Physicochemical properties of 2-ethylhexanoic acid N′,N′- dialkylhydrazides

Article abstract of DOI:10.1134/S1070363206080032

The following characteristics of 2-ethylhexanoic acid N′,N′- dialkyl(C₄-C₈)hydrazides relevant to their potential application as Cu(II) extractants were studied: solubility, acid-base properties, resistance to hydrolysis, loss with the aqueous phase, and distribution ratio in relation to the composition of the medium and length of alkyl chains. Nauka/Interperiodica 2006.

Full text of DOI:10.1134/S1070363206080032

ISSN 1070-3632, Russian Journal of General Chemistry, 2006, Vol. 76, No. 8, pp. 1196 1200.
Pleiades Publishing, Inc., 2006.
Original Russian Text A.V. Radushev, T.D. Batueva, V.Yu. Gusev, 2006, published in Zhurnal Obshchei Khimii, 2006, Vol. 76, No. 8, pp. 1246 1249.
Physicochemical Properties
of 2-Ethylhexanoic Acid N , N -Dialkylhydrazides
A. V. Radushev, T. D. Batueva, and V. Yu. Gusev
Institute of Technical Chemistry, Ural Division, Russian Academy of Sciences,
ul. Lenina 13, Perm, 614990 Russia
phone/fax: (342)2126237
e-mail: cheminst@mpm.ru
Received December 30, 2005
Abstract The following characteristics of 2-ethylhexanoic acid N ,N -dialkyl(C C )hydrazides relevant to
4
8
their potential application as Cu(II) extractants were studied: solubility, acid base properties, resistance to
hydrolysis, loss with the aqueous phase, and distribution ratio in relation to the composition of the medium
and length of alkyl chains.
DOI: 10.1134/S1070363206080032
Carboxylic acid hydrazides, being bidentate lig-
ands, form cationic and chelate complexes with a
number of nonferrous metals [1 4]. Some recent
papers concerned the complexation with N , N -dialkyl-
substituted hydrazides [5 7].
(Alk = C H ). The solubility of N , N -dialkylhydra-
8 17
zides in hydrochloric acid is low, and in water they
are virtually insoluble. In this respect, N , N -dialkyl-
hydrazides differ essentially from their unsubstituted
analogs. For example, octanoic acid hydrazide in in-
soluble in heptane, noticeably soluble in water, and
fairly readily soluble in 0.1 M HCl [8].
N , N -Dialkylhydrazides of aliphatic carboxylic
acids, similarly to N , N -dialkylbenzhydrazides [6, 7],
form extractable complexes with Cu(II) ions in alka-
line solutions. However, to understand the complexa-
tion processes and assess the practical applicability of
these ligands, it is necessary to know their properties.
This paper deals with a physicochemical study of
Like unsubstituted hydrazides of aliphatic acids [8],
the reagents under consideration are amphoteric.
Alk
R C NH N
Alk
Alk
+
H , pK
a1
+
R C NH N H
2
-ethylhexanoic acid N , N -dialkyl(C C )hydrazides
4 8
O
Alk
O
C H CH(C H ) C(O) NH N(Alk) .
4
9
2
5
2
H2L+
HL
The reagents prepared and their solubilities in
Alk
R C=N N
Alk
+
certain solvents are listed in Table 1. It is seen that the
solubility of N , N -dialkylhydrazides in organic solv-
ents increases from 0.7 1 to 3.1 3.3 M in going from
H , pK
a2
(
1)
O
C H to C H , but then decreases to 0.6 1.5 M with
4
9
7
15
L
a further increase in the length of the alkyl substituent
Table 1. Melting points of hydrazides C H CH(C H ) C(O) NH N(Alk) and their solubilities in certain solvents at
4
9
2
5
2
2
0 1 C
Solubility, g l 1 (M)
2-ethylhexanol
Main substance
content, wt %
Alk
T ,
C
m
HCl
kerosene
toluene
C4H9
99.0
99.0
99.0
99.0
75.0 75.5 0.15 (5.5 10 4)
245 (0.9)
653 (2.0)
1098 (3.1)
458 (1.2)
199 (0.7)
620 (1.9)
1099 (3.1)
240 (0.6)
272 (1.0)
718 (2.2)
1169 (3.3)
573 (1.5)
4
C6H13
C7H15
C8H17
56.0 56.5 0.049 (1.5 10 )
3
5
37.0 38.0 3.5 10 (1 10 )
4
35.5 36.0 0.191 (5 10 )
1196

