2
Isotopic Exchange between Hydrogen and
Liquid Ammonia Catalyzed by Alkali
Amides
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ROBERT
DELMAS,
P I E R R E C O U R V O I S I E R , and
Section de Chimie des I s o t o p e s — D P C / S I S , 91—GIF-sur-YVETTE—France
The
catalytic
between
action of alkali amides
hydrogen
and liquid
gated.
It was
change
is first order
dissolved
clear
before
culation
of the
sodium,
potassium,
results
are interpreted
-base catalysis
and
S a c l a y — B . P . n ° 2,
exchange
has been
reinvesti-
that the rate
to the
of ex-
concentration
of the catalytic
coefficients
cesium
have
according
of the been
to the theory
by Bell, the conclusion
ion acts as a catalytic
/ ^ l a e y s , Dayton, and Wilmarth
If
calof the
of the acid-
is that only
species.
with an associative
amides
made.
of
species
A kinetic study and a careful
dissociation
given
are in agreement
^
respect
but the nature
was still under discussion.
RAVOIRE
on the isotopic
ammonia
this work
with
hydrogen,
free amide
C.E.N.
JEAN
The kinetic
the data
mechanism.
have s h o w n that potassium
(4)
d i s s o l v e d i n l i q u i d a m m o n i a is a c a t a l y s t f o r t h e o r t h o - p a r a
amide conver-
sion of hydrogen a n d for the isotopic exchange between molecular h y d r o gen a n d l i q u i d ammonia.
T h i s last r e a c t i o n m a y b e w r i t t e n i n t h e r a n g e
of l o w deuterium concentration: NH
3
+ H D *± N H D + H 2
W i l m a r t h a n d D a y t o n (18)
2
(1)
e s t a b l i s h e d t h a t t h i s r e a c t i o n is h o m o -
geneous a n d t h a t t h e r a t e of e x c h a n g e is first o r d e r w i t h respect t o t h e c o n c e n t r a t i o n of d i s s o l v e d h y d r o g e n .
I n o r d e r to e s t a b l i s h t h e c a t a l y t i c
a c t i o n of p o t a s s i u m a m i d e , t h e y t o o k i n t o c o n s i d e r a t i o n its d i s s o c i a t i o n according to KNH = l o g i ; - t •—LJLJLJI + B i 1 + Ba\/ /x r
0
(12)
where: t\, = *'o[S][C]
(13)
/x is the i o n i c strength, B' is a constant c h a r a c t e r i s t i c of the m e d i u m , a n d Z a n d Z are the electric charges of S a n d C . s
c
T h e e q u a t i o n is s t i l l v a l i d w h e n C or S is a n e u t r a l m o l e c u l e i f the e l e c t r i c a l interactions b e t w e e n a n i o n a n d a n e u t r a l m o l e c u l e are n e g lected. I n o u r p a r t i c u l a r case the substrate, H , is a n e u t r a l m o l e c u l e a n d 2
the t e r m ^ ^ Z c V j * vanishes. S i n c e the i o n i c strength a l w a y s r e m a i n s 1 + BaVn
Spindel; Isotope Effects in Chemical Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
34
ISDTOPE E F F E C T S IN C H E M I C A L PROCESSES
s m a l l ( b e l o w 0.01), it is reasonable to consider the t e r m B'/x as n e g l i g i b l e . F i g u r e 5 shows a p l o t of k as a f u n c t i o n of [ N H " ] 2
[NH "] 2
0
0
at
—45.2°C.
is the c o n c e n t r a t i o n of a m i d e ions c a l c u l a t e d w i t h o u t t a k i n g
i n t o a c c o u n t t h e presence of t r i p l e ions, as e x p l a i n e d p r e v i o u s l y , i n d e p e n d e n t of the c a t i o n a n d p r o p o r t i o n a l to [ N H ~ ] . 2
k is
T h e linear rela-
0
t i o n s h i p is v a l i d i n the w h o l e r a n g e of s o l u b i l i t y i n the case of s o d i u m a n d u p to a b o u t 1.5 X 1 0 " M a n d 5 X 1 0 " M i n the case of c e s i u m a n d 3
3
p o t a s s i u m respectively. T h e d e v i a t i o n a r i s i n g a b o v e these concentrations is a t t r i b u t e d to the presence of the t r i p l e ions, M N H 2
2
+
, w h i c h , as this
has b e e n s h o w n , l e a d to a n increase i n the c o n c e n t r a t i o n of the a m i d e i o n . T h e n , the k i n e t i c l a w m a y be w r i t t e n : Downloaded by TUFTS UNIV on October 28, 2017 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0089.ch002
fc = * [ N H " ] . 0
(14)
2
Figure 5. k as a function of the concentration of the amide ion at —45.2°C. # from sodium amide, Q from potassium amide, • from cesium amide T h i s l a w a l r e a d y has b e e n r e p o r t e d i n b r i e f b y the authors
(5).
