Glycols
J. Chin. Chem. Soc., Vol. 57, No. 2, 2010 169
Scheme III Reactions occurred between phenols and
carbonyl compounds
Fig. 1. Ethoxylation of BPA monitored by FTIR: (a)
120 °C, 2 h; (b) 170 °C, 4 h; (c) 180 °C, 6 h.
in the Scheme II. Carbamates 7 appeared to undergo an
intra-molecular cyclization at a temperature greater than
150 °C in formation of ethylene carbonate (EC). The for-
mation of EC was evidenced by the appearance of absorp-
tions at 1774 and 1802 cm-1 in IR analysis. Potential by-
product oxazolidone 8, which may result from intra-molec-
ular dehydration of 7, was minimized by using ZnO as a
co-catalyst as evidenced by showing little absorption at
1732 in the final ethoxylated product.10,11
period revealed that two methylene NMR peaks of the
4-type carbonate alcohols were observed at 4.07 and 4.43
ppm, while two methylene NMR peaks of ethoxylated
ethers of 3aa-like products appeared as two signals at 3.90
and 4.07 ppm (Fig. 2a). After re-heating the product mix-
tures at about 180 °C for two additional hours, the IR ab-
sorptions of carbonate alcohols disappeared completely
Scheme II Decomposition of urea and formation of
cyclic carbonate
1
through evolution of CO2, and at the same time the H
NMR spectrum becomes simplified into Fig. 2b, indicating
that ethoxylated ether alcohols became the ultimate prod-
Reactions between phenols and the intermediate car-
bonyl compounds seem complicated because many parallel
reactions are proceeding during the reaction. However,
logical routes that phenol or its derivatives react with car-
bonyl intermediates in formation of 2-phenoxyethanol,
3aa, are compiled in Scheme IIIa-b. In the case of ethoxy-
lation of BPA as the example, the formation of 4-type car-
bonate alcohols are observed in IR absorptions at 1736-
1740 cm-1 as shown in Fig. 1b by the first two hours of heat-
ing. Monitoring of the product mixtures at this transition
Fig. 2. 1H NMR spectra. Ethoxylation of BPA: (a) 170
°C, 4 h; (b) 180 °C, 6 h.

170 J. Chin. Chem. Soc., Vol. 57, No. 2, 2010
Lin and Dai
uct. Thus, in the one-pot process, EC converted from cy-
confirmed by HPLC analysis in showing 2°-2° bis-pro-
poxylated diol being predominant component (92%) with
2°-1° (1%) and 1°-1° (< 1%) diols as the minor products. In
short, one-pot urea-propylene glycol propoxylation offered
a very unique route to high secondary ether alcohols of
phenols especially with sodium carbonate as the base-cata-
lyst.
clization of 4- and 7 plays the key role in alkoxylation.
2.3. Product Selectivity in Propoxylation with Urea
and 1,2-Propylene Glycol
One-pot propoxylations of phenols were similarly ac-
complished at 160-185 °C just by replacing ethylene glycol
with 1,2-propylene glycol to react with urea. Although the
reaction pathway and mechanism are basically the same in
both cases, the products are more complex in propoxyla-
tion. This is mainly because propylene carbonate (PPC) is
an asymmetrical compound, and two isomeric propoxy-
lated products for each phenol group were expected. Due to
the steric hindrance of the methyl group in propylene car-
bonate, phenol appears to react preferentially at the less-
hindered carbon of the PPC to form isomeric 2°-alcohol 5
as the major propoxylated isomer of phenol (Scheme IV).
The isomeric molar ratio between secondary (2°) and pri-
mary (1°) propoxylated ether alcohols after propoxylation
of phenol could be conveniently estimated in the product
mixture by checking the relative areas of methyl groups of
propoxylated ethers in 1H NMR signals located12 at ~1.24
and 1.18 ppm corresponding to the secondary and the pri-
mary isomers respectively (see Table 1).
