1284
SHTEINBERG
boric acid and an alcohol is not the decisive factor of
the catalysis in contrast to what has been assumed
previously [2].
ethylene glycol, tetrabutoxytitanium, ammonia, and
PEG-400 were purified and used as described in [1,
2]. The other PEGs, dibenzo-18-crown-6, and sulfuric
acid (technical and pure grade) were used without
additional purification. Glycerol was dried over sodi-
um sulfate and vacuum-distilled.
It is known [3] that alcohols reacting with boric
acid favor its extraction from aqueous solutions into
low-polarity organic phases, such as toluene, decane,
and o-xylene. Low-molecular-weight polyols, which
are readily soluble in water, hardly promote the trans-
fer of boric acid into low-polarity solvents. It is not
improbable that, in the synthesis of 4-nitrobenzamide
too, polyols, in contrast to PEGs, solubilize boric acid
in 1,2,4-trichlorobenzene only weakly, and the cata-
lysis becomes inefficient.
A chromatographic analysis of the mixtures and
evaluation of the product purity were performed on
Silufol UV-254 plates, eluents acetone hexane and
acetone chloroform, development in the UV light.
The IR spectra were recorded on a UR-20 spectrometer
(KBr pellets). The procedures for the synthesis of
4-nitrobenzamide, analysis of the mixtures, and eval-
uation of the product purity were the same as in [1];
an additional amount of the target product was isolated
from the reaction mixture by distilling off the solvent
in a vacuum. Unchanged 4-nitrobenzoic acid was iso-
lated by evaporation of the ammonia mother liquor to
The amidation actually occurs in a multiphase sys-
tem consisting of gaseous ammonia, a low-polarity
solvent, and 4-nitrobenzoic and boric acids, which are
poorly soluble in this solvent. Apparently, the added
PEG initially forms an ester with boric acid, which is
more soluble in the low-polarity medium and is the
true amidation catalyst:
1
0 ml, acidification with concentrated HCl to pH 1,
filtration, and washing of the precipitate with 10 ml
of water.
B OH + H(OCH CH ) OH
2
2 n
The esterification was performed in the amidation
installation as follows. 4-Nitrobenzoic acid (2.0 g,
B O(CH CH O) H + H O.
(4)
2
2
n
2
1
2.0 mmol), triethylene glycol (1.9 g, 12.7 mmol),
At the same time, the role of PEG is hardly re-
stricted to the solubilization of boric acid. We have
shown previously [1] that tetrabutoxytitanium, which
is readily soluble in nonpolar solvents, also catalyzes
the synthesis of 4-nitrobenzamide only in the presence
of PEG-400.
and sulfuric acid (0.03 g, 0.3 mmol) or tetrabutoxy-
titanium (0.1 g, 0.3 mmol) were heated to 160 165 C
in 30 ml of trichlorobenzene containing o-xylene;
the o-xylene water azeotrope was distilled off using
a Dean Stark trap. The reaction was performed until
4-nitrobenzoic acid was fully consumed (TLC moni-
toring). After that, the temperature was elevated to
The boric acid ester formed can solubilize other
molecules (ammonia, 4-nitrobenzoic acid, its ammoni-
um salt) and play the role of a microreactor of a sort,
forming a microphase with a high concentration of the
reactants adsorbed on the PEG chain [7]. Boric acid
can act as a center binding both the reactants and
PEG molecules [8].
170 175 C, and ammonia was bubbled for 4 h.
CONCLUSIONS
(
1) The system constituted by boric acid and poly-
ethylene glycol (PEG-400, PEG-13, PEG-115, PEG-
000, PEG-2000) is an effective catalytic system for
1
It is not improbable that other active catalysts of
-nitrobenzamide synthesis (tetrabutoxytitanium, di-
synthesis of 4-nitrobenzamide by the reaction of 4-ni-
trobenzoic acid with ammonia.
4
methyl hydrogen phosphite, phosphoric and phos-
phorous acids [1]) also form esters with PEG, which
are subsequently involved in the catalysis.
(2) The effect of polyethylene glycols as cocata-
lysts is specific. Lower polyols and dibenzo-18-
crown-6 used instead do not ensure formation of 4-ni-
trobenzamide.
The absence of free hydroxy groups capable of the
reaction with boric acid and the high selectivity to the
size of the transferred particles [9] can be responsible
for the low catalytic activity of dibenzo-18-crown-6 in
the synthesis of 4-nitrobenzamide.
(3) Triethylene glycol esters of 4-nitrobenzoic acid
do not transform into 4-nitrobenzamide under the re-
action conditions.
EXPERIMENTAL
REFERENCES
4
-Nitrobenzoic and boric acids, o-xylene, 1,2,4-tri-
1. Shteinberg, L.Ya., Zh. Org. Khim., 2003, vol. 39,
chlorobenzene, ethylene glycol, diethylene glycol, tri-
no. 7, pp. 1033 1036.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 79 No. 8 2006
Shteinberg
