RADICAL-CHAIN ADDITIONOF HYDOGEN BROMIDE TO PROPYNE
April 20, 1964 [CONTRIBUTION FROM THE
DEPARTMENT O F CHEMISTRY, THEPENNSYLVANIA
I559
STATE UNIVERSITY, UNIVERSITY PARK, PENNSYLVANIA]
Gas Phase Radical-Chain Addition of Hydrogen Bromide to Propyne’
cis-trans Isomerization in Vinyl Radicals. BY
PHILIPs. SKELLAND RICHARDG . L ~ L L E N ~ . ~ RECEIVED NOVEMBER 22, 1963
cis-trans
Radical-chain addition of hydrogen bromide in the gas phase occurs by a stereospecific trans process. Isomerization of the intermediate radical C H 3 k = C < g is an activated process, E,,,
> 17
kcal./mole, and
the half-life for this cis-trans isomerization is > 3 X lo-’ sec.
In liquid phase a t -78 to -60°, propyne and hydrogen bromide react by a stereospecific trans radical chain addition process, producing cis- 1-bromo- l-propene.l These results can best be rationalized by assuming stereospecificity in both chain steps.
propene should be observed, and only a t later stages of conversion should the trans compound appear. CHs
.C=C
A,CH,C?C-H---t
CH3
Br
\
Br.
/
HBr
+
\
H
+
CHs-CZC-H
CH3\ Br .CW< i- HBr H
CHs,
.c=\
c%
-
CH3,
,Br
H’
H
.
,Br
,c=c\ CHS
H
(1)
\H
CC ’,
+
BP
(3)
H
At room temperature cis- and trans-l-bromo-lpropenes are produced in equilibrium or near equilibrium mixtures in both gas4and liquid phase reactions. Since these conditions have been demonstrated4 to be those required for cis-trans isomerizations of the 1-bromo-1-propenes, reactions 1 , 2 , 4 , and 5 can account for the observations, but reaction 3 is not necessarily eliminated as a competing process.
Y
H,
dH3’
B‘ +
c=c
‘H
(4)
I
Br.
e
$”’
/
\
C=C
(2)
,Br
H CH3
/
H H isomerizing vinyl radical
CHs\
Br-
Br
\
(5)
H CH3
The importance of cis-trans isomerization in vinyl radicals, reaction 3, was assessed by examining products from propyne and hydrogen bromide at early stages of the reaction. If reaction 3 occurs in these systems at room temperature, then even at the earliest stages significant quantities of trans-1-bromo-1-propene should be observed (reaction scheme A ) . On the other hand, if (3) makes no contribution and reactions 1, 2, 4, and 5 are sufficient, trans-1-bromo-1-propene is obtained by isomerization of the cis compound (reaction scheme B). Thus a t early stages cis-l-bromo-l(1) Acknowledgment is made t o t h e Air Force Office of Scientific Research for support of this research under Contract A F 49(638)457. (2) This work was done in partial fulfillment of requirements f o r P h . D . degree, 1959. (3) Graduate Pellow. Allied Chemical Corporation, 1958-1959. (4) P. S. Skell a n d R. G Allen, J . Am. Chem. Soc , 80, 5997 (1958)
B, CHa-CEC-H
Br.
CHS, ,Br .C=C \ H
----t
A1
Br a
HBr
,Bf
CHs, C=C,
H’ H nonieomerizing vinyl radical Br
/
CHa
Earlier experiments toward the objective of choosing between A and B were not successful. Satisfactory exclusion of hypothesis A was accomplished by studying gas phase propyne-hydrogen bromide addition reactions in the 5 to 0.1570 conversion regions. Hydrogen bromide and propyne in a 1 : 3 ratio were mixed in a dark container. The mixture obeyed Dalton’s law, indicating that n-complexing and other condensation reactions do not occur on mixing. Samples were admitted to and irradiated in the glass coil of a gas chromatography sampling system. Without delay the irradiated sample was passed through a tube filled with sodium hydroxide pellets connected to the inlet of the gas chromatography column. The gas chromatography trace indicated that the only products were cis- and trans-1-bromo-1-propenes.The results of these experiments are summarized in Table I . At 1lyo conversion of reactants, the cis-trans ratio approaches the equilibrium value of 4.19. However, a t smaller conversions, the cis-trans ratio increases until i t exceeds lo2. The high cis-trans ratio a t lowest conversions leaves little doubt that reactions 1, 2, 4, and 5 are adequate to describe the system in liquid and gas phase within the temperature and concentration limits studied. Further, the experiments most dilute in hydrogen bromide defines an upper limit for the half-life of .. ,Br undergoing cis to trans isomerization as CH, ?c=c \.H
PHILIPS. SKELLA N D RICHARD G. ALLEN
1560
Vol. 86
TABLE I is consistent with this value, and one might RELATIVEAMOUNTSOF cis- A N D ~ ~ U ~ ~ - ~ - B R O M O - ~ AT~ R O that P E S Ethe S activation energy for all cis-trans Low CONVERSION OF HBr A N D PROPYXE TO PRODUCT tions a t double bonded carbon radicals
anticipate isomerizaz\cZx might
cis -
yo conversion t o product
approximate 23 kcal. /mole. Two instances of nonstereospecific radical-chain additions to acetylenes are reported in the l i t e r a t ~ r e . ~ ~ , b Since a t present there is some doubt regarding the meaning of these observation^,^^^^ the stereospecific additions reported here may indeed be general as suggested above
1yans
5 68
11 0 9.0
9 12 15 44 53 95 108 120
5 5 4 0 0 25 18 18 15 15
85 1 2 0 0 0 0 0
Experimental
