Acid-Base Equilibria of I,4-Bis(4'-methylanilino)anthraquinone &
I,4-Bis(2' -sulpho-4' .methylanilino)anthraquinone in Relation to Their Spectral Absorption in Aqueous-Organic Solvent Mixtures
K. A. IDRISS· & M. M. SELEIM
Chemistry Department, Faculty of Science, Assiut University, Assiut, Egypt Received 15 September 1979; revised 14 January 1980; accepted 5 February 1980
The electronic absorption spectra of 1,4-bis(2'-sulph0-4'-methyIanilin0)anthraquinone (qninizarln green) and its non-sulphonated derivative have been studied in aqueous-organic solvent mixtures at different pH.
The solvents used include methanol, ethanol, isopropanol, ethylene glycol, glycerol, acetone and dioxane. The results have been discussed in terms of molecular structures of the compounds and the medium effect. The acid dissociation constants of the two compounds have been determined and discussed in relation to both the concentration and the nature of the organic co-solvent used. The elimination of the proton from the imino group is found to be pH-dependent and becomes more difficult in the non-sulphonated compound.
SEVERAL
and visible spectra of amino- andstudies have been reported on the UVarylamino- anthraquinones 1-9. However, in spite of the significant applications of these compounds in analy- tical chemistry and chelate formation10-13, very fewstudies14-16 have been made on their acid-base equi- libria, in aqueous solutions orin water-organic solvent mixtures1• The effect of organic solvents on the spectra of some 1,4-diarylaminoanthraquinones has been investigated earlier1. The purpose of the present work is to study the effect of pH on the electronic spectra of 1,4-bis-(2'-sulpho-4'-methylani- lino)anthraquinone (quinizarin green, QG) and its non-sulphonated derivative (NSQG) in water-organic solvent mixtures. The pKa values corresponding to the different ionization steps of the two compounds have been determined and correlated in terms of the nature of the medium.
Materials and Methods
Quinizarin green (QG) and its non-sulphonated derivative (NSQG) were purified by recrystallization of commercial reagents. 1O-3M solutions of the anthraquinone derivatives were prepared by dissolv- ing accurately weighed solids in 0.05 M NaOH.
The modified universal buffer series of Britton and Robinson17 was used for pH-adjustment and as supporting electrolytes. The pH measurements were carried out using a Radiometer pH meter (Mb28) having an accuracy of ±0.05 pH units. The pH values in partially aqueous media were corrected according to the method of Bate18• The organic solvents used were purified by recommended proce- dures19• The absorption spectra were recorded on a Unicam SP 8000 spectrophotometer in the range 320-750 nm using 1 cm matched quartz cells.
Results and Discussion
Absorption spectra - The visible spectrum of 1,4- bis (4' -methyl aniline )anthraquinone (NSQG) exhibits three main absorption bands at ",,470, 550 and 590 nm in the pH range 5-11.5 (Fig
n.
These bands are probably due to the absorption by the neutral mole- cules, monovalent and divalent anions of NSQG respectively. The first band undergoes a regular bathochromic shift on increasing th,e pH of the medium as a result of the proton elimination which leads to a lower energy of charge-transfer. The isosbestic point near 495 nm is probably due to the equilibrium set in solution between the neutral and monoionised forms of the compound, whereas the other isosbestic point at 520 denotes the equilibrium between the mono-and divalent anions of NSQG.The shape of the band envelope can be correlated to the nature of the absorbing species. For the neutral molecules and the doubly ionised form of the compound, the arylaminosubstituents can parti- cipate more or less equally in the charge-transfer process, and hence the electronic transition~ would lead to single more or less symmetrical bands. The monoionised form is characterised by a less symmetri- calor split C. T. band.
In alcohol or acetone ~ontaining mixtures, the transformation of the univalent anions to the divalent ones takes place at pH ;;:.10 but occurs at relatively lower pH (""pH 9) in solutions containing >20%
(wJw) of glycol orglycerol. In the presence of lower concentrations of the latter solvents the longer wave- length band (590 nm) overlaps strongly with the preceding one (550 nm) probably due to simultane- ous ionisation of the protons from the imino groups.
The absorption by the non-ionic form of the com- pound attains a maximum value at a certain pH.
771
0.4
:;50 610
3rD 430 490
0.3
0.1
o.
0.3 610
550 490 550 430 370
170 430 490
0.3
C)24.62'I, (w/w) Glycerol
, , ,
,· ••.',
1"I 10.+ \
/.-\ \
I '\\
./ \ I
~ '\ I
_6.71 il _ •.•• I
l' ~.•..."" " .:
0.2~'·
\ . ./ >'h',''::\
-..-.•.../,.,,'-...,.!_.~.....-.,
.••., ... , \\\ I
.I
I~, .•.' /!
I " '., \\ ••O. I~t .. "" .
'\ -.--
./ .//,' .~,"~...•.. '{'"''-. .-;/ ..•...:::.',
...::=.:.-::-,. ....~.
CIIv oC -e.o
~
<t. 0.4 0.3
A)B.2'I,(w/w) Methanol
Fig. 1 - Absorption spectra of NSQG in aqueous-organic solvent mixtures at differentpH.