PHYSICOCHEMICAL PROPERTIES OF 2-ETHYLHEXANOIC ACID
1197
3
2
1
000
000
A
0.21
1
0.17
0
0
.13
.09
000
0
2
3
0
0
.05
.01
2
00 210 220 230 240 250 260 270
nm
,
1
2
3
4
5
6
pH
Fig. 1. UV spectra of 2-ethylhexanoic acid N , N -dibutyl-
Fig. 2. Optical density A of a solution of 2-ethylhexanoic
acid N , N -dibutylhydrazide in a water ethanol mixture
hydrazide in a water ethanol mixture (4 : 1): (1) pH 6.6,
4
(
2) pH 1.7, and (3) 4.8 M KOH. C
3
10 M,
(4 : 1) as a function of the solution acidity. C
DBH
DBH
4
l 1 cm.
5
10 M, l 1 cm,
225 nm.
The basicity of the compounds HL is characterized
by their resistance to hydrolysis, which occurs as
follows:
by the dissociation constants (K ) of the conjugated
a1
+
acids H L , and the acidity, by the dissociation con-
2
H+
+
stants (K ) of the neutral HL molecules (formation of
R COOH + NH N(Alk)
a2
3 2
H O
2
L ). The absorption pattern of the amide form of
-ethylhexanoic acid N , N -dibutylhydrazide (DBH)
R C(O)NHN(Alk)2
(2)
R COO + NH2N(Alk)2
OH
2
H O
2
in the range 205 240 nm strongly differs from that of
the protonated and deprotonated species (Fig. 1).
As seen from Eq. (2), the hydrolysis yields the cor-
responding carboxylic acid or its salt; therefore, the
degree of hydrolysis of N , N -dialkylhydrazides was
calculated from the amount of the released 2-ethyl-
hexanoic acid (Table 2). Table 2 shows that the re-
agents are resistant to hydrolysis in H SO , HCl,
To determine pK , we used a spectrophotometric
a1
method [9]. The pH dependence of the optical density
of aqueous-ethanol solutions of 2-ethylhexanoic acid
N , N -dibutylhydrazide at
225 nm is shown in
Fig. 2. It follows from Fig. 2 that pKa1 of this com-
pound is 3.10 0.03 (n 4, p 0.95). It is seen that at
pH < 2 2-ethylhexanoic acid N , N -dibutylhydrazide
is fully protonated, and at pH > 4.5 5 it occurs in the
amide form. It can be assumed, by analogy with hy-
drazides [8], that an increase in the length of alkyl
groups from C H to C H will affect pKa1 insignif-
2
4
KOH, and NH , considerably surpassing in this re-
3
spect unsubstituted hydrazides [12]. The hydrolysis
resistance of the N , N -dialkylhydrazides is essentially
independent of the length of the alkyl substituent. The
lower resistance to nitric acid is attributable to the
oxidation of alkyl substituents with nitric acid and
formation of additional carboxy groups titrated with
4
9
8
17
icantly. The deprotonation of the N , N -dialkylhydra-
zides starts at the alkali concentration exceeding
an alkali. However, even in HNO the stability of the
3
4
.9 M. We failed to determine pKa2 because at these
N , N -dialkylhydrazides is comparable to that of dial-
kylamides [13]. The possibility of irreversible electro-
chemical oxidation of the NH group in some N , N -
high alkali concentrations the aqueous-alcoholic solu-
tions used for the measurements undergo phase sep-
aration. Our results show that the basicity of the N , N - dialkylhydrazides in nonaqueous solutions was shown
dialkylhydrazides is similar to that of the unsubsti-
tuted hydrazides [8], whereas the acidity is noticeably
weaker.
in [5]; however, the conditions of this process are
incomparable with the conditions of our experiments.
The distribution of the N , N -dialkylhydrazides
between the aqueous and organic phases was evalu-
ated by repeated washing of the extractant solution
To evaluate the chemical stability of the N , N -dial-
kylhydrazides, they were refluxed in acids and alkalis.
By analogy with unsubstituted hydrazides [10, 11], it
can be assumed that the stability of the N , N -dialkyl-
hydrazides in acids and alkalis is mainly determined
with aqueous solutions of 1 M NH or 0.1 M HCl
3
(Table 3). The distribution ratio D was calculated by
formula (3).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 8 2006

