The previous formulation of thc âiiew speciesâ found in aqueous solutions of 1-inetliylpyridiniun~ iodides as ... formation cdn be derived from a study of the varia-.
5838
VOl. 7s
E. LM. KOSOWER A N D J. C. BUREACH [CONTRIBL‘TION FROM THE
DEP.4RTMEST
O F CHEMISTRY,
LEHIGH
USIVERSITY]
Equilibrium Constants for Pyridinium Iodide Charge-Transfer Complex Formation’ BY
EDWARD
A’I.
K O S O W E R ’ .4ND JOIIN
e. BURB.’Cl13
RECEIVED JUSE 14, 1026 The previous formulation of thc “iiew species” found in aqueous solutions of 1-inetliylpyridiniun~iodides as clixge-transfer complexes is fiirtlier confirmed by a study of the equilibrium constants for complex formation. While the K for association [ i f 1-niethylpyridiniuni and iodide ions is 2.3 i 0.3 1. mole-’, the three additional methyl groups present in 1,2,4,6-tetraniethylpyridiiiium ion only reduce the K for complexing with iodide ion to 1.83 i 0.3 1. mole-’. From comparison with the data fur complexing and addition to benzene derivatives ( T- and u-complexes), it is clear that the present case fits the patterti expected for “loose” bonding, or charge-transfer complex formation. In ethanol, there is considerable association t o ion-pairs and 1,2,4,6-tetraniethylpyridinium iodide forms complex in accordance with Beer’s law over certain concentration ranges. .i comparison of “absorption” curves for the complex in ethanol and xvater indicates that the absorption band is shifted to longer wave leiigtlis in solvents oE lower solvating ability, as had bccii suggested previously.
The unusual ultraviolet absorption properties of 1-inethylpyridiniuni iodide solutions in water4 have been attributed to the presence of a chargetransfer comp1ex.j The present paper confirms the interpretation of the “new species”4 as a coniplex and reports several other interesting results. Equilibrium Constant Determinations.---Keefer and Andrews6 and Ross7 have given an equation from which the equilibrium constant for complex formation cdn be derived from a study of the variation in ultraviolet absorption with concentration. The equation for the equilibrium of interest here (1) is given in cq. 2.
”I ‘t
+
4 , 1-
--+ c--
*
\N
.y/ CH3
k+
S,Il
=
L’KE
+
1-
(1)
CtII 1’E
(2)
where S = c,,? (L)C,, - z ) , Z = 1 ( 2 -~ z )~, co~is the initial concentration of the pyridinium iodide, z is the concentration of the complex, E the absorption coeflicient for complex, K the equilibrium constant for complex formation and D the optical density due to complex. Preliminary calculations revealed that equilibrium (1) was sensitive to the ionic strength of the medium. I t was therefore necessary to make measurements of D a t different ionic strengths, followed by extrapolation to a particular ionic strength. Lithium perchlorate was used as the inert salt and pl’*= 0.1 ( f i = 0.01) was chosen as the “standard” ionic strength. Three of the pyridiniuin iodides used previouslya were examined in this work : 1-niethylpyridiniuni iodide 111PI1 1 2.6-trinie th vlDvridiniuni iodide (TMPI) arid I ,2,4,G-tetranietl;;Ipyridiniuiii iodide
(nmq.
alcohol and methyl alcohol) atid 1,2,3,G-tetran~etIi}lpyridiniuin perchlorate, m .p. 204.5-205.5’ (reported 206-1’07 O s ) (recryst. from isopropyl alcohol), were prepared from the corresponding iodides and silver perchlorate.6 Lithium perchlorate ( G . F. S m i t t Co.) was washed with ether and air-dried; m.p. 94.5-96.0 . A solution containing approximately 0.005 df lithium thiosulfate and 0.05 Id lithium chloride was prepared, by grinding sodium thiosulfate pentahydrate with lithiuin chloride, leaching the pasty mass with absolute ethanol and diluting to the desired concentration with absolute ethanol. Spectral Studies.-Solutions of pyridinium iodides in M ) condilute aqueous sodium thiosulfate ( c a . 5 X taining various quantities of lithium perchlorate were prepared. Their ultraviolet absorptions (as D )were measured with a IVarren Spectracord. A sample plot of D veysus for hIPI a t 3100 A. is given in Fig. 1. Solutions of hlPI and T e M P I in ethanol coiltailling lithium thiosulfate were also studied. The data for TeMPI are presented in Fig. 2 in the form of a D versus co plot. The corrections for the ultraviolet absorption due to the pyridinium ions themselves were obtained by esatiii1ii11g solutions of the appropriate perchlorate salt solutions. At
Experimental
The ioditlcs aucl 1-methvlpyridiniuni perchlorate have bcen reported .: 1,2,6-Trimcth~~lpyriditiiumperchlorate, r1i.p. 19L’.t5-193.50 (recryst. from a mixture of isopropyl .
(1) Pyridinium Rings. I V . T h e previous article in this series was I’. AT. Knsower, THISJ O U K N A I , , 78, 3197 (1930). (2) Ilepartment of Chemistry, University of Wisconsin, SIadison fi,
( 3 ) T L L L V I Iiii p a r t Irorn a lIicii\ sul,mitteil b y 1. C . H i o I ) l i i l i i l i ~ ~ l t i l l n l c nolf t h e reiliiircincnts lor t h e R1.S. clrgrre. ( 4 ) 1;. \ I , Kosower, 1’111sJ O U K K A I , , 77, 3983 (IQ5.j). ( 5 ) 1;. 11. Kosower and P. E. Klinedinst, Jr., ibid., 78, 34‘33 (1930). (1;) ll. X I , Keeler and L. 5. Andrews, ;bid,. 7 4 , 1891 (1952). ( 7 ) S U I
