64
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Ozonation of Amines PHILIP S. BAILEY, HAROLD M. WHITE
JOHN
E.
KELLER,
DAVID
A.
MITCHARD,
and
University of Texas at Austin, Austin, Tex. 78712
A review of the literature concerning reactions of ozone with amines has given rise to the working hypothesis that these reactions all involve initially the electrophilic attack of ozone to give an adduct for which at least three different reaction routes are available: loss of molecular oxygen to give an amine oxide or further reaction products thereof, an intramolecular oxidation of a side chain, and dissociation to nitrogen cation radicals and the ozonate anion radical, followed by further reactions of these. Results from ozonations of tert-butylamine and tri-n-butylamine, which furnish additional evidence for the above competitions, are described. >Tphe literature suggests several w a y s i n w h i c h ozone reacts w i t h amines. T h e best k n o w n of these is the o z o n a t i o n of t e r t i a r y amines to a m i n e oxides ( I I )
(I).
H e n b e s t a n d S t r a t f o r d (11)
a n d S h u l m a n (17)
have
s h o w n that c o m p e t i n g w i t h this is an ozone attack o n the a l p h a p o s i t i o n of an a l k y l side c h a i n to p r o d u c e various d e c o m p o s i t i o n p r o d u c t s of III. H e n b e s t (11)
s h o w e d that a m i n e oxide f o r m a t i o n is f a v o r e d i n c h l o r o
f o r m a n d m e t h a n o l , w h i l e side c h a i n o x i d a t i o n is p r e d o m i n a n t i n h y d r o c a r b o n solvents.
A l s o of c o n s i d e r a b l e interest are the r e p o r t e d conver
sions, d u r i n g o z o n a t i o n , of p h e n y l e n e d i a m i n e s to W u r s t e r ' s salts
(VII)
(8, 14), of l i q u i d a m m o n i a to a m m o n i u m ozonate ( V A ) at a l o w t e m perature
(18),
a n d of amines to a m i n e h y d r o c h l o r i d e s ( V B )
n a t e d h y d r o c a r b o n solvents (17, 19). azobenzene
i n chlori
F i n a l l y , a n e a r l y report states that
a n d q u i n o n e are o b t a i n e d u p o n o z o n a t i o n of a n i l i n e
(i5).
T h e e x p e c t e d i n i t i a l r e a c t i o n b e t w e e n ozone a n d amines w o u l d b e a n e l e c t r o p h i l i c ozone attack to g i v e a d d u c t I ( R e a c t i o n 1) ( I ) .
A similar
a d d u c t has a c t u a l l y b e e n o b s e r v e d i n the r e a c t i o n b e t w e e n the n u c l e o p h i l e t r i p h e n y l p h o s p h i t e a n d ozone (20). the reactions
It o c c u r r e d to us that a l l of
d e s c r i b e d above c o u l d be e x p l a i n e d b y c o m p e t i n g fates 58
In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
64.
BAILEY
ET
Ozonization
AL.
of
59
Amines
of a d d u c t I. R e a c t i o n 2 represents the g e n e r a l l y a c c e p t e d d e c o m p o s i t i o n to a n a m i n e o x i d e a n d o x y g e n ( 1 ) , a n d R e a c t i o n 3 is a possible course of the c o m p e t i n g side c h a i n o x i d a t i o n (12).
R e a c t i o n 4 i n v o l v e s the dis
s o c i a t i o n of the a m i n e - o z o n e a d d u c t to n i t r o g e n c a t i o n r a d i c a l s
(IV)
a n d the ozonate a n i o n r a d i c a l ( A ) . T h e n i t r o g e n c a t i o n r a d i c a l f r o m a
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p h e n y l e n e d i a m i n e w o u l d be resonance s t a b i l i z e d ( V I I ) ;
+ -
R N:
+
77
(i)
R °o N — O — O — O :
+ :'6=d—6:
3
....