1198
RADUSHEV et al.
Table 2. Hydrolysis resistance of compounds C H CH(C H ) C(O) NH N(Alk) (refluxing in aqueous solutions
4
9
2
5
2
for 2 h)
Alk
Medium (M)
H SO (1)a
HNO3 (2)
HCl (1)
Hydrolysis, rel. %
Alk
Medium (M)
H SO (1)
HNO3 (2)
NH3 (1)
Hydrolysis, rel. %
C4H9
2.5
11.7
2.5
C6H13
2.4
35.0
1.5
2
4
2
4
KOH (1)
NH3 (1)
1.7
1.6
C7H15
H SO (1)
HNO3 (2)
NH3 (1)
2.3
36.0
1.5
2
4
a
Refluxing for 4 h.
Table 3. Distribution ratios (D ) and loss with the aqueous phase of compounds C H CH(C H ) C(O) NH N(Alk) in
av
4
9
2
5
2
a
relation to their molecular weight and solution composition
Davb in indicated medium
Lossb with indicated medium, mg l 1
c
Alk
M of reagent
nC
1
M NH3
0.1 M HCl
1 M NH3
0.1 M HCl
C4H9
270.1
326.6
354.4
382.6
265
1379
6654
17
299
3.6
0.635
0.2
3241
98
8
C6H13
C7H15
C8H17
12
14
16
1566d
5
0.2
d
d
14791
6761
a
b
V
c
/V = 1 : 3, shaking for 5 min, settling for 10 min; 0.1 M solutions of reagents in kerosene. Averaged over 3 5 washing runs.
Total number of carbon atoms in the alkyl substituents.
org aq
d
Values calculated by Eq. (4).
given and reference compounds, respectively; n, num-
ber of additionally introduced methylene groups; h,
parameter reflecting the contribution of one CH2
group to the distribution ratio of the reagents.
n
C0
C (V /Vorg)
i aq
i=1
D =
,
(
3)
Cn
where D is the distribution ratio of N , N -dialkylhy-
drazides after the nth washing of the organic phase
From the data in Table 3, we calculated for the
compounds C H CH(C H ) C(O) NH N(Alk) the
4
9
2
5
2
with a solution of 1 M NH or 0.1 M HCl; C , initial
3
0
parameters h for 1 M NH and 0.1 M HCl solutions.
3
concentration of N , N -dialkylhydrazides in the organ-
We found that h is 0.32 for 0.1 M HCl and 0.175 for
1 M NH3.
ic phase, M; Vaq and V , volumes of the aqueous
org
and organic phases, ml; C and C , concentrations of
N , N -dialkylhydrazides in the aqueous phase after ith
and nth washing runs, respectively, M.
i
n
EXPERIMENTAL
Unsubstituted 2-ethylhexanoic acid hydrazide was
prepared by the Curtius method [15] involving heating
of methyl 2-ethylhexanoate with hydrazine hydrate in
alcohol at 95 C for 48 h and was purified by recrystal-
lization from hexane. Yield 65%.
Table 3 shows that the loss of the N , N -dialkylhy-
drazides with the aqueous phase and their distribution
ratio strongly depend on the molecular weight of the
N , N -dialkylhydrazide and the aqueous phase compo-
sition. The reagent loss with 0.1 M HCl is larger than
with 1 M NH because of the protonation of the re-
2-Ethylhexanoic acid N , N -dialkyl(C C )hydra-
3
4
8
agents; however, even at Alk = C H the loss is in-
zides were prepared by the reactions of the unsubsti-
tuted hydrazide with a twofold molar excess of the
corresponding alkyl bromide in the presence of KOH.
The synthesis was performed in i-PrOH with addition
of benzene for the azeotropic distillation of water; the
mixture was refluxed for 15 h. The resulting com-
pounds were recrystallized from aqueous acetone.
6
13
significant, both with aqueous ammonia and with
HCl. The following equation is valid for the series of
related compounds [14]:
logDx = logD0 + hn,
(4)
where D and D are the distribution ratios of the
x
0
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 8 2006