A c o n f i r m a t i o n of the fact that the c a t a l y t i c a c t i o n is c a u s e d b y N H " 2
alone has b e e n f o u n d b y experiments o n c o m m o n i o n effects a n d seconda r y salt effects.
If p o t a s s i u m b r o m i d e is a d d e d to a s o l u t i o n of p o t a s s i u m
a m i d e , the c o n c e n t r a t i o n of a m i d e ions changes for t w o reasons.
By
s e c o n d a r y salt effect, the i o n i c strength increases a n d / decreases a c c o r d i n g to the D e b y e - H u c k e l e q u a t i o n ( E q u a t i o n 8 ) .
I n E q u a t i o n 7 of the
Spindel; Isotope Effects in Chemical Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
2.
Isotopic
DELMAS E T A L .
35
Exchange
association constant of p o t a s s i u m a m i d e , [ N H ~ ] , increases. B u t b y c o m 2
m o n i o n effects, the a d d i t i o n of p o t a s s i u m ions f r o m p o t a s s i u m b r o m i d e leads to a decrease i n the c o n c e n t r a t i o n of a m i d e ions. T h e association constant of p o t a s s i u m b r o m i d e b e i n g k n o w n , c a l c u l a t i o n s are possible. T h e p r e c i s i o n is not v e r y g o o d because the c a l c u l a t i o n s h a v e b e e n m a d e a b o v e the range of v a l i d i t y of the D e b y e - H u c k e l e q u a t i o n a n d , moreover, the presence of t r i p l e ions has b e e n i g n o r e d . B u t i t m a y b e satisfactorily estimated for this purpose.
C a l c u l a t i o n s s h o w a slight decrease i n the
c o n c e n t r a t i o n of a m i d e ions. A s l i g h t decrease of k is also observed.
The
same k i n d of c a l c u l a t i o n s a n d measurements h a v e b e e n m a d e w i t h s o d i u m
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a m i d e , the salt b e i n g s o d i u m c h l o r i d e .
I n this case, the c o m m o n
ion
effect is m o r e i m p o r t a n t t h a n the secondary salt effect a n d k decreases m a r k e d l y . T a b l e I shows the results o b t a i n e d at — 45.2 ° C . Table I. Decrease of [NH ~] after addition of salt
Decrease of k ratio 1.16
2
Catalyst
Salt
0
KNH 0.019M
KBr 0.13M
ratio 1.4 ± 0.2
NaNH 0.010M
NaCl 0.15M
ratio 2,5
ratio 2,2
NaNH 0.015M
NaCl 0.075M
ratio 1,3
ratio 1,6
2
2
2
Energy and Entropy
of
Activation
A c c o r d i n g to the t r a n s i t i o n state theory, k m a y be w r i t t e n : Q
, K
=
^
eRT AS* * - R -
e
X
e
E * - R T
a
X
( 1 5 )
where R is the gas constant, T is the absolute t e m p e r a t u r e , N is A v o g a d r o ' s n u m b e r , A S * is the e n t r o p y of a c t i v a t i o n , and E
a
is the energy of a c t i v a t i o n .
T h e energy of a c t i v a t i o n , c a l c u l a t e d b e t w e e n — 4 5 . 2 ° C . a n d —70.0°C. is 5.5 ± 0.2 k c a l . / m o l e . T h e difference w i t h the v a l u e g i v e n b y B a r - E l i
Spindel; Isotope Effects in Chemical Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
36
ISOTOPE E F F E C T S
a n d K l e i n ( I ) , 7.4 ±
IN C H E M I C A L PROCESSES
0.3 k c a l . / m o l e , is o w i n g to the different values u s e d
for the s o l u b i l i t y of h y d r o g e n .
T h e t e m p e r a t u r e coefficient w e h a v e u s e d
is h i g h e r , a n d therefore the t e m p e r a t u r e coefficient of k is smaller. T h e e n t r o p y of a c t i v a t i o n c a l c u l a t e d at — 45.2 ° C . is —18 =t 1 e.u. T h e difference b e t w e e n o u r v a l u e a n d the v a l u e g i v e n b y B a r - E l i a n d K l e i n ( J ) , —9.2 e.u., is m a i n l y because of the difference i n the values of the energy of a c t i v a t i o n . Mechanism
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W i l m a r t h a n d D a y t o n (18), D i r i a n , Botter, Ravoire and Grandcollot (7)
h a v e p r o p o s e d a dissociative m e c h a n i s m slow N H " + H D ±5 N H D + H " 2
(16)
2
H" + N H
3
fast -» NH ~ + H 2
(17)
2
w i t h the i n t e r m e d i a t e f o r m a t i o n of a h y d r i d e i o n . M a n y a r g u m e n t s are against this m e c h a n i s m . A s s h o w n b y B a r - E l i a n d K l e i n ( J ) , the h y d r i d e i o n f r o m L i H is not a catalyst for this r e a c t i o n . I n the s i m i l a r case of w a t e r , S c h i n d e w o l f (16)
has s h o w n that the h y d r i d e
i o n does not a p p e a r i n a s o l u t i o n of p o t a s s i u m h y d r o x i d e i n w a t e r .