2.4. Catalysts Used in Urea-Glycol Alkoxylation
During the course of our yield optimization study,
bases other than sodium carbonate, such as cesium carbon-
ate and potassium carbonate, were examined as possible
catalyst alternatives. Under the identical conditions de-
scribed earlier, IR monitoring reveals that the major prod-
ucts obtained from cesium and potassium carbonates are
still mono-alkoxylated ether alcohols. However, two nota-
ble differences have been observed. First, the yields of
mono-ethoxylated products for each phenol groups in po-
tassium or cesium carbonate mediated reactions were found
to be lower than those run with sodium carbonate. In other
words, higher ethoxylated products were found in the prod-
ucts in greater proportions, which could be readily quanti-
fied from the areas of extra NMR absorptions correspond-
ing to the second extended ether ethylene protons located
at 3.5 ppm and 3.8 ppm. For instance, 11% of higher eth-
oxylated products were present in a reaction in K2CO3 cata-
lyzed reaction of phenol. Second, in the case of propoxyla-
tion, selectivity of secondary alcohols in propoxylation
mixtures became substantially reduced when potassium or
cesium carbonate was used. As an example, the selectivity
Scheme IV Isomeric products in propoxylated ether
alcohols
In the case of reactions involving BPA, the selectivity
of isomeric 2° diol components in the final products was
found to be as high as 95% under sodium carbonate cata-
lyzed reaction at 170-185 °C for 4 hours. As indicated in
Fig. 3a, the methyl peaks of all 2° diols at ~1.25 ppm are
dominant as compared with those of 1° diols at 1.18 ppm.
This high selectivity of propoxylated ether products has
never been observed previously in either PO (85%) or PPC
(90%) propoxylation.13,14 This selectivity has also been
Fig. 3. Propoxylated ether diols of BPA prepared un
-
der different conditions by one-pot propoxyla-
tion. Propoxylation conditions: (a) initially at
100 °C and then slowly raised to 185 °C in 4
hours, 2°/1° = 95/5; (b) initially at 160 °C and
then quickly raised to 185 °C in 3 hours, 2°/1° =
89/11.

One-Pot Alkoxylation of Phenols with Urea and 1,2-Glycols
J. Chin. Chem. Soc., Vol. 57, No. 2, 2010 171
of isomeric products with 2° alcohols of BPA was down to
about 70% in Cs2CO3 (Fig. 4a and 4b, 1.15-1.30 ppm). In
the case of potassium carbonate catalyzed propoxylation,
formation of primary isomeric alcohols also were found to
increase to about 44%. Thus, the yield and the isomer selec-
tivity of mono-alkoxylated product for each phenol group
become reduced when strong bases were applied.
extension of propylene oxide units was even less efficient.
Highest propoxylation extension was found to be about 2
units under 4 mole excess of urea. Therefore, it is reason-
able to conclude that the present one-pot process is more
suited for short alkoxylation extensions of phenols.
2.6. Competitive Reaction between in EG/PG-Urea
Alkoxylation
2.5. Higher Alkoxylation of Phenols with EG/PG-
Urea Alkoxylation
A competitive reaction between EC vs. PPC was run
to alkoxylate phenol simultaneously from equal moles of
excess EG/PG solution mixture. The relative mole ratios
among the key reactants in the competitive alkoxylation are
set at 3.0/3.0/1.5/1.0 for EG/PG/urea/phenol respectively
in the comparative reaction. This controlled reaction cata-
lyzed by Na2CO3-ZnO at 175 °C was designed to differen-
tiate the rate of ethoxylation/propoxylation under the one-
pot condition. From the IR monitoring of reaction mixture,
it appears that the formation of EC (1802, 1774 cm-1) and
PPC (1791 cm-1) were both observed during the course re-
action. The 1H NMR analysis of the isolated alkoxylated
products by the area ratios of propoxylated regions at 1.28
(methyl group) and 6.90-7.33 ppm of the aromatic region
concluded a calculated 70/30 (R = 2.33/1) ratio in favor of
ethoxylation. This ratio indicates that EG has a significant
rate advantage over PG in performing the alkoxylation if
run simultaneously in the mixture. It implies that steric-
hindrance of methyl group in PG and PPC does have a re-
tarding rate impact on both the cyclization (ring closure to
PPC) and the alkoxylation (ring opening) rates as com-
pared to those of EG.