that f r o m a n
i
+ ft f-
+ R N—O + 3
R N—O—O—O 3
0
(2)
2
II
la O—O—OH R N
R N=CHR'
CHR'
2
2
OH
I
—
R N—CHR + 0 2
lb
(3)
2
III 4-
RoN—O—O—O
3
Ic
+
NHR
3
,
:
IV
* or CHC1, N
R
+
R N'
H
-6-6-6:
Rt 3
O—O—O:
(4)
A
£BU
a
(5)
IV
(6)
OH +
VII
In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
60
OXIDATION
a l i p h a t i c a m i n e or a m m o n i a
(IV),
OF
ORGANIC
COMPOUNDS
III
h o w e v e r , m u s t s t a b i l i z e itself
by
a b s t r a c t i n g a h y d r o g e n a t o m f r o m another a m i n e or a m m o n i a m o l e c u l e , or f r o m the solvent ( R e a c t i o n 5 ) . A p o s s i b l e e x p l a n a t i o n for the r e p o r t e d a n i l i n e results i n v o l v e s c o m p e t i n g reactions of the t w o c a n o n i c a l f o r m s of the a n i l i n e c a t i o n r a d i c a l ( R e a c t i o n 6 ) . I n o r d e r to o b t a i n e v i d e n c e f o r or against these ideas w e h a v e b e g u n a n intensive s t u d y of the reactions of ozone w i t h a v a r i e t y of amines.
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T h i s p a p e r s u m m a r i z e s results f r o m the o z o n a t i o n of
tertf-butylamine
and
of t r i - n - b u t y l a m i n e , w h i c h f u r n i s h e v i d e n c e for the c o m p e t i t i o n s
indi
c a t e d b y Reactions 2, 3, a n d 4.
A d d i t i o n a l details w i l l b e p u b l i s h e d
elsewhere. I n a t y p i c a l r u n , 5 m m o l e s of
terf-butylamine
i n chloroform reacted
w i t h 4.1 m m o l e s of ozone i n a n o z o n e - n i t r o g e n stream at a b o u t — 6 0 ° C . to g i v e 0.9 m m o l e ( 1 8 % ) ( 1 8 % ) of
terf-butyl
butylammonium
of 2 - m e t h y l - 2 - n i t r o p r o p a n e
( X I ) , 0.9 m m o l e
isocyanate ( X V I ) , a n d 3.0 m m o l e s ( 6 0 % ) of tert-
chloride ( X I V ) .
I n a separate b u t s i m i l a r r u n ,
3.5
m m o l e s of m o l e c u l a r o x y g e n w e r e d e t e r m i n e d as a p r o d u c t . U n d e r the conditions
of
terf-butylamine
the
occurred
between
a n d c h l o r o f o r m i n the absence of ozone or
reaction
no
significant r e a c t i o n
between
ozone a n d c h l o r o f o r m i n the absence of the a m i n e . T h e p r o b a b l e r e a c t i o n course to the n i t r o a l k a n e
(XI),
after
the
f o r m a t i o n of the a m i n e - o z o n e a d d u c t ( R e a c t i o n 1 ) , is s h o w n b y R e a c t i o n s 7 to 9 a n d s u m m e d u p i n R e a c t i o n 10.
A p r i m a r y amine oxide w o u l d
not b e expected to b e stable a n d s h o u l d rearrange to the h y d r o x y l a m i n e ( I X , Reaction 7).
A s i m i l a r set of reactions ( R e a c t i o n 8) s h o u l d result
i n the n i t r o s o a l k a n e ( X ) , w h i c h s h o u l d t h e n be c o n v e r t e d to the n i t r o a l k a n e ( X I ) as s h o w n i n R e a c t i o n 9. E v i d e n c e for this series of reactions was the o b s e r v a t i o n of the b l u e color of the n i t r o s o a l k a n e ( X ) t h r o u g h o u t the o z o n a t i o n a n d the d e m o n s t r a t i o n , i n separate experiments, that the h y d r o x y l a m i n e ( I X ) reacts w i t h t w o m o l e e q u i v a l e n t s of ozone a n d the n i t r o s o a l k a n e ( X ) w i t h one m o l e e q u i v a l e n t of ozone, e a c h to g i v e the nitroalkane
(XI).