Tfle latter is higher in alcohol or acetone containing mjixtures than in the other solvent mixtures.
iThe spectrum of 1,4-bis (2'-sulpho-4'-methylani- li~o)anthraquinone (QG) shows four absorption b.nds at 400,420, 550 and 610 nm in the pH range 3111.5 (Fig. 2). The first band is probably due to cQ.arge-transfer from the paired electrons on the in),inonitrogen to the C=O group via the anthraqui- n~ne nucleus.oil' NSQG due to the restrainingThis band is ill-defined in the caseof the unshaired p,ir of electrons. Such restraining is affected by tnepH of the medium as well as its composition. In a4idic or weakly alkaline solutions, a hydrogen bond is Iformed by the shairing of the electron pair of the o~ygen atom of the 2'-sulphonic group with the hydrogen of the NH group.
'!The band at 420 nm which corresponds to the a~sorption by the neutral form of QG is lowered in intensity with increase inpH and vanishes completely a~pH ;;>9. The two other bands are due to absorp- ti~n of mono and divalent anionic species of the
cl)mpound.
:The intensity and position of the bands corres- pcj>ndingto the ionised forms of QG and NSQG areaffected by the nature of the organic co-solvent. The inprease in alcohol concentration in solution causes a pecrease in ionisation as indicated by the decrease i~ the characteristic absorptions of the ionised forms.
T~s may be explained on the basis that alcohol mp,lecules interact with the imino nitrogen through Ht-bonding and consequently decreases the tauto- mpric shift of the iminohydrogen. Higher prop or·
ti<l>nsof dioxane lead however to an apparent decrease
in the concentration of the ionised species, probably due to the formation of ionic associates. On the other hand, the separation between the two ionisation steps of both QG and NSQG is favoured by increas- ing [ethylene glycol] or [glycerol] in solution.
Absorbance-pH relations - The values of absor- bance vs pH were plotted at three wavelengths for the different mixtures investigated. The curves obtained (Fig. 3) support the presence of different acid-base equilibria in solutions of QG and NSQG and illustrate the effect of both the nature and com- position of solutions investigated.
The variation of absorbance with pH at ,\=460 nm for QG indicates that increase inpH above 3.5 leads to the liberation of the monoprotonated form of QG from the diprotonated species. The limiting value of absorbance at pH 6-7 confirms the trans- formation of the protonated species to the neutral molecule. The curves obtained at 570, 610 or 640 nm reveal, in all the cases, two inflections within the alkaline side which correspond to the liberation of the mono - and divalent anions of QG. The separation between of the two ionisation steps occurs at relatively higher pH in mixtures containing high proportion of alcohols and acetone than in water- ethylene glycol or-glycerol mixtures.
For the nonsulphonated compound, the absor- bance-pH plots at 1\=450 nm show a decrease in absorbance atpH 8. This is probably due to the conversion of the neutral molecules to the ionic form.
The curves obtained at '\=550 or 590 nm are similar in all the mixtures investigated except the dioxane medium, where increase inpH leads to an apparent
77rJ.
610 670 550 490 430
D)10.21'/,(.,/w) Dia.an.
370 430 490 550 610 670
1.0 0.9 0.8
0.1 0.3 0.2
550 610 490
430
A) 15.5'/0( WlW)Ethanol
370 0.8 0.1 0.6
0.2 0.1 0.2
~
u c0 0.1430610550670370490
.0 .Cl0III~
0.7'B) 12.3 '/,(w/w) Glyc.rol
<l:
Fig. 2 - Absorption spectra of QG in aqueous-organic solvent mixtures at differentpH.
in presen(:tor"'20'/.<w Iw) organic solvents
~=550om
5 6 7 8 9 10 ,', 12 "3 '4
pH
,.O!
0.1
II I,,
II ,, III
inpre~ence of~20·/. M/N>
',~,
of organic solvents
~=610nm.
3 4 5 6 7 e 9 10 I' 12
pH
'"
·m A
rs //( I
.•
1-0
I
0.8
•••u a0.6
.c~o
III
~ 0.5
Fig. 3 - Absorbance vspH plots; A, QG; B, NSQG [(a) methanol, (b) ethanol, (c) iso-propanol, (d) ethylene glycol, (e) glycerol,(f)acetone, (g) dioxane].
decrease in absorbance especially in presence of higher concentration of the solvent.
The lower parts of the absorbance-pH curves denote the equilibria between the protonated forms of the compound and the nonionic species. These results indicate that the predominant form of NSQG in acid solutions is the cationic species. The latter undergoes stepwise ionisation with increase in pH
ofthe medium. From the forgoing results, one can propose the following scheme (Scheme 1) for solu- tions of the different equilibria liable to exist in NSQG.