In traditional EO/PO alkoxylation, long-chained al-
koxylated ether alcohols in molecular weights of several
thousands could readily be achieved by prolonged reaction
with excess EO/PO addition. However, a similar type of
oligomerization seems more difficult to accomplish in a
urea-glycol one-pot approach. The major hindrance is due
to the complication when excess of solid urea is used in the
reaction solution. For instance, while using three-molar ex-
cess of urea per phenol group and K2CO3 as catalyst in eth-
oxylation, the resulting products had an average extension
of only 2.3 repeating units of ethoxyether for each phenol
1
(calculated from H NMR, 3.5-4.1 ppm) (Fig. 5). In the
most elaborated conditions, the repeating units could be ex-
tended to about 3.0 ethylene oxide units per phenol group
with a 4-fold molar excess of urea to phenol in one-pot
urea-ethylene glycol approach. In the case of propoxyla-
tion with urea-propylene glycol catalyzed with K2CO3, the
2.7. Green Chemistry Implication
If NH3 and CO2 released from the process were recap-
tured and recycled back to urea15,16 as indicated in Scheme
I, eq. 2, the overall alkoxylation of one-pot urea-glycol pro-
cess can be regarded as a simple condensation reaction of
phenol with 1,2-glycols yielding alkoxylated alcohol giv-
ing off water as the only by-product (Scheme I, eq. 3). This
overall ethoxylation in theory has a calculated atom-utili-
zation efficiency of 88.5% for phenol and 89.8% for BPA
respectively.17 Thus, urea can be regarded as a recyclable
carrier while the 1,2-glycols become the only consumed re-
agents in the whole process. In the case of propoxylation,
the atom-utilization efficiencies for phenol and BPA are
calculated to be 89.4% and 90.5% respectively.
Fig. 4. Propoxylated ether diols of BPA: catalyzed by
(a) Na2CO3; (b) Cs2CO3.
Fig. 5. Higher ethoxylation of BPA: BPA/urea/EG =
1/8/10, catalyzed by K2CO3.

172 J. Chin. Chem. Soc., Vol. 57, No. 2, 2010
Lin and Dai
3. CONCLUSION
(s, 1H, -OH).
In summary, we have developed a one-pot process for
alkoxylation of phenols using an urea-1,2-glycol mixture
as the reagent. This process is ideal for extending short
alkoxylated alcohols of 1-3 units to each phenol group. It
was found that glycols are not only the idea solvents for
dissolution of urea, but also provide a media with high
enough boiling points for carrying out the alkoxylation in
atmospheric condition. In a propoxylation reaction with
urea-propylene glycol, formation of mono-extended units
with isomeric secondary alcohols were found to be as high
as 95% selectivity, which is the highest among all possible
propoxylation methods. Because the reagents used are safe
and inexpensive, and chemical efficiency is high, this con-
venient one-pot process could have a practical value in syn-
thesis of ether alcohols from phenols in replacing EC and
PPC.
1-Phenoxypropan-2-ol: 1H NMR (200 MHz, CDCl3)12
d 6.8–7.4 (m, 5H), 3.9-4.4 (m, 3H), 2.1 (s, 1H, -OH), 1.2 (d,
3H).
