chemistry of dithiocarbamates -...
TRANSCRIPT
CHAPTER-II
CHEMISTRY OF
DITHIOCARBAMATES
16
CHAPTER - II
CHEMISTRY OF DITHIO CARBAMATES
INTRODUCTION:
The development of the concept of co-
ordination theory by the Switzerland Noble prize
awardee, Werner1 in 1893 revolutionized the
approach to the structure of inorganic compounds
and permeated the entire area of coordination
chemistry. Since then number of ligands have been
introduced in inorganic analysis because of their
selectivity and sensitivity and dimethyl glyoxime
is an example of such organic reagents which is
highly selective for Ni(II)2.
Organic analytical reagents are found to be
advantageous over inorganic reagents for their
reactivity, sensitivity and selectivity and
voluminous literature is available on their use3-
5.The separation of metal ions is simple and more
effective with the organic reagents and hence they
have several applications in gravimetric, titrimetric
and spectrophotometric determinations and also as
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masking and demasking agents in certain
extractive seperations of metals6-9
.
The simplest way of synthesis of these compounds
is as follows:
Ammonium salt of
dithio carbamic acid
N-substituted dithiocarbamic acids,
RNHCSSH or R2NCSSH are formed as their
substituted ammonium salts by the reaction of CS2
with a primary or secondary amine, usually in
alcoholic10
or aqueous solution11
.
The dithio carbamates were developed in
the early history of organo sulphur chemistry.
They are half amides of dithiocarbamic acid and
these amine compounds of dithio carbamates are
prepared by using alkali metal hydroxides12
.
R-NH2 + CS2 + NaOH RNHCSSNa + H2O
18
The sodium or ammonium salts of dithio
carbamates are sufficiently stable than the
corresponding carbamic acids. Sodium salts of
dithiocarbamates are white crystalline solids freely
soluble in water. These salts are stable for a long
period, but the solutions of dithiocarbamates are to
be prepared freshly whenever required. Most of the
heavy metal compounds of the dithiocarbamates
are colored facilitating spectrophotometric studies
in visible or near UV region. They are prepared
simply by the addition of a solution of the heavy
metal ion as its chloride, sulphate, etc., to a
solution of an ammonium or alkali metal salt of the
dithiocarbamic acid13
.
Heavy metal thio
compounds are sparingly soluble in organic
solvents such as chloroform, carbon tetrachloride
and ethyl ether.
Dithiocarbamates are decomposed in acid
solutions and the breakdown of dithiocarbamic
acid into amine and CS2 in the presence of mineral
acids has been used as the basis for quantitative
estimation of dithiocarbamates14
.
19
The CS2 liberated can be estimated by
iodometry as the amount of CS2 liberated is
directly proportional to the concentration of
dithiocarbamate. The detailed study15-18
of various
dithiocarbamates and their derivatives through
infrared spectra confirms the contribution of
various forms equally to the structure. The
significant among several canonical forms for the
dithicarbamates in their metal complexes,
M(S2CNR2)n are as follows :
STRUCTURES:
In the following structures, M is a metal
atom. Although all these forms contribute equally
to the structures of all N,N-dialkyl dithiocarbamic
acid derivatives the canonical form I(a) arises from
the mesomeric electron releasing tendency of the
R2N group. By this electron drift into the sulphur
atoms not only the electron donor capacity
20
increases but the formation tendency of the strong
complexes with the heavy metals also enriches
with reducing tendency of forming dative π-bonds
between the metal and sulphur atoms.
A Number of organic compounds have
been used in spectrophotometric analysis of metal
ions among which dithiocarbamates occupy one of
the prominent positions19
. The abundant
applications of dithiocarbamtes and their
substituted compounds in inorganic analysis are
mainly because of the ease of replacement of
hydrogen from -SH group. A coordinate bond will
be formed through S to form strongly colored
complexes with a number of metals. The strong
metal binding properties of the dithiocarbamates
were recognized by Delepine20
.
21
USES OF DITHIO CARBAMATES IN
VARIOUS FIELDS:
The nitrogen derivatives of dithiocarbamic
acids have got wide applications in the field of
medicine and agriculture because of their
biological activity. The first observation of the
activity of dithio carbamates against human
pathogenic fungi has been ascribed to Hall of
Baker and Baker Co., in 1938. Ferbam, the iron
salt of dimethyl dithio carbamic acid as a first
fungicide, ziram, zinc salt of sodium dimethyl
dithio carbamate as a vegetable fungicide are
particularly effective against the anthracnose
disease. Vapum (sodium methyl dithio carbamate)
is in the market as a soil fungicide, insecticide and
herbicide. The synthesis and anti microbial activity
of piperidine and morpholine dithio carbamates
have also been reported. Another interesting
development has been the discovery of their value
in drugs for the correction of chronic alcoholism.
