Journal of the Pharmaceutical Society of Korea 18, 197-202 C1974)
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Dissociation Constant of the Prim ary Amines and Quaternary Ammonium-methylorang-e Salts.
B ak-K w ang K im
(Received May 8, 1974)
Abstract— —The data of dissociation constants for aliphatic amines and quaternary ammonium-methylorange salts are based on the electrostatic theory of conductance.
Dissociation constants for primary amines and quaternary ammonium-methylorange salts (1 X X 10~® M ) in nitrobenzene solution or water solution was evaluated from the relation of the concentration and the electric conductance at 25®C, Plots of AC against reciprocal conductance were linear; hence the center-to-center distance of this salt was
1.75 A in nitrobenzene solution.
I f the electrolytic materials, A * M O , are in eq u ilibrium w he n dissociated, the dissociation of A » M O can be show n as follows;
A * M O ^ A + + M O ~ (1)
T he dissociation constant and dissociation degree of salts are expressed by the equations
K = ( 7 a V ( 1 —a)C== a ^C / (1 —a ) (2)
a ~ A / A ^ (3)
W here K is the dissociation constant, a is the dissociation degree of salt.효 * S ub stitu tin g (2) w ith (3 ), we find the relations (4) an d (5 ).
T .
A^C
야그 or
A > = K G ^ - / L ) (5)
T hu s, as show n in F ig . 1, the A C vs. 1/A plot was straight line, and the dissociation constant was measured by the equation
K = - ( O X ) COY) (6)
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From the Department of Pharmaceutical Analytical Chemistry, College of Pharmacy, Seoul National University, Seoul, Korea.
197
Kim: Dissociation Constant of Amines Vol. 18, No. 3
T h e values of an d cell constant.
R E S U L T S
AC and 1/A were calculated by using the conductance, . A n d the dissociation constant of salts was calculated
m initial
from
concentration the equation (6 ). Results were illustrated in F ig . 2, F ig. 3 and Table I.
# Fig.
Table
2—Relationship of between AC and 1/A Fig. 3—Relationship of between /Cand \ jA (CHs (CH 2 %NH 2 m o in water). (CHsCCHs). N+CCHaVMO-in N.B.).
I —Differences in dissociation constants of primary amines and quaternary ammonium- methylorange salts in water phase or organic solvent phase.
Compounds
Kh2 0 Kn.b.Butylamine-Methylorange 8.11x10-3
Hexylamine-Methylorange 3.6 xlO-3
Octylamine-Methylorange 0.45x10-3
Laurylamine-Methylorange 0.06x10-3
BTA-Methylorange 8.45x10-3 2.16x10-2
HTA'Methylorange 3.0 xlO-3 3.44X10'^
OT A-Methylorange 1.3 xlO-3 5.05x10"^
LTA-Methylorange 0.1 xl0~3 9.1 xlO-3
BTA; Butyltrimethylammonium. HTA： Hexyltrimethylammonium.
OTA; Octyltrimethylammonium. LTA: Lauryltrimethylammonium. %
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September, 1974 J. Pharm. Soc. Korea 199
T he increase of temperature gives an increm ent in conductance w he n the co ncen tration of salts was in constant. B u t those results from F ig . 4 and F ig. 5 have little difference in their dissociation constant.
출 M o'M 11-*)
Fig. 4—Plot of against l/A for CsHitN (CHs )3 MO at 20% 25°. 30° and 35®.
F ig. 5—Plot of AC against 1/A for C 6 H 13 NH 2 MO at 2(T, 25% 30" and 35°.
It has become apparent in recent years that, w hile Bjerrum^s theorj최 o f ion-pair dissociation is quite adequate for a given solvent system, the variation of the dissociation constant for an ion-pair from solvent to solvent indicates an in a b ility o f the theo ry to describe experim ental results completely. M o st of these samples have different dissociation constants in nitrobenzene solvent h a v in g dielectric constant of 34. 82. T he ratios o f th e dissociation constants were given in T able I.
According to Bjerrum , the reciprocal of the dissociation constant is given by
^ \ 3
K- d R t ' ) (7 )
W h e re b = e ^ /a D K T and values of Q ( 스) have been tabulated as a func tio n o f b냐 , I n F ig . 6, QC&) is plotted against b.
# Fig. 6—Plot of Q O ) against
o*
TROBEN
li II
0-']
Kim: Dissociation Constants of Amines Vol. 18, No. 3
If the ions are not separated by solvent molecules in the ion pair스 the distance of closest approach a of tw o ions should not vary w ith the solvent b u t they are actually in contact.
I f this latter assumption®^ is m ade, Bjerrum^s K is only a function of D T alone. So it is th o u g h t that there are specific solvent effects other than that due to the dielectric constant.