2,2¢-(4,4¢-(Propane-2,2-diyl)bis(4,1-phenylene))-
bis(oxy)diethanol (3ba): 1H NMR (200 MHz, CDCl3)7 d
7.16 (d, J = 8.8 Hz, 4H), 6.83 (d, J = 8.6 Hz, 4H), 4.07 (t,
4H), 3.95 (t, 4H), 2.29 (s, 2H, -OH), 1.63 (s, 6H); mp 110-
111 °C (lit. 112 °C).18
1,1¢-(4,4¢-(Propane-2,2-diyl)bis(4,1-phenylene))-
1
bis(oxy)dipropan-2-ol: H NMR (200 MHz, CDCl3)9 d
7.16 (d, J = 8.8 Hz, 4H), 6.83 (d, J = 8.8 Hz, 4H), 3.71-4.25
(m, 6H), 1.98 (s, 2H, -OH), 1.63 (s, 6H), 1.28 (d, J = 6.4 Hz,
6H).
1
2,2¢-(1,4-Phenylenebis(oxy))diethanol (3ca): H
NMR (200 MHz, Acetone-d6)19 d 6.84 (s, 4H), 4.00 (t, 4H),
3.83 (t, 4H), 2.04 (s, 2H, -OH); mp 102-106 °C (lit. 106-
107 °C).20
4. EXPERIMENTAL
1,1¢-(1,4-Phenylenebis(oxy))dipropan-2-ol: 1H
NMR (200 MHz, CDCl3) d 6.84 (s, 4H), 3.69-4.41 (m, 6H),
2.55 (s, 2H, -OH), 1.28 (d, 6H).
4.1. Representative Procedure for the One-Pot
Alkoxylation: Preparation of BHE-BPA (3ba)
A 250-mL, three-necked, round-bottomed flask equipped
with magnetic stirring bar, nitrogen inlet and reflux con-
denser was charged with 22.8 g (0.1 mol) of bisphenol A,
24.8 g (0.4 mol) of ethylene glycol and 0.2 g (1.89 mmol)
of sodium carbonate. The mixture was then heated to 100-
120 °C until BPA dissolved entirely to give a homogeneous
solution. To this solution was added 0.2 g (2.46 mmol) of
zinc oxide and 18.0 g (0.3 mol) of urea. After that, the reac-
tion mixture was heated to 170-190 °C and maintained for
4-6 hours until no carbonyl absorption can be detected by
FTIR for the reaction mixture. The solution was then cooled
to about 50 °C, diluted with 100 mL of ethyl acetate and
stirred for 10-15 min. After the catalyst was filtered off, the
solution was concentrated on a rotary evaporator. The ex-
cess unreacted ethylene glycol was removed by vacuum
distillation. The resulting viscous oil (29.2 g) was recrys-
tallized from 50% aqueous ethanol solution to give 27.8 g
(83%) of an off-white solid (mp 110-111 °C, lit. mp 112
°C).18 1H NMR analysis confirms that the chemical struc-
ture of the solid is in fact the targeted molecule 2,2¢-(4,4¢-
(propane-2,2-diyl)bis(4,1-phenylene)) bis(oxy)diethanol,
3ba.
1
2,2¢-(1,3-Phenylenebis(oxy))diethanol (3da): H
NMR (200 MHz, CDCl3)19 d 6.4-7.1 (m, 4H), 4.0 (t, 4H),
3.8 (t, 4H), 2.1 (s, 2H, -OH); mp 81-85 °C (lit. 86 °C).21
1,1¢-(1,3-Phenylenebis(oxy))dipropan-2-ol: 1H
NMR (200 MHz, CDCl3) d 6.4-7.1 (m, 4H), 3.8-4.1 (m,
6H), 2.1 (s, 2H, -OH), 1.28 (d, 6H).
1
2-(4-(Prop-1-en-2-yl)phenoxy)ethanol (3ea): H
NMR (200 MHz, CDCl3) d 7.44 (d, J = 9.0 Hz, 2H), 6.90
(d, J = 8.8 Hz, 2H), 5.29 (s, 1H), 5.00 (s, 1H), 4.11 (t, 2H),
3.98 (t, 2H), 2.12 (s, 3H), 1.86 (s, 1H, -OH); mp 84-85 °C.
Received September 7, 2009.
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