Recently Gordon research conference of 2006 on
metals in medicine demonstrated metal dithio
22
carbamates as potential anticancer drugs. A large
volume of biological literature has been built up on
the dithiocarbamates and a number of views have
been published on their activity as fungicides21.
Tertramethyl thiuram disulphide is one such
compound that has greater medicinal value in
controlling various dermatophytes, treating scabies
and correcting chronic alcoholism and because of
its importance it is used as an ingredient in various
soaps and lotions. Much useful research on
insecticidal, acaricidal, fungicidal, bactericidal and
microbicidal activity of these dithiocarbamates has
been published. Similarly these compounds have
played a major role in rubber chemistry as
vulcanization accelerators and anti-oxidants.
Dithiocarbamates play a major role in
analytical chemistry also due to their metal binding
property and chelating tendency and these are
effective in the treatment of effluents arising from
refineries of non ferrous metals22,23
.
Dithiocarbamates are used in the spectrophoto-
metric analysis of metal ions and also in different
23
techniques such as amperometry, pH metry and
catalytic polarography. The application of PDTC
as an analytical reagent has been mostly used in
different techniques such as amperometry24
, pH
metry25,26
, spectrophotometry27-37
and catalytic
polarography38
.
Kinetic and mechanistic studies of metal-
DTC complexes in addition to their analytical
determination by electro analytical methods have
been well documented39-41
. The use of DTC is
mainly for preconcentration of metal ions from
water samples and the metal ions after
preconcentration are determined by atomic
absorption spectrophotometry, pulse polarography,
derivative spectrophotometry and radiochemical
methods42-44
. The complexing ability of DTC is
directly related to the presence of 2 donor sulphur
atoms and stability with its cyclic nature45-51
.
Complexes of Cu(II), Co(II) and Ni(II)
were prepared along with 1,4-piperaizine-bis-
dithiocarbamate sodium salt and analysed by
24
conductivity measurements, magnetic
susceptibilities, thermal analysis and spectroscopic
studies52
. Sadafkhan etal53
have reported the
preparation of a ligand, bis(2,2-dithiopiperazinato-
2,2-diamino diethyl amine) Na2L2 and its transition
metal complexes of the type M2(L2)2 where
M=Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II),
Cd(II) and Hg(II). Some organotin complexes with
Sn were prepared54
as bis(2,2-dithiopiperazinato-
2,2-diamino diethyl amine) and characterized by
UV-visible spectroscopy, NMR spectroscopy and
thermo gravimetric analysis. Oi-Wah Lau and
Sing-Yiu Ho55
have developed a method for the
determination of Fe, Co, Ni, Cu, Hg and Pb in
water by energy – dispersive X-Ray fluorescence
spectrometry after subjecting them to pre
concentration as their piperazino-1,4-bis
(dithiocarbamate) complexes.
The synthesis of tetra dentate piperazine
ligand done56
lead to the determination of the
structures of the ligand N,N1 – bis (2-pyridyl
methyl) piperazine with the transition metal ions
25
Cu++
, Co++
, Mn++
and Cd++
. Adam Hulanicki57
presented a detailed report of complexation
reactions of dithiocarbamates and the properties of
metal-dithiocarbamates were presented as a
function of the nature of the substituents in the
ligand molecule. Complexes of Co(II), Zn(II) and
Cd(II) were prepared by using 1-ethoxy carbonyl-
piperazine -4-carbo dithioate and analysed for their
structure by spectrophotometry58
. Tombeux and
his co-workers59
obtained the spectra of N,N1-bis
(dithiocarboxy) piperazine complexed with
Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II) and
Cd(II) and conducted their studies. Since the
research on dithiocarbamates has been revealing
some promising new avenues in all fields of
sciences including inorganic analysis, the inclusion
of all these references is beyond the scope of this
dissertation.
In view of the importance of
dithiocarbamates, a simple and sensitive extractive
spectrophotometric technique for the determination
of micro quantities of Cu, Ni, Co, Fe and Mn have
26
been developed in the present work using
piperazine dithiocarbamate (PDTC).
Advantages of PDTC as analytical reagent:
1. The preparation of the reagent is simple,
easy and time required is less than 45
minutes.
2. The metal complexes of the ligand are
stable because of the contribution of
resonance hybrid structures.
3. The compound possesses high electric
dipole moment.
4. The electron releasing tendency of N-group
into sulphur atom increases the electron
donor capacity of PDTC to give strong
complexes with number of metal ions.
27
5. Water of crystallization is not known in
PDTC metal complexes and hence are
easily extractable into organic solvents.
28
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