Dehydrohalogenation using magnesium hydroxide

A process comprising contacting a solvent, magnesium hydroxide, and a halogenated hydrocarbon at a pressure sufficient to keep the solvent and the halogenated hydrocarbon under liquid conditions, and a temperature greater than 150.degree. C. under conditions suitable to dehydrohalogenate the halogenated hydrocarbon, and recovering the dehydrohalogenated product.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for dehydrohalogenating a 
halogenated hydrocarbon by contacting the compound with a solution 
containing magnesium hydroxide at high temperatures and pressures. 
Polychlorinated ethanes can all be dehydrochlorinated to corresponding 
chloroethylenes which are used commercially. For example, 
1,2-dichlorethane yields vinyl chloride, the well-known monomer for the 
production of polyvinylchloride; the tetrachloroethanes yield 
trichloroethylene, a widely used industrial degreasing solvent; and 
pentachloroethane yields perchloroethylene, the widely used nonflammable 
dry-cleaning solvent. Depending on the dehydrohalogenation method, 
1,1,2-trichlorethane may yield all three isomeric dichloroethylenes or 
predominantly only one of the dichloroethylenes. Of the three isomers only 
1,1-dichloroethene (vinylidene chloride) has found wide use. In addition 
to its use as a monomer for polymer production, it has also become an 
intermediate for the production of herbicides, latexes, and 
1,1,1-trichloroethane which is used as a degreasing solvent. Therefore, 
the method which yields predominantly vinylidene chloride is preferred. 
The classical method of dehydrochlorinating the polychloroethanes employs 
an aqueous suspension of alkaline earth hydroxides such as calcium or 
barium hydroxide. In general, these reactions are run under ambient 
pressures at a temperature at or below the boiling point of the 
chlorinated reactant. For example, U.S. Pat. No. 2,598,646 discloses a 
continuous process for dehydrochlorinating a polychloroethane by 
contacting an aqueous alkaline earth metal hydroxide slurry in a closed 
chamber with a polychloroethane at a reaction temperature below the 
boiling point of any of the organic azeotropes formed therein. While the 
examples report good yield using calcium hydroxide, data for magnesium 
hydroxide is not provided. It is known, however, that the process produces 
poor conversions to vinylidene chloride when magnesium hydroxide is used 
as the alkaline earth metal hydroxide in such a process. 
More recently, methods employing an aqueous solution of an alkali metal 
hydroxide dispersed in the organic chloroethane phase by means of powerful 
mixing devices have found widespread use. These reactions are typically 
run at or below the boiling point of the chlorinated reactant and under 
ambient pressures. In these cases, extreme care must be taken to avoid an 
excess of the hydroxide. Otherwise, the vinylidene chloride product can 
undergo dehydrochlorination to hazardous acetylenic compounds. Due to the 
potential of producing acetylenic compounds, less than stoichiometric 
amounts of alkali metal hydroxides are typically used. 
Therefore, the existing problem is that the use of magnesium hydroxide by 
itself has been unsuccessful due to the poor solubility of the hydroxide 
ion. Due to the widespread availability, low cost, and other advantages 
described herein, it would be advantageous to the chemical industry and to 
the public to possess a method whereby magnesium hydroxide could be used 
to dehydrohalogenate halogenated organic compounds. Such a process would 
provide another route to vinylidene chloride when, for example, the price 
of caustic is high. It would also be advantageous to have a method whereby 
the product can be selectively formed without precise equivalent mixtures 
of reactant and base, and wherein the formation of potentially explosive 
acetylene compounds does not occur. 
SUMMARY OF THE INVENTION 
It has now been found that magnesium hydroxide can be used successfully in 
a dehydrohalogenation process by contacting dehydrohalogenatable 
halogenated hydrocarbon, a solvent, and magnesium hydroxide at 
temperatures above about 150.degree. C. and pressures sufficient to keep 
both the hydrocarbon and solvent under liquid conditions. 