T h e most l o g i c a l r o u t e f r o m the i n i t i a l ozone a d d u c t ter£-butylammonium
(VIII)
to
c h l o r i d e ( X I V ) is via n i t r o g e n c a t i o n r a d i c a l X I I ,
p r o d u c e d i n m i n u t e , e q u i l i b r i u m amounts t h r o u g h d i s s o c i a t i o n of V I I I (Reaction 11).
I n the case of c h l o r o f o r m solvent, the p r o p o s e d r e a c t i o n
course is s h o w n b y R e a c t i o n s 11 to 15 a n d is s u m m e d u p , w i t h R e a c t i o n 1, i n R e a c t i o n 16.
N i t r o g e n c a t i o n r a d i c a l X I I a p p a r e n t l y stabilizes itself
b y a b s t r a c t i n g h y d r o g e n f r o m a solvent m o l e c u l e to g i v e a m m o n i u m c a t i o n X l V a a n d solvent r a d i c a l X V ( R e a c t i o n 1 2 ) .
T h e ozonate a n i o n
r a d i c a l ( X I I I ) a n d the solvent r a d i c a l ( X V ) t h e n i n t e r a c t to g i v e the chloride anion
(XlVb)
ter£-butylamine
p r o d u c e tert-butyl
a n d phosgene
(Reaction
13).
Phosgene
isocyanate a n d a d d i t i o n a l salt
In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
and
(XIV)
64.
BAILEY
ET
Ozonization
AL.
+ tert-BuNH 0—O—O
• 0
2
of
+ tert-BuNH —O
+
2
61
Amines
2
— »
tert-BuNHOH
VIII
IX H
H
I
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tert-BuNHOH
(7)
+ O
a
I
— * • fert-BuN*—O—O—O" OH
0
+
2
tert-Bu—N*—O" OH (8)
—
tert-BuN(OH)
+ O
tert-Bu—N=0'
—
2
«-
s
+ HOH
tert-Bu—N=0:
tert-Bu—N—O—O—O"
0
II
+
2
° tert-BuNH
2
+ 3 0
3
•
tert-BuN0
(9)
2
XI
ter*-BuN0
2
+ 3 0
2
+ H 0
(10)
2
XI via
Reactions 14 a n d 15.
ter£-butylamine
It was s h o w n separately that phosgene
and
interact to g i v e the isocyanate at the o z o n a t i o n t e m p e r a
ture. R e a c t i o n 17 is a s u m m a t i o n of Reactions 10 a n d 16, a s s u m i n g that the t w o fates of a m i n e - o z o n e a d d u c t V I I I o c c u r e q u a l l y . T h e fact that the e x p e r i m e n t a l results d e s c r i b e d earlier agree w e l l w i t h the s t o i c h i o m e t r y of R e a c t i o n 17 increases confidence i n the v a l i d i t y of the p r o p o s e d mechanisms. I n the
o z o n a t i o n of t r i - n - b u t y l a m i n e at
—40°C.
w i t h an
ozone-
n i t r o g e n stream, 1.2 to 1.6 m o l e e q u i v a l e n t s of ozone w e r e a b s o r b e d , a n d the y i e l d s of t r i - n - b u t y l a m i n e o x i d e w e r e 5 3 % f r o m c h l o r o f o r m a n d 6 % f r o m p e n t a n e solvents. T h e other p r o d u c t s w e r e the side c h a i n o x i d a t i o n p r o d u c t s d e s c r i b e d b y H e n b e s t a n d S t r a t f o r d (11).
T h e s e results e l i m i
nate the p o s s i b i l i t y that the s i d e c h a i n o x i d a t i o n is a n o z o n e - i n i t i a t e d autoxidation.
T h e m e c h a n i s m o u t l i n e d b y R e a c t i o n 3 explains n i c e l y
b o t h the r e q u i r e m e n t of ozone itself as the o x i d i z i n g agent
and
the
solvent effect o b s e r v e d . Solvents s u c h as c h l o r o f o r m w o u l d be e x p e c t e d to solvate the o z o n e - a m i n e a d d u c t ( l b ) a n d m a k e the a b s t r a c t i o n of the p r o t o n i n R e a c t i o n 3 difficult. T h u s , loss of m o l e c u l a r o x y g e n to g i v e the a m i n e o x i d e becomes the major r e a c t i o n ( R e a c t i o n 2 ) .