In a nitrobenzene-carbon tetrachloride m ix ture, it has been fo u n d ®우 that the electrostatic model of the sphere-in-a-continuum of B jerrum fails for tetra-«-butylam m onium brom ide.
Fuoss hypothesized about the B jerrum 's model th at the approach of tw o ions continues u n til they are separated by only one layer of solvent molecules. T h is fact, the breakdown of the latter allow s the intim ate contact of ions, is assumed to be a discrete process, describable by a constant Kg. H e then obtains an expression for the dissociation constant as K = K b / ( l + K a).
Fuoss did not indicate any inform ation for calculating the constant Kg. I t w ill be shown that, attacking the problem from a microscopic point of view , an equation for K m ig h t be derived, w hich could account for the above relationship. In equation (7 ), the reciprocal of the dissociation constant is in proportional to Q ( 스). T he value a, the distance of closest approach, was calculated by using the dissociation constants.
Results obtained for C4H9N(CH3)3MO, C 6 H j 3 N ( C H 3)3 • M O , CgHx7N(CH3)3 • M O and C i 2 H 2 s N (C H 3)3 • M O are presented in Table II.
Table I I — Some parameters of quaternary ammonium-methylorange salts in nitrobenzene solution.
Compounds K Q W b a
Butyltrimethylammonium-methylorange 2. 00 X 10'^ 15.36 12.08 1.33 Hexyltrimethylammonium-methylorange 3.42x10-2 9.09 11.41 1.41 Octyltrimethylammonium-methylorange 5 . 7 5 x 10-3 5.33 10.33 1.56 Lauryltrimethylammonium-methylorange 9.48x10-3 3.13 9.20 1.75
N o w , considering the size of cation and anion used in the experiment, the values of a are too sm all. How ever, w hen the value of conductance^^ is considered, the theoretical value of conductance shall n o t be so large.
T he log arithm of the dissociation constant of a num ber of different salts is plotted against the m ethylene num ber of alkyl groups. T he results for aliphatic am ine and quaternary a m m o n iu m - M O salts were represented linear relationship.
In w ater solution, the dissociation constant for aliphatic am ine and quaternary am m o n iu m salts decreases w ith increasing the m ethylene num ber of alkyl group. B u t, in organic solution, the dissociation constant for salts increases w ith increasing the methylene num ber of alk yl groups.
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September, i9^4 j. Pharm. Soc. Korea 20 i
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F i g . 7 —— P l o t o f l o g K f o r p r i m a r y a m i n e a n d q u a t e r n a r y a m m o n i u m s a l t s a g a i n s t m e t h y l e n e n u m b e r o f a l k y l r a d i c a l s .
( x ) o r g a n i c s o l u t i o n s o f C H3 ( C H 2) n ■ N ( C 1 1 3 ) 3 * M O ( • ) w a t e r s o l u t i o n o f C H3 ( C H 2) » N H g ' M O
( « ) w a t e r s o l u t i o n o f
CH3 (CH2)«N(CH3)3-MO
( « ^ 3 , 5 , 7 . 1 1 ) ( I ) p r i m a r y a m i n e( I V ) q u a t e r n a r y a m m o n i u m s a l t
E X P E R IM E N T A L
R e a g e n ts a n d In stru m en ts— Prim ary am ine and quaternary am m onium - m ethylo rang e salts were prepared by using autoclave. Nitrobenzene was purified by distillatio n; its co n ductance was approxim ately O .S X K T ^ . T he solvent was stored in colored bottle.
A p p a r a tu s — T he measurement of conductance®?®^ for alip hatic am ine an d qu ate rnary am m onium -m ethylorange salt was performed by using conductom etry페 , attached to a therm ostated w ater bath m aintained at 25소 0 . 5°. T he absorption of the alip h atic a m in e or quaternary am m onium -m ethylorange salt was measured by m eans of a S h im a d z u spectrophotometer (M odel Q B 50) in the visible range.
A ll the salts were studied at approxim ately 430 n m in organic solution or appro x im ately 466 n m in a water solution.
M easurem en t o f C o n d u c ta n c e — l x l 0 ~ ^ ~ l x l 0 ~ ^ m o l// of the w ater solution or organic solution of aliphatic am ine and quaternary am m onium -m etliylorange salts were used in m easuring the conductance, and the temperature of a therm ostated w ater b a th was 2 5 + 0 .5 °.
E ffe ct o f T e m p e r a t u r e T he conductances of hexylam ine an d octyltrim ethylam m o niu m -
m ethylorange salts in water solution or organic solution were measured at tem peratures
20% 25°, 30° and 35% respectively.
, , ,
