to form an ester (or an alcoholꢀ, either, or both, mechanisms
could have been operating.
composition of the reaction mixture was similar to that for
3
the other acids, but with a total volume of 5.0 cm , rather than
3
To confirm that the Ad 2 mechanism predominates, the
25.0 cm . The results with this cell were less reliable than those
E
reaction of HCl with 2,3-dimethylbut-2-ene was studied in the
presence of added KCl. When the Cl ion concentration was
obtained with HCl, HBr and HI.
All the experiments were carried out in a water bath, at a
temperature of 25.0 ± 0.04 ЊC.
In the experiments, the alkene was in large excess. Typically
the hydrogen halides were between 0.001 and 0.005 mol dm
Ϫ
increased by 15 times, the rate increased only 1.3 times, a
result similar to that found when HClO4 was added to the
HBr–2-methylbut-1-ene reaction mixture. While the increase
in rate is consistent with some participation by an AdE3
mechanism, it is equally likely that it is due to an increase in
the concentration of undissociated HCl by suppression of
ionisation.
Ϫ3
Ϫ3
and the alkene between 0.05 and 0.3 mol dm . The total
3
volume of the acids was 24.0 cm and the hydrocarbon added
3
3
1.00 cm . If the volume of alkene added was less than 1.00 cm ,
the difference was made up with hexane which had a similar
relative permittivity to the alkenes used. The relative per-
Under anhydrous conditions, phenylethene partially poly-
16
17
merises unless the temperature is kept below Ϫ70 ЊC. In the
presence of water and in the presence of the inhibitor 4-tert-
butylcatechol, polymerisation does not appear to be a problem.
The rate constants for phenylethene are smaller than those for
other terminal alkenes. This can be attributed to the reduction
in the availability of the π electrons of the double bond as a
result of conjugation with the benzene ring.
mittivity, calculated value 7.64, of the reaction mixture was,
therefore, constant.
Analysis by gas chromatography
The products of the reaction between HBr and 2-methylbut-1-
ene were analysed with a Unicam 610 series gas chromatogram,
using a column temperature of 110 ЊC. The products of the
reactions of HCl and HF with 2-methylbut-1-ene and HCl with
Cyclohexene and the E and Z conformers of pent-2-ene all
react at comparable rates. Although they contain two carbon
atoms that can form secondary carbocations, their rates are
slower than those for alkenes with a terminal double bond, that
can form only one secondary carbocation. Possibly steric effects
operate similar to those already discussed with the alkenes that
form tertiary carbocations.
2
,3-dimethylbut-2-ene were analysed with a Perkin Elmer Auto-
system, using a column temperature of 130 ЊC. Both instru-
ments used a flame ionisation detector and the temperatures of
the detectors and injection ports were 200 ЊC. Both instruments
had columns 7Ј long, containing 10% Carbowax 1500 on
Chromosorb WHP.
Experimental
Acknowledgements
Materials
Our thanks to Dave G. Davies for technical help and for writing
the conductivity program, and to Tom Barnet-Lamb for writing
the curve-fitting program. Particular thanks are due to Dr Ed
H. Smith of the Chemistry Department, Imperial College of
Science, Technology and Medicine who spent much time giving
advice and who also gave invaluable help with the analysis using
gas chromatography. Our thanks also to Professor Michael
Spiro of the same department who gave encouragement
throughout the project.
The ethanoic acid and the hydrohalic acids were of AnalaR
quality obtained from Merck. The HF was 48–51% obtained
from Sigma-Aldrich. The alkenes were obtained from Sigma-
Aldrich and most had a quoted purity in excess of 99%. The
water was freshly prepared by glass distillation.
The standards used in the gas chromatography analysis were
mainly obtained from Sigma-Aldrich. As neither 2-fluoro-2-
methylbutane nor 2-methyl-2-butyl ethanoate was obtainable,
impure samples were prepared as follows. 2-Fluoro-2-methyl-
butane was made by mixing 2-methylbutan-2-ol with concen-
trated HF for a week. The gas chromatography trace of the
product showed a new peak with a retention time between that
of the 2-methylbut-1-ene (bp 31 ЊCꢀ and 2-chloro-2-methyl-
butane (bp 86 ЊCꢀ. A sample of 2-methyl-2-butyl ethanoate (bp
References
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1
24 ЊCꢀ was made by reacting 2-methylbutan-2-ol with ethanoyl
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6
3
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1
7
8
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1
Ϫ1
measured cell constant was 0.92(±0.03ꢀ cm . The electrical
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age and current readings were fed to a computer, using a pico
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1
1
1
952, 169, 291.
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1
portion of the curve. The reaction was started by shaking the
alkene with the acid; subsequently no stirring was used.
The experiments with HF were conducted in a PTFE cell.
1
5 R. C. Fahey, C. A. McPherson and R. A. Smith, J. Am. Chem. Soc.,
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1
3
This was made from a 10 cm Teflon centrifuge tube, containing
1
1
6 M. S. Kharasch and M. Kleinman, J. Am. Chem. Soc., 1943, 65, 11.
shiny platinum wires in PTFE tubing secured in a polypropene
7 C. Wohlfarth, in Handbook of Chemistry and Physics, ed. D. R. Lide,
Ϫ1
th
stopper. The measured cell constant was 0.61(±0.06ꢀ cm . The
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J. Chem. Soc., Perkin Trans. 2, 2002, 810–813
813