Historical Use of Chlorinated Solvents and Their Stabilizing Compounds - Environmental Investigation and Remediation - page 36

violent and produce a considerable amount of hydrochloric acid and heat. Heat accelerates the reac-
tion of condensation and evaporates some of the remaining solvent. AlCl 3 has a catalytic effect on
TCE, dichloromethane, and especially methyl chloroform (Kauder, 1960). AlCl 3 is generally con-
sidered to have no catalytic effect on pure perchloroethylene (Solvay SA, 2002a). However, in earlier
decades, impurities in perchloroethylene may have been carried over from production and reacted
with alkali metals.
Methyl chloroform reacts with aluminum (AlCl 3 ), splitting off hydrochloric acid and leaving
1,1-dichloroethylene, shown here in a generalized reaction:

AlCl 3

CH 3 CCl 3 CH 2 CCl 2 + l

Dehydrohalogenation
Methyl chloroform

(1.5)

1,1-dichloroethane Hydrochloric acid

Aluminum surfaces react with oxygen to form a protective aluminum oxide coating. If a machin-
ing, abrasive, or cleaning process removes this coating, the “virgin” metal is available to react with
a solvent. The aluminum reaction with methyl chloroform is most easily observed in the “aluminum
scratch test,” as described in Section 1.2.5.2 . Hydrochloric acid is formed during the scratch test and
dissociates easily. The chlorine ions react with aluminum in the following sequence:
Hydration of oxide i lm:

3

__

2AlO 3 + 3H 2 O → 2Al(OH) 3 +

2 O 2 .

(1.6)

Reaction of hydrated oxide i lm and chlorine (dissolution of i lm):

Al(OH) 3 + 3HCl → AlCl 3 + 3H 2 O. (1.7)

Reaction of aluminum and hydrochloric acid (dissolution of aluminum):

3

__

Al + 3HCl → AlCl 3 +

2 H 2 .

(1.8)

Precipitation of aluminum hydroxide:

AlCl 3 + 3H 2 O → Al(OH) 3 + 3HCl. (1.9)

The compounds produced by the reactions are aluminum hydroxide and hydrochloric acid;
hydrochloric acid is not consumed and acts as a catalyst (Cornell Dubilier, 2003).
TCE, dichloromethane, and methyl chloroform react with aluminum to give aluminum chloride
and oligomers of the solvent (a few solvent molecules bonded together). For TCE, the aluminum
chloride goes on to react with the solvent to form hydrogen chloride, which acts as a catalyst and is
not itself consumed by the reaction. Once initiated, this self-accelerating, “autocatalytic” dehydro-
chlorination (loss of hydrogen chloride) reaction produces a vigorous subsequent reaction between
TCE and aluminum. Aluminum chloride does not have a similar reaction with dichloromethane
(EuroChlor, 2003).
Aluminum chloride promotes a dimerization reaction with TCE to produce hydrochloric acid:

AlCl 3

Cl 2 C —

—

CHCl Cl 2 CHCCl 2 CH CCl 2

(1.10)

Trichloroethylene

— — —

Cl 2 CHCCl 2 CH CCl 2 CCl 2 CClCH CCl 2 + HCl

(1.11)

Dimerization