The
k i n e t i c i s o t o p i c effect o b s e r v e d is also i n c o n t r a d i c t i o n to a dissociative mechanism ( I ) .
T h e l a r g e l y negative v a l u e of the e n t r o p y of a c t i v a t i o n
is a n a r g u m e n t for a h i g h l y o r g a n i z e d a c t i v a t e d c o m p l e x . W e t h e n p r o p o s e d , first, a m e c h a n i s m close to the one a l r e a d y g i v e n by B a r - E l i and Klein ( I ) .
A s s h o w n b y c o n d u c t i m e t r i c d a t a , N H ~ is 2
solvated. A N — H b o n d of the s o l v a t i o n a m m o n i a m o l e c u l e is p o l a r i z e d by N H ~ and then weakened.
T h e exchange occurs b e t w e e n
2
a
HD
m o l e c u l e a n d the solvated N H ~ i o n , t h r o u g h a f o u r center m e c h a n i s m , i n 2
the f o l l o w i n g w a y : H
H 8 8"j . . H — N — H ^>
HH | 8 8~ | H — N .. . H — N — H +
+
H—N\
+ D—H
D...H H
H"
->H—N| + H + D
IN—H
H
Spindel; Isotope Effects in Chemical Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
(18)
2.
Isotopic
DELMAS E T A L .
37
Exchange
A n o t h e r m e c h a n i s m is also possible. T h e s l o w step is a n associative exchange b e t w e e n N H ~ a n d H D , a n d it is f o l l o w e d b y the
exchange
2
b e t w e e n N H D " a n d N H , w h i c h is k n o w n to b e v e r y fast f r o m n u c l e a r S
m a g n e t i c resonance
(15):
H" H—N| + H—D
H ( s o l v
.)
^
H .. . N
Jtt" ( s o l v
' H + |N|
)
H
H. . .D
( s o l v
.
(19)
}
D
N H D " + N H -> N H - + N H D Downloaded by TUFTS UNIV on October 28, 2017 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0089.ch002
3
2
(20)
2
T h i s m e c h a n i s m is i m p o s s i b l e i n the case of the exchange
between,
for instance, h y d r o g e n a n d d i m e t h y l a m i n e , exchange w h i c h is m u c h faster t h a n the exchange
between hydrogen
( 2 , 14).
In contradiction w i t h
B a r - E l i a n d K l e i n , the o p i n i o n of the authors is that w i t h a m m o n i a a n d p r i m a r y amines this m e c h a n i s m m a y c o n t r i b u t e to the exchange to some extent. A n a r g u m e n t w h i c h supports the o c c u r r e n c e of this m e c h a n i s m is t h a t a n isotopic exchange non-solvated potassium amide
does o c c u r b e t w e e n
hydrogen and solid
(17).
Appendix Kinetic
Treatment
L e t us consider a h o m o g e n e o u s system c o m p o s e d of l i q u i d a m m o n i a , d i s s o l v e d h y d r o g e n a n d d i s s o l v e d catalyst. B e c a u s e of a n i m p o r t a n t isot o p i c effect, R , the rate of exchange of the H and ammonia
(expressed
i n atg. c m . "
3
atoms b e t w e e n h y d r o g e n
1
m n . " ) , cannot b e 1
calculated.
H o w e v e r , since o n l y traces of d e u t e r i u m are present, i t is possible c a l c u l a t e Rpu w h e r e p
l9
to
takes i n t o a c c o u n t the isotopic effect a n d is c o n -
stant at constant t e m p e r a t u r e . T h e n , the f o l l o w i n g e q u a t i o n is v a l i d : In y^ZJk y-t/
^ r j _ + 1 |_[H ] ] is the c o n c e n t r a t i o n of the H atoms b e l o n g i n g to the H m o l e c u l e s , i n atg. c m . " , 3
Spindel; Isotope Effects in Chemical Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
2
38
ISOTOPE E F F E C T S IN C H E M I C A L PROCESSES
[ N H ] is the c o n c e n t r a t i o n of the H atoms b e l o n g i n g to the 3
NH
3
m o l e c u l e s , i n atg. c m . " , 3
a is the separation factor b e t w e e n a m m o n i a a n d h y d r o g e n a n d is e q u a l to ^ ? ^ ^
N
N
at e q u i l i b r i u m ,
H 8 ( I )
and t is the t i m e i n m i n u t e s .
