150
V. P a¨ llin et al. / Journal of Organometallic Chemistry 590 (1999) 149–152
In this paper we report the results of a reinvestigation
solution and its further consumption were recorded. The
measured volumes of absorbed acetylene were reduced
to normal conditions and transformed into moles. For
convenient use of the data in kinetic calculations the
molar amounts of consumed acetylene were referred to
of the reaction. A similar experimental technique was
used. It appeared that Grignard’s mechanism had to be
supplemented with the reaction between bromomagne-
siumacetylene and the Grignard reagent.
−
1
the volume of the reaction mixture (mol l ).
Aliquots (about 0.25 ml of the gas phase) were
periodically withdrawn from the reaction flask and
analyzed by means of GLC. The concentration of
benzene in the solution was calculated by making use of
the calibration data obtained from analyses of samples
with known benzene content. Then kinetic curves of the
evolution of benzene were plotted.
After the absorption of acetylene had practically
ceased, 0.25 ml of triethylamine was added to the
reaction mixture for the completion of the reaction. This
accelerated the reaction sufficiently to read the final
amounts of benzene and consumed acetylene after some
2
. Experimental
2.1. Materials
All the operations with carefully purified reagents and
solutions were carried out under dry argon. Phenylmag-
nesium bromide was prepared and analyzed in the
conventional manner [9].
2.2. Kinetic measurements
1
0 min. The final concentration of benzene in the
The reaction of phenylmagnesium bromide with
reaction mixture corrected for the benzene concentra-
tion in the initial solution was taken for the initial
concentration of the active Grignard reagent. The acidi-
metrically determined concentration of basic magne-
sium in the initial reagent exceeded the active Grignard
concentration by 4–6%.
acetylene was carried out in a thermostated 100 ml glass
vessel at 20°C. The reaction cell was provided with a
magnetic stirrer and inlets for the injection of the
reagents and the sampling of the vapor above the
reaction mixture.
The reaction vessel was purged thoroughly with pure
argon. Then 50 ml of diethyl ether solution of phenyl-
magnesium bromide (0.9–1.1 M), 5 ml of pure toluene
3
. Results and discussion
(
internal standard for GLC analyses) and a calculated
amount of triethylamine (up to 5.6 mg) were introduced.
The reaction mixture was refluxed for 5 min to displace
the inert gas, then the condenser was quickly replaced
with a silicon septum, the flask was cooled and placed
into the thermostat. After the thermal equilibrium was
set, the acetylene flow was turned on. A constant
pressure of acetylene in the reaction cell was provided
by an automatic gasometer with a continuous registra-
tion of the volume of consumed gas. First the initial
volume of acetylene in the flask was registered, then the
consumption of acetylene for the saturation of the
The reaction between acetylene and phenylmagne-
sium bromide in diethyl ether was investigated. Both the
consumption of acetylene and the evolution of benzene
during the reaction were registered. Kinetic measure-
ments were also carried out in the presence of small
−4
−1
additions of triethylamine (up to 9.6×10 mol mol
of the Grignard reagent).
It was found that under constant acetylene concentra-
tion in the solution, the evolution of benzene during the
reaction was a first-order process. The consumption of
acetylene appeared to be of more complicated kinetics,
the kinetic order of the process changing during the
reaction. In this context plots of the molar amount
of benzene evolved versus that of acetylene consumed
seem to be informative as regards the reaction
mechanism.
As is seen in Fig. 1 (plot A), at the beginning of the
reaction one mole of benzene was evolved per mole of
acetylene consumed in the absence of the catalyst.
Further, 2 mol of benzene were evolved per mol of
acetylene up to the end of the reaction. Apart from the
enormous acceleration of the reaction, additions of
triethylamine greatly alter the kinetic picture (Fig. 1, B).
During the first half-period of the reaction exactly 2 mol
of benzene are formed per mol of consumed acetylene.
Moreover, the plot declines slightly from the straight
line.
Fig. 1. Plots of benzene evolution vs. acetylene consumption (mol
−
1
l
) for noncatalytic reaction (A) and in the presence of 0.03 mol%
of triethylamine (B). The numerals indicate the slopes of the lines.