3902
J. Org. Chem. 1980,45, 3902-3906
be obtained from solvolysis in acetic acid. The enol ether products were identified by 'H NMR spectroscopy and GLC.
Acknowledgment. The continued financial support of the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged as are the awards of NRC S c h o l a r s h i p s to G.M. Registry No. 4-Chloro-2-methylacetophenone, 37074-38-7; 5chloro-2-methylacetophenone, 58966-35-1;2,6-dimethylbenzoic acid, 632-46-2; 2,6-dimethylbenzoyl chloride, 21900-37-8; 2,6-dimethylacetophenone, 2142-76-9; 4-bromo-2,6-dimethylbenzonitrile, 575766-4; 4-bromo-2,6-dimethylbenzoic acid, 74346-19-3; 4-bromo-2,6dimethylbenzoyl chloride, 74346-20-6;4-bromo-3,5-dimethylphenol, 7463-51-6; 4-bromo-3,5-dimethylanisole, 6267-34-1; 2,6-dimethyl-4methoxyacetophenone, 60999-76-0; 2,6-dimethyl-4-methoxybenzoic acid, 37934-89-7; 2,6-dimethyl-4-methoxybenzoylchloride, 3124759-3; 1-(2-methylphenyl)vinyl chloride, 38379-19-0; 1,l-dichloro-l(2-methylphenyl)ethae, 74346-21-7;o-methylphenylacetylene, 76647-2; 1-(2-methylpheny1)vinyl bromide, 74346-22-8; l-(2-methylpheny1)vinyl tosylate, 74331-76-3; 1-(4-methylphenyl)vinylbromide, 74331-69-4;451270-89-4; l-(4-methoxy-2-methylphenyl)acetylene, hydroxy-2-methylacetophenone, 875-59-2; 1-(4-methoxy-2-methylpheny1)vinyl chloride, 74346-23-9; 1-(4-methoxy-2-methylphenyl)vinyl bromide, 74346-24-0; l-(4-methoxy-2-methylphenyl)vinyltosylate, 74331-92.3; l,l-dichloro-l-(4-chloro-2-methylphenyl)ethane,
74346-25-1;l-(4-chloro-2-methylphenyl)vinyl bromide, 74346-26-2; l-(4-chloro-2-methylphenyl)vinyl tosylate, 74346-27-3; 1-(5-chloro2-methylphenyl)acetylene, 74331-72-9; 1-(5-chloro-2-methylpheny1)vinyl tosylate, 74346-28-4; 1-(5-chloro-2-methylphenyl)vinyl bromide, 74346-29-5; 1-(2,4-dimethylphenyI)acetylene,16017-30-4; 2,4-dimethylacetophenone,89-74-7; 1-(2,4-dimethylphenyl)vinyl chloride, 74346-30-8; 1-(2,4-dimethylphenyl)vinylbromide, 5127074331-74-1; 1-(2,6-dimethyl86-1; 1-(2,6-dimethylphenyl)acetylene, pheny1)vinyl chloride, 74331-79-6; l,l-dichloro-l-(2,6-dimethylphenyl)ethane, 74346-31-9; 1-(2,6-dirnethylphenyl)vinylbromide, 74331-78-5; 1-(2,6-dimethylphenyl)vinyltosylate, 74331-77-4; (4bromo-2,6-dimethylphenyl)acetylene,74331-75-2; 4-bromo-2,6-dimethylacetophenone, 53379-63-8; 1-(4-bromo-2,6-dimethylphenyl)vinyl chloride, 74346-32-0; l-(4-bromo-2,6-dimethylphenyl)vinyl bromide, 74346-33-1;l-(4-bromo-2,6-dimethylphenyl)vinyl tosylate, 74346-34-2; l-(2,6-dimethyl-4-methoxyphenyl)vinyi chloride, 7433191-2; l-(2,6-dimethyl-4-methoxyphenyl)vinyl bromide, 74346-35-3; l-(2,6-dimethyl-4-methoxyphenyl)acetylene, 74331-73-0; 1-(2,4-dimethylpheny1)vinyl tosylate, 51270-87-2; 1-(4-bromo-2-methylpheny1)vinyl tosylate, 74346-36-4; 2,6-dimethylaniline, 87-62-7; omethylacetophenone, 577-16-2; 1-(4-methylphenyl)vinyl tosylate, 51270-88-3; l-(4-chloro-2-methylphenyl)acetylene, 74331-71-8. S u p p l e m e n t a r y Material Available: Listing of rate constants for all compounds studied (6 pages). Ordering information is given on any current masthead page.
