Ind. Eng. Chem. hod. Res. Dev. 1982, 27, 120-122
120
CompatibHtty of Aluminum Alkyls and Chlorinated Hydrocarbons WlHord H. Thomas Ethyl ~
a
~Baton n Rouge, , Louisiana 70821
Mixing aluminum alkyls with chlorinated hydrocarbons (e.g., CCi,) has been known to be a hazardous procedure capable of leading to uncontrolled decompositions. To get a broader feel for the relative reactivity of various alkyl halides with the m e common akrmlnum alkyls and to allow assignment of m e quantitative values to the energies invoked, a series of experiments were conducted on mlxtures of triethylaluminum, diethylaiuminum chloride, ethylaluminum dichloride, trlmethylaluminum, dimethylaluminum chloride, and triisobutylaluminum with such chiorocarbons as methylene chlorkle, chloroform, and ethylene dichloride. Trends found in the order of alkyl halide reactivity wlth organoalumhum compounds generally are: primary < secondary < tertiary. Addiinally, reactivity of chlorinated methanes increases with increasing chlorine content. Chloroform and carbon tetrachloride are especlally hazardous and probably should never be mixed with aluminum alkyls.
Introduction Much of the knowledge of the reactions of aluminum alkyls with alkyl halidea has evolved from extensive studies of the mechanism of cationic polymerization using R3AlRCl initiator systems (Kennedy, 1975), as well as in a specific search for RaAl-compatible solvents for use in organic synthesis (Reinheckel et al., 1979). Although them studies were carried out generally without serious problems from uncontrolled runaway reactions, aluminum alkylalkyl halide systems must be considered potentially hazardous. As a manufacturer of aluminum alkyls, Ethyl Corporation is constantly aware of the potential hazard in both small-scale lab experiments and large-scale commercial processes in which mixtures of aluminum alkyls and chlorinated hydrocarbons are used. This article is an attempt to define the scope and nature of the hazards involved in the use of such combinations. Thermal Considerations One can write balanced chemical equations for the reactions of chlorinated hydrocarbons with aluminum alkyls to give "maximum energy release" products (Stull, 1977) for which the heats of reaction can be estimated. For example, the reactions of carbon tetrachloride, chloroform, and methylene chloride with triethylaluminum (TEA)are given in Table I together with the corresponding heats of reaction. The heats of formation (AHof)of aluminum alkyls used in these and similar calculations are those determined by Smith (1974). The heats of reaction in Table I are given both in terms of kcal/mol of TEA as well as in cal/g of reactant mixture. This allows a direct comparison of the maximum energy of reaction of TEA/RCl mixtures with the detonation energy of high explosives such as TNT. Thus the reaction of TEA with CCl.,, CHC13, and CHzClzgives a maximum energy release of 707,719, and 685 cal/g of reactant mixture, respectively, compared to the measured heat of decomposition of TNT of about 1100 cal/g (Stull, 1977). The above is not intended to imply that ~ A . l / R C mixtures l are, or even might be, high explosives. Rather such maximum energy calculations serve the useful purpose of indicating that there exists a real probability (however small if may be) that under some set of conditions an uncontrolled, runaway reaction may occur. That some mixturea may be used safely in chemical processes is a consequence of the kinetic parametersprobably a high activation energy barrier-of the decomposition reaction. The work reported here was conducted in order to better establish the conditions under which
Table I. Triethylaluminum Reaction with Chloromethanes (C,H,),Al
+ '/,CCl,
AlCl,(s)
+ 's/,CH,(g) + 3C(s)
AH= -162.4 kcal/mol of TEA
= -707 cal/g of reaction mixture
AlCl,(s) + 4CH,(g) + 3C(s) AH = -168.0 kcal/mol of TEA = -719 c d / g of reactant mixture
(C,H,),AI
+ CHCl,
(C,H,),Al
+ 3/ICH,Cl,
3AiCl, + "/,CH, + 13/,C AH = -165.5 kcal/mol of TEA = -685 cd/g of reactant mixture