The process of the present invention makes it possible to use magnesium 
hydroxide as the sole base to dehydrohalogenate halogenated hydrocarbons. 
Excess magnesium hydroxide does not produce acetylenes and, therefore, a 
stoichiometric amount of magnesium hydroxide is not required in the 
process of the present invention. Thus, the method of the present 
invention is simple and safe to use. The public, moreover, is benefited by 
a new dehydrohalogenation method using an inexpensive alternative to 
caustic which is commonly employed in dehydrohalogenation processes today.

DETAILED DESCRIPTION OF THE INVENTION 
The dehydrohalogenation process of the present invention is generally 
employed to dehydrohalogenate halogenated hydrocarbon using a solution 
containing magnesium hydroxide. 
The temperature at which the method of the present invention will operate 
is preferably above about 150.degree. C. More preferably, the temperature 
is between about 150.degree. C. and 300.degree. C., and most preferably 
between about 175.degree. C. and 225.degree. C. 
The pressure at which the present invention will operate is any pressure 
effective to keep both the halogenated hydrocarbon and solvent in the 
liquid phase. Sufficient pressure may thus be any pressure at or above the 
combined vapor pressure of the solvent and reactants at a given 
temperature. Preferably, the pressure is above about 175 psig. As used 
herein "psig" is defined as pounds per square inch gauge. More preferably, 
the pressure is between about 175 and 500 psig, most preferably between 
about 250 and 400 psig. 
The apparatus which are useful in the present invention are known in the 
art and are any which will enable the invention to be carried out at 
elevated pressures and temperatures so as to dehydrohalogenate halogenated 
alkanes using a solution containing magnesium hydroxide. For example, a 
plug flow or stirred tank reactor may be used. Likewise, a useful 
apparatus can be employed which runs batch, semicontinuous, or continuous 
reactions; preferably a continuous reaction is run. Preferably, the 
apparatus allows the dehydrohalogenated products of the present invention 
to be continuously removed from the reaction zone. Methods of recovering 
the products formed by the method of the present invention are well known 
to those skilled in the art. For example, distillation is often employed 
to separate and further purify the product. 
The process of the present invention is carried out in the liquid phase. In 
a preferred embodiment, the solution containing the magnesium hydroxide is 
contacted with the halogenated hydrocarbon under vigorous agitation. 
The solvent used to dissolve magnesium hydroxide is typically water. Other 
solvents may, however, be used as co-solvents if the halogenated 
hydrocarbon has low solubility in water during the process of the present 
invention. Examples of such cosolvents include glycols and alcohols such 
as methanol, ethanol, the propyl alcohols, and the like. Thus, 
multi-component solvents may be used in the present invention such as 
mixed water-alcohol or glycol-water solvent systems. The most preferred 
solvent is water. 
In its broadest application, the present invention is useful to allow bases 
which do not have appreciable solubility in water at temperatures up to 
about 150.degree. C. and under ambient pressures to enter solution in 
amounts sufficient to dehydrohalogenate halogenated hydrocarbons. Most 
preferably, magnesium hydroxide is the base. 
The concentration of base in the solvent may be in the range between about 
0.1 and 75 percent by weight. Preferably, the concentration of base in the 
solvent is between about 10 and 50 percent by weight, more preferably, 
between about 15 and 35 percent by weight. Importantly, the molar ratio of 
base to halogenated hydrocarbon can be less than, equal to, or more than 
the stoichiometric amount. Preferably, the molar ratio of base to 
halogenated hydrocarbon is between about 1:1 and 10:1, more preferably 
between about 2:1 and 5:1. 
