Isotopic Exchange between Hydrogen and Liquid Ammonia Catalyzed

2

Isotopic Exchange between Hydrogen and

Liquid Ammonia Catalyzed by Alkali

Amides

Downloaded by TUFTS UNIV on October 28, 2017 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0089.ch002

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


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


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


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


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


(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


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 .


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'


^

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.


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.


Isotopic Exchange between Hydrogen and Liquid Ammonia Catalyzed

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