PHYSICOCHEMICAL PROPERTIES OF 2-ETHYLHEXANOIC ACID
1199
1
Their purity was confirmed by TLC and by H NMR
and IR spectroscopy. The main substance content was
determined by potentiometric titration with 0.1 M
HCl in i-PrOH [16] and by volumetric heterophase
titration with sodium lauryl sulfate [17]. The optical
density of the solutions was measured with an SF-26
spectrophotometer, and pH, with an I-160M pH meter
equipped with glass and calomel electrodes.
2. Popel’, A.A. and Shchukin, V.A., Zh. Neorg. Khim.,
1975, vol. 20, no. 7, p. 1917.
3
. Shvelashvili, A.E. and Machkhoshvili, R.I., Zh. Neorg.
Khim., 1996, vol. 41, no. 4, p. 570.
4. Radushev, A.V., Gusev, V.Yu., and Bogomazova, G.S.,
Zh. Neorg. Khim., 1998, vol. 43, no. 12, p. 2108.
5
6
. Shul’gin, V.F., Zub, V.Ya., Strizhakova, N.G., Zales-
skaya, A.G., and Maletin, Yu.A., Koord. Khim., 2000,
vol. 26, no. 6, p. 446.
The negative logarithm of the acid dissociation
constant of the protonated species of 2-ethylhexanoic
acid N , N -dibutylhydrazide (pK ) was calculated
a1
. Gusev, V.Yu., Batueva, T.D., Radushev, A.V., Shaba-
lina, L.S., Bogomazova, G.S., Tyryshkina, V.N., and
Karmanov, V.I., Abstracts of Papers, XIII Rossiiskaya
konferentsiya po ekstraktsii. Simpozium Ekstraktsiya
v gidrometallurgii, radiokhimii, tekhnologii neorgani-
cheskikh i organicheskikh veshchestv (XIII Russian
Conf. on Extraction. Symp. Extraction in Hydro-
metallurgy, Radiochemistry, and Technology of In-
organic and Organic Substances ), Moscow, 2004,
part I, p. 173.
from the UV spectra by formula (5) [9]:
DHL
D
H2L+
pKa1 = log
+ pH,
(5)
D
D
where DHL and D
H2L+
are the optical densities of the
solutions containing, respectively, the neutral and
protonated species of 2-ethylhexanoic acid N , N -di-
butylhydrazide; D, optical density of the N , N -di-
butylhydrazide solution in the given point of the D =
f(pH) curve (at a given pH).
7
. Gusev, V.Yu., Baigacheva, E.V., and Radushev, A.V.,
Abstracts of Papers, XIII Rossiiskaya konferentsiya
po ekstraktsii. Simpozium Ekstraktsiya v gidrometal-
lurgii, radiokhimii, tekhnologii neorganicheskikh i
organicheskikh veshchestv (XIII Russian Conf. on
Extraction. Symp. Extraction in Hydrometallurgy,
Radiochemistry, and Technology of Inorganic and
Organic Substances ), Moscow, 2004, part I, p. 174.
The solubility of N , N -dialkylhydrazides was deter-
mined gravimetrically. To this end, the solution was
equilibrated with the solid phase in a thermostat at
2
0 0.5 C for 24 h, and an aliquot of the saturated
solution was evaporated in an oven at 105 110 C.
To evaluate the hydrolysis resistance, a 0.001-mol
portion of the reagents was refluxed for 0.5 4 h in
1
00 ml of 1 2 M mineral acid or in 100 ml of 1 M
8. Radushev, A.V., Gusev, V.Yu., Chernova, G.V., Les-
nov, A.E., and Karmanov, V.I., Zh. Obshch. Khim.,
1998, vol. 68, no. 10, p. 1674.
KOH or NH OH. After cooling, the 2-ethylhexanoic
acid formed by hydrolysis was extracted with chloro-
4
form (3 10 ml; samples with KOH and NH OH
4
9
. Bernshtein, I.Ya. and Kaminskii, Yu.L., Spektrofoto-
metricheskii analiz v organicheskoi khimii (Spectro-
photometric Analysis in Organic Chemistry), Lenin-
grad: Khimiya, 1975, p. 116.
were preliminarily acidified with 0.5 M H SO to
2
4
pH 5). Then the combined extract was washed with
distilled water to remove traces of the mineral acid
and titrated with 0.1 M KOH in 50% EtOH against
phenolphthalein.
1
0. Grekov, A.P. and Sukhorukova, S.A., Polimery na
osnove gidrazina (Polymers Derived from Hydrazine),
Kiev: Naukova Dumka, 1976, p. 214.
To determine the loss of the hydrazides with the
aqueous phase, 10 ml of a 0.1 M solution of an N , N -
dialkylhydrazide in kerosene was shaken for 5 min
11. Grekov, A.P., Mavrenik, O.V., and Malyushenko, S.A.,
with 30 ml of 0.1 M HCl or 1 M NH . After 10-min
Zh. Org. Khim., 1970, vol. 6, no. 1, p. 94.
3
phase separation, a 10-ml aliquot of the aqueous phase
was titrated with 0.015 M sodium lauryl sulfate to
determine N , N -dialkylhydrazides [17], and the con-
tent of the reagents in the aqueous phase was calcu-
lated.
1
1
2. Radushev, A.V., Gusev, V.Yu., Bogomazova, G.S.,
Kolpakov, A.M., and Kulmukhamedov, G.K., Zh.
Prikl. Khim., 1996, vol. 69, no. 8, p. 1283.
3. Blake, C. and Schmitt, J., in Doklady mezhdunarodnoi
konferentsii (Proc. Int. Conf.), Harwell (the United
Kingdom), September 27 30, 1965, Moscow, 1969,
p. 132.
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