The halogenated hydrocarbons which are useful in the present invention 
include halogenated alkanes having one or more halogen atoms and having 
two or more carbon atoms. The halogenated hydrocarbons useful in the 
present invention must be capable of being dehydrohalogenated. Compounds 
containing more than one halogen may have two or more types of halogen 
atoms. The preferred halogens are bromine, iodine, and chlorine. The 
halogenated hydrocarbons may be halo-alkanes, aliphatic compounds 
containing a halo-alkyl group, or aromatic compounds containing a 
halo-alkyl group. Preferably, the hydrocarbon is a straight-chain, 
branched, or cyclic organic compound having from 2 to 6 carbon atoms and 
the halogen atom is chlorine. Most preferred halogenated hydrocarbons 
include 1,1,2-trichloroethane; 1,1-dichloroethane; 1,1,1-trichloroethane; 
1,2-dichloroethane; chloroethane, 1,1,1,2-tetrachloroethane, 
1,1,2,2-tetrachloroethane, pentachloroethane, pentachloroethane or 
mixtures thereof. 
SPECIFIC EMBODIMENTS OF THE INVENTION 
The following examples are given to illustrate the invention and should not 
be construed as limiting its scope. All parts and percentages are by 
weight unless otherwise indicated. 
EXAMPLE 1 
Batch Synthesis of Vinylidene Chloride 
In a batch reaction apparatus, 27.3 grams of 1,1,2-trichloroethane is added 
to 202.7 grams of a 7.8 weight percent aqueous magnesium hydroxide 
solution. With continuous stirring, the mixture is heated to maintain a 
temperature in the range between about 190.degree. to 200.degree. C. and 
is pressurized to maintain about a 200 psig nitrogen atmosphere. After 45 
minutes from time of addition, the product is collected and analyzed. The 
analysis data shows 86.5% conversion of the 1,1,2,-trichloroethane and 
98.6% selectivity to vinylidene chloride. 
EXAMPLE 2 
Continuous Synthesis of Vinylidene Chloride 
In an apparatus designed to run continuous high pressure and temperature 
reactions, the following data is generated in Table I for reactions using 
water as the solvent, magnesium hydroxide as the base, and 
1,1,2-trichloroethane as the halogenated hydrocarbon. Acetylenic compounds 
are not detected in any run. 
TABLE I 
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Molar 
ratio of 
Mg(OH.sub.2) 
1,1,2- 
Concentration 
trichloro- 
Residence 
Temperature 
Pressure 
in Solvent 
ethane to 
Time Conversion 
Selectivity 
.degree.C. 
(psig) 
(% by weight) 
Mg(OH).sub.2 
(Minutes) 
(%) (%) 
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185 275 0.15 .5 5.0 55 99 
200 375 0.15 1.5 12.5 70 97 
220 375 0.15 1.5 12.5 67 96 
180 375 0.15 1.5 12.5 40 98 
200 375 0.15 0.6 12.3 86 97 
220 375 0.15 0.6 12.3 78 96 
220 375 0.32 0.26 19.7 82 96 
201 375 0.32 0.26 19.7 85 97 
200 375 0.32 0.64 12.5 62 97 
220 375 0.32 0.24 12.6 77 96 
222 375 0.31 0.63 19.7 64 96 
221 375 0.31 1.0 12.5 45 97 
202 275 0.31 0.26 19.5 75 97 
199 275 0.31 1.0 19.9 43 97 
179 275 0.31 0.6 19.9 58 98 
201 275 0.31 0.6 19.9 45 97 
182 275 0.31 0.9 13.1 34 97 
201 275 0.31 0.6 12.2 47 98 
180 275 0.34 0.26 12.6 60 99 
200 275 0.33 0.28 20.5 67 96 
200 275 0.33 0.26 12.4 62 99 
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From the examples it can be seen that halogenated hydrocarbons may be 
converted to alkenyl compounds using magnesium hydroxide as the sole base. 
The reaction produces product free of acetylenic compounds despite the use 
of a large excess of base. The reaction produces high conversions and high 
selectivity toward vinylidene chloride from 1,1,2-trichloroethane. While 
selectivity to vinylidene chloride remains relatively constant as 
variables are changed, conversion increases using longer residence times, 
higher ratios of base to halogenated hydrocarbon, and temperatures at or 
above 200.degree. C.