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Since N H
3
is i n m u c h larger a m o u n t t h a n H , E q u a t i o n 21 becomes 2
l
n
y ^ y-y*
*Pi
=
t
( 2 2 )
H
[ 2]
T h e a c t u a l k i n e t i c l a w is of the f o r m : R
=
P l
fc [H ] 0
[catalyst]"
2
(23)
Because of the w e l l - e s t a b l i s h e d first o r d e r r e l a t i o n w i t h respect to the c o n c e n t r a t i o n of h y d r o g e n ,
[H ]
no longer appears i n the k i n e t i c
2
equations a n d k is defined as (24)
k = ^ = fc [catalyst]" [H J 0
2
k appears as a p s e u d o rate constant.
( N o t e K i n E q u a t i o n 24 is e q u i v a l e n t
to k' i n References 1 a n d 18 a n d K i n Reference
7.)
k w o u l d b e d i r e c t l y c a l c u l a t e d i f the system was a c t u a l l y h o m o g e n e ous.
T h e presence
of gaseous h y d r o g e n , at a l l t i m e i n isotopic
equi-
l i b r i u m w i t h d i s s o l v e d h y d r o g e n , results i n r e p l a c i n g E q u a t i o n 22 b y
y-ye in which [ H ] 2
g
H
[ 2]
g
is a p s e u d o - c o n c e n t r a t i o n of h y d r o g e n , e q u a l to the t o t a l
a m o u n t of h y d r o g e n d i v i d e d b y the v o l u m e of l i q u i d a m m o n i a ,
k' is
defined as
* = - n r r -
(
2
6
)
k' c a n b e d i r e c t l y c a l c u l a t e d a n d is r e l a t e d to k b y Rp = 1
fc[H ]=fc'[H ] . 2
2 g
(27)
P r a c t i c a l l y , k is d e t e r m i n e d first a n d k is c a l c u l a t e d f r o m f
^
total amount of hydrogen amount of dissolved hydrogen'
Spindel; Isotope Effects in Chemical Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
^
2.
DELMAS E T A L .
Isotopic
Exchange
39
Acknowledgments T h e authors w i s h to a c k n o w l e d g e the generous assistance of P . G r a n d collot i n w r i t i n g i n t h e c o m p u t e r p r o g r a m , F . B o t t e r , I . L a m b e r t , M . M a g a t , E . R o t h , G . D i r i a n , a n d E . R o c h a r d f o r m a n y h e l p f u l discussions.
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Literature
Cited
(1) Bar-Eli, K., Klein, F . S., J. Chem. Soc. 1962, 1378. (2) Ibid., 1962, 3083. (3) Bell, R. P., "Acid-Base Catalysis," Oxford University Press, London, England, 1949. (4) Claeys, Y., Dayton, J. C., Wilmarth, W. K., J. Chem. Phys. 18, 759 (1950). (5) Delmas, R., Courvoisier, P., Ravoire, J., J. Chim. Phys. 62, 1423 (1965). (6) Delmas, R., thesis, Paris (1967). (7) Dirian, G . , Botter, F . , Ravoire, J., Grandcollot, P., J. Chim. Phys. 60, 138 (1963). (8) Franklin, E . C., Z. für Phys. Chem. 69, 290 (1909). (9) Fuoss, R. M . , J. Am. Chem. Soc. 81, 2659 (1959). (10) Fuoss, R. M . , Accascina, F . , "Electrolytic Conductance," Interscience Publishers, Inc., New York, N . Y., 1959. (11) Hawes, W . W., J. Am. Chem. Soc. 55, 4422 (1933). (12) Nief, G . , Botter, R., "Advances in Mass Spectrometry," Waldron, Pergamon Press, London, 1959. (13) Ravoire, J., Grandcollot, P., Dirian, G., J. Chim. Phys. 60, 130 (1963). (14) Rochard, E . (private communication). (15) Swift, T. J., Marks, S. A., Sayre, W . G., J. Chem. Phys. 44, 2797 (1966). (16) Schindewolf, U . , Ber. Bunsenges, Phys. Chem. 67, 219 (1963). (17) Schindewolf, U . (private communication). (18) Wilmarth, W. K., Dayton, J. C . ,J.Am. Chem. Soc. 75, 4553 (1953). R E C E I V E D August 28,
1967.
Spindel; Isotope Effects in Chemical Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.