| dc.description.abstract | The pH dependence of formation of
tris(2,4-pentanediono)chromium(III)J, Cr(acac), in the
absence -end presence" of carboxy1ate ions" ha s been investigated to determine"f'actors;other than the pH" affecting
the extent of reaction completion. The reactions were
followed by spectroscopic techniques. The Cr(acao)3
formation in the absence of carboxylate ions was used
as< a -\asis t'or comparison in all the other studies. The " study was carried out over the pH range ot' 1.0 to 12.0
in view of continuing literature statements about the
quantitative formation and extraction of Cr(acac)3
species.
These of sodium hydroxide and ammonia t'or the
pH adjustment show~ that the formation of Cr(acac)3
was only 93% complete in the case of sodium hydroxide
and 95% complete in the case of ammonia at the optimum
pH values when the reaction w~s carried out at the
boiling temperature (900
) of the mixture for 30 minutes.
However. the reaction as greatly inhibited in the pH
range 9.5 to 10.5 then ammonia used. There
appears to be little inhibition by sodium h.,vclroxide
over this pITz-ange, 'ilio z-eaotLon inhibition by ammonia
has been attributed to the formation of stable polymeric
~ecbromium(III) speoies wh.i ch are mer-e inert to 3ubstitut~
on by 2,4-pentanedione (acetylacetone), Hacae.
'!he extent of r-eact Lon completion in the r>:resence
of caa-boxyl.at e ions at the boiling podrrt of the mixture
has been ~tudied over the pTIrange 1.0 to 12.0 for
othm10ate, trichloroothanoate and trifluoroethanoate
iOM. A pH of 1.0 to 8.0 tras used for monocbloroethanoatc
and dichloroethanoate ions. Sodiun bydronde
,.•as used for _the :::>Hadjustment for all the car-boxyl.at e
ions studied. A 100% reaction com~letion for ethanoate
and monochloroethanoate ions in -1;hepITrange of 4.0 to
6.0 ahd for triehlorocthanoate ion in the pH r~ of
1.25 to 5.00 traa found. Di.cb.Lor-oc thanoat e and trifluor-
oe'bhanoate gave about 95 and 96%reaction compl.et.Lon
respectively at the optimum initiGl pH value of 6.0
'!he )00% reaction completion in the :;:resence of . / . othanoate m1dmon~chloroeth~!oate ions has been attributed
to the _fOI'Illation of,'the carbo:qlio/carbaxylate buffer system.
· .
(vii)
In the case of the trichloroethanoate ion the reaction proceeds
to 100% completion by virtue of its decomposition to
yield the bicarbonate/carbonate buffer system. This
system has the widest initial pH range over which the
reaction is 100% complete. However, above pH 7..0 the
reaction in the presence of the trichloroethanoe.te ion
system, was found to be very slow. The slowness of the
reaction in this pH region was attributed to the formation
of basic trichloroethanoatochromium(III) species.
The use of trichloroethanoate ion as a labilizer
in the formation of Cr(acac)3 has resulted in the
reduction of the time required to get 100% reaction
completion f'rointhe previously claimed 30 - 60 to.10 minu-
:teitw.hen. it! concentration i.e·;heldbetween 0.08 and 0 •?.8 M".
A violet-green extractable species analysed as
bis (2,4-pentanedl one )trl.chlo;o~tl'''.an·oatoaquochroummi(III)
, ~
has be,en"isolated and characterized when the reaction
was conducted in presence of the trichloroethanoate ion.
This compound has some i.r. and uv-vis. absorption peaks
similar to those o£ Cr(acac)3 but with reduced molar
extinction ccef'f'Lcf.enst, Therefore, it was concluded
r.:
that the fqrmation of Cr(acac)3 in presence of the trich-
I ", "'
loroethanoate ion goe~'through an intermediate species
where the ~cid anion is coordinated to chromium(III). A
further proof of the '.carbo~late-chrom~um(III) coordination
Was obtained from an ether oxtr~ct of chromium(III)/
trichloroethanoate system. The isolated green species
(viii)
had. some i.r. peaks similar to those of the violet-green
oompound.
The rat~B of Qr(acac)3 f()rmation. at temperature and,
00· .
pHranges of 60 to 15 and 1.50 to 3.50 respeotively,
llere determined to elucidate the labilizing or deactivating
effects of car-boxyf.ato ions. The rate constants
obtained for the Qr(acac)~ formation in ~he absence of
the acid anions uere used for corrpar-Lson, The follmTing
kinetic model 1!a5 employed to describe the rate constants.
(a.)
trhcr-e k2 is the observed ra"te constant Irith respect to
Qr3+ in the absence of' carboxy-late ions I 1;;:3 is the rate
constant rrhen tho reaction is conducted in the presence
of much excess of' £H+], k4 is the rate constant due to
the ~~se of fH+] (or baae ) catalyzation of.the
reacti~nt Ka is the ionization constant of' Hacao; In tho
presence of carboxylato ions Equ~tion (a) tcl(es the form
of Equation (b) trher-ek:is the observed rate oonst arrt
dl.n£'cr;+J/dt=f:~=k3-ilC4KaL"ffaeacJILff+J+k~RCO;J (b)
Hith r-espect tg' er->+ in the prosenoe of car-boxy'lat e ions,
~,
k5 is the la;{iliz~t-aon.,o:r: deact~vQ;Cionr2.to oonstant
't'Tithre~poct * to the ~'ci.rb?xylatet ~C02t ion. The rolD.tionship
in ill:l~ation (b) nas, hosovor-, modified as shotrn in
TIiLuation(c) trhor-ekt is th_e labiliz2.t.ion or deactivation
dll'1C(:r~+Jlat=k~=k3-tklJ.K;-~cacJj £ir+J+k5' (c)
rato oonst-ant trhon fRc0'2J has a"constant naximumeffect.
(ix)
It was f'ound that cthanoatcr monochloroethanoate,
dichloroothcmol1to, trichloroothanoatc, trifluoroothnnol1to
and succinate ions labilizcd tho roaction at e pH of 2.50
o
and temperatures of 70 and 75. The trichloroethanoate
ion had the geea-ce::;-l; labili7.::..ie.flf.egct 1Tith a re>.teabout
tuice that in the absence oi' any carboxylate ion. On the
other hand, the oxalate and malonate ions deaotivated the
reaction. The oxalate ion decreased the reaction rate
by aboirt five. times and the malonate by tHico' the
standard rate.
The labilizing effect of the monocarboxylate ions
~nd succinate ion Bay be attributed to formation of
easily reDlaceable carboxyl~te ligands in the coordination
sphere of chromium (III) • 'Ihe slmmess of the reaction
in the presence of oxalate and malonate ions, on the other
hand, may be due to formation. of stable five- and sixmembE7~
rings 1Tith the metal.
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