rapid, high energy-release reactions might be encountered. Reactivity Trends It is generally accepted that aluminum alkyls and chlorinated hydrocarbons tend to form complexes. One mechanism postulates an electrophilic process generating what may be looked on as a carbonium counter ion pair, a free carbonium ion, or a polar complex EhA1+ RX + [R]+[Et3AlXIwhich then may react further depending on the reaction conditions and the nature of the reactants (Miller, 1966). If aromatic compounds are present, electrophilic attack on the aromatic ring (Friedel-Crafts reaction) is likely. When aromatics are absent or the Friedel-Crafts reaction is inhibited, electrophilic attack then proceeds at the tetracoordinated aluminum complex via several alternative pathways. Attack on halogen forms a halohydrocarbon (starting material or an isomer). Alkylation occurs by attack of R+ on the A1-C bond. The transfer of a 8-hydrogen from the complex to R+ results in reduction, and transfer of a proton to the complex from R+ results in dehydrohalogenation. Within the context of the above reaction scheple, the order of increasing reactivity of triethylaluminum with dkyl halides is primary < secondary < tertiary, and for the tertiary halides it is I < Br < C1. In general, higher alkyl chlorides are more reactive than ethyl chloride, which in turn is more reactive than methyl chloride. For example, the following order of reactivity with %Al compounds has been demonstrated: MeCl < EtCl< iPrCl< n-BuC1 < M%EtCCl. Also, for a given alkyl chloride, the reactivity of organoaluminum compounds increases with increasing Lewis acidity (Pasynkiewin and Kuran, 1969;Pozamantir and Genusov, 1962; Kennedy, 1970): RBAl< RzAICl < RAlClp 0 1982 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 1, 1982
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Table 11. Aluminum Alkyl-Chlorocarbon Mixture Reaction Data increasing temp, 4-5 "C/min alum. alkyla TMA DMAC MADC TEA DEAC EADC DEAC DEAC DEAC DEAC DEAC TEA TEA TEA TEA TIBA
temp, "C chlorocarbon CH,Cl, CH,Cl, CH,Cl, CH,Cl, CH,Cl, CH,Cl, CH,Cl, CHC1, CCl, C,H,Cl CCl,=CCl, CH,Cl, C,H,Cl CH ,Cl-CH ,C1 C,H,Cl CH,Cl,
initb 237 235 148 183 170 147 170 125 23 145
peak 365 352 192 292 280 248 280 300
183 190 212
292 357 317
183
252
234
press., Psig > 2000 > 2000 320 2950 >1300 200 > 1300 2500
constant temp rate, psils > 5000 >1600 80 13000 5000 15 5000 20000
temp, "C init b peak
press., Psig
148
275
150
142 142
310 193
500 120
142 94
193 300
120 850
310
500
275 10 no reaction to 260 "C 2950 13000 142 1250 370 1800 2860 no reaction to 265 "C 590 310
Abbreviations for aluminum alkyls used in this paper: TMA, (CH,),Al, trimethylaluminum; DMAC, (CH,),AlCl, dimethylaluminum chloride; MADC, CH,AlCl,, methylaluminum dichloride; TEA, (C,H,),Al, triethylaluminum; DEAC, ~C2H5),AlCl, diethylaluminum chloride; EADC, C,H,AlCl,, ethylaluminum dichloride; TIBA, (i-Bu),Al, triisobutylaluminum. Temperature at which decomposition process becomes self-sustaining, i.e., the exotherm temperature. a
Table 111. Examples of the Stoichiometry of Aluminum Alkyl-Chlorocarbon Mixtures and Assumed Reaction Products
----
(CH,),Al t 3CH,Cl, -AlCl, t 3HC1 t 3CH, t 3C (CH,),AlCl t 2CH,Cl, AlCl, t 2HC1 t 2CH, t 2C (CH,)AlCl, t CH,Cl, AlC1, t HCl t CH, t C (C,H,),Al t 3CH,Cl, AlCl, t 3HC1 t g/,CH, t 9/,C AlCl, t 2HC1 t 3CH, t 3C (C,H,),AlCl t 2CH,Cl, (C,H,)AlCl, t CH,Cl, -AlCl, t HC1 + 3/,CH, t 3/,C (C,H,),Al t 3CH,Cl, +AlCl, t 3HC1 t "/aCH, t q 4 c
The reaction of vinyl chloride with methylaluminum (Kennedy, 1970) and ethylaluminum (Raxuvaev et al., 1966) compounds has been studied. Vinyl chloride appears to be somewhat less reactive toward triethylaluminum than is ethyl chloride. Trimethylaluminum (TMA) is relatively more strongly associated (dimerized) than is triethylaluminum, and triisobutylaluminum (TIBA) exists mostly as a monomer. Thus TMA forms complexes with alkyl halides less readily than do other trialkylaluminum compounds and tends to be less reactive (Kennedy, 1970; Kennedy and Milliman, 1969; Kennedy and Melby, 1975): TMA < TEA < TIBA. The relative order of reactivity of the chloromethanes with aluminum alkyls increases with chlorine content (Reinheckle and Gensike, 1968; Priola et al., 1972): CH&l < CH2C12< CHC13