Determination ofpKa values - ThepKa1 and pKaa values of QG and NSQG were determined from the variation of absorbance vspH by applying the follow- ing methods: (i) The half-height method20 where
773
QG
Dielectric NSQG constant of the medium
organic solvent ~~ methanol 4.10
74.32 10.04 11.248.71 10.61
8.20 11.2410.438.659.9473.29
16.60 11.2110.678.909.9771.06
30.08 9.2110.721O.oI* 67.13
organic solvent
=
ethanol 3.8574.48 11.51S.479.94 10.52
7.76 11.1810.728.669.7673.63
15.90 11.2510.858.829.8771.69
29.90 11.5011.079.069.9568.13
organic solvent = isopropanol 3.98
74.58 10.839.009.89 10.92
8.43 10.8810.619.159.5673.81
16.44 11.0410.459.169.6372.06
19.77
68.75 9.88 11.349.31 10.62 organic solvent
=
ethylene glycol5.58
74.94 (lO·06)t9.04 11.05
11.00 11.04 ( 9.97)t74.579.23
11.70 11.0311.149.5610.0073.71
37.48
72.06 9.63 10.9810.989.52 organic solvent = glycerol6.30
74.88(9.82)t 9.89 11.12
11.30 11.109.8174.41(9.96):l:
24.60
73.36 10.00 11.0411.089.26 40.60
71.40 11.0510.649.019.48
"'pK values could not be determined.
toverall pK value.
organic solvent
=
acetone 4.1774.59 9.72 11.2410.608.45 7.80
73.83 9.64 10.9810.588.69 16.60
72.10 9.82 11.0010.619.12 30.12
68.86 10.03 11.109.32 10.80 organic solvcnt ,=dioxane 5.15
74.48 9.25 10.1810.1310.97 10.27
n60
9.40 10.4110.769.72 25.60
70.69
'"8.51 9.0810.54
% (wfw) of organic solvent
TABLE 1- MEAN VALUES OF pK1 AND pK2 FOR 1.5x 1O-.M QUINIZARIN GREEN (QG) AND ITs NON-SULPHONATED DERIVATIVE
(NSQG) IN AQUEOUS-ORGANIC SOLVENT MIXTURES.
quite apparent for alcohols and acetone and this in turn leads to decreased ionisation of the imino- hydrogen in QG or NSQG. Though glycerol a.nd ethylene glycol are less basic than water, they can 8')lvate the organic molecules and their ions more than water and hence increase the solvolysis constant.
This leads to a lower acid dissociation constant22•
Correlation ofpKa values with molecular structures - The pKa1 and pKa2 values of QG are lower than those of NSQG. This can be attributed to the electron withdrawing property of the sulphonic group in QG which decreases the electron density on the iminonitrogen and accordingly leads to easier ionisation. The lowerpKo. values of QG or NSQG as compared to 1,4-diamino-anthraquinone23 may be ascribed to the higher donating character of the arylamino-substituents in the former compounds
divalent anion!>(pH> 10.5) (pink violet) monoprotonated (pH 4 -6 )
( orange yellow)
"
diprotonated (pH <4) (pale yellow)
Scheme
I1lClrlOIICIlentanions(pH 8 -10 )
( red)
pKa is equal to thepH at half height of absorbance- pH curve, (ii) the limiting absorbance method2o,2l
where Eq. (1) is used,
pH=(pKa-l--Iog y)+log A-AoIAl-A ..(1)
~here Al is the limiting absorbance, A is the absor- il>anceat a givenpH and Ao is the absorbance at the
¢inimum of the curve. Thus,pKa can be determined trom the plot of logA-AoIAl--A against pH.
The mean values ofpKal andpK"2 as calculated py the above methods at different wavelengths are tiven in Table 1.
Effect of organic solvent 0/1pKa values -The pKa values obtained in aqueous solutionl5 are higher than those obtained in the presence of organic solvents Which thus favour ionisation of QG or its NSQG.
'The elimination of the proton is facilitated by in- creasing the [ethylene glycol] or [glycerol], whereas the increase in the concentration of methyl, ethyl and fso-propyl alcohols or acetone decreases ionisation.
'The increased ionisation in the case of the former solvents may be explained on the basis that they act as proton acceptors rather than donors, leading to an easier dissociation of the proton from the -NH groups. The decrease in ionisation in the presence of high concentrations of alcohols or acetone can be ascribed to the blocking of the n-electrons of the carbonyl groups by the solvent molecules which tender the excitation of the n-electrons more difficult, resulting in higher pKa values.
The plots of pKa against liD, are not strictly Jinear. This indicates that changes in pKa with isolvent concentration, though mainly governed by lthe dielectric constant, are influenced by solvent ibasicities too. The decrease in solvent basicity is 174
which increases the charge-transfer from the substi- tuent to the
c=o
group via the anthraquinone nucleus. On the other hand, the 1,4-dihydroxyan- thraquinone24 exhibits higher pKa values as com- pared to QG or NSQG. This may be attributed to the higher mesomeric interaction in the latter com- pounds and the effect of the stronger intramolecular H-bond in the l,4-dihydroxyanthraquinone.References
1. ALLEN,C. F., WILSON,C. V.&FARM,G.F., J. org. Chem., 7 (1942), 169.
2. Peter, R. H. & Summer, H. H., J. chem. Soc., (1953), 2101.
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4. SODA, Y., Bull. chem. Soc. Japan, 32 (1959), 1384.
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