I n pentane s o l u -
In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
62
OXIDATION
O F ORGANIC
COMPOUNDS
III
t i o n , n o s o l v a t i o n of l b occurs, a n d the major r e a c t i o n course is as d e s c r i b e d b y R e a c t i o n 3.
+ ^ r f - B u N H 0 0 0 " Z=±
tert-BuNK
2
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O—0—0:
+
2
XII
(H)
XIII
+ ter*-BuNH
4- C H C 1
+
2
-
3
terf-BuNH
XII
-CC1
XlVa
XIII
2
+
3
XV
XV
terf-BuNH
XlVb
+ C1 C=0
2
(12)
3
- tert-BuN—C=0
2
+
tert-BuNH Cl
(14)
3
I I
H
CI XIV
+
tert-BuNK
2
terf-BuN—C=0
» tert-BuN=C=Q
+
+ _
ter*-BuNH Cl 3
(15)
I I H
CI XVI
4
ter*-BuNH
2
+ 0
+ CHC1
3
-
3
XIV
ter*-BuN=G=0
+ 3
+ -
ter*-BuNH Cl 3
+ 0
2
(16) 5 tert-BuNH
2
+ 40
3
ter*-BuNH Cl-
3
+ CHC1
+
3
-
3
+ 40
2
ter*-BuN0
2
+ tert-BuN=C=Q
+
(17)
+H 0 2
Experimental T h e o z o n a t i o n setup a n d p r o c e d u r e s a n d the m e t h o d of d e t e r m i n i n g m o l e c u l a r o x y g e n y i e l d s are d e s c r i b e d i n earlier p u b l i c a t i o n s ( 2 , 3, 4 5, 16). T h e tert-butylamine, t r i - n - b u t y l a m i n e , a n d solvents w e r e t h e best 9
In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
64.
BAILEY
ET AL.
Ozonizotion
of
Amines
63
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grades a v a i l a b l e c o m m e r c i a l l y ; the amines w e r e f u r t h e r p u r i f i e d b y d i s tillation. ter^Butylhydroxylamine ( I X ) ( 9 ) , 2-methyl-2-nitrosopropane ( X ) (10), 2 - m e t h y l - 2 - n i t r o p r o p a n e ( X I ) (13), a n d tert-butyl isocyanate ( X V I ) (6) w e r e p r e p a r e d b y k n o w n literature p r o c e d u r e s . O z o n a t i o n o f tert-Butylamine. I n a t y p i c a l experiment, a s o l u t i o n of 5.0 m m o l e s of ter£-butylamine i n 50 m l . of c h l o r o f o r m was c o o l e d to — 6 0 ° C . a n d treated w i t h a n o z o n e - n i t r o g e n stream c o n t a i n i n g 6 m m o l e s of o z o n e ; the exit gases w e r e a n a l y z e d f o r m o l e c u l a r o x y g e n (4, 5, 16). F r o m the b e g i n n i n g of the o z o n a t i o n a s l i g h t b l u e c o l o r a r i s i n g f r o m the presence of 2 - m e t h y l - 2 - n i t r o s o p r o p a n e ( X ) a p p e a r e d , b u t it was r e p l a c e d at the e n d b y the p u r p l e c o l o r of u n r e a c t e d ozone. A p r e c i p i t a t e of terf-butylammonium c h l o r i d e f o r m e d d u r i n g the o z o n a t i o n . Excess ozone was p u r g e d w i t h n i t r o g e n , a n d it was f o u n d , b y t i t r a t i n g the i o d i d e t r i p , that 4.1 m m o l e s of ozone h a d r e a c t e d . T h e s o l u t i o n above the p r e c i p i t a t e was a n a l y z e d f o r a l l p r o d u c t s except the £er£-butylammonium c h l o r i d e b y a n F a n d M gas c h r o m a t o g r a p h , u s i n g a 1/4-inch X 20 foot c o l u m n containing C a r b o w a x 2 0 M on Chromosorb P. T h e column temperature was 1 2 5 ° C . f o r a l l p r o d u c t s except tert-butyl isocyanate, f o r w h i c h i t was 7 5 ° C . T h e ter£-butylammonium c h l o r i d e y i e l d was d e t e r m i n e d b y d i s s o l v i n g the p r e c i p i t a t e i n w a t e r a n d t i t r a t i n g for c h l o r i d e i o n . O z o n a tions of terf-butylhydroxylamine ( I X ) and 2-methyl-2-nitrosopropane ( X ) were carried out similarly. A q u a n t i t y of 10 m m o l e s of ozone was passed i n t o 50 m l . of c h l o r o f o r m at — 60 ° C , a n d the exit gases w e r e passed into 1 0 % s o d i u m car b o n a t e s o l u t i o n . T h e c h l o r o f o r m was also w a s h e d w i t h the carbonate s o l u t i o n , w h i c h was t h e n a c i d i f i e d w i t h n i t r i c a c i d a n d t r e a t e d w i t h silver nitrate. O n l y a f a i n t t u r b i d i t y was p r o d u c e d . W h e n a s o l u t i o n of 50 m l . of c h l o r o f o r m a n d 20 m m o l e s of ter£-butylamine was a l l o w e d to s t a n d at r o o m t e m p e r a t u r e f o r 3 hours, o n l y 5 m g . of ter£-butylammonium c h l o r i d e were produced. O z o n a t i o n o f T r i - w - b u t y l a m i n e . I n a t y p i c a l experiment, solutions of 4.0 grams (21.6 m m o l e s ) of t r i b u t y l a m i n e i n 40 m l . of p u r e p e n t a n e or c h l o r o f o r m w e r e t r e a t e d w i t h 21.6 m m o l e s of ozone at — 4 5 ° C . T h e ozone a b s o r p t i o n was q u a n t i t a t i v e . F o r pentane solvent, the r e a c t i o n m i x t u r e was extracted w i t h w a t e r ( 3 X 1 5 m l . ) at r o o m t e m p e r a t u r e a n d an a l i q u o t of the w a t e r extract was u s e d to d e t e r m i n e the a m i n e o x i d e b y the t i t a n i u m c h l o r i d e m e t h o d ( 7 ) . F o r the c h l o r o f o r m r e a c t i o n m i x ture, the solvent was r e m o v e d u n d e r r e d u c e d pressure, the r e s i d u e w a s d i s s o l v e d i n pentane, a n d the a m i n e o x i d e was d e t e r m i n e d as just de s c r i b e d . T h e u n r e a c t e d s t a r t i n g m a t e r i a l a n d side c h a i n o x i d a t i o n p r o d ucts w e r e d e t e r m i n e d w i t h a n A e r o g r a p h M o d e l 1520B gas c h r o m a t o g r a p h , e q u i p p e d w i t h a h y d r o g e n flame detector a n d a d i s k integrator. A 1/8-inch X 10-foot c o l u m n of 3 0 % silicone g u m r u b b e r S E - 3 0 o n C h r o m o s o r b W was u s e d w i t h t e m p e r a t u r e p r o g r a m m i n g f r o m 7 5 ° to 2 5 0 ° C . T h e p r i n c i p a l side c h a i n o x i d a t i o n p r o d u c t s w e r e , i n d e c r e a s i n g o r d e r of i m p o r t a n c e , d i - n - b u t y l a m i n e , 1 - d i - n - b u t y l a m i n o - l - b u t e n e , N,Nd i b u t y l b u t y r a m i d e , a n d N , N - d i b u t y l f o r m a m i d e . T h e r a t i o of ozone re a c t i n g to a m i n e r e a c t i n g was 1.2 to 1.6.
In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
64
OXIDATION
OF ORGANIC
COMPOUNDS
III
Acknowledgment T h i s w o r k w a s s u p p o r t e d b y grants f r o m the N a t i o n a l Science F o u n d a t i o n , the R o b e r t A . W e l c h F o u n d a t i o n , a n d the P e t r o l e u m R e s e a r c h F u n d o f the A m e r i c a n C h e m i c a l Society, f o r w h i c h the authors are v e r y grateful.
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Literature Cited
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20)
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RECEIVED
October 20, 1967.
In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.