Historical Use of Chlorinated Solvents and Their Stabilizing Compounds - Environmental Investigation and Remediation - page 50

1.66 V, that is, it is strongly reduc-
ing. The insoluble oxide coating on aluminum will by itself prevent reaction with methyl chloro-
form; clean but oxide-coated aluminum dipped into methyl chloroform produces no reaction.
However, any scratches in the aluminum oxide surface produce sites for microcorrosion to begin,
which leads to the degradation of the solvent and the consumption of aluminum in the catastrophic
decomposition reaction.
Methyl chloroform is relatively immune to the free radical mechanism for oxidation reactions;
however, methyl chloroform can react violently with exposed aluminum or aluminum chloride
salts. Methyl chloroform will also react with iron or tin in a similar fashion, but the corrosion rates
are much slower than the reaction with aluminum (Archer and Stevens, 1977). Dichloromethane
and, to some degree, TCE are also susceptible to degradation in the presence of aluminum chloride
salts or aluminum swarf (aluminum i nes), particularly in the vapor phase. Perchloroethylene
does not appear to have any signii cant reactivity toward aluminum (EuroChlor, 2003). A variety
of metal stabilizers that are Lewis bases are added to chlorinated solvents to prevent or inhibit
these reactions.
For inhibition to occur, there should be one or preferably two electronegative functional
groups in the molecular structure of the selected metal inhibitor. Nitrogen functional groups
have greater activity than suli de groups, which in turn have greater activity than ether com-
pounds. The activity of the functional group in this hierarchy is in reference to the ability to
complex with Lewis acids such as aluminum chloride. Cyclic structures such as 1,4-dioxane and
1,3-dioxolane are generally more active than their straight-chain analogues. Compounds
commonly used as inhibitors incorporate ether linkages, suli de linkages, carbonyl groups,
nitriles, amines, and alcohols (Archer, 1982). Because 1,4-dioxane has two ether linkages, it is
considered “difunctional” and exhibits greater activity than a compound with a single ether link-
age such as tetrahydropyran.
Compounds found to be effective metal inhibitors for methyl chloroform are not necessarily
effective for other chlorinated solvents. For example, the hierarchy of functional groups in a mixture
of 90% dichloromethane and 10% toluene (a common aerosol spray formulation) is opposite the
hierarchy for methyl chloroform metal inhibitors (Archer, 1982). Other solvents require larger quan-
tities of the same stabilizers that are effective for methyl chloroform. For example, 1,1,2-trichloro-
ethane is easily stabilized by furfuryl alcohol and dimethyl oxalate, whereas 1,1-dichloroethane can
be stabilized by furfuryl alcohol, dimethyl oxalate, pyrazine, glycidol, or 1,4-dioxane. Stabilizers
preferred for the commonly used dichloromethane-toluene aerosol aluminum paint formulation
contain an oxygen functional group, such as dimethoxymethane, dimethyl carbonate, or 1,4-dioxane
(Archer, 1979).
The ability of solvent stabilizers selected to inhibit the aluminum

Aluminum is a highly reactive metal; its standard potential is −

methyl chloroform reaction
to protect the solvent from breakdown, ordered from best to worst, is 1,4-dioxane

+

> nitromethane > tert -

1,2-butylene oxide * (van Gemert, 1982). However, when aluminum powder is added
to a commercial formulation of methyl chloroform containing all these stabilizers, the i rst stabi-
lizer to react is 1,2-butylene oxide, followed by tert -amyl alcohol. The reaction of aluminum with
1,4-dioxane and nitromethane is slow in comparison, but all of these stabilizers are more reactive
toward aluminum than toward methyl chloroform. An explanation for this difference notes that
epoxides † and alcohols form reaction products with aluminum chloride and are consumed, whereas
1,4-dioxane and nitromethane form complexes and are preserved. The 1,4-dioxane-aluminum
chloride complex is highly insoluble; however, 1,2-butylene oxide will dissolve it and release the

amyl alcohol >

* 1,2-Butylene oxide, the most widely used acid acceptor for methyl chloroform, can also be considered a Lewis base (van
Gemert, 1982).
† 1,2-Butylene oxide is an epoxide.