Vinyl Cation Intermediates in Solvolytic and Electrophilic Reactions. 2. Bromination of Arylacetylenes Keith Yates* and George Mandrapilias Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 1Al Received M a y 14, 1980 The kinetics of bromination of a series of ten ring-substituted phenylacetylenes were investigated in anhydrous acetic acid a t 25 "C. All substrates were substituted with one or two methyl groups a t the ortho position(s) for comparison with the behavior of analogously substituted a-arylvinyl tosylates, bromides, and chlorides, which generate formally very similar vinyl cation intermediates. The phenylacetylenes show wide variations in bromination rate, with second-order rate constants in the range 10-3-104 M-' s-l. Activation parameters were also measured, with AS' showing little variation with the number of ortho substituents. The rates of bromination correlate much better with ut than with u, yielding p values of -6.9 and -6.7 for the 2-methyl and 2,6-dimethyl series, respectively. The magnitudes of these p values are compared with those for the parent phenylacetylene series and with the corresponding values for a-arylvinyl-X solvolysis. The product distributions and stereochemistry are very similar to those found for phenylacetylenes with no o-methyl substituents.
As mentioned in the preceding paper,' vinyl cations are now well established as organic reaction intermediates. The present paper describes the generation of this type of intermediate b y electrophilic a d d i t i o n of b r o m i n e to substituted phenylacetylenes for comparison with the behavior of s t r u c t u r a l l y similar cationic i n t e r m e d i a t e s f o r m e d d u r i n g the solvolysis of a-arylvinyl derivatives. A l t h o u g h bromination of some substituted phenylacetylenes has a l r e a d y been investigated,2 the present study deals exclusively w i t h o-methyl and 2,6-dimethyl s u b s t i t u t e d phenylacetylenes for d i r e c t comparison w i t h the solvolysis of similarly s u b s t i t u t e d a-arylvinyl systems, described in the preceding paper. In the latter systems, o-methyl groups were necessary t o accelerate the solvolytic
reactions to a conveniently measurable rate r a n g e with typical leaving groups.
Results and Discussion Although it is well-known3 that the kinetics of bromine addition to acetylenes generally follows the three-term
expression of eq 1, the experimental conditions were adV = -d[Brz]/dt = [acetylene](kl[Brzl + kz[Br2I2+ k3[Brzl[Br-l) (1) j u s t e d i n the present case so that eq 1 would reduce to a simpler pseudo-first-order rate equation (eq 2), where koM -d[Brzl /dt = kobsd[Br21
(2)
= kl[acetylene]. This was done b y using low initial bro(1)K. Yates and G. Mandrapilias, J . Org. Chem., preceding paper in this issue. (2) J. A. Pincock and K. Yates, Can. J . Chem., 48, 3332 (1970).
(3) G. H. Schmid, "The Chemistry of the Carbon-Carbon Triple Bond", S. Patai, Ed., Wiley, New York, 1978, Chapter 8.
0022-3263/80/1945-3902$01.00/0 0 1980 American Chemical Society
J. Org. Chem., Vol. 45, No. 19, 1980 3903
Bromination of Arylacetylenes
Table 11. Activation Parameters for Bromination of Arylacetylenes in Acetic Acid
Table I. Rate Constants for the Bromination of Ring Substituted Arylacetylenes in Acetic Acid ( 2-Methylpheny1)acetylenes
h , , M-' s-'
substituent T, C 4-OMeb 4-Meb
24.8
(4.3
39.2 29.9
(2.97 f (2.02 t (1.67 t (1.30 i (4.22 t 113.10 t i(2.36 i(1.58 t 111.49c
24.8 5-Me
21.9 35.9 30.5
24.8 18.6
H 441
5-C1
39.9 30.8
Hb
4-Br
*
0.40) X
a+"
lo4
0.03) X 10 0.05) x 10 0.05) x 10
0.04) x 10 0.20) x 0.15) X 0.10) X
0.04) x
0.02) X
(8.00 t 0.40) x 0.13) x 0.05) x
24.8 29.6 24.8 18.6 16.0 69.6 59.6
(5.55 t (2.08 t i(1.60 t 111.11t 118.27 t i(1.25 c
4.9.1
i(3.74 t 0.18) x i(7.19 e 0.50) X
24.8 4-OMeb 4-Meb
t
0.04) x
0.02) x
0.04) x 0.06) X (6.80 c 0.34) x
lo-' lo-' lo-' lo-' lo-' lo-' lo-' lo-' lo-' lo-' lo-' lo-'
A H # , kcal
AS#,eu
mol-'
mol-'
substrate Me
-0.778
7.3 c 1.4a
-24
9.5
0.3
- 3 O t 1.2
* 0.1
- 2 6 t 2.0
t
4.6"
-0.311
-0.069
Me'
&LCH
11.5
0
t
10.8 ?: 1.0
+0.114
t
2.5
-30
?:
0.ga
C 3 C H
6.8
Me
t0.373
(2,6-Dimethylphenyl)acetylenes 24.8 >lo5 -0.778 4:5.5 r5.25 c 0.22) X l o 2 4:0.3 (3.43 t 0.15) x lo2 (3.13 * 0.10) x 10' 35.9 -0.311 214.8 (2.07 t 0.10) x 10' 2!4.8 1.51 0 24.8 0.16 t0.15
-31
* 0.3"
12.3 c 0.3
- 3 2 ? 2.1
11.7b
- 31b
CI PhClCH
a Based on stopped-flow measurements. ref 2.
Determined by stopped-flow Taken from ref 4. measurements. Extrapolated from values at other temperatures.
Taken from
a
mine concentrations (
